Bandwidth Extension Encoder, Bandwidth Extension Decoder and Phase Vocoder
Description
The present invention relates to audio signal processing and, in particular, to a bandwidth
extension encoder, a method for encoding an audio signal, a bandwidth extension decoder,
a method for decoding an encoded audio signal, a phase vocoder and an audio signal.
Moreover, embodiments of the present invention relate to an application of a phase
vocoder for pure time stretching, independent of a bandwidth extension.
Storage or transmission of audio signals is often subject to strict bit rate constraints. These
constraints are usually accounted for by the use of encoders/decoders ("codecs") that
efficiently compress the audio signal in terms of the information rate necessary to store or
transmit the signal. In the past, coders were forced to drastically reduce the audio
bandwidth when only a very low bit rate was available. Modern audio codecs are able to
code wide-band signals by using bandwidth extension (BWE) methods, as described in M.
Dietz, L. Liljeryd, K. KjÖrling and O. Kunz, "Spectral Band Replication, a novel approach
in audio coding," in 112th AES Convention, Munich, May 2002; S. Meltzer, R. Btfhm and
F. Henn, "SBR enhanced audio codecs for digital broadcasting such as "Digital Radio
Mondiale" (DRM)," in 112th AES Convention, Munich, May 2002; T. Ziegler, A. Ehret,
P. Ekstrand and M. Lutzky, "Enhancing mp3 with SBR: Features and Capabilities of the
new mp3PRO Algorithm," in 112th AES Convention, Munich, May 2002; International
Standard ISO/IEC 14496-3:2001/FPDAM 1, "Bandwidth Extension," ISO/IEC, 2002;
"Speech bandwidth extension method and apparatus", Vasu Iyengar et al. US Patent
5,455,888; E. Larsen, R. M. Aarts, and M. Danessis. Efficient high-frequency bandwidth
extension of music and speech. In AES 112th Convention, Munich, Germany, May 2002;
R. M. Aarts, E. Larsen, and O. Ouweltjes. A unified approach to low- and high frequency
bandwidth extension. In AES 115th Convention, New York, USA, October 2003; K.
KSyhkÖ. A Robust Wideband Enhancement for Narrowband Speech Signal. Research
Report, Helsinki University of Technology, Laboratory of Acoustics and Audio Signal
Processing, 2001; E. Larsen and R. M. Aarts. Audio Bandwidth Extension - Application to
psychoacoustics, Signal Processing and Loudspeaker Design. John Wiley & Sons, Ltd,
2004; E. Larsen, R. M. Aarts, and M. Danessis. Efficient high-frequency bandwidth
extension of music and speech. In AES 112th Convention, Munich, Germany, May 2002;
J. Makhoul. Spectral Analysis of Speech by Linear Prediction. IEEE Transactions on
Audio and Electroacoustics, AU-21(3), June 1973; United States Patent Application
08/951,029, Ohmori, et al. Audio band width extending system and method; United States

Patent 6895375, Malah, D & Cox, R. V.: System for bandwidth extension of Narrow-band
speech and Frederik Nagel, Sascha Disch, "A harmonic bandwidth extension method for
audio codecs," ICASSP International Conference on Acoustics, Speech and Signal
Processing, IEEE CNF, Taipei, Taiwan, April 2009.
These algorithms rely on a parametric representation of the high-frequency content (HF).
This representation is generated from the low-frequency part (LF) of the decoded signal by
means of transposition into the HF spectral region ("patching") and application of a
parameter driven post processing.
In the art, methods of bandwidth extension such as spectral band replication (SBR) or
harmonic bandwidth extension (HBE) are known. In the following, these two BWE
methods are briefly described.
On the one hand, spectral band replication (SBR), as described in M. Dietz, L. Liljeryd, K.
KjÖrling and O. Kunz, "Spectral Band Replication, a novel approach in audio coding," in
112th AES Convention, Munich, May 2002, uses a quadrature mirror filterbank (QMF) for
generating the HF information. Applying a so-called "patching" algorithm, lower QMF
band signals are copied into higher QMF bands, leading to a replication of the information
of the LF part in the HF part. Subsequently, the generated HF part is adapted to closely
match the original HF part with the help of parameters that adjust the spectral envelope and
the tonality.
On the other hand, harmonic bandwidth extension (HBE) is an alternative bandwidth
extension scheme based on phase vocoders. HBE enables a harmonic continuation of the
spectrum as opposed to SBR, which relies on a non-harmonic spectral shift. It may be
utilized to replace or amend the SBR patching algorithm.
US Provisional Patent Application with the application number US 61/079,841 discloses a
BWE method, which may choose between alternative patching algorithms that operate
either in frequency domain or in time domain. In the time-frequency transform by the
filterbank, a certain predetermined analysis window is applied. Moreover, classic phase
vocoder implementations according to the state-of-the-art use one predefined window
shape such as a raised-cosine window or a Bartlett window.
However, choosing one predetermined analysis window for vocoder applications always
encompasses a trade-off to be made by the application designer in terms of overall
perceptual audio quality achieved for different classes of audio signals. Thus, although the

mean audio quality can be optimized by the initial choice of a certain window, the audio
quality for each individual class of signals remains to be sub-optimal.
Moreover, it was found that certain signals benefit from using specialized analysis
windows for a phase vocoder, which may especially be used for temporally spreading the
audio signal without modifying the pitch of the same.
Therefore, a concept for selecting the optimal analysis windows such as within a BWE
scheme is required. However, measures against the just-mentioned degradation of the
perceptional audio quality should preferably not result in a significantly increased
computational complexity of the employed codecs.
It is the object of the present invention to provide an encoding and/or decoding concept or
phase vocoder concept providing an improved audio quality.
This object is achieved by a bandwidth extension encoder according to claim 1, a
bandwidth extension decoder according to claim 2, a phase vocoder according to claim 12,
a method for encoding according to claim 13, a method for decoding according to claim
14, an encoded audio signal according to claim 15 or a computer program according to
claim 16.
An idea underlying the present invention is that an improved perceptual quality can be
achieved when the audio signal having a block of audio samples with a specified length in
time is analyzed in order to determine from a plurality of analysis windows an analysis
window to be used for performing a bandwidth extension in a bandwidth extension
decoder. By this measure, the reduction of the audio quality resulting from the application
of a predetermined analysis window may be prevented and, consequently, the perceptual
audio quality may be improved with relatively low efforts as compared to prior art BWE
methods.
According to an embodiment of the present invention, a bandwidth extension encoder for
encoding an audio signal comprises a signal analyzer, a core encoder and a parameter
calculator. The audio signal comprises a low frequency signal comprising a core frequency
band and a high frequency signal comprising an upper frequency band. The signal analyzer
is configured for analyzing the audio signal, the audio signal having a block of audio
samples, the block having a specified length in time. The signal analyzer is furthermore
configured for determining from a plurality of analysis windows an analysis window to be
used for performing a bandwidth extension in a bandwidth extension decoder. The core

encoder is configured for encoding the low frequency signal to obtain an encoded low
frequency signal. The parameter calculator is configured for calculating bandwidth
extension parameters from the high frequency signal.
According to another embodiment of the present invention, a bandwidth extension decoder
for decoding an encoded audio signal comprises a core decoder, a patch module and a
combiner. The encoded audio signal comprises an encoded low frequency signal and upper
band parameters. The core decoder is configured for decoding the encoded low frequency
signal, wherein the decoded low frequency signal comprises a core frequency band. The
patch module is configured to generate a patched signal based on the decoded low
frequency signal and the upper band parameters, wherein the patched signal comprises an
upper frequency band generated from the core frequency band. The combiner is configured
to combine the patched signal and the decoded low frequency signal to obtain a combined
output signal.
According to another embodiment, a phase vocoder processor for processing an audio
signal comprises an analysis windower, a time/spectrum converter, a frequency domain
processor, a frequency/time converter, a synthesis windower, a comparator and an overlap
adder. The analysis windower is configured for applying a plurality of analysis window
functions to the audio signal or a signal derived from the audio signal, the audio signal
having a block of audio samples, the block having a specified length in time, to obtain a
plurality of windowed audio signals. The time/spectrum converter is configured for
converting the windowed audio signals into spectra. The frequency domain processor is
configured for processing the spectra in a frequency domain to obtain modified spectra.
The frequency/time converter is configured for converting the modified spectra into
modified time domain signals. The synthesis windower is configured for applying a
plurality of synthesis window functions to the modified time domain signals, wherein the
synthesis window functions are matched to the analysis window functions, to obtain
windowed modified time domain signals. The comparator is configured to determine a
plurality of comparison parameters based on a comparison of the plurality of windowed
modified time domain signals and the audio signal or a signal derived from the audio
signal, wherein the plurality of comparison parameters corresponds to the plurality of
analysis window functions. The comparator is furthermore configured to select an analysis
window function and a synthesis window function for which a comparison parameter
satisfies a predetermined condition. The overlap adder is configured for adding
overlapping blocks of a windowed modified time domain signal to obtain a temporally
spread signal. The overlap adder is furthermore configured for processing blocks of the

