COMPACT SIDE INFORMATION FOR PARAMETRIC CODING OF SPATIAL
AUDIO
BACKGROUND OF THE INVENTION
Cross-Reference to Related Applications
The subject matter of this application is related to the
subject matter of the following U.S. applications, the
teachings of all of which are incorporated herein by
reference:
o U.S. application Ser. No. 09/848,877, filed on May 4,
2001 as attorney docket no. Faller 5;
o U.S. application Ser. No. 10/045,458, filed on Nov. 7,
2001 as attorney docket no. Baumgarte 1-6-8, which
itself claimed the benefit of the filing date of U.S.
provisional application No. 60/311,565, filed on Aug.
10, 2001;
o U.S. application Ser. No. 10/155,437, filed on May 24,
2002 as attorney docket no. Baumgarte 2-10;
o U.S. application Ser. No. 10/246,570, filed on Sep. 18,
2002 as attorney docket no. Baumgarte 3-11;
o U.S. application Ser. No. 10/815,591, filed on Apr. 1,
2004 as attorney docket no. Baumgarte 7-12;
o U.S. application Ser. No. 10/936,464, filed on Sep. 8,
2004 as attorney docket no. Baumgarte 8-7-15;
o U.S. application Ser. No. 10/762,100, filed on Jan. 20,
2004 (Faller 13-1);
o U.S. application Ser. No. 11/006,492, filed on Dec. 7,
2004 as attorney docket no. Allamanche 1-2-17-3; and
o U.S. application Ser. No. 11/006, , filed on Dec.
7, 2004 as attorney docket no. Allamanche 2-3-18-4.
The subject matter of this application is also related to
subject matter described in the following papers, the
teachings of all of which are incorporated herein by
reference:

— 2 —
o F. Baumgarte and C. Faller, "Binaural Cue Coding--Part
I: Psychoacoustic fundamentals and design principles,"
IEEE Trans, on Speech and Audio Proc, vol. 11, no. 6,
November 2003;
o C. Faller and F. Baumgarte, "Binaural Cue Coding--Part
II: Schemes and applications," IEEE Trans, on Speech
and Audio Proc, vol. 11, no. 6, November 2003; and
o C. Faller, "Coding of spatial audio compatible with
different playback formats," Preprint 117th Conv. Aud.
Eng. Soc, October 2004.
Field of the Invention
The present invention relates to the encoding of audio
signals and the subsequent synthesis of auditory scenes
from the encoded audio data.
Description of the Related Art
When a person hears an audio signal (i.e., sounds)
generated by a particular audio source, the audio signal
will typically arrive at the person's left and right ears
at two different times and with two different audio (e.g.,
decibel) levels, where those different times and levels are
functions of the differences in the paths through which the
audio signal travels to reach the left and right ears,
respectively. The person's brain interprets these
differences in time and level to give the person the
perception That the received audio signal is being
generated by an audio source located at a particular
position (e.g., direction and distance) relative to the
person. An auditory scene is the net effect of a person
simultaneously hearing audio signals generated by one or
more different audio sources located at one or more
different positions relative to the person.

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The existence of this processing by the brain can be used
to synthesize auditory scenes, where audio signals from one
or more different audio sources are purposefully modified
to generate left and right audio signals that give the
perception that the different audio sources are located at
different positions relative to the listener.
FIG. 1 shows a high-level block diagram of conventional
binaural signal synthesizer 100, which converts a single
audio source signal (e.g., a mono signal) into the left and
right audio signals of a binaural signal, where a binaural
signal is defined to be the two signals received at the
eardrums of a listener. In addition to the audio source
signal, synthesizer 100 receives a set of spatial cues
corresponding to the desired position of the audio source
relative to the listener. In typical implementations, the
set of spatial cues comprises an inter-channel level
difference (ICLD) value (which identifies the difference in
audio level between the left and right audio signals as
received at the left and right ears, respectively) and an
inter-channel time difference (ICTD) value (which
identifies the difference in time of arrival between the
left and right audio signals as received at the left and
right ears, respectively) . In addition or as an
alternative, some synthesis techniques involve the modeling
of a direction-dependent transfer function for sound from
the signal source to the eardrums, also referred to as the
head-related transfer function (HRTF). See, e.g., J.
Blauert, The Psychophysics of Human Sound Localization, MIT
Press, 1983, the teachings of which are incorporated herein
by reference.
Using binaural signal synthesizer 100 of FIG. 1, the mono
audio signal generated by a single sound source can be
processed such that, when listened to over headphones, the
sound source is spatially placed by applying an appropriate
set of spatial cues (e.g., ICLD, ICTD, and/or HRTF) to
generate the audio signal for each ear. See, e.g., D. R.

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Begault, 3-D Sound for Virtual Reality and Multimedia,
Academic Press, Cambridge, Mass., 1994.
Binaural signal synthesizer 100 of FIG. 1 generates the
simplest type of auditory scenes: those having a single
audio source positioned relative to the listener. More
complex auditory scenes comprising two or more audio
sources located at different positions relative to the
listener can be generated using an auditory scene
synthesizer that is essentially implemented using multiple
instances of binaural signal synthesizer, where each
binaural signal synthesizer instance generates the binaural
signal corresponding to a different audio source. Since
each different audio source has a different location
relative to the listener, a different set of spatial cues
is used to generate the binaural audio signal for each
different audio source.
SUMMARY OF THE INVENTION
According to one embodiment, the present invention is a
method, apparatus, and machine-readable medium for encoding
audio channels. One or more cue codes are generated for two
or more audio channels, wherein at least one cue code is a
combined cue code generated by combining two or more
estimated cue codes, and each estimated cue code is
estimated from a group of two or more of the audio
channels.
According to another embodiment, the present invention is
an apparatus for encoding C input audio channels to
generate E transmitted audio channel(s). The apparatus
comprises a code estimator and a downmixer. The code
estimator generates one or more cue codes for two or more
audio channels, wherein at least one cue code is a combined
cue code generated by combining two or more estimated cue
codes, and each estimated cue code is estimated from a

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group of two or more of the audio channels. The downmixer
downmixes the C input channels to generate the E
transmitted channel(s), where C>E>1, wherein the apparatus
is adapted to transmit information about the cue codes to
enable a decoder to perform synthesis processing during
decoding of the E transmitted channel(s).
According to another embodiment, the present invention is
an encoded audio bitstream generated by encoding audio
channels, wherein one or more cue codes are generated for
two or more audio channels, wherein at least one cue code
is a combined cue code generated by combining two or more
estimated cue codes, and each estimated cue code is
estimated from a group of two or more of the audio
channels. The one or more cue codes and E transmitted audio
channel(s) corresponding to the two or more audio channels,
where E≥1, are encoded into the encoded audio bitstream.
According to another embodiment, the present invention is
an encoded audio bitstream comprising one or more cue codes
and E transmitted audio channel (s) . The one or more cue
codes are generated for two or more audio channels, wherein
at least one cue code is a combined cue code generated by
combining two or more estimated cue codes, and each
estimated cue code is estimated from a group of two or more
of the audio channels. The E transmitted audio channel(s)
correspond to the two or more audio channels.
According to another embodiment, the present invention is a
method, apparatus, and machine-readable medium for decoding
E transmitted audio channel(s) to generate C playback audio
channels, where C>E≥1. Cue codes corresponding to the E
transmitted channel(s) are received, wherein at least one
cue code is a combined cue code generated by combining two
or more estimated cue codes, and each estimated cue code
estimated from a group of two or more audio channels
corresponding to the E transmitted channel(s). One or more
of the E transmitted channel(s) are upmixed to generate one

