APPARATUS AND METHOD FOR ERROR CONCEALMENT IN LOW- DELAY UNIFIED SPEECH AND AUDIO
CODING (USAC)
Description
5
The present invention relates to audio signal processing and, in particular, to an apparatus
and method for error concealment in Low-Delay Unified Speech and Audio Coding (LDUSAC).
0
Audio signal processing has advanced in many ways and becomes increasingly important.
In audio signal processing, Low-Delay Unified Speech and Audio Coding aims to provide
coding techniques suitable for speech, audio and any mixture of speech and audio.
Moreover, LD-USAC aims to assure a high quality for the encoded audio signals.
Compared to USAC (Unified Speech and Audio Coding), the delay in LD-USAC is
reduced.
When encoding audio data, a LD-USAC encoder examines the audio signal to be encoded.
The LD-USAC encoder encodes the audio signal by encoding linear predictive filter
coefficients of a prediction filter. Depending on the audio data that is to be encoded by a
particular audio frame, the LD-USAC encoder decides, whether ACELP (Advanced Code
Excited Linear Prediction) is used for encoding, or whether the audio data is to be encoded
using TCX (Transform Coded Excitation). While ACELP uses LP filter coefficients (linear
predictive filter coefficients), adaptive codebook indices and algebraic codebook indices
and adaptive and algebraic codebook gains, TCX uses LP filter coefficients, energy
parameters and quantization indices relating to a Modified Discrete Cosine Transform
(MDCT).
On the decoder side, the LD-USAC decoder determines whether ACELP or TCX has been
employed to encode the audio data of a current audio signal frame. The decoder then
decodes the audio signal frame accordingly.
From time to time, data transmission fails. For example, an audio signal frame transmitted
by a sender is arriving with errors at a receiver or does not arrive at all or the frame is late.
In these cases, error concealment may become necessary to ensure that the missing or
erroneous audio data can be replaced. This is particularly true for applications having realtime
requirements, as requesting a retransmission of the erroneous or the missing frame
might infringe low-delay requirements.
However, existing concealment techniques used for other audio applications often create
artificial sound caused by synthetic artefacts.
It is therefore an object of the present invention to provide improved concepts for error
concealment for an audio signal frame. The object of the present invention is solved by an
apparatus according to claim 1, by a method according to claim 15 and by a computer
program according to claim 16.
An apparatus for generating spectral replacement values for an audio signal is provided.
The apparatus comprises a buffer unit for storing previous spectral values relating to a
previously received error-free audio frame. Moreover, the apparatus comprises a
concealment frame generator for generating the spectral replacement values, when a
current audio frame has not been received or is erroneous. The previously received errorfree
audio frame comprises filter information, the filter information having associated a
filter stability value indicating a stability of a prediction filter. The concealment frame
generator is adapted to generate the spectral replacement values based on the previous
spectral values and based on the filter stability value.
The present invention is based on the finding that while previous spectral values of a
previously received error-free frame may be used for error concealment, a fade out should
be conducted on these values, and the fade out should depend on the stability of the signal.
The less stable a signal is, the faster the fade out should be conducted.
In an embodiment, the concealment frame generator may be adapted to generate the
spectral replacement values by randomly flipping the sign of the previous spectral values.
According to a further embodiment, the concealment frame generator may be configured to
generate the spectral replacement values by multiplying each of the previous spectral
values by a first gain factor when the filter stability value has a first value, and by
multiplying each of the previous spectral values by a second gain factor being smaller than
the first gain factor, when the filter stability value has a second value being smaller than
the first value.
In another embodiment, the concealment frame generator may be adapted to generate the
spectral replacement values based on the filter stability value, wherein the previously
received error-free audio frame comprises first predictive filter coefficients of the
prediction filter, wherein a predecessor frame of the previously received error-free audio
frame comprises second predictive filter coefficients, and wherein the filter stability value
depends on the first predictive filter coefficients and on the second predictive filter
coefficients.
According to an embodiment, the concealment frame generator may be adapted to
determine the filter stability value based on the first predictive filter coefficients of the
previously received error-free audio frame and based on the second predictive filter
coefficients of the predecessor frame of the previously received error-free audio frame.
In another embodiment, the concealment frame generator may be adapted to generate the
spectral replacement values based on the filter stability value, wherein the filter stability
value depends on a distance measure LSF j t , and wherein the distance measure L S j is
defined by the formula:
wherein u+1 specifies a total number of the first predictive filter coefficients of the
previously received error-free audio frame, and wherein u+1 also specifies a total number
of the second predictive filter coefficients of the predecessor frame of the previously
received error- ree audio frame, wherein f i specifies the i-fh filter coefficient of the first
predictive filter coefficients and wherein f specifies the i-th filter coefficient of the
second predictive filter coefficients.
According to an embodiment, the concealment frame generator may be adapted to generate
the spectral replacement values furthermore based on frame class information relating to
the previously received error-free audio frame. For example, the frame class information
indicates that the previously received error-free audi frame is classified as "artificial
onset", "onset", "voiced transition", "unvoiced transition", "unvoiced" or "voiced".
In another embodiment, the concealment frame generator may be adapted to generate the
spectral replacement values furthermore based on a number of consecutive frames that did
not arrive at a receiver or that were erroneous, since a last error-free audio frame had
arrived at the receiver, wherein no other error-free audio frames arrived at the receiver
since the last error-free audio frame had arrived at the receiver.
According to another embodiment, the concealment frame generator may be adapted to
calculate a fade out factor and based on the filter stability value and based on the number
of consecutive frames that did not arrive at the receiver or that were erroneous. Moreover,
the concealment frame generator may be adapted to generate the spectral replacement
values by multiplying the fade out factor by at least some of the previous spectral values,
or by at least some values of a group of intermediate values, wherein each one of the
intermediate values depends on at least one of the previous spectral values.
In a further embodiment, the concealment frame generator may be adapted to generate the
spectral replacement values based on the previous spectral values, based on the filter
stability value and also based on a prediction gain of a temporal noise shaping.
According to a further embodiment, an audio signal decoder is provided. The audio signal
decoder may comprise an apparatus for decoding spectral audio signal values, and an
apparatus for generating spectral replacement values according to one of the abovedescribed
embodiments. The apparatus for decoding spectral audio signal values may be
adapted to decode spectral values of an audio signal based on a previously received errorfree
audio frame. Moreover, the apparatus for decoding spectral audio signal values may
furthermore be adapted to store the spectral values of the audio signal in the buffer unit of
the apparatus for generating spectral replacement values. The apparatus for generating
spectral replacement values may be adapted to generate the spectral replacement values
based on the spectral values stored in the buffer unit, when a current audio frame has not
been received r is erroneous.