windowed modified time domain signal having been modified by an analysis window
function and a synthesis window function selected by the comparator.
Embodiments of the present invention are based on the concept that a plurality of patched
signals may be generated from a plurality of analysis window functions applied to the
audio signal comprising the core frequency band. The plurality of patched signals may be
compared with a reference signal being the original audio signal or a signal derived from
the audio signal. This will result in a plurality of comparison parameters, which may be
related to measures of the audio quality. Furthermore, from the plurality of analysis
window functions, an analysis window function may be selected for which a comparison
parameter satisfies a predetermined condition. Therefore, the use of the selected analysis
window function may ensure minimal reduction of the audio quality, leading to optimal
perceptual audio quality in the context of a BWE scenario.
Other embodiments of the present invention relate to a signal analyzer comprising a signal
classifier, wherein the signal classifier is configured to analyze/classify the audio signal or
a signal derived from the audio signal. In this case, the analysis window function to be
used for performing a bandwidth extension in the bandwidth extension decoder is selected
based on a signal characteristic of the analyzed/classified signal.
Therefore, embodiments provide a method of selecting the optimal analysis window for the
bandwidth extension in the decoder. Control parameters may be evaluated in order to
decide which analysis window is the most appropriate. To achieve this, an analysis-by-
synthesis scheme may be used; i.e. a set of windows may be applied and the best according
to a suitable objective is chosen. In the preferred mode of the invention, the objective is to
ensure optimal perceptual audio quality of the restitution. In alternative modes, an
objective function may be optimized. For example, the objective may be to preserve the
spectral flatness of the original HF as close as possible.
On the one hand, the window selection can be done only at the encoder by considering the
original signal, the synthesized signal or both of them. A decision (window indication) is
then transmitted to the decoder. On the other hand, the selection may be performed
synchronously at the encoder and the decoder side considering only the core bandwidth of
the decoded signal. The latter method is not in need to generate additional side
information, which is favorable in terms of bitrate efficiency of the codec.
The invention is advantageous in that it optimizes the perceptual quality of the vocoder
output signal. Embodiments provide a signal adaptive choosing of appropriate analysis and

synthesis windows for the vocoding process, wherein different time responses or frequency
responses of the analysis and/or synthesis windows are possible.
Another advantage of the invention is that it enables a better trade-off between reduction of
the above-mentioned degradation and the computational complexity such as within a BWE
scheme.
In the following, embodiments of the present invention are explained with reference to the
accompanying drawings, in which:
Fig. 1 shows a block diagram of an embodiment of a bandwidth extension
encoder;
Fig. 2 shows a block diagram of an embodiment of a bandwidth extension
decoder;
Fig. 3 shows a block diagram of a further embodiment of a bandwidth extension
encoder;
Fig. 4 shows a block diagram of a further embodiment of a bandwidth extension
decoder;
Fig. 5 shows a block diagram of a further embodiment of a bandwidth extension
encoder;
Fig. 6 shows a block diagram of a further embodiment of a bandwidth extension
decoder;
Fig. 7 shows a block diagram of an implementation of a comparator;
Fig. 8 shows a block diagram of a further embodiment of a bandwidth extension
encoder;
Fig. 9 shows a block diagram of an implementation of a signal classifier;
Fig. 10 shows a block diagram of a further embodiment of a bandwidth extension
encoder;

Fig. 11 shows a block diagram of a further embodiment of a bandwidth extension
decoder;
Fig. 12 shows a block diagram of an embodiment of a phase vocoder processor;
Fig. 13 shows a block diagram of an embodiment of an apparatus for switching
between different analysis and synthesis windows dependent on control
information; and
Fig. 14 shows an overview of an embodiment of a phase vocoder driven bandwidth
extension decoder.
Fig. 1 shows a block diagram of a bandwidth extension encoder 100 for encoding an audio
signal 101-1 according to an embodiment of the present invention. The audio signal 101-1
comprises a low frequency signal 101-2 comprising a core frequency band 101-3 and a
high frequency signal. 101-4 comprising an upper frequency band 101-5. The bandwidth
extension encoder 100 comprises a signal analyzer 110, a core encoder 120 and a
parameter calculator 130. The signal analyzer 110 is configured for analyzing the audio
signal 101-1, the audio signal 101-1 having a block 101-6 of audio samples, the block 101-
6 having a specified length in time. The signal analyzer 110 is furthermore configured for
determining from a plurality 111-1 of analysis windows an analysis window 111-2 to be
used for performing a bandwidth extension such as in the bandwidth extension decoder
200. The core encoder 120 is configured for encoding the low frequency signal 101-2 to
obtain an encoded low frequency signal 121. Finally, the parameter calculator 130 is
configured for calculating bandwidth extension parameters 131 from the high frequency
signal 101-4. The bandwidth extension parameters 131, the analysis window 111-2 to be
used in the bandwidth extension decoder 200 and the encoded low frequency signal 121
constitute an encoded audio signal 103-1 provided by the bandwidth extension encoder
100.
Fig. 2 shows a block diagram of a bandwidth extension decoder 200 for decoding an
encoded audio signal 201-1 according to another embodiment of the present invention. The
encoded audio signal 201-1 comprises an encoded low frequency signal 201-2 and upper
band parameters 201-3. Here, the encoded audio signal 201-1 may correspond to the
encoded audio signal 103-1 as provided by the bandwidth extension encoder 100 shown in
Fig. 1. The bandwidth extension decoder 200 comprises a core decoder 210, a patch
module 220 and a combiner 230. The core decoder 210 is configured for decoding the
encoded low frequency signal 201-2 to obtain a decoded low frequency signal 211-1. The

decoded low frequency signal 211-1 comprises a core frequency band 211-2. The patch
module 220 is configured to generate a patched signal 221-1 based on the decoded low
frequency signal 211-1 and the upper band parameters 201-3, wherein the patched signal
221-1 comprises an upper frequency band 221-2 generated from the core frequency band
211-2. Finally, the combiner 230 is configured to combine the patched signal 221-1 and the
decoded low frequency signal 211-1 to obtain a combined output signal 231-1. In
particular, the patched signal 221-1 may be a signal in a target frequency range of a
bandwidth extension algorithm, while the combined output signal 231-1 provided by the
bandwidth extension decoder 200 may be a manipulated signal with an extended
bandwidth (231-2).
Fig. 3 shows a block diagram of a further embodiment of a bandwidth extension encoder
300. The bandwidth extension encoder 300 may comprise a low pass (LP) filter and a high
pass (HP) filter. The filters may be implemented to generate a low pass filtered version of
the audio signal 101-1 being the low frequency signal 101-2 and a high pass filtered
version of the audio signal 101-1 being the high frequency signal 101-4. As shown in Fig.
3, the bandwidth extension encoder 300 may further comprise a window controller 310 for
providing window control information 311 to be used by a parameter calculator 320 and a
patch module 330. The window control information 311 provided by the window
controller 310 may indicate a plurality 111-1 of analysis window functions to be applied to
the block 101-6 of audio samples derived from the audio signal 101-1. The parameter
calculator 320, in particular, may comprise a windower controlled by the window
controller 310, wherein the windower of the parameter calculator 320 is configured to
apply the plurality 111-1 of analysis window functions and an analysis window function
111-2 to be selected by a comparator 340 to the high frequency signal 101-4. Here,
bandwidth extension parameters 321-1, 321-2 corresponding to the plurality 111-1 of
analysis window functions as indicated by the window control information 311 and
corresponding to the selected analysis window function 111-2 as provided by a window
indication 340-1 at the output of the comparator 340 are obtained, respectively.
In the embodiment shown in Fig. 3, the signal analyzer 110 comprises a patch module 330,
which is configured to generate a plurality 331-1 of patched signals based on the low
frequency signal 101-2, the window control information 311 and the bandwidth extension
parameters 321-1. Here, the patched signals 331-1 comprise upper frequency bands 331-2
generated from the core frequency band 101-3. The patch module 330, in particular,
comprises a windower controlled by the window controller 310, wherein the windower of
the patch module 330 is configured for applying the plurality 111-1 of analysis window
functions to the low frequency signal 101-2.