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or more upmixed channels. One or more of the C playback
channels are synthesized by applying the cue codes to the
one or more upmixed channels, wherein two or more derived
cue codes are derived from the combined cue code, and each
derived cue code is applied to generate two or more
synthesized channels.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects, features, and advantages of the present
invention will become more fully apparent from the
following detailed description, the appended claims, and
the accompanying drawings in which like reference numerals
identify similar or identical elements.
FIG. 1 shows a high-level block diagram of conventional
binaural signal synthesizer;
FIG. 2 is a block diagram of a generic binaural cue coding
(BCC) audio processing system;
FIG. 3 shows a block diagram of a downmixer that can be
used for the downmixer of FIG. 2;
FIG. 4 shows a block diagram of a BCC synthesizer that can
be used for the decoder of FIG. 2;
FIG. 5 shows a block diagram of the BCC estimator of FIG.
2, according to one embodiment of the present invention;
FIG. 6 illustrates the generation of ICTD and ICLD data for
five-channel audio;
FIG. 7 illustrates the generation of ICC data for five-
channel audio;

- 7 -
FIG. 8 shows a block diagram of an implementation of the
BCC synthesizer of FIG. 4 that can be used in a BCC decoder
to generate a stereo or multi-channel audio signal given a
single transmitted sum signal s(n) plus the spatial cues;
FIG. 9 illustrates how ICTD and ICLD are varied within a
subband as a function of frequency;
FIG. 10 shows a block diagram of a BCC synthesizer that can
be used for the decoder of FIG. 2 for a 5-to-2 BCC scheme;
and
FIG. 11 shows a flow diagram of the processing of a BCC
system, such as that shown in FIG. 2, related to one
embodiment of the present invention.
DETAILED DESCRIPTION
In binaural cue coding (BCC), an encoder encodes C input
audio channels to generate E transmitted audio channels,
where C>E≥1. In particular, two or more of the C input
channels are provided in a frequency domain, and one or
more cue codes are generated for each of one or more
different frequency bands in the two or more input channels
in the frequency domain. In addition, the C input channels
are downmixed to generate the E transmitted channels. In
some downmixing implementations, at least one of the E
transmitted channels is based on two or more of the C input
channels, and at least one of the E transmitted channels is
based on only a single one of the C input channels.
In one embodiment, .a BCC coder has two or more filter
banks, a code estimator, and a downmixer. The two or more
filter banks convert two or more of the C input channels
from a time, domain into a frequency domain. The code
estimator generates one or more cue codes for each of one
or more different frequency bands in the two or more

- 8 -
converted input channels. The downmixer downmixes the C
input channels to generate the E transmitted channels,
where C>E≥1.
In BCC decoding, E transmitted audio channels are decoded
to generate C playback audio channels. In particular, for
each of one or more different frequency bands, one or more
of the E transmitted channels are upmixed in a frequency
domain to generate two or more of the C playback channels
in the frequency domain, where C>E≥1. One or more cue codes
are applied to each of the one or more different frequency
bands in the two or more playback channels in the frequency
domain to generate two or more modified channels, and the
two or more modified channels are converted from the
frequency domain into a time domain. In some upmixing
implementations, at least one of the C playback channels is
based on at least one of the E transmitted channels and at
least one cue code, and at least one of the C playback
channels is based on only a single one of the E transmitted
channels and independent of any cue codes.
In one embodiment, a BCC decoder has an upmixer, a
synthesizer, and one or more inverse filter banks. For each
of one or more different frequency bands, the upmixer
upmixes one or more of the E transmitted channels in a
frequency domain to generate two or more of the C playback
channels in the frequency domain, where C>E≥1. The
synthesizer applies one or more cue codes to each of the
one or more different frequency bands in the two or more
playback channels in the frequency domain to generate two
or more modified channels. The one or more inverse filter
banks convert the two or more modified channels from the
frequency domain into a time domain.
Depending on the particular implementation, a given
playback channel may be based on a single transmitted
channel, rather than a combination of two or more
transmitted channels. For example, when there is only one

- 9 -
transmitted channel, each of the C playback channels is
based on that one transmitted channel. In these situations,
upmixing corresponds to copying of the corresponding
transmitted channel. As such, for applications in which
there is only one transmitted channel, the upmixer may be
implemented using a replicator that copies the transmitted
channel for each playback channel.
BCC encoders and/or decoders may be incorporated into a
number of systems or applications including, for example,
digital video recorders/players, digital audio
recorders/players, computers, satellite
transmitters/receivers, cable transmitters/receivers,
terrestrial broadcast transmitters/receivers, home
entertainment systems, and movie theater systems.
Generic BCC Processing
FIG. 2 is a block diagram of a generic binaural cue coding
(BCC) audio processing system 200 comprising an encoder 202
and a decoder 204. Encoder 202 includes downmixer 206 and
BCC estimator 208.
Downmixer 206 converts C input audio channels xi(n) into E
transmitted audio channels yi(n), where C>E≥1. In this
specification, signals expressed using the variable n are
time-domain signals, while signals expressed using the
variable k are frequency-domain signals. Depending on the
particular implementation, downmixing can be implemented in
either the time domain or the frequency domain. BCC
estimator 208 generates BCC codes from the C input audio
channels and transmits those BCC codes as either in-band or
out-of-band side information relative to the E transmitted
audio channels. Typical BCC codes include one or more of
inter-channel time difference (ICTD), inter-channel level
difference (ICLD), and inter-channel correlation (ICC) data
estimated between certain pairs of input channels as a

- 10 -
function of frequency and time. The particular
implementation will dictate between which particular pairs
of input channels, BCC codes are estimated.
ICC data corresponds to the coherence of a binaural signal,
which is related to the perceived width of the audio
source. The wider the audio source, the lower the coherence
between the left and right channels of the resulting
binaural signal. For example, the coherence of the binaural
signal corresponding to an orchestra spread out over an
auditorium stage is typically lower than the coherence of
the binaural signal corresponding to a single violin
playing solo. In general, an audio signal with lower
coherence is usually perceived as more spread out in
auditory space. As such, ICC data is typically related to
the apparent source width and degree of listener
envelopment. See, e.g., J. Blauert, The Psychophysics of
Human Sound Localization, MIT Press, 1983.
Depending on the particular application, the E transmitted
audio channels and corresponding BCC codes may be
transmitted directly to decoder 204 or stored in some
suitable type of storage device for subsequent access by
decoder 204. Depending on the situation, the term
"transmitting" may refer to either direct transmission to a
decoder or storage for subsequent provision to a decoder.
In either case, decoder 204 receives the transmitted audio
channels and side information and performs upmixing and BCC
synthesis using the BCC codes to convert the E transmitted
audio channels into more than E (typically, but not
necessarily, C) playback audio channels xi(n) for audio
playback. Depending on the particular implementation,
upmixing can be performed in either the time domain or the
frequency domain.
In addition to the BCC processing shown in FIG. 2, a
generic BCC audio processing system may include additional
encoding and decoding stages to further compress the audio