Moreover, an audio signal decoder according to another embodiment is provided. The
audio signal decoder comprises a decoding unit for generating first intermediate spectral
values based on a received error-free audio frame, a temporal noise shaping unit for
conducting temporal noise shaping on the first intermediate spectral values to obtain
second intermediate spectral values, a prediction gain calculator for calculating a
prediction gain of the temporal noise shaping depending on the first intermediate spectral
values and depending on the second intermediate spectral values, an apparatus according to
one f the above-described embodiments for generating spectral replacement values when
a current audio frame has not been received or is erroneous, and a values selector for
storing the first intermediate spectral values in the buffer unit of the apparatus for
generating spectral replacement values, if the prediction gain is greater than or equal to a
threshold value, or for storing the second intermediate spectral values in the buffer unit of
the apparatus for generating spectral replacement values, if the prediction gain is smaller
than the threshold value.
Furthermore, another audio signal decoder is provided according to another embodiment.
The audio signal decoder comprises a first decoding module for generating generated
spectral values based on a received error-free audio frame, an apparatus for generating
spectral replacement values according to one o the above-described embodiments, a
processing module for processing the generated spectral values by conducting temporal
noise shaping, applying noise- filling and/or applying a global gain, to obtain spectral audio
values of the decoded audio signal. The apparatus for generating spectral replacement
values may be adapted to generate spectral replacement values and to feed them into the
processing module when a current frame has not been received or is erroneous.
Preferred embodiments will be provided in the dependent claims.
In the following preferred embodiments of the present invention will be described with
respect to the figures, in which
Fig. 1 illustrates an apparatus for obtaining spectral replacement values for an
audio signal according to an embodiment,
Fig. 2 illustrates an apparatus for obtaining spectral replacement values for an
audio signal according to another embodiment,
Fig. 3a - 3c illustrate the multiplication of a gain factor and previous spectral values
according to an embodiment,
Fig. 4a illustrates the repetition of a signal portion which comprises an onset in a
time domain,
Fig. 4b illustrates the repetition of a stable signal portion in a time domain,
Fig. 5a - 5b illustrate examples, where generated gain factors are applied on the spectral
values of Fig. 3a, according to an embodiment,
Fig. 6 illustrates an audio signal decoder according to a embodiment,
Fig. 7 illustrates an audio signal decoder according to another embodiment, and
Fig. 8 illustrates an audio signal decoder according to a further embodiment.
Fig. 1 illustrates an apparatus 100 for generating spectral replacement values for an audio
signal. The apparatus 100 comprises a buffer unit 10 for storing previous spectral values
relating to a previously received error-free audio frame. Moreover, the apparatus 100
comprises a concealment frame generator 120 for generating the spectral replacement
values, when a current audio frame has not been received or is erroneous. The previously
received error-free audio frame comprises filter information, the filter information having
associated a filter stability value indicating a stability of a prediction filter. The
concealment frame generator 120 is adapted to generate the spectral replacement values
based on the previous spectral values and based on the filter stability value.
The previously received error-free audio frame may, for example, comprise the previous
spectral values. E.g. the previous spectral values may be comprised i the previously
received error-free audio frame in an encoded form.
Or, the previous spectral values may, for example, be values that may have been generated
by modifying values comprised in the previously received error-free audio frame, e.g.
spectral values of the audio signal. For example, the values comprised in the previously
received error-free audio frame may have been modified by multiplying each one of them
with a gain factor to obtain the previous spectral values.
Or, the previous spectral values may, for example, be values that may have been generated
based on values comprised in the previously received error-free audio frame. For example,
each one of the previous spectral values may have been generated by employing at least
some of the values comprised in the previously received error-free audio frame, such that
each one of the previous spectral values depends on at least some of the values comprised
in the previously received error-free audio frame. E.g., the values comprised in the
previously received error-free audio frame may have been used to generate an intermediate
signal. For example, the spectral values of the generated intermediate signal may then be
considered as the previous spectral values relating to the previously received error-free
audio frame.
Arrow 105 indicates that the previous spectral values are stored in the buffer unit 10.
The concealment frame generator 120 may generate the spectral replacement values, when
a current audio frame has not been received in time or is erroneous. For example, a
transmitter may transmit a current audio frame to a receiver, where the apparatus 100 for
obtaining spectral replacement values, may for example be located. However, the current
audio frame does not arrive at the receiver, e.g. because of any kind of transmission error.
Or, the transmitted current audio frame is received by the receiver, but, for example,
because of a disturbance, e.g. during transmission, the current audio frame is erroneous. In
such or other cases, the concealment frame generator 1 0 is needed for error concealment.
For this, the concealment frame generator 120 is adapted to generate the spectral
replacement values based on at least some of the previous spectral values, when a current
audio frame has not been received or is erroneous. According to embodiments, it is
assumed that the previously received error-free audio frame comprises filter information,
the filter information having associated a filter stability value indicating a stability of a
prediction filter defined by the filter information. For example, the audio frame may
comprise predictive filter coefficients, e.g. linear predictive filter coefficients, as filter
information.
The concealment frame generator 120 is furthermore adapted to generate the spectral
replacement values based on the previous spectral values and based on the filter stability
value.
For example, the spectral replacement values may be generated based on the previous
spectral values and based on the filter stability value in that each one of the previous
spectral values are multiplied by a gain factor, wherein the value of the gain factor depends
on the filter stability value. E.g., the gain factor may be smaller in a second case than in a
first case, when the filter stability value in the second case is smaller than in the first case.
According to another embodiment, the spectral replacement values may be generated based
on the previous spectral values and based on the filter stability value. Intermediate values
may be generated by modifying the previous spectral values, for example, by randomly
flipping the sign of the previous spectral values, and by multiplying each one of the
intermediate values by a gain factor, wherein the value of the gain factor depends on the
filter stability value. For example, the gain factor may be smaller in a second case than in a
first case, when the filter stability value in the second case is smaller than in the first case.
According to a further embodiment, the previous spectral values may be employed to
generate an intemediate signal, and a spectral domain synthesis signal may be generated
by applying a linear prediction filter on the intemediate signal. Then, each spectral value
of the generated synthesis signal may be multiplied by a gain factor, wherein the value of
the gain factor depends on the filter stability value. As above, the gain factor may, for
example, be smaller in a second case than in a first case, if the filter stability value in the
second case is smaller than in the first case.
