Digital encoding process - Patent Review 5579430

What is claimed is:

1. A digital coding process for at least one of transmitting and storing acoustical signals, the process comprising the steps of:

sampling an acoustical signal to obtain samples;

transforming the samples of said acoustical signal using a transform/filter bank into a sequence of second samples to thereby reproduce a spectral composition of said acoustical signal;

quantizing said sequence of second samples in accordance with the requirements with varying precision;

at least partially coding said sequence of second samples using an optimum encoder, and in which a corresponding decoding and inverse transformation takes place during reproduction of the acoustical signal;

correlating the occurrence probability of the quantized spectral coefficient to the length of the code utilizing a code in such a way that the more frequently said spectral coefficient occurs, the shorter the code word; and

allocating a code word to several elements of said sequence or to a value range to thereby reduce the size of the table of said encoder;

wherein the allocating step further comprising the steps of:

directly assigning a code word to only one part of the value range of an element of said sequence; and

assigning all values lying outside said partial range a common identifier and a special code.

2. A coding process according to claim 1, wherein a Huffman code is utilized as said encoder.

3. A coding process according to claim 2, wherein said special code is carried out using a PCM code.

4. A coding process according to claim 1, further comprising the step of:

combining n spectral coefficients with n being greater than or equal to 2 up to a n-tupel; and

encoding together the n spectral coefficients by assigning a code word.

5. A coding process according to claim 4, wherein the spectral coefficients of successive blocks, in particular, having the same number are combined from successive blocks to form a pair, respectively an n-tupel, and are encoded together by assigning a code word.

6. A coding process operating in accordance with an optimum coding in a frequency domain (OCF) process according to claim 1, wherein an overall amplification factor which forms side information that is transmitted to a receiver is calculated from the values of the number of iterations for an initial quantization step level, from a value for the non-uniformity of the spectrum, and from other level information from the calculation.

7. A coding process according to claim 6, further comprising the steps of:

gathering and transmitting together side information on changes in the quantization step in order to stay within a permissible disturbance for more than one block.

8. A coding process according to claim 7, wherein the encoding of said side information occurs with a variable length code.

9. A coding process according to claim 1, further comprising the steps of:

utilizing different code tables depending on the sequence;

transmitting the number of the utilized code tables; and

storing or transmitting the number together with the encoded values.

10. A coding process according to claim 9, further comprising the steps of:

utilizing different code tables for different spectral ranges; and

setting borders between the different spectral ranges dependent upon the signal.

11. A coding process according to claim 10, wherein a table is utilized for decoding, in which pairs of values are stored, of which the first value of the pair contains an address which is the next pointer into the table in case a "0" has been decoded, the second value of said pair contains the next pointer into the table in case a "1" has been decoded and entries flagged not to contain an address contain the target codeword itself.

12. A coding process according to claim 1, further comprising the steps of:

transmitting, in order to determine the beginning of a next block of information in the event of a transmission error, a block commencement mark and a length of an entropy code.

13. A coding process according to claim 1, further comprising the step of adapting quantization and reconstruction to each other in such a manner that the mean quantization error is minimized.

14. A coding process according to claim 13, wherein the calculation of the reconstruction values takes place in a receiver using a table, which was compiled by determining expectancy value of the input values of a quantization interval.

15. A coding process according to claim 13, wherein said quantization takes place via a table, which was calculated from the characteristic curve of the reconstruction and the probability distribution of the input data.

16. A coding process according to claim 13, wherein, for reconstructing by means of the characteristics curve of a quantization and a probability distribution of the input data, a reconstruction value is calculated in a decoder for each individual quantized value in such a manner that the quantization error is minimized.

17. A process according to claim 1 further comprising the step of reproducing the stored signal using a corresponding decoding and a reverse transformation or a reverse filter bank.

18. A digital coding process for at least one of transmitting and storing acoustical signals, the process comprising the steps of:

sampling an acoustical signal to obtain samples;

transforming the samples of said acoustical signal using a transform/filter bank into a sequence of second samples to thereby reproduce a spectral composition of said acoustical signal;

quantizing said sequence of second samples in accordance with the requirements with varying precision;

at least partially encoding said sequence of second samples using an optimum encoder, and in which a corresponding decoding and inverse transformation takes place during reproduction of the acoustical signal;

arranging one part of the variable length code words in a raster, and

distributing the remaining code words in the remaining gaps so that the beginning of a code word is more easily found;

wherein the process further comprises the steps of:

utilizing an equidistant raster in order to improve reliability of a transmission of continuously successive information of varying degrees of importance, a raster length of which corresponds to a mean length of the information to be transmitted;

arranging most important portions of said information in the raster; and

transmitting a position of less important information in said raster in addition to said most important portions of information if necessary.