Furthermore, the signal analyzer 110 of the bandwidth extension encoder 300 comprises a
comparator 340, which is configured to determine a plurality 341-2 of comparison
parameters based on a comparison of the patched signals 331-1 and a reference signal
being the audio signal 101-1 or a signal derived from the audio signal such as the high
frequency signal 101-4 indicated by the dashed line, wherein the plurality 341-2 of
comparison parameters corresponds to the plurality 111-1 of analysis window functions.
The comparator 340 is furthermore configured to provide a window indication 341-1
corresponding to an analysis window function 111-2, for which a comparison parameter
satisfies a predetermined condition. Finally, the bandwidth extension encoder 300
comprises an output interface 350 for providing an encoded audio signal 351, the encoded
audio signal 351 comprising the window indication 341-1.
With regard to an implementation of the above comparison, Fig. 7 shows a block diagram
of an embodiment of a comparator 700, which may comprise a spectral flatness measure
(SFM) parameter calculator 710, an SFM parameter comparator 720 and a window
indication extractor 730. The SFM parameter calculator 710 may be implemented to
calculate, for example, a plurality 703-1 of SFM parameters from a plurality 701-1 of input
signals and a reference SFM parameter 703-2 from a reference input signal 701-2. In
particular, each SFM parameter may be calculated by dividing the geometric mean of the
power spectrum by the arithmetic mean of the power spectrum derived from the
corresponding input signal, wherein a relatively high SFM parameter indicates that the
spectrum has a similar amount of power in all spectral bands, while a relatively low SFM
parameter indicates that the spectral power is concentrated in a relatively small number of
bands. In addition, the SFM parameter can also be measured within a certain partial band
(subband) rather than across the whole band of the input signal. The SFM parameter
comparator 720 may be implemented to compare the SFM parameters 703-1 with the
reference SFM parameter 703-2 to obtain a plurality 705 of comparison parameters,
wherein the comparison parameters 705 may, for example, be based on the deviations in
the compared SFM parameters. The window indication extractor 730 may be implemented
to select, from the plurality of comparison parameters 705, a comparison parameter, for
which a predetermined condition will be satisfied. The predetermined condition may, for
example, be chosen such that the selected comparison parameter will be a rninimum of the
plurality of comparison parameters 705. In this case, the selected comparison parameter
will correspond to an input signal from the plurality of input signals 701-1, which is
characterized by a minimum deviation from the reference input signal 701-2 in terms of
spectral flatness.

Specifically, the input signals 701-1 may correspond to the patched signals 331-1, the
patched signals 331-1 having been obtained after applying the plurality 111-1 of analysis
window functions to the audio signal 101-1 or a signal derived from the audio signal 101-1
such as the low frequency signal 101-2, while the reference input signal 701-2 may
correspond to the original audio signal 101-1. Furthermore, the plurality 705 of comparison
parameters of the comparator 700 may correspond to the plurality 341-2 of comparison
parameters of the bandwidth extension encoder 300. Therefore, an analysis window
function 111-2 may be selected corresponding to the selected comparison parameter in that
a deviation in the SFM parameters of the patched signals 331-1 and the original audio
signal 101-1, for example, will be minimal. The selected analysis window function 111-2
may also be referenced to by a window indication 707, which may correspond to the
window indication 341-1, provided at the output of the comparator 700 or the comparator
340, respectively. Consequently, the perceptual audio quality as measured by a spectral
flatness, for example, will be changed or reduced as less as possible when the selected
analysis window function 111-2 is chosen for performing a bandwidth extension such as
within a bandwidth extension decoder.
Moreover, the plurality 111-1 of analysis window functions indicated by the window
control information 311 at the output of the window controller 310 may comprise different
analysis window functions having different window characteristics having the same
window length as the block 101-6 in time. In particular, the different analysis window
functions may be characterized by different frequency response functions ("transfer
functions") obtained from a spectral analysis. The transfer functions, in turn, can be
distinguished by characteristic features such as their main lobe widths, side lobe levels or
side lobe fall-offs. The different analysis window functions may also be divided into
several groups with regard to their performance characteristics such as spectral resolution
or dynamic range. For example, high and moderate resolution windows may be represented
by rectangular, triangular, cosine, raised-cosine, Hamming, Hann, Bartlett, Blackman,
Gaussian, Kaiser or Bartlett-Hann window functions, while low resolution or high dynamic
range windows may be represented by flat-top, Blackman-Harris or Tukey window
functions. In alternative embodiments, it may also be possible to use window functions
having a different number of samples (i.e. windows of different window lengths).
Specifically, applying different analysis window functions 111-1, which may belong to
different groups of analysis window functions, to the block 101-6 of audio samples by the
use of the patch module 330, for example, will result in patched signals 331-1 having
different characteristic features such as different SFM parameters.

Fig. 4 shows a block diagram of a further embodiment of a bandwidth extension decoder
400, which can explicitly make use of the window indication 341-1 as provided, for
example, by the bandwidth extension encoder 300 shown in Fig. 3. The bandwidth
extension decoder 400, in particular, is implemented to be operative on an encoded audio
signal 401-1 comprising, besides an encoded low frequency signal 401-2 and upper band
parameters 401-3, a window indication 401-4. Here, the encoded low frequency signal
401-2, the upper band parameters 401-3 and the window indication 401-4 may correspond
to the encoded low frequency signal 121, the bandwidth extension parameters 321-2 and
the window indication 341-1 output from the output interface 350 of the bandwidth
extension encoder 300, respectively. In the embodiment shown in Fig. 4, the bandwidth
extension decoder 400 comprises a core decoder 410, which may correspond to the core
decoder 210 of the bandwidth extension decoder 200, the core decoder 410 being
configured for decoding the encoded low frequency signal 401-2, wherein the decoded low
frequency signal 411-1 comprises a core frequency band 411-2. Furthermore, the
bandwidth extension decoder 400 comprises a patch module 420, which may correspond to
the patch module 220 of the bandwidth extension decoder 200, wherein the patch module
420 comprises a controllable windower for selecting an analysis window function from a
plurality of analysis window functions based on the window indication 401-4 and for
applying the selected analysis window function to the decoded low frequency signal 411-1.
In this way, a patched signal 421 will be obtained at the output of the patch module 420.
The patched signal 421 may further be combined with the low frequency signal 411-1 by a
combiner 430 such that a combined output signal 431 will be output from the bandwidth
extension decoder 400. Here, the patched signal 421, the decoded low frequency signal
411-1, the combiner 430 and the combined output signal 431 may correspond to the
patched signal 221-1, the decoded low frequency signal 211-1, the combiner 230 and the
combined output signal 231-1, respectively. As before, the combined output signal 431
may be a manipulated signal with an extended bandwidth.
With regard to Figs. 3 and 4, it may be advantageous that the window indication 341-1;
401-4 corresponding to an optimum analysis window function having been obtained with a
signal analysis on the encoder side (Fig. 3), can be transmitted within the encoded audio
signal 351; 401-1 and subsequently be used by the patch module 420 such that a bandwidth
extension can be performed without requiring a further signal analysis on the decoder side
(Fig. 4).
Fig. 5 shows a block diagram of a further embodiment of a bandwidth extension encoder
500. The bandwidth extension encoder 500 essentially comprises the same blocks as the
bandwidth extension encoder 300 in Fig. 3. Therefore, identical blocks having similar