A particular embodiment illustrated in Fig. 2 is now explained in detail. A first frame 101
arrives at a receiver side, where an apparatus 100 for obtaining spectral replacement values
may be located. On the receiver side, it is checked, whether the audio frame is error-free or
not. For example, an error-free audio frame is an audio frame where all the audio data
comprised in the audio frame is error-free. For this purpose, means (not shown) may be
employed on the receiver side, which determine, whether a received frame is error-free or
not. To this end, state-of-the art error recognition techniques may be employed, such as
means which test, whether the received audio data is consistent with a received check bit or
a received check sum. Or, the error-detecting means may employ a cyclic redundancy
check (CRC) to test whether the received audio data is consistent with a received CRCvalue.
Any other technique for testing, whether a received audio frame is error-free or not,
may also be employed.
The first audio frame 101 comprises audio data 102. Moreover, the first audio frame
comprises check data 103. For example, the check data may be a check bit, a check sum or
a CRC-value, which may be employed on the receiver side to test whether the received
audio frame 101 is error-free (is an error-free frame) or not.
If it has been determined that the audio frame 101 is error-free, then, values relating to the
error-free audio frame, e.g. to the audio data 102, will be stored in the buffer unit 110 as
"previous spectral values". These values may, for example, be spectral values of the audio
signal encoded in the audio frame. Or, the values that are stored in the buffer unit may, for
example, be intermediate values resulting from processing and/or modifying encoded
values stored in the audio frame. Alternatively, a signal, for example a synthesis signal in
the spectral domain, may be generated based on encoded values of the audio frame, and the
spectral values of the generated signal may be stored in the buffer unit 110. Storing the
previous spectral values in the buffer unit 110 is indicated by arrow 105.
Moreover, the audio data 102 of the audio frame 101 is used on the receiver side to decode
the encoded audio signal (not shown). The part of the audio signal that has been decoded
may then be replayed on a receiver side.
Subsequently after processing audio frame 101, the receiver side expects the next audio
frame 11 (also comprising audio data 112 and check data 113) to arrive at the receiver
side. However, e.g., while the audio frame 111 is transmitted (as shown in 115), something
unexpected happens. This is illustrated by 116. For example, a connection may be
disturbed such that bits of the audio frame 111 may be unintentionally modified during
transmission, or, e.g., the audio frame 1 may not arrive at all at a receiver side.
In such a situation, concealment is needed. When, for example, an audio signal is replayed
on a receiver side that is generated based on a received audio frame, techniques should be
employed that mask a missing frame. For example, concepts should define what to do,
when a current audio frame of an audio signal that is needed for play back, does not arrive
at the receiver side or is erroneous.
The concealment frame generator 120 is adapted to provide error concealment. In Fig. 2,
the concealment frame generator 120 is informed that a current frame has not been
received or is erroneous. On the receiver side, means (not shown) may be employed to
indicate to the concealment frame generator 120 that concealment is necessary (this is
shown by dashed arrow 117).
To conduct error concealment, the concealment frame generator 120 may request some or
all of the previous spectral values, e.g. previous audio values, relating to the previously
received error-free frame 101 from the buffer unit 10. This request is illustrated by arrow
118. As in the example of Fig. 2, the previously received error-free frame may, for
example, be the last error-free frame received, e.g. audio frame 101. However, a different
error-free frame may also be employed on the receiver side as previously received errorfree
frame.
The concealment frame generator then receives (some or all of) the previous spectral
values relating to the previously received error-free audio frame (e.g. audio frame 101)
from the buffer unit 1 , as shown in 119. E.g., in case of multiple frame loss, the buffer is
updated either completely or partly. In an embodiment, the steps illustrated by arrows 18
and 119 may be realized in that the concealment frame generator 120 loads the previous
spectral values from the buffer unit 10.
The concealment frame generator 1 0 then generates spectral replacement values based on
at least some of the previous spectral values. By this, the listener should not become aware
that one or more audio frames are missing, such that the sound impression created by the
play back is not disturbed.
A simple way to achieve concealment would be, to simply use the values, e.g. the spectral
values of the last error-free frame as spectral replacement values for the missing or
erroneous current frame.
However, particular problems exist especially i case onsets, e.g., when the sound
volume suddenly changes significantly. For example, in case of a noise burst, by simply
repeating the previous spectral values of the last frame, the noise burst would also be
repeated.
In contrast, if the audio signal is quite stable, e.g. its volume does not change significantly,
or, e.g. its spectral values do not change significantly, then the effect of artificially
generating the current audio signal portion based on the previously received audio data,
e.g., repeating the previously received audio signal portion, would be less disturbing for a
listener.
Embodiments are based on this finding. The concealment frame generator 120 generates
spectral replacement values based on at least some f the previous spectral values and
based on the filter stability value indicating a stability of a prediction filter relating to the
audio signal. Thus, the concealment frame generator 1 0 takes the stability of the audio
signal into account, e.g. the stability of the audio signal relating to the previously received
error-free frame.
For this, the concealment frame generator 1 0 might change the value of a gain factor that
is applied on the previous spectral values. For example, each of the previous spectral
values is multiplied by the gain factor. This is illustrated with respect to Figs. 3a - 3c.
In Fig. 3a, some of the spectral lines of an audio signal relating to a previously received
error-free frame are illustrated before an original gain factor is applied. For example, the
original gain factor may be a gain factor that is transmitted i the audio frame. On the
receiver side, if the received frame is error-free, the decoder may, for example, be
configured to multiply each of the spectral values of the audio signal by the original gain
factor g to obtain a modified spectrum. This is shown in Fig. 3b.
In Fig. 3b, spectral lines that result from multiplying the spectral lines of Fig. 3a by an
original gain factor are depicted. For reasons of simplicity it is assumed that the original
gain factor g is 2.0. (g = 2.0). Fig. 3a and 3b illustrate a scenario, where no concealment
has been necessary.
In Fig. 3c, a scenario is assumed, where a current frame has not been received or is
erroneous. In such a case, replacement vectors have to be generated. For this, the previous
spectral values relating to the previously received error-free frame, that have been stored in
a buffer unit may be used for generating the spectral replacement values.
In the example of Fig. 3c, it is assumed that the spectral replacement values are generated
based on the received values, but the original gain factor is modified.
