Note: Descriptions are shown in the official language in which they were submitted.
CA 02482866 2004-10-18
DEVICE AND METHOD FOR CONCEALING AN ERROR
Description
The present invention relates to the transmission of data via an erroneous
transmis-
sion channel, and in particular to a concept of concealing an error in an
erroneous or poten-
dally erroneous information unit which has been transmitted via the erroneous
channel.
In particular with audio and/or video coding there is a need to make the cod-
ing/decoding concepts robust against transmission errors. It has been known,
in particular
from wireless transmission technology, to perform a forward error correction
(FEC) on the
coder side. By means of this concept, a redundancy is introduced into the data
stream by a
coder. This redundancy may then be exploited in a decoder, e.g. using a
Viterbi decoding
block, so as to correct any transmission errors that developed in the
transmission. This
method is disadvantageous in that by adding a redundancy, the transmission
rate via. the
channel is increased in the coder. Particularly with highly disturbed
transmission chan-
nels, such as wireless transmission channels, however, there is often no
alternative choice
so as to enable reliable reception in the receiver/decoder with non-optimal
channel condi-
tions.
On the other hand, a main goal, especially with audio or video compression
meth-
ods, is to compress audio or video data as highly as possible to enable a
transmission via
channels which are typically not highly disturbed, such as line-conducted
channels, which
allow a limited data rate. This is why such prior-art compression methods, as
are standard-
ized in the MPEG family, often utilize entropy codes for entropy-coding
quantized data,
such as spectral values. A known representative of an entropy-coding method is
so called
Huffinan coding which enables coding of a set of values with a nearly minimum
redun-
dancy. Prior to assigning the Huffman code words to the individual information
units, a
statistic of the information units is determined so as to associate the most
frequently occur-
ring information unit with a code word having as short a length as possible,
whereas an
information unit which occurs very rarely is assigned a code word with a
longer length
(with regard to the number of bits).
Huffinan codes are problematic in that one cannot see, from a data stream of
Huff
man code words, where a code word starts (aside from the first code word), and
where a
CA 02482866 2004-10-18
2
code word ends. A data stream of Huffman code words typically consists of a
string of
binary ones and zeros. A decoder decodes such a data stream of Huffman code
words
while knowing the table of codes on which the coding was based. The table of
codes,
which may also be represented as a code tree, is configured such that the end
of a code
word inherently results because the code is free from prefixes (in the code
tree, only leaves
are valid code words). The Huffman code has the characteristic that all
"branches" of the
tree are complete, i.e. lead to valid code words.
If during the transmission of such a data stream of Huffman code words a
transmis-
sion error arises, this will almost inevitably mean that even though all code
words have
been decoded correctly up to the occurrence of the error, all code words are
decoded in an
erroneous manner after the occurrence of the error, which may relate to, e.g.,
only one bit.
If a code is selected wherein all branches have been terminated with valid
code words, the
decoder will keep on decoding irrespective of the error and will not establish
before the
end of the data stream that somewhere further upstream an error has occurred,
if for the
last code words, the bits in the data stream run out or if any bits remain.
Thus, the Huff
man code as an example of an entropy code is favorable from the point of view
of data rate
compression. However, error robustness is minimal, since as little as one bit
error in the
very first code word is very likely to lead to the fact that all other
subsequent code words
will be recognized incorrectly by the decoder even though they have not been
affected by
the bit error.
A more error-robust approach has been described in US patent No. 5,852,469. In-
stead of using Huffman codes, said US patent suggests using reversible codes
(i.e. codes
which may be decoded on both sides) having code words of variable lengths
(referred to
below as reversible code words), such as symmetrical code words. These codes
which in
addition to the property of the freedom from prefixes also have the property
of being free
from suffixes, are also referred to as RVL (reversible variable length).
Moreover, an addi-
tional backward decoder is used instead of the forward decoder which is
sufficient for a
Huffinan decoder. The forward decoder performs a decoding from a starting
point of a
block of reversible code words of variable lengths, whereas the backward
decoder be-
CA 02482866 2004-10-18
3
comes effective/starts from an end point of the block of reversible code words
of variable
lengths.