implementations and/or functions are denoted by the same numerals. However, contrary to
the embodiment shown in Fig. 3, the bandwidth extension encoder 500 comprises a
comparator 510, which is configured to compare the plurality of patched signals 333-1
with a reference low frequency signal derived from the audio signal 101-1. The bandwidth
extension encoder 500 may optionally also comprise a core decoder 520, which is
implemented to provide a decoded low frequency signal 521 by decoding the encoded low
frequency signal 121 from the output of the core encoder 120. For the reference low
frequency signal, for example, the low frequency signal 101-2 being a low pass filtered
version of the audio signal 101-1 or the decoded low frequency signal 521 from the output
of the core decoder 520, may be used. Furthermore, the comparator 510 is configured to
provide a window indication 511 corresponding to a selected (optimum) analysis window
function, wherein, in this case, the window selection is based on the comparison of the
patched signals 331-1 with the reference low frequency signal 101-2 or 521. As with the
window indication 341-1 in the embodiment shown in Fig. 3, the window indication 511
can be supplied to the parameter calculator 320 such that only the BWE parameters 321-2
corresponding to the window indication 511 will be obtained. The BWE parameters 321-2,
together with the encoded low frequency signal 121, may be supplied to an output interface
530. Here, the window indication 511, however, may not be supplied to the output
interface 530. Finally, the output interface 530 is configured for providing an encoded
audio signal 531, the encoded audio signal 531 not comprising the window indication 511.
Fig. 6 shows a block diagram of a further embodiment of a bandwidth extension decoder
600. The bandwidth extension decoder 600, in particular, is implemented to be operative
on an encoded audio signal 601-1 comprising an encoded low frequency signal 601-2 and
upper band parameters 601-3. Here, the encoded audio signal 601-1, the encoded low
frequency signal 601-2 and the upper band parameters 601-3 may correspond to the
encoded audio signal 201-1, the encoded low frequency signal 201-2 and the upper band
parameters 201-3, respectively. Especially in the embodiment shown in Fig. 6, the encoded
audio signal 601-1, which is fed into the bandwidth extension decoder 600, does not
comprise a window indication. For this reason, a signal analysis with the objective of
selecting an appropriate window function to be applied such as within a bandwidth
extension scheme is required on the decoder side in this case (Fig. 6).
As shown in Fig. 6, the patch module 220 of the bandwidth extension decoder 600
comprises an analysis windower 610, a time/spectrum converter 620, a frequency domain
processor 630, a frequency/time converter 640, a synthesis windower 650, a comparator
660 and a bandwidth extension module 670. In addition, the bandwidth extension decoder
600 comprises a core decoder 680 for decoding the encoded low frequency signal 601-2,

wherein the decoded low frequency signal 681-1 comprises a core frequency band 681-2.
Here, the core decoder 680 and the decoded low frequency signal 681-1 may correspond to
the core decoder 210 and the decoded low frequency signal 211-1, respectively.
The analysis windower 610 is configured for applying a plurality of analysis window
functions such as the analysis window functions 111-1 in the embodiments of the
bandwidth extension encoders 300; 500 to the decoded low frequency signal 681-1 to
obtain a plurality 611 of windowed low frequency signals. The time/spectrum converter
620 is configured for converting the windowed low frequency signals 611 into spectra 621.
The frequency domain processor 630 is configured for processing the spectra 621 in a
frequency domain to obtain modified spectra 631. The frequency/time converter 640 is
configured for converting the modified spectra 631 into modified time domain signals 641.
The synthesis windower 650 is configured for applying a plurality of synthesis window
functions to the modified time domain signals 641, wherein the synthesis window
functions are matched to the analysis window functions, to obtain windowed modified time
domain signals 651. In particular, the synthesis window functions can be matched to the
analysis window functions such that applying the synthesis window functions will
compensate for the effect of the corresponding analysis window functions. The comparator
660 is configured to determine a plurality of comparison parameters based on a
comparison of the plurality 651 of windowed modified time domain signals and the
decoded low frequency signal 681-1, wherein the plurality of comparison parameters
corresponds to the plurality 111-1 of analysis window functions having been applied to the
decoded low frequency signal 681-1 by the analysis windower 610. The comparator 660 is
furthermore configured to select an analysis window function and a synthesis window
function for which a comparison parameter satisfies a predetermined condition. Here, the
comparator 660 may especially be configured as discussed before in the context of Fig. 7.
The selected analysis window function and synthesis window function may constitute a
window indication 661 provided at the output of the comparator 660. However, opposed to
the embodiment of the bandwidth extension decoder 400 shown in Fig. 4, wherein the
window indication 401-4 used for performing a bandwidth extension on the decoder side is
contained in the encoded audio signal 401-1, the window indication 661 of the bandwidth
extension decoder 600 shown in Fig. 6 is not available in the encoded audio signal 601-1
such that the window indication 661 has to be determined from analyzing the decoded low
frequency signal 681-1 derived from the encoded audio signal 601-1 first. Furthermore, the
patch module 220 of the bandwidth extension decoder 600 may comprise a bandwidth
extension module 670, which is configured to carry out a bandwidth extension algorithm in
that the patch module 220 will generate a patched signal 671 based on the decoded low
frequency signal 681-1, the analysis window function and the synthesis window function

selected by the comparator 660 and the upper band parameter 601-3. Finally, the patched
signals 671 and the decoded low frequency signal 681-1 may be combined by a combiner
690 to obtain a combined output signal 691 having an extended bandwidth. Here, the
patched signal 671, the decoded low frequency signal 681-1, the combiner 690 and the
combined output signal 691 may correspond to the patched signal 221-1, the decoded low
frequency signal 211-1, the combiner 230 and the combined output signal 231-1 of the
bandwidth extension decoder 200 shown in Fig. 2, respectively.
In the embodiments of the bandwidth extension encoders/decoders presented before, the
employed comparators may correspond to the comparator 700 as described in Fig. 7.
Specifically, the comparator 700 may be implemented to receive, as the plurality of input
signals 701-1, the plurality 331-1 of patched signals of the bandwidth extension encoders
300 and 500 in Figs. 3 and 5 or the plurality 651 of windowed modified time domain
signals of the bandwidth extension decoder 600 in Fig. 6 and, as the reference input signal
701-2, the audio signal 101-1 denoted by 'reference signal' in Fig. 3 or the high frequency
signal 101-4 indicated by the dashed line in Fig. 3, the low frequency signal 101-2 denoted
by 'reference low frequency signal' in Fig. 5 or the decoded low frequency signal 521
indicated by the dashed line in Fig. 5 or the decoded low frequency signal 681-1 of the
bandwidth extension decoder 600 in Fig. 6. The comparator 700 is furthermore configured
to provide the window indication 707, which may correspond to the window indication
341-1 of the bandwidth extension encoder 300 in Fig. 3, the window indication 511 of the
bandwidth extension encoder 500 in Fig. 5 or the window indication 661 of the bandwidth
extension decoder 600 in Fig. 6. As discussed before, the comparison may, for example, be
based on a calculation of the SFM parameters of the input signals. Alternatively, the input
signals 701-1 may also be compared with the reference input signals 701-2 based on a
sample-wise calculation of the differences in their audio signal values.
In the previous embodiments, the window selection is performed by a signal analysis in
that a plurality of different analysis window functions is applied to the audio signal or a
signal derived from the audio signal, generating a plurality of different patched
(synthesized) signals. From this plurality of synthesized signals, an optimum window
function is selected based on a predefined criterion based on a comparison of the
synthesized signals with the original audio signal or a signal derived from the audio signal.
The selected window function is then applied to the audio signal or a signal derived from
the audio signal such as within a bandwidth extension scheme so that a specific patched
(synthesized) signal will be generated. The above procedure, in particular, corresponds to a
closed loop and can be referred to as an 'analysis-by-synthesis' scheme. Alternatively, the
window selection can also be performed by a direct analysis of an input signal being the

audio signal or a signal derived from the audio signal, wherein the original input signal is
analyzed/classified with regard to a certain signal characteristic such as a measure of the
tonality. This alternative analysis scheme corresponding to an open loop will be presented
in the following embodiments.
Fig. 8 shows a block diagram of a further embodiment of a bandwidth extension encoder
800. Here, the basic structure of the bandwidth extension encoder 800 corresponds to that
of the bandwidth extension encoder 300 shown in Fig. 3. Therefore, identical blocks shown
in Figs. 3 and 8 may be denoted by the same numerals.
The signal analyzer 110 of the bandwidth extension encoder 800 comprises a signal
classifier 810, wherein the signal classifier 810 is configured to classify the audio signal
101-1 or a signal derived from the audio signal such as the high frequency signal 101-4
(dashed line) for determining a window indication 811 corresponding to an analysis
window function based on a signal characteristic of the classified signal. For example, the
signal classifier 810 may be implemented to determine the window indication 811 by
calculating a tonality measure from the audio signal 101-1 or the high frequency signal
101-4, wherein the tonality measure may indicate how the spectral energy is distributed in
their bands. In case the spectral energy is distributed relatively uniformly in a band, a
rather non-tonal signal ('noisy signal') exists in this band and the window indication 811
may be related to a first window function having a first characteristic adapted to be applied
to the non-tonal signal, while in case the spectral energy is relatively strongly concentrated
at a certain location in this band, a rather tonal signal exists for this band and the window
indication 811 may be related to a second window function having a second characteristic
adapted to be applied to the tonal signal. Furthermore, the encoder 800 comprises a
window controller 820 for providing window control information 821 based on the
window indication 811 determined by the signal classifier 810. The parameter calculator
830 of the encoder 800 comprises a windower controlled by the window controller 820,
wherein the windower of the parameter calculator 830 is configured to apply an analysis
window function based on the window control information 821 to the high frequency
signal 101-4 to obtain B WE parameters 831. The window controller 820 may, for example,
be implemented to provide the window control information 821 for the parameter
calculator 830 so that a first window characterized by a transfer function with a first width
of a main lobe will be applied by the windower of the parameter calculator 830, when the
determined tonality measure is below a predefined threshold, or a second window
characterized by a transfer function with a second width of a main lobe will be applied by
the windower of the parameter calculator 830, when the determined tonality measure is
equal or above the predefined threshold, wherein the first width of the main lobe of the