A different, smaller, gain factor is used to generate the spectral replacement values than the
gain factor that is used to amplify the received values in the case of Fig. 3b. By this, a fade
out is achieved.
For example, the modified gain factor used in the scenario illustrated by Fig. 3c may be
75% of the original gain factor, e.g. 0.75 · 2.0 = 1.5. By multiplying each of the spectral
values by the (reduced) modified gain factor, a fade out is conducted, as the modified gain
factor gact=1.5 that is used for multiplication of the each one of the spectral values is
smaller than the original gain factor (gain factor gprev=2.0) used for multiplication of the
spectral values in the error-free case.
The present invention is inter alia based on the finding, that repeating the values of a
previously received error-free frame is perceived as more disturbing, when the respective
audio signal portion is unstable, then in the case, when the respective audio signal portion
is stable. This is illustrated in Figs. 4a and 4b.
For example, if the previously received error-free frame comprises an onset, then the onset
is likely to be reproduced. Fig. 4a illustrates an audio signal portion, wherein a transient
occurs in the audio signal portion associated with the last received error-free frame. In
Figs. 4a and 4b, the abscissa indicates time, the ordinate indicates an amplitude value of
the audio signal.
The signal portion specified by 410 relates to the audio signal portion relating to the last
received error-free frame. The dashed line in area 420 indicates a possible continuation of
the curve in the time domain, if the values relating to the previously received error-free
frame would simply be copied and used as spectral replacement values of a replacement
frame. As can be seen, the transient is likely to be repeated what may be perceived as
disturbing by the listener.
n contrast, Fig. 4b illustrates an example, where the signal is quite stable. In Fig. 4b, an
audio signal portion relating to the last received error-free frame is illustrated. In the signal
portion of Fig. 4b, no transient occurred. Again, the abscissa indicates time, the ordinate
indicates an amplitude of the audio signal. The area 430 relates to the signal portion
associated with the last received error-free frame. The dashed line in area 440 indicates a
possible continuation of the curve in the time domain, if the values of the previously
received error-free frame would be copied and used as spectral replacement values of a
replacement frame. In such situations where the audio signal is quite stable, repeating the
last signal portion appears to be more acceptable for a listener than in the situation where
an onset is repeated, as illustrated in Fig. 4a.
The present invention is based on the finding that spectral replacement values may be
generated based on previously received values of a previous audio frame, but that also the
stability of a prediction filter depending on the stability of an audio signal portion should
be considered. For this, a filter stability value should be taken into account. The filter
stability value may, e.g., indicate the stability of the prediction filter.
In LD-USAC, the prediction filter coefficients, e.g. linear prediction filter coefficients,
may be determined on an encoder side and may be transmitted to the receiver within the
audio frame.
On the decoder side, the decoder then receives the predictive filter coefficients, for
example, the predictive filter coefficients of the previously received error-free frame.
Moreover, the decoder may have already received the predictive filter coefficients of the
predecessor frame of the previously received frame, and may, e.g., have stored these
predictive filter coefficients. The predecessor frame of the previously received error-free
frame Is the frame that immediately precedes the previously received error-free frame. The
concealment frame generator may then determine the filter stability value based on the
predictive filter coefficients of the previously received error-free frame and based on the
predictive filter coefficients of the predecessor frame of the previously received error-free
frame.
In the following, determination of the filter stability value according an embodiment is
presented, which is particularly suitable for LD-USAC. The stability value considered
depends on predictive filter coefficients, for example, 10 predictive filter coefficients f in
case of narrowband, or, for example, 16 predictive filter coefficients f - in case of
wideband, which may have been transmitted In a previously received error-free frame.
Moreover, predictive filter coefficients of the predecessor frame of the previously received
error-free frame are also considered, for example 10 further predictive filter coefficients
_ p in case of narrowband (or, for example, 1 further predictive filter coefficients f in
case of wideband).
For example, the k-th prediction filter f may have been calculated on an encoder side by
computing an autocorrelation, such that:
å s'(n)s'(n -k)
n=k
wherein s' is a windowed speech signal, e.g. the speech signal that shall be encoded, after a
window has been applied on the speech signal t may for example be 383. Alternatively, t
may have other values, such as 191 or 95.
In other embodiments, instead of computing an autocorrelation, the Levinson-Durbinalgorithm,
k own from the state of the art, may alternatively be employed, see, for
example,
[3]: 3GPP, "Speech codec speech processing functions; Adaptive Multi-Rate - Wideband
(AMR-WB) speech codec; Transcoding functions", 2009, V9.0.0, 3GPP TS 26.190.
As already stated, the predictive filter coefficients f and may have been transmitted
to the receiver within the previously received error-free frame and the predecessor of the
previously received error-free frame, respectively.
On the decoder side, a Line Spectral Frequency distance measure (LSF distance measure)
LSF ist may then be calculated employing the formula:
u may be the number of prediction filters in the previously received error-free frame minus
1. E.g. if the previously received error-free frame had 10 predictive filter coefficients, then,
for example, u=9. The number of predictive filter coefficients in the previously received
error-free frame is typically identical to the number of predictive filter coefficients in the
predecessor frame of the previously received error-free frame.
The stability value may then be calculated according to the formula:
q = 0 if (1.25 - LSF i / v) < ()
= 1 if ( 1.25 LSFdlsl / v) >
q = 1.25 - LSFdist / v 0 < ( 1.25 - LSF / v) < 1
v may be an integer. For example, v may be 156250 in case of narrowband. In another
embodiment, v may be 400000 in case of wideband.
Qis considered to indicate a very stable prediction filter, if Qis 1 or close to 1.
Qis considered to indicate a very unstable prediction filter, if Qis 0 or close to 0.
The concealment frame generator may be adapted to generate the spectral replacement
values based on previous spectral values of a previously received error-free frame, when a
current audio frame has not been received or is erroneous. Moreover, the concealment
frame generator may be adapted to calculate a stability value Q based on the predictive
filter coefficients f of the previously received error-free frame and also based on the
predictive filter coefficients p of the previously received error-free frame, as has been
described above.
In an embodiment, the concealment frame generator may be adapted to use the filter
stability value to generate a generated gain factor, e.g. by modifying an original gain
factor, and to apply the generated gain factor on the previous spectral values relating to the
audio frame to obtain the spectral replacement values. In other embodiments, the
concealment frame generator is adapted to apply the generated gain factor on values
derived from the previous spectral values.