19. A digital coding process for at least one transmitting and storing acoustical signals, the process comprising the steps of:

sampling an acoustical signal to obtain scanned values;

transforming the samples of said acoustical signal using a transform/filter bank into a sequence of second samples to thereby reproduce a spectral composition of said acoustical signal;

quantizing said sequence of second samples in accordance with the requirements with varying precision;

at least partially encoding said sequence of second samples using an optimum encoder, and in which a corresponding decoding and inverse transformation takes place during reproduction of the acoustical signal; and

wherein the sequence of second samples are divided into data blocks corresponding to time periods of a given length, and further comprising calculating a permissible disturbance of the data blocks by using a time period which is one of: 1) longer than the given length; and 2) a function of results of previous time periods; and

wherein, in order to calculate said permissible disturbance for a block, an analysis of the signal is conducted in different frequency groups, and respective values of a preceding block, which have been corrected by an omission factor, and values of the current block, are both drawn upon for calculating said permissible disturbance.

20. A coding process according to claim 19, wherein, in order to calculate said permissible disturbance for a block, energy values of the last block are utilized in such a manner that values of lower amplitude following values of higher amplitude require less precise quantization.

21. A coding process according to claim 19, wherein if a sharp rise in energy toward high frequencies is detected, the calculated permissible disturbance is reduced for a frequency group below the rise.

22. A digital coding process for at least one of transmitting and storing acoustical signals, the process comprising the steps of:

sampling an acoustical signal to obtain samples;

transforming the samples of said acoustical signal using a transform/filter bank into a sequence of second samples to thereby reproduce a spectral composition of said acoustical signal;

quantizing said sequence of second samples in accordance with the requirements with varying precision;

at least partially encoding said sequence of second samples using an optimum encoder, and in which a corresponding decoding and inverse transformation takes place during reproduction of the acoustical signal; and

wherein, although a constant data rate is maintained, the number of bits for use by a block may deviate from an average data rate depending on the signal characteristics, on the capacity of the transmission channel, or in order to simplify the encoding; and

wherein a number of bits at disposal for encoding a data block is altered contingent on the signal characteristics in such a manner, that on the average a constant data rate is maintained, and the deviation of sums from this mean value is not higher than a specific previously fixed value and not less than another previously fixed value.

23. A process according to claim 19 further comprising the step of reproducing the stored signal using a corresponding decoding and a reverse transformation or a reverse filter bank.

24. A coding process according to claim 22, wherein in order to synchronize coding processes having a random ratio of input to output bit timing, a load level of a buffer read out with an output bit timing is used to determine the number of bits to be allocated.

25. A coding process according to claim 24, wherein a number of bits at disposal for encoding a data block is altered contingent on the signal characteristics in such a manner that signal blocks having a greater distance between the signal energy in the individual frequency groups and the respective permissible disturbance are given a greater number of bits (current data rate) at disposal than signal blocks having a smaller distance.

26. A coding process according to claim 25, wherein the number of bits transmitted on the same channel required by the side information is subtracted from the bits at disposal for a block.

27. A coding process according to claim 26, wherein, if a special bit combination is employed for a synchronizing word for block synchronization, said synchronizing word and all bit combinations, which by chance are identical to the synchronizing word, are distinguished from one another with an intentionally added supplementary bit.

28. A coding process according to claim 25, wherein any ranges, in which said permissible disturbance is greater than said signal energy, are deleted.

29. A coding process according to claim 28, wherein a resulting sequence of deleted values is encoded with a bit in the side information.

30. A coding process according to claim 29, wherein said resulting sequence of deleted values is encoded in said side information with a value in the table of possible quantization step levels for each frequency group.

31. A coding process according to claim 22, wherein an outer iteration loop is interrupted in the event an inner loop cannot be ended with certainty within the maximum number of iterations.

32. A process according to claim 22 further comprising the step of reproducing the stored signal using a corresponding decoding and a reverse transformation or a reverse filter bank.

Description Submit all comments and votes

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a digital encoding process for transmitting and/or storing accoustical signals and, in particular, music signals which scanned values of the acoustical signal are transferred by a transformation or a filter bank into a sequence of second scanned values which reproduce the spectral composition of the acoustical signal. The sequence of second scanned values is quantized in accordance with the requirements with varying precision and is partially or entirely encoded by an optimum encoder. A corresponding decoding and inverse transformation takes place during the transformation.