transfer function is larger than the second width of the main lobe of the transfer function.
In particular, in the context of a bandwidth extension scheme, it may be advantageous to
use a window function having a rather large main lobe of the transfer function in case of a
non-tonal signal and a rather small main lobe of the transfer function in case of a tonal
signal.
The core encoder 120 of the bandwidth extension encoder 800 is configured to encode the
low frequency signal 101-2 to obtain an encoded low frequency signal 121. As in the
embodiment shown in Fig. 3, the encoded low frequency signal 121, the window indication
811 and the BWE parameters 831 may be supplied to an output interface 840 for providing
an encoded audio signal 841 comprising the window indication 811.
Fig. 9 shows a block diagram of an implementation of a signal classifier 900, which may
be used for the direct analysis of the audio signal 101-1 in the embodiment of Figs. 8, 10
and 11. The signal classifier 900 may comprise a tonality measurer 910, a signal
characterizer 920 and a window selector 930. The tonality measurer 910 may be
configured to analyze the audio signal 101-1 in order to determine a tonality measure 911
of the audio signal 101-1. The signal characterizer 920 may be configured to determine a
signal characteristic 921 of the audio signal 101-1 based on the tonality measure 911
provided by the tonality measurer 910. In particular, the signal characterizer 920 is
configured to determine whether the audio signal 101-1 corresponds to a noisy signal or
rather to a tonal signal. Finally, the window selector 930 is implemented to provide the
window indication 811 based on the signal characteristic 921.
Fig. 10 shows a block diagram of a further embodiment of a bandwidth extension encoder
1000, which may correspond to the bandwidth extension encoder 500 shown in Fig. 5.
Correspondingly, identical blocks in the embodiments shown in Figs. 5 and 10 are denoted
by the same numerals. The signal analyzer 110 of the bandwidth extension encoder 1000
comprises a signal classifier 1010, wherein the signal classifier 1010 is configured to
classify the low frequency signal 101-2 derived from the audio signal 101-1 for
determining a window indication 1011 corresponding to an analysis window function
based on a signal characteristic of the classified signal provided by the signal classifier
1010. Furthermore, the encoder 1000 comprises a window controller 1020 for providing
window control information 1021 based on the window indication 1011 determined by the
signal classifier 1010. The parameter calculator 1030 of the bandwidth extension encoder
1000 comprises a windower controlled by the window controller 1020, wherein the
windower of the parameter calculator 1030 is configured to apply an analysis window
function based on the window control information 1021 to the high frequency signal 101-4

to obtain BWE parameters 1031. The bandwidth extension encoder 1000 may comprise a
core encoder 120 for encoding the low frequency signal 101-2 to obtain an encoded low
frequency signal 121. Moreover, the bandwidth extension encoder 1000 may also
optionally comprise a core decoder 1050 indicated by the dashed block, which is
configured to decode the encoded low frequency signal 121 to obtain a decoded low
frequency signal 1051 (dashed arrow). Correspondingly, the signal classifier 1010 may
optionally be configured to analyze/classify the decoded low frequency signal 1051 in
order to determine the window indication 1011. The encoded low frequency signal 121 and
the BWE parameters 1031 may further be supplied to an output interface 1040, which is
configured for providing an encoded audio signal 1041 not comprising the window
indication 1011. Here, the encoded audio signal 1041 may correspond to the encoded audio
signal 531 shown in Fig. 5.
In this case, the window indication is not contained in the encoded audio signal on the
encoder side (Fig. 10), which means that the window indication has to be determined on
the decoder side (Fig. 11) as well, as will be illustrated in the following.
Fig. 11 shows a block diagram of a further embodiment of a bandwidth extension decoder
1100, which may correspond to the bandwidth extension decoder 600 shown in Fig. 6.
Correspondingly, identical blocks in the embodiments of Figs. 6 and 11 are denoted by the
same numerals. In particular, the bandwidth extension decoder 1100 comprises a core
decoder 680 for decoding the encoded low frequency signal 601-2 to obtain a decoded low
frequency signal 681-1. The patch module 220 of the bandwidth extension decoder 1100
comprises a signal classifier 1110, which is configured to analyze/classify the decoded low
frequency signal 681-1 for determining a window indication 1111 corresponding to an
analysis window function based on a signal characteristic of the analyzed signal.
Furthermore, the decoder 1100 comprises a window controller 1120 for providing window
control information 1121 based on the window indication 1111 determined by the signal
classifier 1110. In addition, the decoder 1100 may comprise a BWE module 1130, which
may be configured such that the patch module 220 will generate a patched signal 671
based on the decoded low frequency signal 681-1, the analysis window function based on
the window control information 1121 and the upper band parameter 601-3. The patched
signal 671 and the decoded low frequency signal 681-1 may be further combined by a
combiner 690 to obtain a combined output signal 691.
The analysis-by-synthesis scheme of the previous embodiments may also be used in the
context of a phase vocoder implementation. Accordingly, Fig. 12 shows a block diagram of
an embodiment of a phase vocoder processor 1200. The phase vocoder processor 1200 for

processing an audio signal 1201 may comprise an analysis windower 1210, a
time/spectrum converter 1220, a frequency domain processor 1230, a frequency/time
converter 1240, a synthesis windower 1250, a comparator 1260 and an overlap adder 1270.
Specifically, the analysis windower 1210 may be configured for applying a plurality 111-1
of analysis window functions to the audio signal 1201 or a signal derived from the audio
signal such as the decoded low frequency signal 1202 indicated by the dashed arrow, the
audio signal 1201 having a block of audio samples, the block having a specified length in
time, to obtain a plurality 1211 of windowed audio signals. The time/spectrum converter
1220 may be configured for converting the windowed audio signals 1211 into spectra
1221. The frequency domain processor 1230 may be configured for processing the spectra
1221 in a frequency domain to obtain modified spectra 1231. The frequency/time converter
1240 may be configured for converting the modified spectra 1231 into modified time
domain signals 1241. The synthesis windower 1250 may be configured for applying a
plurality of synthesis window functions to the modified time domain signals 1241, wherein
the synthesis window functions are matched to the analysis window functions, to obtain
windowed modified time domain signals 1251. The comparator 1260 may furthermore be
configured to determine a plurality of comparison parameters based on a comparison of the
plurality of windowed modified time domain signals 1251 and the audio signal 1201 or a
signal derived from the audio signal such as the decoded low frequency signal 1202
(dashed line), wherein the plurality of comparison parameters corresponds to the plurality
of analysis window functions, and wherein the comparator 1260 is furthermore configured
to select an analysis window function and a synthesis window function for which a
comparison parameter satisfies a predetermined condition. Here, it is to be noted that the
analysis window function and the synthesis window function selected by the comparator
1260 may be determined in a similar way as has been described before in the context of the
previous embodiments. In particular, the comparator 1260 may be implemented as in the
embodiment shown in Fig. 7. Subsequently, the selected analysis window function and the
synthesis window function may be used for a signal path starting at the analysis windower
1210 and ending with the synthesis windower 1250 before the comparator 1260 in the
processing chain as shown in Fig. 12 such that a specific (optimized) windowed modified
time domain signal 1255 will be obtained at the output of the synthesis windower 1250.
Finally, the overlap adder 1270 may be configured for adding overlapping consecutive
blocks of the windowed modified time domain signal 1255 having been modified by the
analysis window function and synthesis window function selected by the comparator 1260
to obtain a temporally spread signal 1271.
In particular, the temporally spread signal 1271 can be obtained by spacing the overlapping
consecutive blocks of the windowed modified time domain signal 1255 further apart from