Reversible code words, such as symmetrical code words, may also have the prop-
erty that they lead to a code tree wherein not all branches are terminated by
valid code
words. Therefore, it is during the course of the decoding already that a
decoder will rec-
ognize whether an error has occurred, if the decoder comes across such an
invalid code
word, i.e. a code word not provided in the table of codes. However, the
decoder cannot
determine with certainty whether the error was located precisely in the code
word which
has been recognized as invalid. Since the number of valid code words of this
table of
codes is selected to be as small as possible for reasons of data compression,
only a few
invalid code words exist, so that a decoder may not come across an invalid
code word until
several code words after the occurrence of the transmission error in the bit
stream. There-
fore, an error in the bit stream leads to continuation errors which, however,
do not relate to
the entire remaining bit stream anymore, as they do with the Huffinan code,
but which
typically propagate only via several code words after the occurrence of the
error. For error
limitation, the backward decoder is provided in addition to the forward
decoder, which
backward decoder will still output a few code words as seemingly correct code
words in
the backward direction, also due to the continuation errors, if it is assumed
that only one
bit error exists in the data stream. An error may be recognized if the forward
decoder ,and
the backward decoder output different decoded information units for
information units of
the same ordinal number.
Therefore, US patent No. 5,852,469 suggests discarding all outputs in this
error
area and using only information units that have been coded, starting from the
starting point
of the block of code words up to the beginning of the error or overlap area,
for receiver-
side further processing, and using, in addition, decoded information units
only which have
been decoded from the end point of the block of code words up to the end (seen
in the for-
ward direction) of the overlap area.
DE 198 40 835 A1 also discloses a device and a method for entropy-coding infor-
mation words, and a device and method for decoding entropy-coded information
words.
For information units decoded in the error or overlap area it is suggested to
utilize an error
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concealment technique. Potential error concealment techniques consist in
simply replacing
an erroneous value by its adjacent, intact value. If both intact values which
are adjacent to
an error are known, weighted mean values from the left and right edges may
also be used
to artificially replace, i.e. conceal, the erroneous value. Further error
concealment tech-
S niques that have been mentioned use interpolation using two adjacent values
which have
an error sandwiched between them. Similarly, a one-sided prediction may be
performed
from the front or from the back, relating to the overlap area, so as to
replace an erroneous
value by a value which is "possibly relatively intact".
A disadvantage of this concept is the fact that it is problematic if several
successive
information units have been affected by continuation errors. Interpolation or
prediction-
based concealing techniques in this case will very soon find their
limitations, and the qual-
ity of the error concealment will decrease more and more strongly.
US patent No. 6,104,754 discloses a moving-picture coder/decoder system using
a
coding with code words of variable lengths. What is worked with, in
particular, are re-
f S versible code words of variable lengths which may be decoded from the
front or from the
back. If a forward decoder determines an error and if a backward decoder also
establishes
an error, the area including the two errors is discarded if it does not
overlap. If, on the
other hand, the area overlaps, what is taken is the output of forward decoder
up to the er-
ror, excluding the error. From the error onwards, the output of the backward
decoder is
taken. Alternatively, the output of the backward decoder up to the error may
also be taken,
and then the output of the forward decoder from the error onwards. If an error
is found
only in the forward decoder, the output of the forward decoder up to the error
is taken, and
the output of the backward decoder from the error is taken. If both decoders
determine an
error in the same code word, the erroneous code word is discarded, the output
of the for-
ward decoder up to the error is taken, and the output of the backward decoder
up to the
error is taken.
DE 19959038 A1 discloses a method of decoding digital audio data, wherein
error
recognition is performed in dependence on reference values transmitted,
preferably scale
factors. Here, reference values of a frequency range are compared with
previous reference
values of the same frequency range so as to generate a feature which is
compared with a
CA 02482866 2004-10-18
threshold value. If the feature exceeds the preset threshold value, this is
indicated by
means of signaling. For error concealment, reference values marked as errors
are replaced
by previous reference values which have been stored.
It is the object of the present invention to provide an improved concept for
conceal-
s ing an error in an erroneous or potentially erroneous information unit.
This object is achieved by a device for concealing an error as claimed in
claim 1,
by a method of concealing an error as claimed in claim 24, or by a computer
program as
claimed in claim 25.
The present invention is based on the findings that - if, for reasons of
clarity, only
a single bit error is assumed - one of the two different values for the
information unit
which are output by the forward decoder and the backward decoder is the
correct value.