Processes such as those described above are known, by way of illustration, from DE-PS 33 10 480 or from WO 88/01811. Moreover, the cited publications are to be referred to for the explanation of all the terms not made more apparent herein.

In particular, the present invention relates to the OCF process, which was proposed for the first time in WO 88/01811.

The object of the present invention is to improve digital encoding processes and, in particular, the OCF process known from WO 88/01811 in such a manner that encoding of music of a quality comparable to compact-disc quality is possible at data rates of approximately 2 bit/ATW and of music of good FM-radio quality at data rates of 1.5 bit/ATW.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a spectrum with a distinct maximum,

FIG. 2 depicts encoding words in a fixed raster,

FIG. 3 depicts the arrangement of important information portions in a fixed raster,

FIG. 4 depicts schematically the ring buffer serving as the "bit bank", and

FIG. 5 depicts the frequency distribution of the spectrum.

DETAILED DESCRIPTION OF THE DRAWINGS

The procedure with encoding processes for transmitting and/or storing accoustical signals and, in particular, music signals is usually as follows: first the scanned values of the accoustical signal are transformed into a sequence of second scanned values reproducing the spectral composition of the acoustical signal. This sequence of second scanned values is then quantized with varying precision and partially or completely encoded using an optimal encoder depending on the requirements. In reproduction, a corresponding decoding and reverse transformation takes place.

The translation of the scanned values of the acoustical signal into a sequence of second scanned values occur by means of a transformation or a filter bank, with the output signal of the filter bank, if need be, being "under"-scanned in such a way than a block is created as in a transformation.

An element of the present invention is, according to an embodiment of the present invention, that a known encoder, i.e. an encoding allocation, is used where the occurrence probability of the quantized spectral coefficient correlates with the length of the code in such a manner that the code word is shorter in proportion to the frequency of the spectral coefficient. Encoders of this type are, by way of illustration, as Huffman codes.

A further element of the present invention is that, in order to reduce the size of the table of the encoder, a code word and, if need be, a supplementary code is allocated either to several elements of the sequence or to a value range.

The joint encoding of spectral coefficients can take place in different ways: by way of ilustration, it can occur in the frequency range or in the time axis, with the same coefficients from successive blocks being encoded together. Furthermore, a code word can be allocated to a value range in a kind of rough quantization. A subrange may also be directly encoded, with the remainder of the value range being provided with a special identifier and in addition the offset being encoded up to the limit of the value range.

In this event, the code is usually taken from a table, the length of which corresponds to the number of code words. If a large number of code words with a word length, which is longer than the average length of a word, are of a similar length, all these code words can be described by a common identifier and a subsequent special code, which is adapted to the specific case of application, with minimum loss in encoding effectivity. This code may, by way of illustration, be a PCM code (pulse code modulanion). This process is especially efficient if only a few values possess a large occurrence probabilty as is, for example, the case when encoding music in spectral representation.

In the following section, this shall be made apparent using an example, with the following probability distribution being given:

______________________________________ Value Probability ______________________________________ 0 50% 1 30% 2 15% 3 . . . 15 all together 5%, i.e. 0.38% each ______________________________________

The entropy, i.e. the shortest possible mean code length is in this case 1.83275 bit.

In such a case of application, it is advantageous to determine a Huffman code containing the values 0,1,2 and an identifier (called ESC in the following section), in which the values 3 to 15 are encoded.

______________________________________ Huffman Code Huffman Code Value with ESC without ESC ______________________________________ 0 0 0 1 10 10 2 110 110 3 111 + 0011 111101 4 111 + 0100 111110 5 111 + 0101 111111 6 111 + 0110 1110000 7 111 + 0111 1110001 8 111 + 1000 1110010 9 111 + 1001 1110011 10 111 + 1010 1110100 11 111 + 1011 1110101 12 111 + 1100 1110110 13 111 + 1101 1110111 14 111 + 1110 1111000 15 111 + 0111 1111001 ______________________________________

A pure Huffmann encoding yields a mean code length of 1.89 bit, an encoding with ESC, on the other hand, yields a mean code length of 1.9 bit. The code efficiency is a little worse with the ESC encoding, but the size of the table for the encoder and the decoder is smaller by about the factor 4, so that the speed of the encoding and decoding process increases considerably.

If a modified PCM code is utilized as the code according to the ESC value, even values up to 18 can be encoded without changing the mean code length.