each other than the corresponding blocks of the original audio signal 1201 or the decoded
low frequency signal 1202. Additionally, the overlap adder 1270 here acting as a signal
spreader may also be configured to temporally spread the audio signal 1201 or the decoded
low frequency signal 1202 in that the pitch of the same will not be changed, leading to a
scenario of "pure time stretching".
Alternatively, the comparator 1260 may also be placed after the overlap adder 1270 in the
processing chain such that the latter will also be included in the analysis-by-synthesis
scheme, which may be advantageous insofar as in this case, effects of the different
windowed modified time domain signals 1251 processed by the overlap adder 1270 may
also be accounted for by a subsequent comparison/window selection.
In further alternative embodiments, the phase vocoder processor 1200 may also comprise a
decimator in form of, for example, a simple sample rate converter, wherein the decimator
may be configured to decimate (compress) the spreaded signal such that a decimated signal
in a target frequency range of a bandwidth extension algorithm will be obtained.
In further alternative embodiments, a phase vocoder processor may also be implemented to
perform a direct analysis of an input audio signal with the aim to select an optimal analysis
window function adapted to the signal characteristic of the analyzed audio signal.
Particularly, it was found that certain signals benefit from using specialized analysis
windows for the phase vocoder. For instance, noisy signals are better analyzed by
application of, for example, a Tukey window, while predominantly tonal signals benefit
from a small main lobe of the transfer function as provided by, e.g., the Bartlett window.
In summary, it can be seen that the procedure of selecting the optimum window function
can either be performed only on the encoder side such as within the bandwidth extension
encoders 300 and 800 of Figs. 3 and 8, wherein then the provided window indication is
transmitted to the decoder side such as the bandwidth extension decoder 400 of Fig. 4, or
both at the encoder and the decoder side such as with regard to the bandwidth extension
encoders/decoders 500 and 600 of Figs. 5 and 6 or the bandwidth extension
encoders/decoders 1000 and 1100 of Figs. 10 and 11.
In this context, it may be of advantage that in the latter case, the window indication is not
to be stored as additional side-information within the encoded audio signal such that the bit
rate for storage or transmission of the encoded audio signal may be reduced.

Fig. 13 illustrates an embodiment of an apparatus 1300, which may be used for switching
between different analysis and synthesis windows dependent on control information in the
context of time-frequency transforms applicable for phase vocoder applications. The
incoming bitstream 1301-1 may be interpreted by a datastream interpreter, which is
implemented to separate the control information 1301-2 from the audio data 1301-3.
Furthermore, depending on the control information 1301-2, an analysis window function
1311-1 from a plurality 1311-2 of analysis windows may be applied to the audio data
1301-3. Here, exemplarily, the plurality 1311-2 of analysis windows comprises four
different analysis windows denoted by the blocks "analysis window 1" to "analysis
window 4", wherein the block "analysis window 1" refers to the applied analysis window
1311-1. The control information 1301-2, in particular, may have been obtained by a direct
calculation of the signal characteristic or an analysis-by-synthesis scheme as described
correspondingly before. In case of a noisy signal, for example, a Tukey window may be
chosen, while in case of a tonal signal, for example, a Bartlett window may be chosen. The
Tukey window, which may also be referred to as a cosine-tapered window, may be imaged
as a cosine lobe of width (a ■ 2) N convolved with a rectangular window of width (1.0 -
a-2) N. The Tukey window may be defined by

wherein the window evolves from the rectangular window to the Hanning window as the
parameter a varies from 0 to unity. The Bartlett window representing a triangular window
may be defined as

In Eqs. (1) and (2), n is an integer value and N the width (in samples) of the time-discrete
window functions w(n).
The windowed audio signal obtained after applying the analysis window 1311-1 may
further be transformed in a block 1320 denoted by "time-frequency transformation" from
the time domain to a frequency domain. The obtained spectrum may then be processed in a
block 1330 denoted by "frequency domain processing". In particular, the block 1330 may

comprise a phase modifier for modifying phases of spectral values of the spectrum. Then,
the processed spectrum may be transformed in a block 1340 denoted by "frequency-time
transformation" back into the time domain to obtain a modified time domain signal.
Finally, depending on the control information 1301-2, a synthesis window 1351-1 from a
plurality of synthesis windows 1351-2 denoted by "synthesis window 1" to synthesis
window 4", wherein the synthesis window 1351-1 compensates for the effect of the
analysis window 1311-1, may be applied to the modified time domain signal to obtain,
after adding contributions from all possible signal paths in a block 1360 indicated by a plus
symbol, the windowed modified time domain signal 1361 at the output of the apparatus
1300.
Fig. 14 shows an overview of an embodiment of a phase vocoder driven bandwidth
extension decoder 1400. In particular, a data audio stream 1411-1 may be separated into an
encoded low frequency signal 1411-2 and HBE/SBR data 1411-3. The encoded low
frequency signal 1411-2 may be decoded by a core decoder 1420 to obtain a decoded low
frequency signal 1421 comprising a core frequency band 1425. The decoded low
frequency signal 1421 may, for example, represent PCM (pulse code modulation) data
having a frame size of 1024. The decoded low frequency signal 1421 is further supplied to
a delay stage 1430 to obtain a delayed signal 1431. Subsequently, the delayed signal 1431
is input into a 32-band QMF (quadrature mirror filter) analysis bank 1440, generating, for
example, 32 frequency subbands 1441 of the delayed signal 1431. The HBE/SBR data
1411-3 may comprise control information for controlling a patch switch 1450, wherein the
patch switch 1450 is configured for switching between a SBR patching algorithm and an
HBE patching algorithm. In case of the SBR patching algorithm, the frequency subbands
1441 are supplied to a SBR patching device 1460-1 in order to obtain patched QMF data
1461. The patched QMF data 1461 present at the output of the SBR patching device 1460-
1 are supplied to an HBE/SBR tool 1470-1 comprising, for example, a noise filling unit
1470-2, a missing harmonics reconstruction unit 1470-3 or an inverse filtering unit 1470-4.
In particular, the HBE/SBR tool 1470-1 may implement known spectral band replication
techniques to be used on the patched QMF data 1461. The patching algorithm used by the
SBR patching device 1460-1 may, for example, use a mirroring or copying of the spectral
data within the frequency domain. Furthermore, the HBE/SBR tool 1470-1 is controlled by
the HBE/SBR data 1411-3. The patched QMF data 1461 and the output 1471 of the
HBE/SBR tool 1470-1 are supplied to an envelope formatter 1470. The envelope formatter
1470 is implemented to adjust the envelope for the generated patch such that an envelope-
adjusted patched signal 1471 comprising an upper frequency band is generated. The
envelope-adjusted signal 1471 is supplied to a QMF synthesis bank 1480, which is
configured to combine the components of the upper frequency band with the audio signal

in the frequency domain 1441. Finally, a synthesis audio signal 1481 denoted by
"waveform" is obtained.
In case of the HBE patching algorithm (block 1460-2), the decoded low frequency signal
1421 may be down-sampled by a down sampler 1490 by, for example, a factor of 2 to
obtain a down-sampled version of the decoded low frequency signal 1491. The down-
sampled signal 1491 may further be processed in an advanced processing scheme of a
harmonic bandwidth extension algorithm using a phase vocoder.
On the one hand, a signal dependent processing scheme may be employed, making use of
the switching between a standard algorithm as illustrated by a signal path 1500 denoted by
"no" when a transient event is not detected in a block of the decoded low frequency signal
1421 by a transient detector 1485 and an advanced algorithm as illustrated by a signal path
1510 denoted by "yes" starting from a zero padding operation (block 1515) when a
transient event is detected in the block.
On the other hand, essentially, a signal dependent switching of analysis window
characteristics within a phase vocoder in a time-frequency transform implementation may
be performed as has been described in detail before. In particular, in Fig. 14, the boxes
referenced by 1520; 1530 with dotted borders indicate the windows that can be altered by
the signaling. Basically, Fig. 14 shows the application of the embodiment of Fig. 13 within
a phase vocoder driven bandwidth extension.
Here, the blocks denoted by "FFT" (Fast Fourier Transform), "phase adaption" and "iFFT"
(inverse Fast Fourier Transform) may correspond to the blocks 1320, 1330 and 1340
shown in Fig. 13, respectively. Specifically, the FFT and iFFT processing blocks may be
implemented to apply a short-time Fourier transform (STFT) or a discrete Fourier
transform (DFT) and an inverse short-time Fourier transform (iSTFT) or an inverse
discrete Fourier transform (iDFT) to a block of the decoded low frequency signal 1421,
respectively. In addition, the bandwidth extension decoder 1400 shown in Fig. 14 may also
comprise an up-sampling stage 1540, an overlap add (OLA) stage 1550 and a decimation
stage 1560.
It is to be noted that with the above concept, it is possible to switch between different
windows on arbitrary positions in the audio signal.
Although the present invention has been described in the context of block diagrams where
the blocks represent actual or logical hardware components, the present invention can also