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signals at the encoder and then decompress the audio
signals at the decoder, respectively. These audio codecs
may be based on conventional audio
compression/decompression techniques such as those based on
pulse code modulation (PCM), differential PCM (DPCM), or
adaptive DPCM (ADPCM).
When downmixer 206 generates a single sum signal (i.e.,
E=l), BCC coding is able to represent multi-channel audio
signals at a bitrate only slightly higher than what is
required to represent a mono audio signal. This is so,
because the estimated ICTD, ICLD, and ICC data between a
channel pair contain about two orders of magnitude less
information than an audio waveform.
Not only the low bitrate of BCC coding, but also its
backwards compatibility aspect is of interest. A single
transmitted sum signal corresponds to a mono downmix of the
original stereo or multi-channel signal. For receivers that
do not support stereo or multi-channel sound reproduction,
listening to the transmitted sum signal is a valid method
of presenting the audio material on low-profile mono
reproduction equipment. BCC coding can therefore also be
used to enhance existing services involving the delivery of
mono audio material towards multi-channel audio. For
example, existing mono audio radio broadcasting systems can
be enhanced for stereo or multi-channel playback if the BCC
side information can be embedded into the existing
transmission channel. Analogous capabilities exist when
downmixing multi-channel audio to two sum signals that
correspond to stereo audio.
BCC processes audio signals with a certain time and
frequency resolution. The frequency resolution used is
largely motivated by the frequency resolution of the human
auditory system. Psychoacoustics suggests that spatial
perception is most likely based on a critical band
representation of the acoustic input signal. This frequency

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resolution is considered by using an invertible filterbank
(e.g., based on a fast Fourier transform (FFT) or a
quadrature mirror filter (QMF)) with subbands with
bandwidths equal or proportional to the critical bandwidth
of the human auditory system.
Generic Downmixing
In preferred implementations, the transmitted sum signal(s)
contain all signal components of the input audio signal.
The goal is that each signal component is fully maintained.
Simply summation of the audio input channels often results
in amplification or attenuation of signal components. In
other words, the power of the signal components in a
"simple" sum is often larger or smaller than the sum of the
power of the corresponding signal component of each
channel. A downmixing technique can be used that equalizes
the sum signal such that the power of signal components in
the sum signal is approximately the same as the
corresponding power in all input channels.
FIG. 3 shows a block diagram of a downmixer 300 that can be
used for downmixer 206 of FIG. 2 according to certain
implementations of BCC system 200. Downmixer 300 has a
filter bank (FB) 302 for each input channel xi(n), a
downmixing block 304, an optional scaling/delay block 306,
and an inverse FB (IFB) 308 for each encoded channel yi(n) .
Each filter bank 302 converts each frame (e.g., 20 msec) of
a corresponding digital input channel Xi(n) in the time
domain into a set of input coefficients 5c x1 (k) in the
frequency domain. Downmixing block 304 downmixes each sub-
band of C corresponding input coefficients into a
corresponding sub-band of E downmixed frequency-domain
coefficients. Equation (1) represents the downmixing of the
kth sub-band of input coefficients (x1 (k) , x2 (k) , ...xc (k) ) to
generate the kth sub-band of downmixed coefficients
( y1 (k) , y2 (k) ,..., yE(k)) as follows:

- 13 -

where DCE is a real-valued C-by-E downmixing matrix.
Optional scaling/delay block 306 comprises a set of
multipliers 310, each of which multiplies a corresponding
downmixed coefficient y1(k) by a scaling factor ei(k) to
generate a corresponding scaled coefficient y (k) • The
motivation for the scaling operation is equivalent to
equalization generalized for downmixing with arbitrary
weighting factors for each channel. If the input channels
are independent, then the power Py1(k) of the downmixed
signal in each sub-band is given by Equation (2) as
follows:

where DCE is derived by squaring each matrix element in
the C-by-E downmixing matrix DCE and px(k) is the power of
sub-band k of input channel i.
If the sub-bands are not independent, then the power values
Py(k) of the downmixed signal will be larger or smaller than
that computed using Equation (2), due to signal

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amplifications or cancellations when signal components are
in-phase or out-of-phase, respectively. To prevent this,
the downmixing operation of Equation (1) is applied in sub-
bands followed by the scaling operation of multipliers 310.
The scaling factors ei(k) (l≤i≤E) can be derived using
Equation (3) as follows:

where Py(k) is the sub-band power as computed by Equation
(2), and py(t) is power of the corresponding downmixed sub-
band signal y1(k) .
In addition to or instead of providing optional scaling,
scaling/delay block 306 may optionally apply delays to the
signals.
Each inverse filter bank 308 converts a set of
corresponding scaled coefficients yi(k) in the frequency
domain into a frame of a corresponding digital, transmitted
channel yi (n) .
Although FIG. 3 shows all C of the input channels being
converted into the frequency domain for subsequent
downmixing, in alternative implementations, one or more
(but less than C-l) of the C input channels might bypass
some or all of the processing shown in FIG. 3 and be
transmitted as an equivalent number of unmodified audio
channels. Depending on the particular implementation, these
unmodified audio channels might or might not be used by BCC
estimator 208 of FIG. 2 in generating the transmitted BCC
codes.
In an implementation of downmixer 300 that generates a
single sum signal y(n), E=l and the signals xc(k) of each

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subband of each input channel c are added and then
multiplied with a factor e(k), according to Equation (4) as
follows:

the factor e(k) is given by Equation (5) as follows:

where p? (k) is a short-time estimate of the power of xc(k)
at time index k, and px (k) is a short-time estimate of the
power of
The equalized subbands are transformed
back to the time domain resulting in the sum signal y(n)
that is transmitted to the BCC decoder.
Generic BCC Synthesis
FIG. 4 shows a block diagram of a BCC synthesizer 400 that
can be used for decoder 204 of FIG. 2 according to certain
implementations of BCC system 200. BCC synthesizer 400 has
a filter bank 402 for each transmitted channel yi(n), an
upmixing block 404, delays 406, multipliers 408,
correlation block 410, and an inverse filter bank 412 for
each playback channel xi(n).
Each filter bank 402 converts each frame of a corresponding
digital, transmitted channel yi(n) in the time domain into
a set of input coefficients yi (k) in the frequency domain.
Upmixing block 404 upmixes each sub-band of E corresponding