For example, the concealment frame generator may generate the modified gain factor by
multiplying a received gain factor by a fade out factor, wherein the fade out factor depends
on the filter stability value.
Let us, for example, assume that a gain factor received in an audio signal frame has, e.g.
the value 2.0. The gain factor is typically used for multiplying the previous spectral values
to obtain modified spectral values. To apply a fade out, a modified gain factor is generated
that depends on the stability value Q.
For example, if the stability value = 1, then the prediction filter is considered to be very
stable. The fade out factor may then be set to 0.85, if the frame that shall be reconstructed
is the first frame missing. Thus, the modified gain factor is 0.85 · 2.0 = 1.7. Each one of the
received spectral values of the previously received frame is then multiplied by a modified
gain factor of 1.7 instead o 2.0 (the received gain factor) to generate the spectral
replacement values.
Fig. 5a illustrates an example, where a generated gain factor 1.7 is applied on the spectral
values of Fig. 3a .
However, if, for example, the stability value = 0, then the prediction filter is considered
to be very unstable. The fade out factor may then be set to 0.65, if the frame that shall be
reconstructed is the first frame missing. Thus, the modified gain factor is 0.65 2.0 = 1.3.
Each one of the received spectral values of the previously received frame is then multiplied
by a modified gain factor of 1.3 instead of 2.0 (the received gain factor) to generate the
spectral replacement values.
Fig. 5b illustrates an example, where a generated gain factor 1.3 is applied on the spectral
values of Fig. 3a. As the gain factor in the example of Fig. 5b is smaller than in the
example o Fig. 5a, the magnitudes i Fig. 5b are also smaller than n the example Fig.
5a.
Different strategies may be applied depending on the value Q, wherein Q might be any
value between 0 and 1.
For example, a value Q> 0.5 may be interpreted as 1 such that the fade out factor has the
same value as if Qwould be 1, e.g. the fade out factor is 0.85. A value Q< 0.5 may be
interpreted as 0 such that the fade out factor has the same value as if 0 would be 0, e.g. the
fade out factor is 0.65.
According to another embodiment, the value of the fade out factor might alternatively be
interpolated, if the value of Qis between 0 and 1. For example, assuming that the value of
the fade out factor is 0.85 if Q is 1, and 0.65 if Q is 0, then the fade out factor may be
calculated according to the formula:
fade utJactor = 0.65 + Q· 0.2; for 0 0 < 1.
n another embodiment, the concealment frame generator is adapted to generate the
spectral replacement values furthermore based o frame class information relating to the
previously received error-free frame. The information about the class may be determined
by an encoder. The encoder may then encode the frame class information in the audio
frame. The decoder might then decode the frame class information when decoding the
previously received error-free frame.
Alternatively, the decoder may itself determine the frame class information by examining
the audio frame.
Moreover, the decoder may be configured to determine the frame class information based
on information from the encoder and based on an examination of the received audio data,
the examination being conducted by the decoder, itself.
The frame class may, for example indicate whether the frame is classified as "artificial
onset", "onset", "voiced transition", unvoiced transition", "unvoiced" and "voiced.
For example, "onset" might indicate that the previously received audio frame comprises an
onset. E.g., "voiced" might indicate that the previously received audio frame comprises
voiced data. For example, "unvoiced" might indicate that the previously received audio
frame comprises unvoiced data. E.g., "voiced transition" might indicate that the previously
received audio frame comprises voiced data, but that, compared to the predecessor of the
previous received audio frame, the pitch did change. For example, "artificial onset" might
indicate that the energy of the previously received audio frame has been enhanced (thus,
for example, creating an artificial onset). E.g. "unvoiced transition" might indicate that the
previously received audio frame comprises unvoiced data but that the unvoiced sound is
about to change.
Depending on the previously received audio frame, the stability value Qand the number of
successive erased frames, the attenuation gain, e.g. the fade out factor, may, for example,
be defined as follows:
Last good received frame Number of successive Attenuation gain
erased frames (e.g. fade out factor)
ARTIFICIAL ONSET 0.6
ONSET < 3 0.2 Q+ 0.8
ONSET > 3 0.5
VOICED TRANSITION 0.4
UNVOICED TRANSITION > 1 0.8
UNVOICED TRANSITION = 0.2 Q+ 0.75
UNVOICED = 2 0.2 · Q+ 0.6
UNVOICED > 2 0.2 Q+ 0.4
UNVOICED = 0.2 Q+ 0.8
VOICED = 2 0.2 · Q+ 0.65
VOICED > 2 0.2 Q+ 0.5
According to an embodiment, the concealment frame generator may generate a modified
gain factor by multiplying a received gain factor by the fade out factor determined based
on the filter stability value and on the frame class. Then, the previous spectral values may,
for example, be multiplied by the modified gain factor to obtain spectral replacement
values.
The concealment frame generator may again be adapted to generate the spectral
replacement values furthermore also based on the frame class information.
According to an embodiment, the concealment frame generator may be adapted to generate
the spectral replacement values furthermore depending on the number of consecutive
frames that did not arrive at the receiver or that were erroneous.
In an embodiment, the concealment frame generator may be adapted to calculate a fade out
factor based on the filter stability value and based on the number of consecutive frames
that did not arrive at the receiver or that were erroneous.
The concealment frame generator may moreover be adapted to generate the spectral
replacement values by multiplying the fade out factor by at least some of the previous
spectral values.
Alternatively, the concealment frame generator may be adapted to generate the spectral
replacement values by multiplying the fade out factor by at least some values of a group of
intermediate values. Each one of the intermediate values depends on at least one of the
previous spectral values. For example, the group of intermediate values may have been
generated by modifying the previous spectral values. Or, a synthesis signal in the spectral
domain may have been generated based o the previous spectral values, and the spectral
values of the synthesis signal may form the group of intermediate values.
In another embodiment, the fade out factor may be multiplied by an original gain factor to
obtain a generated gain factor. The generated gain factor is then multiplied by at least some
of the previous spectral values, or by at least some values of the group of intermediate
values mentioned before, to obtain the spectral replacement values.
The value of the fade out factor depends on the filter stability value and on the number of
consecutive missing or erroneous frames, and may, for example, have the values:
Here, "Number of consecutive missing/erroneous frames = " indicates that the immediate
predecessor of the missing/erroneous frame was error-free.