______________________________________ Huffman Code with ESC ______________________________________ 10 110 111 + 0000 111 + 0001 111 + 0010 111 + 0011 111 + 0100 111 + 0101 111 + 0110 111 + 0111 111 + 1000 111 + 1001 111 + 1010 111 + 1011 111 + 1100 111 + 1101 111 + 1110 111 + 0111 ______________________________________

An embodiment of the present invention provides an improvement according to which the n spectral coefficients are combined with n.gtoreq.2 a n-tupel and are encoded together by means of an optimum encoder. Optimum encoders, which assign a code word of different length to each spectral value, are only in exceptional cases "optimum in the information theory sense". A further reduction of the code redundance can be achieved by the invented encoding in that at least a pair of spectral values is allocated to a word. A decrease in redundancy results, firstly, due to the fact that both spectral values encoded together are not statistically independent, secondly due to the fact that a finer adaption of the code book, or of the code table, to the statistics of the signal can ensue by encoding pairs of values.

This shall be made more apparent in the following section using an example.

First an entropy encoder (optimum encoder), which allocates individual values to individual code words, is looked at:

______________________________________ Data word Frequency Code word Frequency * Code length ______________________________________ 0 70% 0 0.7 1 30% 1 0.3 ______________________________________

A mean code word length of 1 is yielded.

Looking at the probabilities of pairs of scanned values yields the following optimum encoder:

______________________________________ Data word Frequency Code word Frequency * Code length ______________________________________ 00 50% 0 0.50 * 1 01 20% 10 0.20 * 2 10 20% 110 0.20 * 3 11 10% 111 0.10 * 3 = 1.8 ______________________________________

The mean code word length per single value is yielded by the sum of the terms "frequency*code word length" divided by 2 (due to the encoding of pairs of values). In the example, it is 0.9. This is less than what can be attained by encoding single values and assuming the same signal statistics. Encoding pairs of spectral values occurs, e.g. by employing the first spectral value as the line number and the second value of a pair as the column number in order to address the respective code word in a code cable.

If the number of values encoded together is further increased, on the average a shorter code word length is yielded, e.g. for 4 tupel in which every value originates from the interval [0.1]:

______________________________________ Data word Frequency Code word Frequency * Code length ______________________________________ 0000 25% 10 0.25 * 2 = 0.5 0001 10% 000 0.1 * 3 = 0.3 0010 10% 010 0.1 * 3 = 0.3 0011 5% 11101 0.05 * 5 = 0.2 0100 10% 011 0.1 * 3 = 0.3 0101 4% 11100 0.04 * 5 = 0.2 0110 4% 11001 0.04 * 5 = 0.2 1111 2% 00100 0.02 * 5 = 0.1 1000 10% 1111 0.1 * 4 = 0.4 1001 4% 11011 0.04 * 5 = 0.2 1010 4% 11010 0.04 * 5 = 0.2 1011 2% 110001 0.02 * 6 = 0.12 1100 5% 0011 0.05 * 4 = 0.2 1101 2% 110000 0.02 * 6 = 0.12 1110 2% 001011 0.02 * 6 = 0.12 1111 1% 001010 0.01 * 6 = 0.06 3.57 ______________________________________

The mean code length in this example thus is 3.57/4=0.89

Furthermore, joint encoding can also be carried out, in accordance with another embodiment of the present invention by combining spectral coefficients of the same number from successive blocks and encoding them together. This will be made more apparent using an example, in which for reasons of simplicity two data blocks are encoded together; however, more data blocks can also be combined in the same manner.

x(1), x(2), . . . , x(n) are the frequency coefficients of a block, with y(1), y(2), . . . , y(n) the ones of the next block.

1) The spectral values to be encoded of two successive data blocks are encoded together. For this purpose, the quantized spectral value having the same number is taken from each of the two blocks and this pair is encoded, i.e. temporally successive values of the same frequency are encoded together. The correlation between these is very great in quasi-stationary signals, i.e. their value varies very little. Due to the joint encoding, the collateral quantization information is only required once for both data blocks.

The pairs (x(1) y(1), (x(2)), . . . , (x(n) y (n)) being encoded together.

2) The values of two successive spectral values of a block are correlated together in the case of "even" spectra. For signals of this kind, it is useful to encode two spectral values of a block together.

The pairs (x(1) x(2)), (x(3)x(4)), . . . , (x(n-1)x(n)) being encoded together. Depending on the transformation, other combinations of values are also useful.

3) Switching between 1) and 2) can, for example, be transmitted by an identification bit.

For more than two jointly encoded values, both possible ways can be combined: for 4-tupel, e.g. the following possible ways are useful:

a) one value each from four successive blocks

b) two values each from two successive blocks

c) four values from one block

In cases a) and b) supplementary information can be reduced.