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transmitted-channel coefficients into a corresponding sub-
band of C upmixed frequency-domain coefficients. Equation
(4) represents the upmixing of the kth sub-band of
transmitted-channel coefficients {y](k),y2{k),...,yl,{k)) to
generate the kth sub-band of upmixed coefficients
as follows:

where UEC is a real-valued E-by-C upmixing matrix.
Performing upmixing in the frequency-domain enables
upmixing to be applied individually in each different sub-
band.
Each delay 406 applies a delay value di(k) based on a
corresponding BCC code for ICTD data to ensure that the
desired ICTD values appear between certain pairs of
playback channels. Each multiplier 408 applies a scaling
factor a1(k) based on a corresponding BCC code for ICLD
data to ensure that the desired ICLD values appear between
certain pairs of playback channels. Correlation block 410
performs a decorrelation operation A based on corresponding
BCC codes for ICC data to ensure that the desired ICC
values appear between certain pairs of playback channels.
Further description of the operations of correlation block
410 can be found in U.S. patent application Ser. No.
10/155,437, filed on May 24, 2002 as Baumgarte 2-10.
The synthesis of ICLD values may be less troublesome than
the synthesis of ICTD and ICC values, since ICLD synthesis
involves merely scaling of sub-band signals. Since ICLD
cues are the most commonly used directional cues, it is
usually more important that the ICLD values approximate

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those of the original audio signal. As such, ICLD data
might be estimated between all channel pairs. The scaling
factors ai(k) (l≤i≤C) for each sub-band are preferably
chosen such that the sub-band power of each playback
channel approximates the corresponding power of the
original input audio channel.
One goal may be to apply relatively few signal
modifications for synthesizing ICTD and ICC values. As
such, the BCC data might not include ICTD and ICC values
for all channel pairs. In that case, BCC synthesizer 400
would synthesize ICTD and ICC values only between certain
channel pairs.
Each inverse filter bank 412 converts a set of
corresponding synthesized coefficients x,(k) in the
frequency domain into a frame of a corresponding digital,
playback channel x1,. (n)
Although FIG. 4 shows all E of the transmitted channels
being converted into the frequency domain for subsequent
upmixing and BCC processing, in alternative
implementations, one or more (but not all) of the E
transmitted channels might bypass some or all of the
processing shown in FIG. 4. For example, one or more of the
transmitted channels may be unmodified channels that are
not subjected to any upmixing. In addition to being one or
more of the C playback channels, these unmodified channels,
in turn, might be, but do not have to be, used as reference
channels to which BCC processing is applied to synthesize
one or more of the other playback channels. In either case,
such unmodified channels may be subjected to delays to
compensate for the processing time involved in the upmixing
and/or BCC processing used to generate the rest of the
playback channels.
Note that, although FIG. 4 shows C playback channels being
synthesized from E transmitted channels, where C was also

- 18 -
the number of original input channels, BCC synthesis is not
limited to that number of playback channels. In general,
the number of playback channels can be any number of
channels, including numbers greater than or less than C and
possibly even situations where the number of playback
channels is equal to or less than the number of transmitted
channels.
"Perceptually Relevant Differences" Between Audio Channels
Assuming a single sum signal, BCC synthesizes a stereo or
multi-channel audio signal such that ICTD, ICLD, and ICC
approximate the corresponding cues of the original audio
signal. In the following, the role of ICTD, ICLD, and ICC
in relation to auditory spatial image attributes is
discussed.
Knowledge about spatial hearing implies that for one
auditory event, ICTD and ICLD are related to perceived
direction. When considering binaural room impulse responses
(BRIRs) of one source, there is a relationship between
width of the auditory event and listener envelopment and
ICC data estimated for the early and late parts of the
BRIRs. However, the relationship between ICC and these
properties for general signals (and not just the BRIRs) is
not straightforward.
Stereo and multi-channel audio signals usually contain a
complex mix of concurrently active source signals
superimposed by reflected signal components resulting from
recording in enclosed spaces or added by the recording
engineer for artificially creating a spatial impression.
Different source signals and their reflections occupy
different regions in the time-frequency plane. This is
reflected by ICTD, ICLD, and ICC, which vary as a function
of time and frequency. In this case, the relation between
instantaneous ICTD, ICLD, and ICC and auditory event

- 19 -
directions and spatial impression is not obvious. The
strategy of certain embodiments of BCC is to blindly
synthesize these cues such that they approximate the
corresponding cues of the original audio signal.
Filterbanks with subbands of bandwidths equal to two times
the equivalent rectangular bandwidth (ERB) are used.
Informal listening reveals that the audio quality of BCC
does not notably improve when choosing higher frequency
resolution. A lower frequency resolution may be desired,
since it results in less ICTD, ICLD, and ICC values that
need to be transmitted to the decoder and thus in a lower
bitrate.
Regarding time resolution, ICTD, ICLD, and ICC are
typically considered at regular time intervals. High
performance is obtained when ICTD, ICLD, and ICC are
considered about every 4 to 16 ms. Note that, unless the
cues are considered at very short time intervals, the
precedence effect is not directly considered. Assuming a
classical lead-lag pair of sound stimuli, if the lead and
lag fall into a time interval where only one set of cues is
synthesized, then localization dominance of the lead is not
considered. Despite this, BCC achieves audio quality
reflected in an average MUSHRA score of about 87 (i.e.,
"excellent" audio quality) on average and up to nearly 100
for certain audio signals.
The often-achieved perceptually small difference between
reference signal and synthesized signal implies that cues
related to a wide range of auditory spatial image
attributes are implicitly considered by synthesizing ICTD,
ICLD, and ICC at regular time intervals. In the following,
some arguments are given on how ICTD, ICLD, and ICC may
relate to a range of auditory spatial image attributes.
Estimation of Spatial Cues

- 20 -
In the following, it is described how ICTD, ICLD, and ICC
are estimated. The bitrate for transmission of these
(quantized and coded) spatial cues can be just a few kb/s
and thus, with BCC, it is possible to transmit stereo and
multi-channel audio signals at bitrates close to what is
required for a single audio channel.
FIG. 5 shows a block diagram of BCC estimator 208 of FIG.
2, according to one embodiment of the present invention.
BCC estimator 208 comprises filterbanks (FB) 502, which may
be the same as filterbanks 302 of FIG. 3, and estimation
block 504, which generates ICTD, ICLD, and ICC spatial cues
for each different frequency subband generated by
filterbanks 502.
Estimation of ICTD, ICLD, and ICC for Stereo Signals
The following measures are used for ICTD, ICLD, and ICC for
corresponding subband signals x, (k) and x2 (k) of two (e.g.,
stereo) audio channels:
with a short-time estimate of the normalized cross-
correlation function given by Equation (8) as follows:
where
o ICTD [samples] :



- 21 -

and is a short-time estimate of the mean of
xx(k-d,)x2(k-d2).
o ICLD [dB]:

o ICC:

Note that the absolute value of the normalized cross-
correlation is considered and C12(k) has a range of [0,1].
Estimation of ICTD, ICLD, and ICC for Multi-Channel Audio
Signals
When there are more than two input channels, it is
typically sufficient to define ICTD and ICLD between a
reference channel (e.g., channel number 1) and the other
channels, as illustrated in FIG. 6 for the case of C=5
channels, where I1c(k) and ∆L1c(k) denote the ICTD and ICLD,
respectively, between the reference channel 1 and channel
c.
As opposed to ICTD and ICLD, ICC typically has more degrees
of freedom. The ICC as defined can have different values

- 22 -
between all possible input channel pairs. For C channels,
there are C(C-l)/2 possible channel pairs; e.g., for 5
channels there are 10 channel pairs as illustrated in FIG.
7(a). However, such a scheme requires that, for each
subband at each time index, C(C-l)/2 ICC values are
estimated and transmitted, resulting in high computational
complexity and high bitrate.
Alternatively, for each subband, ICTD and ICLD determine
the direction at which the auditory event of the
corresponding signal component in the subband is rendered.
One single ICC parameter per subband may then be used to
describe the overall coherence between all audio channels.
Good results can be obtained by estimating and transmitting
ICC cues only between the two channels with most energy in
each subband at each time index. This is illustrated in
FIG. 7 (b) , where for time instants k-1 and k the channel
pairs (3, 4) and (1, 2) are strongest, respectively. A
heuristic rule may be used for determining ICC between the
other channel pairs.
Synthesis of Spatial Cues
FIG. 8 shows a block diagram of an implementation of BCC
synthesizer 400 of FIG. 4 that can be used in a BCC decoder
to generate a stereo or multi-channel audio signal given a
single transmitted sum signal s(n) plus the spatial cues.
The sum signal s (n) is decomposed into subbands, where
s (k) denotes one such subband. For generating the
corresponding subbands of each of the output channels,
delays dc, scale factors ac, and filters hc are applied to
the corresponding subband of the sum signal. (For
simplicity of notation, the time index k is ignored in the
delays, scale factors, and filters.) ICTD are synthesized
by imposing delays, ICLD by scaling, and ICC by applying
de-correlation filters. The processing shown in FIG. 8 is
applied independently to each subband.

- 23 -
ICTD Synthesis
The delays dc are determined from the ICTDs I1c(k),
according to Equation (12) as follows:

The delay for the reference channel, d1, is computed such
that the maximum magnitude of the delays dc is minimized.
The less the subband signals are modified, the less there
is a danger for artifacts to occur. If the subband sampling
rate does not provide high enough time-resolution for ICTD
synthesis, delays can be imposed more precisely by using
suitable all-pass filters.
ICLD Synthesis
In order that the output subband signals have desired ICLDs
ALi2(k) between channel c and the reference channel 1, the
gain factors ac should satisfy Equation (13) as follows:

Additionally, the output subbands are preferably normalized
such that the sum of the power of all output channels is
equal to the power of the input sum signal. Since the total
original signal power in each subband is preserved in the
sum signal, this normalization results in the absolute
subband power for each output channel approximating the

- 24 -
corresponding power of the original encoder input audio
signal. Given these constraints, the scale factors ac are
given by Equation (14) as follows:

ICC Synthesis
In certain embodiments, the aim of ICC synthesis is to
reduce correlation between the subbands after delays and
scaling have been applied, without affecting ICTD and ICLD.
This can be achieved by designing the filters hc in FIG. 8
such that ICTD and ICLD are effectively varied as a
function of frequency such that the average variation is
zero in each subband (auditory critical band).
FIG. 9 illustrates how ICTD and ICLD are varied within a
subband as a function of frequency. The amplitude of ICTD
and ICLD variation determines the degree of de-correlation
and is controlled as a function of ICC. Note that ICTD are
varied smoothly (as in FIG. 9 (a)), while ICLD are varied
randomly (as in FIG. 9(b)). One could vary ICLD as smoothly
as ICTD, but this would result in more coloration of the
resulting audio signals.
Another method for synthesizing ICC, particularly suitable
for multi-channel ICC synthesis, is described in more
detail in C. Faller, "Parametric multi-channel audio
coding: Synthesis of coherence cues," IEEE Trans, on Speech
and Audio Proc, 2003, the teachings of which are
incorporated herein by reference. As a function of time and
frequency, specific amounts of artificial late

- 25 -
reverberation are added to each of the output channels for
achieving a desired ICC. Additionally, spectral
modification can be applied such that the spectral envelope
of the resulting signal approaches the spectral envelope of
the original audio signal.
Other related and unrelated ICC synthesis techniques for
stereo signals (or audio channel pairs) have been presented
in E. Schuijers, W. Oomen, B. den Brinker, and J.
Breebaart, "Advances in parametric coding for high-quality
audio," in Preprint 114th Conv. Aud. Eng. Soc, March 2003,
and J. Engdegard, H. Purnhagen, J. Roden, and L. Liljeryd,'
"Synthetic ambience in parametric stereo coding," in
Preprint 117th Conv. Aud. Eng. Soc, May 2004, the
teachings of both of which are incorporated here by
reference.
C-to-E BCC
As described previously, BCC can be implemented with more
than one transmission channel. A variation of BCC has been
described which represents C audio channels not as one
single (transmitted) channel, but as E channels, denoted C-
to-E BCC. There are (at least) two motivations for C-to-E
BCC:
o BCC with one transmission channel provides a backwards
compatible path for upgrading existing mono systems for
stereo or multi-channel audio playback. The upgraded
systems transmit the BCC downmixed sum signal through
the existing mono infrastructure, while additionally
transmitting the BCC side information. C-to-E BCC is
applicable to E-channel backwards compatible coding of
C-channel audio.
o C-to-E BCC introduces scalability in terms of different
degrees of reduction of the number of transmitted

- 26 -
channels. It is expected that the more audio channels
that are transmitted, the better the audio quality will
be. Signal processing details for C-to-E BCC, such as
how to define the ICTD, ICLD, and ICC cues, are
described in U.S. application Ser. No. 10/762,100,
filed on Jan. 20, 2004 (Faller 13-1) .
Compact Side Information
As described above, in a typical BCC scheme, the encoder
transmits to the decoder ICTD, ICLD, and/or ICC codes
estimated between different pairs or groups of audio
channels. This side information is transmitted in addition
to the (e.g., mono or stereo) downmix signal(s) in order to
obtain a multi-channel audio signal after BCC decoding.
Thus, it is desirable to minimize the amount of side
information while not degrading subjective quality of the
decoded sound.
Since ICLD and ICTD values typically relate to one
reference channel, C-l ICLD and ICTD values are sufficient
to describe the characteristics of C encoded channels). On
the other hand, ICCs are defined between arbitrary pairs of
channels. As such, for C encoded channels, there are C(C-
l)/2 possible ICC pairs. For 5 encoded channels, this would
correspond to 10 ICC pairs. In practice, in order to limit
the amount of transmitted ICC information, only ICC
information for certain pairs are transmitted.
FIG. 10 shows a block diagram of a BCC synthesizer 1000
that can be used for decoder 204 of FIG. 2 for a 5-to-2 BCC
scheme. As shown in FIG. 10, BCC synthesizer 1000 receives
two input signals y1(n) and y2(n) and BCC side information
(not shown) and generates five synthesized output signals
x1 (n) ,..., x5 (n) , where, first, second, third, fourth, and
fifth output signals correspond to the left, right, center,