As can be seen, in the above example, the fade out factor may be updated each time a
frame does not arrive or is erroneous based on the last fade out factor. For example, if the
immediate predecessor of a missing/erroneous frame is error-free, then, i the above
example, the fade out factor is 0.8. If the subsequent frame is also missing or erroneous,
the fade out factor is updated based on the previous fade out factor by multiplying the
previous fade out factor by an update factor 0.65: fade out factor = 0.8 · 0.65 = 0.52, and so
on.
Some or all of the previous spectral values may be multiplied by the fade out factor itself.
Alternatively, the fade out factor may be multiplied by an original gain factor to obtain a
generated gain factor. The generated gain factor may then be multiplied by each one (or
some) of the previous spectral values (or intermediate values derived from the previous
spectral values) to obtain the spectral replacement values.
It should be noted, that the fade out factor may also depend on the filter stability value. For
example, the above table may also comprise definitions for the fade out factor, if the filter
stability value is 1.0, 0.5 or any other value, for example:
Fade out factor values for intermediate filter stability values may be approximated.
In another embodiment, the fade out factor may be determined by employing a formula
which calculates the fade out factor based on the filter stability value and based on the
number of consecutive frames that did not arrive at the receiver or that were erroneous.
As has been described above, the previous spectral values stored in the buffer unit may be
spectral values. To avoid that disturbing artefacts are generated, the concealment frame
generator may, as explained above, generate the spectral replacement values based on a
filter stability value.
However, the such generated signal portion replacement may still have a repetitive
character. Therefore, according to an embodiment, it is moreover proposed to modify the
previous spectral values, e.g. the spectral values of the previously received frame, by
randomly flipping the sign of the spectral values. E.g. the concealment frame generator
decides randomly for each of the previous spectral values, whether the sign of the spectral
value is inverted or not, e.g. whether the spectral value is multiplied by - 1 or not. By this,
the repetitive character of the replaced audio signal frame with respect to its predecessor
frame is reduced.
n the following, a concealment in a LD-USAC decoder according to an embodiment is
described. In this embodiment, concealment is working on the spectral data just before the
LD-USAC-decoder conducts the final frequency to time conversion.
In such an embodiment, the values of an arriving audio frame are used to decode the
encoded audio signal by generating a synthesis signal in the spectral domain. For this, an
intermediate signal in the spectral domain is generated based on the values of the arriving
audio frame. Noise filling conducted on the values quantized to zero.
The encoded predictive filter coefficients define a prediction filter which is then applied on
the intermediate signal to generate the synthesis signal representing the decoded/
reconstructed audio signal in the frequency domain.
Fig. 6 illustrates an audio signal decoder according to an embodiment. The audio signal
decoder comprises an apparatus for decoding spectral audio signal values 610, and an
apparatus for generating spectral replacement values 620 according to one of the above
described embodiments.
The apparatus for decoding spectral audio signal values 610 generates the spectral values
o the decoded audio signal as just described, when a error-free audio frame arrives.
In the embodiment of Fig. 6, the spectral values of the synthesis signal may then be stored
in a buffer unit of the apparatus 620 for generating spectral replacement values. These
spectral values of the decoded audio signal have been decoded based on the received errorfree
audio frame, and thus relate to the previously received error-free audio frame.
When a current frame is missing or erroneous, the apparatus 620 for generating spectral
replacement values is informed that spectral replacement values are needed. The
concealment frame generator of the apparatus 620 for generating spectral replacement
values then generates spectral replacement values according to one of the above-described
embodiments.
For example, the spectral values from the last good frame are slightly modified by the
concealment frame generator by randomly flipping their sign. Then, a fade out is applied
on these spectral values. The fade out may depend on the stability the previous
prediction filter and on the number of consecutive lost frames. The generated spectral
replacement values are then used as spectral replacement values for the audio signal, and
then a frequency to time transformation is conducted to obtain a time-domain audio signal.
In LD-USAC, as well as in USAC and MPEG-4 (MPEG = Moving Picture Experts
Group), temporal noise shaping (TNS) may be employed. By temporal noise shaping, the
fine time structure of noise is controlled. O a decoder side, a filter operation is applied on
the spectral data based on noise shaping information. More information on temporal noise
shaping can, for example, be found in:
[4]: ISO/IEC 14496-3:2005: Information technology - Coding of audio-visual objects -
Part 3: Audio, 2005
Embodiments are based on the finding that in case of an onset / a transient, TNS is highly
active. Thus, by determining whether the TNS is highly active or not, it can be estimated,
whether an onset / a transient is present.
According to an embodiment, a prediction gain that TNS has, is calculated on receiver
side. On receiver side, at first, the received spectral values of a received error-free audio
frame are processed to obtain first intermediate spectral values ai. Then, TNS is conducted
and by this, second intermediate spectral values b are obtained. A first energy value Ei is
calculated for the first intermediate spectral values and a second energy value E2 is
calculated for the second intermediate spectral values. To obtain the prediction gain g s of
the TNS, the second energy value may be divided by the first energy value.
For example, g may be defined as:
number f considered spectral values)
According to an embodiment, the concealment frame generator is adapted to generate the
spectral replacement values based on the previous spectral values, based on the filter
stability value and also based on a prediction gain of a temporal noise shaping, when
temporal noise shaping is conducted on a previously received error-free frame. According
to another embodiment, the concealment frame generator is adapted to generate the
spectral replacement values furthermore based on the number of consecutive missing or
erroneous frames.
The higher the prediction gain is, the faster should the fade out be. For example, consider a
filter stability value of 0.5 and assume that the prediction gain is high, e.g. = 6; then a
fade out factor, may, for example be 0.65 (= fast fade out). In contrast, again, consider a
filter stability value of 0.5, but aussume that the prediction gain is low, e.g. 1.5; then a fade
out factor may, for example be 0.95 (= slow fade out).
The prediction gain of the TNS may also influence, which values should be stored in the
buffer unit of an apparatus for generating spectral replacement values.
If the prediction gain gT s is lower than a certain threshold (e.g. threshold = 5.0), then the
spectral values after the TNS has been applied are stored in the buffer unit as previous
spectral values. In case of a missing or erroneous frame, the spectral replacement values
are generated based on these previous spectral values.
Otherwise, if the prediction gain s is greater than or equal to the threshold value, the
spectral values before the TNS has been applied are stored in the buffer unit as previous
spectral values. In case of a missing or erroneous frame, the spectral replacement values
are generated based on these previous spectral values.
TNS is not applied in any case on these previous spectral values.