However, the classical prior-art error concealment techniques do not take
these properties
into consideration. Here it is assumed that in the case of two different
values for the same
information unit, a problem has arisen which is to be concealed by eliminating
both values
at once. In accordance with the invention, however, "discarding" correct
values is dis-
pensed with, and instead, both values are examined so as to select that value
which seems
more plausible as a concealed information unit. In other words, an inventive
error con-
cealment device includes a means for selecting the value output by the forward
decoder or
the value output by the backward decoder as an information unit, dependent on
which
value meets a predetermined criterion, i.e. is plausible, so that a potential
error in the in-
formation unit is concealed.
The inventive concept has the advantage that by forward decoding, on the one
hand, and by backward decoding, on the other hand, continuation errors with
only one er-
roneous code word in the transmission stream may be completely eliminated if
the selec-
tion based on the predetermined criterion leads to a correct value. If
differential coding
has been applied to the information units, even this one erroneous code word
may also be
completely reconstructed due to the difference information from forward and/or
from
backward.
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What is remarkable here is that the error concealment is, in the event of the
occur-
rence of a single error (provided that there is a correct selection based on
the predeter-
mined criterion) is an exact reconstruction which is achieved, in addition,
without increas-
ing the data rate, e.g. by means of FEC techniques etc., it being possible to
also correctly
reconstruct the incorrect code word in the event of underlying differential
coding. The
expense incurred for error concealment is effected only in the decoder.
However, this ex-
panse incurred in the decoder is justifiable if the transmission rate of the
transmission
channel is a more decisive design option than a decoder which is low in
expense. In addi-
tion it shall be pointed out that even in the event of FEC coding in the
coder, a decoder
must be employed which is more expensive than a normal code word decoder.
The plausibility check, i.e. the selection of a value of the pair of values of
the two
decoders, is performed by means of a previously known property of the
information signal,
determined by the information units, which may be, e.g., an audio andlor video
signal.
Scale factors of an audio signal, for example, have the characteristic that
they generally do
not change very much from one scale factor band to the next. In this case, the
predeter-
mined criterion is that the decoder output value which differs less from a
latest intact scale
factor, or from a scale factor which has already been reconstructed, than the
other decoder
output value.
In a further embodiment of the present invention, the predetermined criterion
used
is that the scale factors do not change very much even from block to block, so
that the de-
cision as to which of the two decoder output values is the better one is based
on a respec-
tive scale factor of a preceding block.
However, the predetermined criterion may also be configured in terms of
energy.
In this case it is assumed that the change in energy from one scale factor
band of a previ-
ous block to the same scale factor band in the subsequent block or from a
scale factor band
of the current block to the scale factor band of the next block is relatively
small. Unlike
the previously described embodiments, in this case the energy of the spectral
values in the
respective scale factor bands must be determined to be able to perform the
plausibility
check.
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The predetermined criterion may also be selected such that one proceeds not
only
from one pair of values to the next in the overlap area, but that several or
all pairs of values
in the overlap area are used. In this case, all potential paths may be
constructed through all
pairs of values to then select that path which overall comes closest to the
predetermined
criterion, i.e. has, e.g., the smallest overall change. The overall change is
a weighted or
unweighted sum of individual changes, i.e. from one pair of values to the
next. Alterna-
tively or additionally, a selection of paths may also be performed to discard,
for example,
the paths wherein a change from, e.g., a forward-decoding value to a backward-
decoding
value is performed more than once.
Generally speaking, there are many properties previously known of many informa-
tion signals, to the effect that the signal must, e.g., ascend monotonically,
must descend
monotonically, changes, or does not change, erratically or gradually or by a
certain amount
etc. from one point to the next. One of these criteria or several criteria may
be jointly used
for picking out the correct, or better, value among the two values made
available by the
forward decoder and the backward decoder.
As has already been set forth, the advantage of the inventive concept is that
in the
event of only one single bit error, a complete construction may, in principle,
be achieved.
However, even in the occurrence of several bit errors, concealment may be
achieved if the
decoder generates a sufficient number of continuation errors in the forward
direction and
in the backward direction, so that an overlap area, i.e. an error concealment
area, develops,
wherein two different "suggestions" of the two decoders exist for an
information unit of a
specific ordinal number. However, a complete reconstruction is not possible in
this case,
since decoded values which are located between several bit errors are
generally not cor-
rect.