- 27 -
rear left, and rear right surround signals, respectively,
shown in FIGS. 6 and 7.
Delay, scaling, and de-correlation parameters derived from
the transmitted ICTD, ICLD, and ICC side information are
applied at elements 1004, 1006, and 1008, respectively, to
synthesize the five output signals x,(n) from the five
"upmixed" signals s1. (k) generated by upmixing element 1002.
As shown in FIG. 10, de-correlation is performed only
between the left and left rear channels (i.e., channels 1
and 4) and between the right and right rear channels (i.e.,
channels 2 and 5) . As such, no more than two sets of ICC
data need to be transmitted to BCC synthesizer 1000, where
those two sets characterize the ICC values between the two
channel pairs for each subband. While this is already a
considerable reduction in the amount of ICC side
information, a further reduction is desirable.
According to one embodiment of the present invention, in
the context of the 5-to-2 BCC scheme of FIG. 10, for each
subband, the corresponding BCC encoder combines the ICC
value estimated for the "left/left rear" channel pair with
the ICC value estimated for the "right/right rear" channel
pair to generate a single, combined ICC value that
effectively indicates a global amount of front/back de-
correlation and which is transmitted to the BCC decoder as
the ICC side information. Informal experiments indicated
that this simplification results in virtually no loss in
audio quality, while reducing transmitted ICC information
by a factor of two.
In general, embodiments of the present invention are
directed to BCC schemes in which two or more different ICCs
estimated between different channel pairs, or groups of
channels, are combined for transmission, as indicated by
Equation (15) as follows:

- 28 -

where f is a function that combines N different ICCs.
In order to obtain a combined ICC measure that is
representative of the spatial image, it may be advantageous
to use a weighted average for function f that considers the
importance of the individual channels, where channel
importance may be based on the channel powers, as
represented by Equation (16) as follows:

where pi is the power of the corresponding channel pair in
the subband. In this case, ICCs estimated from stronger
channel pairs are weighted more than ICCs estimated from
weaker channel pairs. The combined power pi of a channel
pair may be computed as the sum of the individual channel
powers for each subband.
In the decoder, given ICCtransmitted, ICCs may be derived for
each channel pair. In one possible implementation, the
decoder simply uses ICCtransmitted as the derived ICC code for
each channel pair. For example, in the context of the 5-to-
2 BCC scheme of FIG. 110 ICCtransmitted can be used directly
for the de-correlation of both the left/left rear channel
pair and the right/right rear channel pair.
In another possible implementation, if the decoder
estimates channel pair powers from the synthesized signals,
then the weighting of Equation (16) can be estimated and
the decoder process can optionally use this information and
other perceptual and signal statistics arguments for

- 29 -
generating a rule for deriving two individual, perceptually
optimized ICC codes.
Although the combination of ICC values has been described
in the context of a particular 5-to-2 BCC scheme, the
present invention can be implemented in the context of any
C-to-E BCC scheme, including those in which E=l.
FIG. 11 shows a flow diagram of the processing of a BCC
system, such as that shown in FIG. 2, related to one
embodiment of the present invention. FIG. 11 shows only
those steps associated with ICC-related processing.
In particular, a BCC encoder estimates ICC values between
two or more groups of channels (step 1102), combines two or
more of those estimated ICC values to generate one or more
combined ICC values (step 1104), and transmits the combined
ICC values (possibly along with one or more "uncombined"
ICC values) as BCC side information to a BCC decoder (step
1106). The BCC decoder derives two or more ICC values from
the received, combined ICC values (step 1108) and de-
correlates groups of channels using the derived ICC values
(and possibly one or more received, uncombined ICC values)
(step 1110) .
Further Alternative Embodiments
The present invention has been described in the context of
the 5-to-2 BCC scheme of FIG. 10. In that example, a BCC
encoder (1) estimates two ICC codes for two channel pairs
consisting of four different channels (i.e., left/left rear
and right/right rear) and (2) averages those two ICC codes
to generate a combined ICC code, which is transmitted to a
BCC decoder. The BCC decoder (1) derives two ICC codes from
the transmitted, combined ICC code (note that the combined
ICC code may simply be used for both of the derived ICC
codes) and (2) applies each of the two derived ICC codes to

- 30 -
a different pair of synthesized channels to generate four
de-correlated channels (i.e., synthesized left, left rear,
right, and right rear channels).
The present invention can also be implemented in other
contexts. For example, a BCC encoder could estimate two ICC
codes from three input channels A, B, and C, where one
estimated ICC code corresponds to channels A and B, and the
other estimated ICC code corresponds to channels A and C.
In that case, the encoder could be said to estimate two ICC
codes from two pairs of input channels, where the two pairs
of input channels share a common channel (i.e., input
channel A). The encoder could then generate and transmit a
single, combined ICC code based on the two estimated ICC
codes. A BCC decoder could then derive two ICC codes from
the transmitted, combined ICC code and apply those two
derived ICC codes to synthesize three de-correlated
channels (i.e., synthesized channels A, B, and C) . In this
case, each derived ICC code may be said to be applied to
generate a pair of de-correlated channels, where the two
pairs of de-correlated channels share a common channel
(i.e., synthesized channel A).
Although the present invention has been described in the
context of BCC coding schemes that employ combined ICC
codes, the present invention can also be implemented in the
context of BCC coding schemes that employ combined BCC cue
codes that are generated by combining two or more BCC cue
codes other than ICC codes, such as ICTD codes and/or ICLD
codes, instead of or in addition to employing combined ICC
codes.
Although the present invention has been described in the
context of BCC coding schemes involving ICTD, ICLD, and ICC
codes, the present invention can also be implemented in the
context of other BCC coding schemes involving only one or
two of these three types of codes (e.g., ICLD and ICC, but
not ICTD) and/or one or more additional types of codes.