be implemented by a computer-implemented method. In the latter case, the blocks
represent corresponding method steps where these steps stand for the functionalities
performed by corresponding logical or physical hardware blocks.
The described embodiments are merely illustrative for the principles of the present
invention. It is understood that modifications and variations of the arrangements and the
details described herein will be apparent to others skilled in the art. It is the intent,
therefore, to be limited only by the scope of the impending patent claims and not by the
specific details presented by way of description and explanation of the embodiments
herein.
Dependent on certain implementation requirements of the inventive methods, the inventive
methods can be implemented in hardware or in software. The implementation can be
performed using a digital storage medium, in particular a disc, a DVD or a CD having
electronically, readable control signals stored thereon, which co-operate with
programmable computer systems, such that the inventive methods are performed.
Generally, the present invention can therefore be implemented as a computer program
product with the program code stored on a machine-readable carrier, the program code
being operated for performing the inventive methods when the computer program product
runs on a computer. In other words, the inventive methods are, therefore, a computer
program having a program code for performing at least one of the inventive methods when
the computer program runs on a computer. The inventive encoded audio signal can be
stored on any machine-readable storage medium, such as a digital storage medium.
The advantages of the novel processing are that the above-mentioned embodiments, i.e.
apparatus, methods or computer programs, described in this application allow improving
the perceptual audio quality of bandwidth extension applications. In particular, it utilizes a
signal-dependent switching of analysis window characteristics such as within a phase
vocoder driven bandwidth extension.
The novel processing can also be used in other phase vocoder applications such as pure
time stretching whenever it is beneficial to take into account signal characteristics for the
choice of an optimal analysis or synthesis window.
The presented concept allows the bandwidth extension to take into account signal
characteristics for the patching process. The decision for the best-suited analysis window
can be done within an open or within a closed loop. Therefore, the restitution quality can
be optimized and, thus, further enhanced.

Most prominent applications are audio decoders based on bandwidth extension principles.
However, the inventive processing may also enhance phase vocoder applications for music
production or audio post-processing.

WE CLAIM:
1. A bandwidth extension encoder (100; 300; 500; 800; 1000) for encoding an audio
signal (101-1), the audio signal (101-1) comprising a low frequency signal (101-2)
comprising a core frequency band (101-3) and a high frequency signal (101-4)
comprising an upper frequency band (101-5), the encoder (100; 300; 500; 800;
1000) comprising:
a signal analyzer (110) for analyzing the audio signal (101-1), the audio signal
(101-1) having a block (101-6) of audio samples, the block (101-6) having a
specified length in time, wherein the signal analyzer (110) is configured for
determining, from a plurality (111-1) of analysis windows, an analysis window
(111-2) to be used for performing a bandwidth extension in a bandwidth extension
decoder (200; 400; 1400);
a core encoder (120) for encoding the low frequency signal (101-2) to obtain an
encoded low frequency signal (121); and
a parameter calculator (130; 320; 830; 1030) for calculating bandwidth extension
parameters (131; 321-2; 831; 1031) from the high frequency signal (101-4).
2. A bandwidth extension decoder (200; 400; 600; 1100; 1400) for decoding an
encoded audio signal (201 -1; 401 -1; 601 -1; 1411 -1), the encoded audio signal (201 -
1; 401-1; 601-1; 1411-1) comprising an encoded low frequency signal (201-2; 401-
2; 601-2; 1411-2) and upper band parameters (201-3; 401-3; 601-3; 1411-3), the
decoder (200; 400; 600; 1100; 1400) comprising:
a core decoder (210; 410; 680; 1420) for decoding the encoded low frequency
signal (201-2; 401-2; 601-2; 1411-2), wherein the decoded low frequency signal
(211-1; 411-1; 681-1; 1421) comprises a core frequency band (211-2; 411-2; 681-2;
1425);
a patch module (220; 420; 1460-2) which is configured to generate a patched signal
(221-1; 421; 671; 1461) based on the decoded low frequency signal (211-1; 411-1;
681-1; 1421) and the upper band parameters (201-3; 401-3; 601-3; 1411-3),
wherein the patched signal (221-1; 421; 671; 1461) comprises an upper frequency
band (221-2) generated from the core frequency band (211-2; 411-2; 681-2; 1425);
and

a combiner (230; 430; 690; 1480) which is configured to combine the patched
signal (221-1; 421; 671; 1461) and the decoded low frequency signal (211-1; 411-1;
681-1; 1421) to obtain a combined output signal (231-1; 431; 691; 1481).
3. A bandwidth extension encoder (300) according to claim 1, further comprising:
a window controller (310) for providing window control information (311)
indicating a plurality (111-1) of analysis window functions, the parameter
calculator (320) comprising a windower controlled by the window controller (310),
wherein the windower is configured to apply the plurality (111-1) of analysis
window functions and an analysis window function (111-2) to be selected by a
comparator (340) to the high frequency signal (101-4), the signal analyzer (110)
comprising a patch module (330), which is configured to generate a plurality (331-
1) of patched signals based on the low frequency signal (101-2), the window
control information (311) and BWE parameters (321-1), wherein the patched
signals (331-1) comprise upper frequency bands (331-2) generated from the core
frequency band (101-3);
a comparator (340) which is configured to determine a plurality (341-2) of
comparison parameters based on a comparison of the patched signals (331-1) and a
reference signal being the audio signal (101-1) or a signal (101-4) derived from the
audio signal, wherein the plurality (341-2) of comparison parameters corresponds
to the plurality (111-1) of analysis window functions, and wherein the comparator
(340) is furthermore configured to provide a window indication (341-1)
corresponding to an analysis window function (111-2) for which a comparison
parameter satisfies a predetermined condition; and
an output interface (350) for providing an encoded audio signal (351), the encoded
audio signal (351) comprising the window indication (341-1).
4. A bandwidth extension decoder (400) according to claim 2, wherein the encoded
audio signal (401-1) comprises a window indication (401-4), and wherein the patch
module (420) comprises a controllable windower for selecting an analysis window
function from a plurality of analysis window functions based on the window
indication (401-4) and for applying the selected analysis window function to the
decoded low frequency signal (411-1).

5. A bandwidth extension encoder (500) according to claim 1, further comprising:
a window controller (310) for providing window control information (311)
indicating a plurality (111-1) of analysis window functions, the parameter
calculator (320) comprising a windower controlled by the window controller (310),
wherein the windower is configured to apply the plurality (111-1) of analysis
window functions and an analysis window function (111-2) to be selected by a
comparator (510) to the high frequency signal (101-4), the signal analyzer (110)
comprising a patch module (330), which is configured to generate a plurality (331-
1) of patched signals based on the low frequency signal (101-2), the window
control information (311) and bandwidth extension parameters (321-1), wherein the
patched signals (331-1) comprise upper frequency bands (331-2) generated from
the core frequency band (101-3), and wherein the patch module (330) comprises a
windower controlled by the window controller (310), wherein the windower is
configured for applying the plurality (111-1) of analysis window functions to the
low frequency signal (101-2);
a comparator (510) which is configured to determine a plurality of comparison
parameters based on a comparison of the patched signals (331-1) and a reference
low frequency signal (101-2) derived from the audio signal, wherein the plurality of
comparison parameters corresponds to the plurality (111-1) of analysis window
functions, and wherein the comparator (510) is furthermore configured to provide a
window indication (511) corresponding to an analysis window function for which a
comparison parameter satisfies a predetermined condition; and
an output interface (530) for providing an encoded audio signal, the encoded audio
signal (531) not comprising the window indication (511).
6. A bandwidth extension decoder (600) according to claim 2, wherein the patch
module (220) comprises:
an analysis windower (610) for applying a plurality (111-1) of analysis window
functions to the decoded low frequency signal (681-1) to obtain a plurality (611) of
windowed low frequency signals;
a time/spectrum converter (620) for converting the windowed low frequency
signals (611) into spectra (621);