Accordingly, Fig. 7 illustrates an audio signal decoder according to a corresponding
embodiment. The audio signal decoder comprises a decoding unit 710 for generating first
intermediate spectral values based on a received error-free frame. Moreover, the audio
signal decoder comprises a temporal noise shaping unit 720 for conducting temporal noise
shaping on the first intermediate spectral values to obtain second intermediate spectral
values. Furthermore, the audio signal decoder comprises a prediction gain calculator 730
for calculating a prediction gain of the temporal noise shaping depending on the first
intermediate spectral values and the second intermediate spectral values. Moreover, the
audio signal decoder comprises an apparatus 740 according to one of the above-described
embodiments for generating spectral replacement values when a current audio frame has
not been received or is erroneous. Furthermore, the audio signal decoder comprises a
values selector 750 for storing the first intermediate spectral values in the buffer unit 745
of the apparatus 740 for generating spectral replacement values, if the prediction gain is
greater than or equal to a threshold value, or for storing the second intermediate spectral
values in the buffer unit 745 of the apparatus 740 for generating spectral replacement
values, if the prediction gain is smaller than the threshold value.
The threshold value may, for example, be a predefined value. E.g. the threshold value may
be predefined in the audio signal decoder.
According to another embodiment, concealment is conducted on the spectral data just after
the first decoding step and before any noise-filling, global gain and/or TNS is conducted.
Such a embodiment is depicted in Fig. 8. Fig. 8 illustrates a decoder according to a further
embodiment. The decoder comprises a first decoding module 810. The first decoding
module 810 is adapted to generate generated spectral values based on a received error-free
audio frame. The generated spectral values are then stored in the buffer unit of an
apparatus 820 for generating spectral replacement values. Moreover, the generated spectral
values are input into a processing module 830, which processes the generated spectral
values by conducting TNS, applying noise-filling and/or by applying a global gain to
obtain spectral audio values of the decoded audio signal. If a current frame is missing or
erroneous, the apparatus 820 for generating spectral replacement values generates the
spectral replacement values and feeds them into the processing module 830.
According to the embodiment illustrated in Fig. 8, the decoding module or the processing
module conduct some or all of the following steps in case of concealment:
The spectral values, e.g. from the last good frame, are slightly modified by randomly
flipping their sign. In a further step, noise-filling is conducted based on random noise on
the spectral bins quantized to zero. In another step, the factor of noise is slightly adapted
compared to the previously received error-free frame.
In a further step, spectral noise-shaping is achieved by applying the LPC-coded (LPC =
Linear Predictive Coding) weighted spectral envelope in the frequency-domain. For
example, the LPC coefficients of the last received error-free frame may be used. In another
embodiment, averaged LPC-coefficients may be used. For example, an average of the last
three values of a considered LPC coefficient of the last three received error-free frames
may be generated for each LPC coefficient of a filter, and the averaged LPC coefficients
may be applied.
In a subsequent step, a fade out may be applied on these spectral values. The fade out may
depend on the number of consecutive missing or erroneous frames and on the stability of
the previous LP filter. Moreover, prediction gain information may be used to influence the
fade out. The higher the prediction gain is, the faster the fade out may be. The embodiment
of Fig. 8 is slightly more complex than the embodiment of Fig. 6, but provides better audio
quality.
Although some aspects have been described in the context of an apparatus, it is clear that
these aspects also represent a description of the corresponding method, where a block or
device corresponds to a method step or a feature of a method step. Analogously, aspects
described in the context of a method step also represent a description of a corresponding
block or item or feature of a corresponding apparatus.
Depending on certain implementation requirements, embodiments of the invention can be
implemented in hardware or in software. The implementation can be performed using a
digital storage medium, for example a floppy disk, a DVD, a CD, a ROM, a PROM, an
EPROM, an EEPROM or a FLASH memory, having electronically readable control
signals stored thereon, which cooperate (or are capable of cooperating) with a
programmable computer system such that the respective method is performed.
Some embodiments according to the invention comprise a data carrier having
electronically readable control signals, which are capable of cooperating with a
programmable computer system, such that one of the methods described herein is
performed.
Generally, embodiments of the present invention can be implemented as a computer
program product with a program code, the program code being operative for performing
one of the methods when the computer program product runs on a computer. The program
code may for example be stored on a machine readable carrier.
Other embodiments comprise the computer program for performing one of the methods
described herein, stored on a machine readable carrier or a non-transitory storage medium.
In other words, an embodiment of the inventive method is, therefore, a computer program
having a program code for performing one of the methods described herein, when the
computer program runs on a computer.
A further embodiment of the inventive methods is, therefore, a data carrier (or a digital
storage medium, or a computer-readable medium) comprising, recorded thereon, the
computer program for performing one of the methods described herein.
A further embodiment of the inventive method is, therefore, a data stream or a sequence of
signals representing the computer program for performing one of the methods described
herein. The data stream or the sequence of signals may for example be configured to be
transferred via a data communication connection, for example via the Internet or over a
radio channel.
A further embodiment comprises a processing means, for example a computer, or a
programmable logic device, configured to or adapted to perform one of the methods
described herein.
A further embodiment comprises a computer having installed thereon the computer
program for performing one of the methods described herein.
In some embodiments, a programmable logic device (for example a field programmable
gate array) may be used to perform some or all of the functionalities of the methods
described herein. In some embodiments, a field programmable gate array may cooperate
with a microprocessor in order to perform one of the methods described herein. Generally,
the methods are preferably performed by any hardware apparatus.
The above 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.
Literature:
[1]: 3GPP, "Audio codec processing functions; Extended Adaptive Multi-Rate - Wideband
(AMR-WB+) codec; Transcoding functions", 2009, 3GPP TS 26.290.
[2]: USAC codec (Unified Speech and Audio Codec), ISO/IEC CD 23003-3 dated
September 24, 2010
[3]: 3GPP, "Speech codec speech processing functions; Adaptive Multi-Rate - Wideband
(AMR-WB) speech codec; Transcoding functions", 2009, V9.0.0, 3GPP TS 26. 190.
[4]: ISO/IEC 14496-3:2005: Information technology - Coding of audio-visual objects -
Part 3: Audio, 2005
[5] : ITU-T G.718 (06-2008) specification
Claims
An apparatus (100) for generating spectral replacement values for an audio signal
comprising:
a buffer unit ( 110) for storing previous spectral values relating to a previously
received error-free audio frame, and
a concealment frame generator (1 0) for generating the spectral replacement values
when a current audio frame has not been received or is erroneous, wherein the
previously received error-free audio frame comprises filter information, the filter
information having associated a filter stability value indicating a stability of a
prediction filter, and wherein the concealment frame generator (120) is adapted to
generate the spectral replacement values based on the previous spectral values and
based on the filter stability value.