In a preferred embodiment of the invention, the predetermined criterion
depends on
a previously known property of an information signal represented by
information units
from a current, preceding and/or subsequent block, the predetermined criterion
being met
if a property of the information signal is closer to the previously known
property, while
considering the value selected, than a property of the information signal,
while considering
a non-selected value.
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Preferred embodiments of the present invention will be explained below in more
detail with reference to the accompanying figures, wherein:
Fig. 1 is a block diagram of an inventive device for error concealment;
Fig. 2 shows a schematic diagram for depicting the coming into being of an
error
concealment area by continuation errors both in the forward decoder and in the
backward
decoder;
Fig. 3 shows an elementary schematic diagram for depicting the inventive error
concealment, wherein the predetermined criterion is performed on the basis of
an informa-
tion unit which was last correctly decoded or of an information unit which was
last con-
cealed and is possibly correct; and
Fig. 4 an elementary schematic diagram in accordance with an alternative em-
bodiment of the present invention, wherein a block of decoded information
units, which is
preceding in time, provides a basis for the predetermined criterion.
Fig. 1 shows a preferred embodiment of an inventive device for error
concealment.
The device has a memory 10 associated with it, wherein a sequence of bits 12
is stored
which represent a block of reversible code words of variable lengths. The
first code word
CW1 could include, e.g., three bits, whereas the second code word includes
only two bits,
and the last code word CWn includes, for example, five bits.
Both to a forward decoder 14 and to a backward decoder, bit stream 12 presents
it-
self merely as a sequence of bits, since the start and the end of a code word
are contained
inherently, but not explicitly, as has been set forth. The forward decoder is
configured to
start decoding of bit stream 12 starting from a starting point SP, which has
been drawn, in
Fig. 1, at the left-hand end of the stream, whereas the backward decoder 16 is
configured
to start decoding of bit stream 12 from an end point EP, drawn, in Fig. 1, on
the right-hand
end of bit stream 12.
Both forward decoder 14 and backward decoder 16 feed means 18 for determining
an error concealment area. In particular, the forward decoder provides a
forwaxd-decoding
value for the first code word CW1, then a next forward-decoding value for code
word
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CW2, etc. By analogy therewith, the backward decoder provides a backward-
decoding
value initially for code word CWn and then for code word CW(n-1), etc.
An error concealment area is given if forward decoder 14 and backward decoder
16
provide different output values for one and the same code word. In other
words, the error
concealment area exists if two different "suggestions" for one and the same
information
unit are fed to the means for determining an error concealment area.
In this case, a means 22 for selecting, for one code word, either the output
of for-
ward decoder 14, i.e. of the forward-decoding value, or the output of backward
decoder 16,
i.e. of the backward-decoding value, becomes active.
Alternatively, the overlap area is created in that initially only one decoder,
i.e. the
forward decoder, is operating. Not until it comes across an error and aborts
decoding, the
other decoder, in the example the backward decoder, is activated, so that even
though the
backward decoder aborts its work due to an error, an overlap area may have
formed.
Means 22 for selecting is configured to select that value of the two
alternatives
which meets a predetermined criterion, which causes an error or a potential
error to be
concealed in the information unit and/or, correspondingly, in the code word.
Thus, means
22 provides error-concealed information units at its output 24, it being
pointed out, how-
ever, that this information is error-concealed in an ideal manner, i.e. has
been completely
reconstructed, if only a single erroneous code word was present in bit stream
12, and if the
selection has provided correct values due to the predetermined criterion.
As will be explained below, in a preferred embodiment a criterion indicator
exists
which is fed to means 22 so as to switch from one predetermined criterion to
another pre-
determined criterion depending on a property of the information signal
determined by the
information units, the property being indicated by the indicator. Generally,
the predeter-
mined criterion will depend on the previously known property of an information
signal
determined by the information units, the information signal possibly being an
audio signal
and/or a video signal.