- 31 -
In the 5-to-2 BCC scheme represented in FIG. 10, the two
transmitted channels y1(n) and y2n) are typically
generated by applying a particular one-stage downmixing
scheme to the five channels shown in FIGS. 6 and 7, where
channel y1 is generated as a weighted sum of channels 1, 3,
and 4, and channel y2 is generated as a weighted sum of
channels 2, 3, and 5, where, for example, in each weighted
sum, the weight factor for channel 3 is one half of the
weight factor used for each of the two other channels. In
this one-stage BCC scheme, the estimated BCC cue codes
correspond to different pairs of the original five input
channels. For example, one set of estimated ICC codes is
based on channels 1 and 4 and another set of estimated ICC
codes is based on channels 2 and 5.
In an alternative, multi-stage BCC scheme, channels are
downmixed sequentially, with BCC cue codes potentially
corresponding to different groups of channels at each stage
in the downmixing sequence. For example, for the five
channels in FIGS. 6 and 7, at a BCC encoder, the original
left and rear left channels could be downmixed to form a
first-downmixed left channel with a first set of BCC cue
codes generated corresponding to those two original
channels. Similarly, the original right and right rear
channels could be downmixed to form a first-downmixed right
channel with a second set of BCC cue codes generated
corresponding to those two original channels. In a second
downmixing stage, the first-downmixed left channel could be
downmixed with the original center channel to form a
second-downmixed left channel with a third set of BCC cue
codes generated corresponding to the first-downmixed left
channel and the original center channel. Similarly, the
first-downmixed right channel could be downmixed with the
original center channel to form a second-downmixed right
channel with a fourth set of BCC cue codes generated
corresponding to the first-downmixed right channel and the
original center channel. The second-downmixed left and

- 32 -
right channels could then be transmitted with all four sets
of BCC cue codes as the side information. In an analogous
manner, a corresponding BCC decoder could then sequentially
apply these four sets of BCC cue codes at different stages
of a two-stage, sequential upmixing scheme to synthesize
five output channels from the two transmitted "stereo"
channels.
Although the present invention has been described in the
context of BCC coding schemes in which combined ICC cue
codes are transmitted with one or more audio channels
(i.e., the E transmitted channels) along with other BCC
codes, in alternative embodiments, the combined ICC cue
codes could be transmitted, either alone or with other BCC
codes, to a place (e.g., a decoder or a storage device)
that already has the transmitted channels and possibly
other BCC codes.
Although the present invention has been described in the
context of BCC coding schemes, the present invention can
also be implemented in the context of other audio
processing systems in which audio signals are de-correlated
or other audio processing that needs to de-correlate
signals.
Although the present invention has been described in the
context of implementations in which the encoder receives
input audio signal in the time domain and generates
transmitted audio signals in the time domain and the
decoder receives the transmitted audio signals in the time
domain and generates playback audio signals in the time
domain, the present invention is not so limited. For
example, in other implementations, any one or more of the
input, transmitted, and playback audio signals could be
represented in a frequency domain.
BCC encoders and/or decoders may be used in conjunction
with or incorporated into a variety of different

- 33 -
applications or systems, including systems for television
or electronic music distribution, movie theaters,
broadcasting, streaming, and/or reception. These include
systems for encoding/decoding transmissions via, for
example, terrestrial, satellite, cable, internet,
intranets, or physical media (e.g., compact discs, digital
versatile discs, semiconductor chips, hard drives, memory
cards, and the like). BCC encoders and/or decoders may also
be employed in games and game systems, including, for
example, interactive software products intended to interact
with a user for entertainment (action, role play, strategy,
adventure, simulations, racing, sports, arcade, card, and
board games) and/or education that may be published for
multiple machines, platforms, or media. Further, BCC
encoders and/or decoders may be incorporated in audio
recorders/players or CD-ROM/DVD systems. BCC encoders
and/or decoders may also be incorporated into PC software
applications that incorporate digital decoding (e.g.,
player, decoder) and software applications incorporating
digital encoding capabilities (e.g., encoder, ripper,
recoder, and jukebox).
The present invention may be implemented as circuit-based
processes, including possible implementation as a single
integrated circuit (such as an ASIC or an FPGA) , a multi-
chip module, a single card, or a multi-card circuit pack.
As would be apparent to one skilled in the art, various
functions of circuit elements may also be implemented as
processing steps in a software program. Such software may
be employed in, for example, a digital signal processor,
micro-controller, or general-purpose computer.
The present invention can be embodied in the form of
methods and apparatuses for practicing those methods. The
present invention can also be embodied in the form of
program code embodied in tangible media, such as floppy
diskettes, CD-ROMs, hard drives, or any other machine-
readable storage medium, wherein, when the program code is

- 34 -
loaded into and executed by a machine, such as a computer,
the machine becomes an apparatus for practicing the
invention. The present invention can also be embodied in
the form of program code, for example, whether stored in a
storage medium, loaded into and/or executed by a machine,
or transmitted over some transmission medium or carrier,
such as over electrical wiring or cabling, through fiber
optics, or via electromagnetic radiation, wherein, when the
program code is loaded into and executed by a machine, such
as a computer, the machine becomes an apparatus for
practicing the invention. When implemented on a general-
purpose processor, the program code segments combine with
the processor to provide a unique device that operates
analogously to specific logic circuits.
It will be further understood that various changes in the
details, materials, and arrangements of the parts which
have been described and illustrated in order to explain the
nature of this invention may be made by those skilled in
the art without departing from the scope of the invention
as expressed in the following claims.
Although the steps in the following method claims, if any,
are recited in a particular sequence with corresponding
labeling, unless the claim recitations otherwise imply a
particular sequence for implementing some or all of those
steps, those steps are not necessarily intended to be
limited to being implemented in that particular sequence.

35
CLAIMS
1. A method for encoding audio channels, the method
comprising:
generating one or more cue codes for two or more audio
channels, wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code is estimated from a group of
two or more of the audio channels; and transmitting or
storing the one or more cue codes.
2. The method of claim 1, further comprising transmitting
E transmitted audio channel(s) corresponding to the
two or more audio channels, where E>1.
3. The method of claim 2, wherein:
the two or more audio channels comprise C input audio
channel(s), where C>E; and the C input channels are
downmixed to generate the E transmitted channel(s).
4. The method of claim 1, wherein the one or more cue
codes are transmitted to enable a decoder to perform
synthesis processing during decoding of E transmitted
channel(s) based on the combined cue code, wherein the
E transmitted audio channel(s) correspond to the two
or more audio channels, where E>1.
5. The method of claim 1, wherein the one or more cue
codes comprise one or more of a combined inter-channel
correlation (ICC) code, a combined inter-channel level

36
difference (ICLD) code, and a combined inter-channel
time difference (ICTD) code.
6. The method of claim 1, wherein the combined cue code
is generated as an average of the two or more
estimated cue codes.
7. The method of claim 6, wherein the combined cue code
is generated as a weighted average of the two or more
estimated cue codes.
8. The method of claim 7, wherein:
each estimated cue code used to generate the combined
cue code is associated with a weight factor used in
generating the weighted average; and
the weight factor for each estimated cue code is based
on power in the group of channels corresponding to the
estimated cue code.
9. The method of claim 1, wherein the combined cue code
is a combined ICC code.
10. The method of claim 9, wherein:
the two or more audio channels comprise a left
channel, a left rear channel, a right channel, and a
right rear channel;
a first estimated ICC code is generated from the left
and left rear channels;
a second estimated ICC code is generated from the
right and right rear channels; and
the combined ICC code is generated by combining the
first and second estimated ICC codes.