An apparatus (100) according to claim 1, wherein the concealment frame generator
(120) is adapted to generate the spectral replacement values by randomly flipping
the sign of the previous spectral values.
An apparatus (100) according to claim 1 or 2, wherein the concealment frame
generator (120) is configured to generate the spectral replacement values by
multiplying each of the previous spectral values by a first gain factor when the filter
stability value has a first value, and by multiplying each of the previous spectral
values by a second gain factor, being smaller than the first gain factor, when the
filter stability value has a second value being smaller than the first value.
An apparatus according to one of the preceding claims, wherein the concealment
frame generator (120) is adapted to generate the spectral replacement values based
on the filter stability value, wherein the previously received error-free audio frame
comprises first predictive filter coefficients of the prediction filter, wherein a
predecessor frame of the previously received error-free audio frame comprises
second predictive filter coefficients, and wherein the filter stability value depends
on the first predictive filter coefficients and on the second predictive filter
coefficients.
An apparatus according to claim 4, wherein the concealment frame generator (120)
is adapted to determine the filter stability value based on the first predictive filter
coefficients of the previously received error-free audio frame and based on the
second predictive filter coefficients of the predecessor frame of the previously
received error-free audio frame.
An apparatus according to claim 4 or 5, wherein the concealment frame generator
(120) is adapted to generate the spectral replacement values based on the filter
stability value, wherein the filter stability value depends on a distance measure
LSFdist, and wherein the distance measure LSFjj is defined by the formula:
wherein u+1 specifies a total number of the first predictive filter coefficients f the
previously received error- free audio frame, and wherein u+1 also specifies a total
number of the second predictive filter coefficients of the predecessor frame of the
previously received error-free audio frame, wherein f specifies the i-th filter
coefficient of the first predictive filter coefficients and wherein f p specifies the
i-th filter coefficient of the second predictive filter coefficients.
An apparatus (100) according to one of the preceding claims, wherein the
concealment frame generator (120) is adapted to generate the spectral replacement
values furthermore based on frame class information relating to the previously
received error- free audio frame.
An apparatus ( 00) according to claim 7, wherein the concealment frame generator
(120) is adapted to generate the spectral replacement values based on the frame
class information, wherein the frame class information indicates that the previously
received error-free audio frame is classified as "artificial onset", "onset", "voiced
transition", "unvoiced transition", "unvoiced" or "voiced".
An apparatus (100) according to one of the preceding claims, wherein the
concealment frame generator (120) is adapted to generate the spectral replacement
values furthermore based on a number of consecutive frames that did not arrive at a
receiver or that were erroneous, since a last error-free audio frame had arrived at the
receiver, wherein no other error-free audio frames arrived at the receiver since the
last error-free audio frame had arrived at the receiver.
10. An apparatus (100) according to claim 9,
wherein the concealment frame generator (120) is adapted to calculate a fade out
factor, based on the filter stability value and based on the number of consecutive
frames that did not arrive at the receiver or that were erroneous, and
wherein the concealment frame generator (120) is adapted to generate the spectral
replacement values by multiplying the fade out factor by at least some of the
previous spectral values, or by at least some values of a group of intermediate
values, wherein each one of the intermediate values depends on at least one of the
previous spectral values.
11. An apparatus (100) according to one of the preceding claims, wherein the
concealment frame generator (120) is adapted to generate the spectral replacement
values based on the previous spectral values, based on the filter stability value and
also based on a prediction gain of a temporal noise shaping.
12. An audio signal decoder comprising:
an apparatus (610) for decoding spectral audio signal values, and
an apparatus (620) for generating spectral replacement values according to one of
claims 1 to 11,
wherein the apparatus (610) for decoding spectral audio signal values is adapted to
decode spectral values of an audio signal based on a previously received error-free
audio frame, wherein the apparatus (610) for decoding spectral audio signal values
is furthermore adapted to store the spectral values of the audio signal in the buffer
unit of the apparatus (620) for generating spectral replacement values, and
wherein the apparatus (620) for generating spectral replacement values is adapted
to generate the spectral replacement values based on the spectral values stored in
the buffer unit, when a current audio frame has not been received or is erroneous.
3. An audio signal decoder, comprising:
a decoding unit (710) for generating first intermediate spectral values based on a
received error-free audio frame,
a temporal noise shaping unit (720) for conducting temporal noise shaping on the
first intermediate spectral values to obtain second intermediate spectral values,
a prediction gain calculator (730) for calculating a prediction gain of the temporal
noise shaping depending on the first intermediate spectra! values and depending on
the second intermediate spectral values,
an apparatus (740) according to one of claims 1 to 11, for generating spectral
replacement values when a current audio frame has not been received or is
erroneous, and
a values selector (750) for storing the first intermediate spectral values in the buffer
unit (745) of the apparatus (740) for generating spectral replacement values, if the
prediction gain is greater than or equal to a threshold value, or for storing the
second intermediate spectral values in the buffer unit of the apparatus for
generating spectral replacement values, if the prediction gain is smaller than the
threshold value.
An audio signal decoder, comprising:
a first decoding module (810) for generating generated spectral values based on a
received error-free audio frame,
an apparatus (820) for generating spectral replacement values according to one of
claims 1 to 11, and
a processing module (830) for processing the generated spectral values by
conducting temporal noise shaping, applying noise-filling or applying a global gain,
to obtain spectral audio values of the decoded audio signal,
wherein the apparatus (820) for generating spectral replacement values is adapted
to generate spectral replacement values and to feed them into the processing
module (830), when a current frame has not been received or is erroneous.
15, A method for generating spectral replacement values for an audio signal
comprising:
storing previous spectral values relating to a previously received error-free audio
frame, and
generating the spectral replacement values when a cuixent audio frame has not been
received or is erroneous, wherein the previously received error-free audio frame
comprises filter information, the filter information having associated a filter
stability value indicating a stability of a prediction filter defined by the filter
information, wherein the spectral replacement values are generated based on the
previous spectral values and based on the filter stability value.
16. A computer program for implementing the method of claim 15, when the computer
program is executed by a computer or signal processor.