CA 02482866 2004-10-18
Fig. 2 is a more detailed depiction of the starting situation where the
inventive error
concealment technique becomes relevant. A first line of Fig. 2 depicts the
block of re-
versible code words of variable lengths 12 with a starting point SP and an end
point EP,
wherein ordinal numbers for the code words of variable lengths are
additionally plotted
S above the code words symbolized by small squares. In the example shown in
Fig. 2, the
block of code words of variable lengths includes 23 code words in total. The
depiction
shown in Fig. 2 is only schematic, since the length, in bits, of the
individual code words of
variable lengths is, of course, not identical for each code word, but differs
from code word
to code word. In addition, it shall be assumed that in the code word bearing
the ordinal
10 number 12, an error has occurred in the transmission of bit stream 12 via
an erroneous
channel, as is symbolized by a cross in Fig. 2.
Starting from starting point SP, the forward decoder will output forward-
decoding
values which are correct information units up to the ordinal number 12 and are
erroneous
only from the ordinal number 12 onwards, as is symbolized by small dots in the
respective
squares of Fig. 2. Not before ordinal number 18 will the forward decoder come
across an
invalid, e.g. non-symmetrical, code word which has been caused by bit error 12
but which
has led to an invalid code word only for the code word bearing the ordinal
number 18, due
to the error propagation. The forward decoder will therefore end its output at
code word
No. 18.
The backward decoder works analogously, it starts at end point EP of block 12
and
will provide correct results up to the code word bearing the ordinal number 13
and will
provide erroneous results no earlier than from there. In addition, it is
assumed that it is not
before the code word with the ordinal number 7 that the backward decoder will
come
across an invalid code word and recognize that somewhere further upstream an
error has
occurred.
Of course, the decoder is unaware that the error has occurred in code word No.
12.
Therefore, it must assume that, in the extreme case, no error propagation has
occurred. In
other words, this means that the error in code word No. 12 may also have
occurred in code
word No. 7, on the one hand, or in code word No. 18, on the other hand. The
entire area
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11
from the code word bearing the ordinal number 7 up to the code word bearing
the ordinal
number 18, this area representing the error concealment area, is therefore
ambiguous.
This may be recognized by means 22 for selecting in that the forward-decoding
value differs from the backward-decoding value for a code word of the same
ordinal num-
ber. Nevertheless, the schematic diagram in Fig. 2 clearly shows that the
forward decoder
has worked correctly from code word No. 7 up to code word No. 11, and that the
backward
decoder also has worked correctly from code word No. 18 up to code word No.
13. These
findings are utilized, in accordance with the invention, to no longer discard,
interpolating
or otherwise replacing the entire error concealment area, but to examine the
individual
pairs of values for ordinal numbers 7 to 18 so as to select, in each case,
that value which
meets the predetermined criterion.
Figs. 3 and 4, which will be set forth below, show two different predetermined
cri-
teria which may be used, e.g., depending on the criterion indicator of Fig. 1
so as to
achieve error concealment. In a preferred embodiment of the present invention,
the infor-
mation units coded by the code words of variable lengths are scale factors of
an MPEG-
standard coded audio signal. A predetermined property of audio signals is that
scale fac
tors generally do not change very much from one scale factor band to the next,
i.e. from
one scale factor to the next scale factor which is adjacent to it in terms of
frequency. In
other words this means that small changes between successive scale factors are
more likely
than big changes.
The predetermined criterion in the example shown in Fig. 3 thus is that both
the
forward-decoding value for the code word bearing the ordinal number 7 and/or
for the in-
formation unit bearing the ordinal number 7, and the backward-decoding value
for the
code word and/or the information unit bearing the ordinal number 7 are
compared with a
threshold value as a reference, the threshold value used being the latest
scale factor desig-
nated by 34 in Fig. 3. This means that that value which is closer to the scale
factor 34 for
ordinal number 6 is used as an "error-concealed" scale factor 36.
After this comparison, a first step is terminated, and a second step is
started so as to
perform error concealment also for the code word bearing ordinal number 8. For
this pur-
CA 02482866 2004-10-18
12
pose, both suggestions of the forward decoder and of the backward decoder for
code word
8 are taken and compared with the scale factor 36 just reconstructed so as to
generate a
further error-concealed, i.e. reconstructed scale factor 38. The same is
performed analo
gously by the other side of the error-concealment area until both sides come
across a mean
scale factor m if same exists.