a frequency domain processor (630) for processing the spectra (621) in a frequency
domain to obtain modified spectra (631);
a frequency/time converter (640) for converting the modified spectra (631) into
modified time domain signals (641);
a synthesis windower (650) for applying a plurality of window functions to the
modified time domain signals (641), wherein the synthesis window functions are
matched to the analysis window functions to obtain windowed modified time
domain signals (651); and
a comparator (660) which is configured to determine a plurality of comparison
parameters based on a comparison of the plurality (651) of windowed modified
time domain signals and the decoded low frequency signal (681-1), wherein the
plurality of comparison parameters corresponds to the plurality (111-1) of analysis
window functions, and wherein the comparator (660) is furthermore configured to
select an analysis window function and a synthesis window function for which a
comparison parameter satisfies a predetermined condition, and wherein the patch
module (220) is configured for generating a patched signal (671) based on the
decoded low frequency signal (681-1), the analysis window function and the
synthesis window function selected by the comparator (660) and the upper band
parameters (601-3).
7. A bandwidth extension encoder (300; 500) or decoder (600) according to one of the
claims 3, 5 or 6, wherein the comparator (340; 510; 660; 700) is configured for
calculating a plurality (703-1) of SFM parameters for the patched signals (331-1) or
the windowed modified time domain signals (651) and a reference SFM parameter
(703-2) derived from the audio signal (101-1) or the decoded low frequency signal
(681-1) and for determining the plurality (705) of comparison parameters based on
a comparison of the SFM parameters (703-1) and the reference SFM parameter
(703-2).
8. A bandwidth extension encoder (800) according to claim 1, the signal analyzer
(110) comprising a signal classifier (810; 900), wherein the signal classifier (810;
900) is configured to classify the audio signal (101-1) or a signal derived from the
audio signal (101-4) for determining a window indication (811) corresponding to an
analysis window function based on a signal characteristic of the classified signal,
the encoder (800) comprising a window controller (820) for providing window

control information (821) based on the window indication (811) determined by the
signal classifier (810), the parameter calculator (830) comprising a windower
controlled by the window controller (820), wherein the windower is configured to
apply an analysis window function based on the window control information (821)
to the high frequency signal (101-4), and the encoder (800) further comprising an
output interface (840) for providing an encoded audio signal (841), the encoded
audio signal (841) comprising the window indication (811).
9. A bandwidth extension encoder (1000) according to claim 1, the signal analyzer
(110) comprising a signal classifier (900; 1010), wherein the signal classifier (900;
1010) is configured to classify a low frequency signal (101-2) derived from the
audio signal (101-1) for determining a window indication (1011) corresponding to
an analysis window function based on a signal characteristic of the classified signal,
the encoder (1000) comprising a window controller (1020) for providing window
control information (1021) based on the window indication (1011) determined by
the signal classifier (900; 1010), the parameter calculator (1030) comprising a
windower controlled by the window controller (1020), wherein the windower is
configured to apply an analysis window function based on the window control
information (1021) to the high frequency signal (101-4), and the encoder (1000)
further comprising an output interface (1040) for providing an encoded audio signal
(1041), the encoded audio signal (1041) not comprising the window indication
(1011).
10. A bandwidth extension encoder (500; 1000) according to claims 5 or 9, further
comprising:
a core decoder (520; 1050) for decoding the encoded low frequency signal (121) to
obtain a decoded low frequency signal (521; 1051).
11. A bandwidth extension decoder (1100) according to claim 2, wherein the patch
module (220) comprises:
a signal classifier (900; 1110) which is configured to classify the decoded low
frequency signal (681-1) for determining a window indication (1111)
corresponding to an analysis window function based on a signal characteristic of
the classified signal, the decoder (1100) comprising a window controller (1120) for
providing window control information (1121) based on the window indication
(1111) determined by the signal classifier (900; 1110), and wherein the patch

module (220) is configured for generating a patched signal (671) based on the
decoded low frequency signal (681-1), an analysis window function based on the
window control information (1121) and the upper band parameters (601-3).
12. A phase vocoder processor (1200) for processing an audio signal (1201),
comprising:
an analysis windower (1210) for applying a plurality (111-1) of analysis window
functions to the audio signal (1201) or a signal (1202) derived from the audio
signal, the audio signal (1201) having a block (101-6) of audio samples, the block
(101-6) having a specified length in time, to obtain a plurality (1211) of windowed
audio signals;
a time/spectrum converter (1220) for converting the windowed audio signals (1211)
into spectra (1221);
a frequency domain processor (1230) for processing the spectra (1221) in a
frequency domain to obtain modified spectra (1231);
a frequency/time converter (1240) for converting the modified spectra (1231) into
modified time domain signals (1241);
a synthesis windower (1250) for applying a plurality of synthesis window functions
to the modified time domain signals (1241), wherein the synthesis window
functions are matched to the analysis window functions, to obtain windowed
modified time domain signals (1251);
a comparator (1260) which is configured to determine a plurality of comparison
parameters based on a comparison of the plurality (1251) of windowed modified
time domain signals and the audio signal (1201) or a signal (1202) derived from the
audio signal, wherein the plurality of comparison parameters corresponds to the
plurality of analysis window functions, and wherein the comparator (1260) is
furthermore configured to select an analysis window function and a synthesis
window function for which a comparison parameter satisfies a predetermined
condition; and
an overlap adder (1270) for adding overlapping blocks of a windowed modified
time domain signal (1255) to obtain a temporally spreaded signal (1271), wherein

the overlap adder (1270) is configured for processing blocks of the windowed
modified time domain signal (1255) having been modified by an analysis window
function and a synthesis window function selected by the comparator (1260).
13. A method (100; 300; 500; 1000) for encoding an audio signal (101-1), the audio
signal (101-1) comprising a low frequency signal (101-2) comprising a core
frequency band (101-3) and a high frequency signal (101-4) comprising an upper
frequency band (101-5), the method (100; 300; 500; 1000) comprising:
analyzing (110) the audio signal (101-1), the audio signal (101-1) having a block
(101-6) of audio samples, the block (101-6) having a specified length in time, for
determining, from a plurality (111-1) of analysis windows, an analysis window
(111-2) to be used for performing a bandwidth extension in a bandwidth extension
decoder (200; 400; 1400);
encoding (120) the low frequency signal (102-2) to obtain an encoded low
frequency signal (121); and
calculating (130; 320; 830; 1030) bandwidth extension parameters from the high
frequency signal (101-4).
14. A method (200; 400; 600; 1100; 1400) for decoding an encoded audio signal (201-
1; 401-1; 601-1; 1411-1), the encoded audio signal (201-1; 401-1; 601-1; 1411-1)
comprising an encoded low frequency signal (201-2; 401-2; 601-2; 1411-2) and
upper band parameters (201-3; 401-3; 601-3; 1411-3), the method (200; 400; 600;
1100; 1400) comprising:
decoding (210; 410; 680; 1420) the encoded low frequency signal (201-2; 401-2;
601-2; 1411-2), wherein the decoded low frequency signal (211-1; 411-1; 681-1;
1421) comprises a core frequency band (211-2; 411-2; 681-2; 1425);
generating (220; 420; 1460-2) a patched signal (221-1; 421; 671; 1461) based on
the decoded low frequency signal (211-1; 411-1; 681-1; 1421) and the upper band
parameters (201-3; 401-3; 601-3; 1411-3), wherein the patched signal (221-1; 421;
671; 1461) comprises an upper frequency band (221-2) generated from the core
frequency band (211-2; 411-2; 681-2; 1425); and

combining (230; 430; 690; 1480) the patched signal (221-1; 421; 671; 1461) and
the decoded low frequency signal (211-1; 411-1; 681-1; 1421) to obtain a combined
output signal (231-1; 431; 691; 1481).
15. An encoded audio signal (103-1; 351; 841) comprising:
an encoded low frequency signal (121);
bandwidth extension parameters (131; 321-2; 831); and
an analysis window (111-2) to be used for performing a bandwidth extension in a
bandwidth extension decoder (200; 400; 1400).
16. A computer program having a program code for performing the method of claim 13
or claim 14 when the computer program is executed on a computer.

A bandwidth extension encoder for encoding an audio signal comprises a signal analyzer, a
core encoder and a parameter calculator. The audio signal comprises a low frequency
signal comprising a core frequency band and a high frequency signal comprising an upper
frequency band. The signal analyzer is configured for analyzing the audio signal, the audio
signal having a block of audio samples, the block having a specified length in time. The
signal analyzer is furthermore configured for determining from a plurality of analysis
windows an analysis window to be used for performing a bandwidth extension in a
bandwidth extension decoder. The core encoder is configured for encoding the low
frequency signal to obtain an encoded or frequency signal. The parameter calculator is
configured for calculating bandwidth extension parameters from the high frequency signal.