-37—
11. Apparatus for encoding audio channels, the apparatus
comprising:
means for generating one or more cue codes for two or
more audio channels, wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code is estimated from a group of
two or more of the audio channels; and means for
transmitting or storing the one or more cue codes.
12. Apparatus for encoding C input audio channels to
generate E transmitted audio channel(s), the apparatus
comprising:
a code estimator adapted to generate one or more cue
codes for two or more audio channels, wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code is estimated from a group
of two or more of the audio channels; and
a downmixer adapted to downmix the C input channels to
generate the E transmitted channel(s), where C>E≥1,
wherein the apparatus is adapted to transmit
information about the cue codes to enable a decoder to
perform synthesis processing during decoding of the E
transmitted channel(s).
13. The apparatus of claim 12, wherein:

38
the apparatus is a system selected from the group
consisting of a digital video recorder, a digital
audio recorder, a computer, a satellite transmitter, a
cable transmitter, a terrestrial broadcast
transmitter, a home entertainment system, and a movie
theater system; and
the system comprises the code estimator and the
downmixer.
14. A machine-readable medium, having encoded thereon
program code, wherein, when the program code is
executed by a machine, the machine implements a method
for encoding audio channels, the method comprising:
generating one or more cue codes for two or more audio
channels, wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code is estimated from a group of
two or more of the audio channels; and transmitting or
storing the one or more cue codes.
15. An encoded audio bitstream generated by encoding audio
channels, wherein:
one or more cue codes are generated for two or more
audio channels, wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code is estimated from a group
of two or more of the audio channels; and

39
the one or more cue codes and E transmitted audio
channel (s) corresponding to the two or more audio
channels, where E≥l, are encoded into the encoded
audio bitstream.
16. An encoded audio bitstream comprising one or more cue
codes and E transmitted audio channel(s), wherein:
the one or more cue codes are generated for two or
more audio channels, wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code is estimated from a group
of two or more of the audio channels; and
the E transmitted audio channel(s) correspond to the
two or more audio channels.
17. A method for decoding E transmitted audio channel(s)
to generate C playback audio channels, where C>E≥1,
the method comprising:
receiving cue codes corresponding to the E transmitted
channel(s), wherein:
at least one cue code is a combined cue code
generat'ed by combining two or more estimated cue
codes; and
each estimated cue code estimated from a group of
two or more audio channels corresponding to the E
transmitted channel(s);

40
upmixing one or more of the E transmitted channel (s)
to generate one or more upmixed channels; and
synthesizing one or more of the C playback channels by
applying the cue codes to the one or more upmixed
channels, wherein:
two or more derived cue codes are derived from
the combined cue code; and
each derived cue code is applied to generate two
or more synthesized channels.
18. The method of claim 17, wherein the cue codes comprise
one or more of a combined ICC code, a combined ICLD
code, and a combined ICTD code.
19. The method of claim 17, wherein the combined cue code
is an average of the two or more estimated cue codes.
20. The method of claim 19, wherein the combined cue code
is a weighted average of the two or more estimated cue
codes.
21. The method of claim 20, wherein:
each estimated cue code used to generate the combined
cue code is associated with a weight factor used in
generating the weighted average; and
the weight factor for each estimated cue code is based
on power in the group of channels corresponding to the
estimated cue code.
22. The method of claim 17, wherein the two or more
derived cue codes are derived by:

41
deriving a weight factor for each group of two or more
channels associated with an estimated cue code; and
deriving the two or more derived cue codes as a
function of the combined cue code and two or more
derived weight factors.
23. The method of claim 22, wherein each derived weight
factor is derived by:
estimating power in the group of channels
corresponding to an estimated cue code; and
deriving the weight factor based on the estimated
powers for different groups of channels corresponding
to different estimated cue codes.
24. The method of claim 17, wherein the combined cue code
is a combined ICC code.
25. The method of claim 24, wherein:
the two or more audio channels comprise a left
channel, a left rear channel, a right channel, and a
right rear channel;
a first estimated ICC code is generated from the left
and left rear channels;
a second estimated ICC code is generated from the
right and right rear channels; and
the combined ICC code is generated by combining the
first and second estimated ICC codes.
26. The method of claim 25, wherein:

42
the combined ICC code is used to de-correlate
synthesized left and left rear channels; and
the combined ICC code is used to de-correlate
synthesized right and right rear channels.
27. Apparatus for decoding E transmitted audio channel(s)
to generate C playback audio channels, where C>E>1,
the apparatus comprising:
means for receiving cue codes corresponding to the E
transmitted channel(s), wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code estimated from a group of
two or more audio channels corresponding to the E
transmitted channel(s);
means for upmixing one or more of the E transmitted
channel(s) to generate one or more upmixed channels;
and
means for synthesizing one or more of the C playback
channels by applying the cue codes to the one or more
upmixed channels, wherein:
two or more derived cue codes are derived from
the combined cue code; and
each derived cue code is applied to generate two
or more synthesized channels.
28. Apparatus for decoding E transmitted audio channel(s)
to generate C playback audio channels, where C>E≥1,
the apparatus comprising:

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a receiver adapted to receive cue codes corresponding
to the E transmitted channel(s), wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code estimated from a group of
two or more audio channels corresponding to the E
transmitted channel(s);
an upmixer adapted to upmix one or more of the E
transmitted channel(s) to generate one or more upmixed
channels; and
a synthesizer adapted to synthesize one or more of the
C playback channels by applying the cue codes to the
one or more upmixed channels, wherein:
two or more derived cue codes are derived . from
the combined cue code; and
each derived cue code is applied to generate two
or more synthesized channels.
29. The apparatus of claim 28, wherein:
the apparatus is a system selected from the group
consisting of a digital video player, a digital audio
player, a computer, a satellite receiver, a cable
receiver, a terrestrial broadcast receiver, a home
entertainment system, and a movie theater system; and
the system comprises the receiver, the upmixer, and
the synthesizer.

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30. A machine-readable medium, having encoded thereon
program code, wherein, when the program code is
executed by a machine, the machine implements a method
for decoding E transmitted audio channel(s) to
generate C playback audio channels, where C>E>1, the
method comprising:
receiving cue codes corresponding to the E transmitted
channel(s), wherein:
at least one cue code is a combined cue code
generated by combining two or more estimated cue
codes; and
each estimated cue code estimated from a group of
two or more audio channels corresponding to the E
transmitted channel(s);
upmixing one or more of the E transmitted channel (s)
to generate one or more upmixed channels; and
synthesizing one or more of the C playback channels by
applying the cue codes to the one or more upmixed
channels, wherein:
two or more derived cue codes are derived from
the combined cue code; and
each derived cue code is applied to generate two or
more synthesized channels.

At an audio encoder, cue codes are generated for one or
more audio channels, wherein a combined cue code (e.g., a
combined inter-channel correlation (ICC) code) is generated
by combining two or more estimated cue codes, each
estimated cue code estimated from a group of two or more
channels. At an audio decoder, E transmitted audio
channel(s) are decoded to generate C playback audio
channels. Received cue codes include a combined cue code
(e.g., a combined ICC code). One or more transmitted
channel (s) are upmixed to generate one or more upmixed
channels. One or more playback channels are synthesized by
applying the cue codes to the one or more upmixed channels,
wherein two or more derived cue codes are derived from the
combined cue code, and each derived cue code is applied to
generate two or more synthesized channels.