Sometimes the error-concealed value generated from the left-hand side
approxima-
tion will differ from the error-concealed value generated by the right-hand
side approxima-
tion. One of the two values could be chosen at random. Alternatively, however,
it is pre-
ferred in this case to take a mean value from these two values, or to take any
value located
between those two values. This value for the mean ordinal number m is very
likely to be
an incorrect value which, however, will not lead to any major deviation,
since, after all, it
was formed on the basis of reconstructed values to the left and to the right
of the ordinal
number m.
As has been set forth, the predetermined criterion may be performed on the
basis of
the previous or subsequent scale factor in the direction of frequency. An
alternative possi-
bility is depicted in Fig. 4. In Fig. 4, the predetermined criterion is not
determined on the
basis of the scale factor, which is adjacent in terms of frequency, from the
current block,
but on the basis of the scale factor with the same ordinal number, from the
previous block.
Again, both suggestions made by the forward decoder and by the backward
decoder are
compared with the scale factor of the same ordinal number of the previous
block, so as to
then accept that suggestion that differs less from the scale factor of the
previous block,
since, as has been indicated, in very many cases scale factors only change to
a limited ex-
tent from block to block.
A selection between the predetermined criterion depicted in Fig. 3 and the
prede-
termined criterion depicted in Fig. 4 may be performed on the basis of the
criterion indica-
tor. For example, the criterion indicator may be given as side information to
the bit stream
generated by the coder, so as to indicate, e.g. for each block, whether or not
the scale fac-
tors greatly change, in terms of scale factor bands, in this block or with
regard to a block
adjacent in time. If the scale factors greatly change in a current block, the
predetermined
criterion depicted in Fig. 3 will not work so well. In this case, the
criterion depicted in
CA 02482866 2004-10-18
13
Fig. 4 will possibly work better, so that for such a block, the criterion
indicator will be
such that means 22 is initialized in such a manner that it applies the error-
concealment
criterion depicted in Fig. 4 which is based on the previous block rather than
on scale fac-
tors adjacent in terms of frequency, as in Fig. 3.
In an alternative embodiment, the predetermined criterion is solved, by the
scale
factors, in that the scale factors alone are no longer considered the
threshold value for the
predetermined criterion, but energies are looked at in scale factor bands.
Means 22 for
selecting is therefore configured to determine the energies of scale factor
bands which are
either adjacent to one another or successive in time, using the spectral
values which have
also been decoded, so as to then take that scale factor suggestion which
results in the small
change in energy.
In alternative embodiments, the predetermined criterion is not only determined
for
a pair of values in the error-concealment area, but for several pairs in
common. Here, the
means for selecting is configured to determine an overall property for a
plurality of differ-
ent paths in the error-concealment area. Each path passes between different
forward and
backward-decoding values in different "zigzag paths". The means for selecting
here is
configured to select the decoding values located on the path having an overall
property
which corresponds to the predetermined criterion. Here, the overall property
is a weighted
or unweighted sum of individual properties of the path.
Alternatively or additionally, only such a path will be taken into
consideration
which precisely exhibits a change from a forward-decoding value to a backward-
decoding
value or vice versa. This embodiment takes into account that when only one
single error is
present, only one single change can occur.
If the operation is not based on paths, but is effected from pair of values to
pair of
values, the above findings may be exploited to not perform a second change in
the case of
an existing change, but to select such decoding values which require no
further change,
regardless of the predetermined criterion.
It shall be pointed out that the predetermined criterion may be based on any
infor-
mation which, in modern coding methods, is contained in the data stream
anyhow, the in-
CA 02482866 2004-10-18
14
formation allowing a statement to be made about a property of the quantity to
be recon-
structed. In the example of audio signals, such information is LlR and/or MJS
stereo in-
formation which is also an indicator of whether the information in both stereo
channels
differs very much or is similar and therefore also reflects a rough measure of
signal statis-
tics and therefore defines a previously known property of the information
signal.
Depending on the circumstances, the inventive error-concealment method may be
implemented in hardware or in software. The implementation may be effected on
a digital
storage medium, in particular a disc or CD with electronically readable
control signals
which may cooperate with a programmable computer system such that the
respective
method is performed. Generally, the invention thus also consists in a computer
program
product with a program code, which is stored on a machine-readable carrier,
for perform-
ing the inventive method when the computer program product runs on a computer.
n other
words, the invention may thus be realized as a computer program having a
program code
for performing the method when the computer program runs on a computer.