Naturally, the encoding cannot only occur by forming pairs or n-tupels of one spectral value of each data block or by forming n-tupels of more than one spectral value from each data block, but also by switching between pairs or n-tupels of successive data blocks and pairs or n-tupels of successive spectral values in the counting of frequency values in order to form pairs or n-tupels of spectral values.

In accordance with another embodiment of the invention, the following information should be considered. In the OCF processes described in WO 88/01811, separate values are transmitted from encoder to decoder for one factor of level control, for the number of iterations, which occurred in the inner loop, and for a measured value for the uneven spectral distribution (spectral flatness measure sfm). An element of the present invention is that a common "overall amplification factor" is determined from these values and transmitted to the receiver. The overall amplification factor is calculated by expressing all individual values as exponents of a specific numerical value and the factors are added to each other.

This shall be made more apparent in the following section using an example:

The following signal amplification operations are possible (a,b,c are integers):

1) Level adaption: amplification steps with 2.sup.a

2) Quantization:

a) Start value for quantizers in steps of (.sup.4 .sqroot.8).sup.b =1.682.sup.b

b) Roughening the quantizers in steps of (.sup.4 .sqroot.2).sup.c =1.189.sup.c

The quantization corresponds to a division, i.e. a diminishment. For this reason, factors formed in this manner have to be taken negatively.

The common factor herefor is therefore f=.sup.4 .sqroot.2.

for 1) f.sup.4a =2.sup.a

for 2a f.sup.3b =(.sup.4 .sqroot.8).sup.b

for 2b f.sup.c =(.sup.4 .sqroot.2).sup.c

The overall amplification factor is therefore f.sup.4a-3b-c, only the integer exponent is transmitted to the decoder. The number of required bits is determined by the word length of the input data (in general 16 bit) and the transformation length (results in maximum dynamics).

Furthermore, variable data rates may be assigned to the supplementary information.

The correction factors, which make it possible to remain within the permissible level of disturbance, have to be transmitted to the receiver as supplementary level information per frequency group. An element of the present invention is that a reduction of the mean data rates required for this purpose is achieved by encoding the length of the succeeding data words in one trigger word and only utilizing the word length necessary for the respective transmission. This also will be made more apparent in the following section using an example:

The assumptions being: the number of frequency groups: 3

maximum number of iterations 8

Transmitted are the number of amplifications per frequency group. Without the variable data rate of the supplementary information 3*3=9 bit would be required. The maximum number of amplifications (in the example) are encoded as follows:

______________________________________ no amplification 0 at most one amplification 1 at most three amplifications 2 at most seven amplifications 3 ______________________________________

The respective code word directly gives the number of bits required in order to encode the maximum amplification value.

The result of the psychoacoustical iteration loop is (0 0 2) in the example, i.e. the frequency group 3 was amplified twice, the other frequency groups were not. This can be encoded with the following bit sequence:

10 00 00 10, thus altogether 8 bits.

In accordance with an embodiment of the present invention, an encoder operating according to a so-called Huffman code is also employed in an as such known manner. An element of the present invention is that now, however, n-code tables with n.gtoreq.1 and with varying length are utilized, which are adapted to the respective acoustical signal to be encoded. The number of the used code table is transmitted, or stored, with the coded values.

The mean code length, namely, depends on the number of different characters in the code. For this reason, it is useful to select a Huffman code which does not contain more than the required number of values. If one chooses the maximum value to be encoded as the criterium for selection for the code table, all the currently present values can be encoded.

If several code books, or code tables, are at disposal, the best table can be chosen using the values to be encoded and the code-table number can be transmitted as supplementary information. A pre-selection from the code tables may occur via the maximum value to be encoded.

Just to round things off, it must be added that, by way of illustration, very rough spectra like those generated by brass wind-instruments have different statistics, in which small values occur more frequently than even spectra like those generated by string instruments or wooden wind-instruments.

An embodiment of the present invention provides an improvement, in which different spectral ranges are allocated to different code tables in addition to or in place of the afore-mentioned allocation to various tables. In the case of spectra with a distinct maximum, it is, namely, a gain to divide the maximum into separate ranges and to select an optimum Huffman code for each section.

FIG. 1 shows a spectrum of this type, in which the maximum of the spectrum is about in the center of the spectral range. Here, by way of illustration, the range can be divided into four ranges.

In the first range, a Huffman code having 16 values is used, in the second a code having more than 32 values, in the third a code having again 16 values and in the fourth range a code having 8 values. It is advantageous if in the case of the code table having more than 32 values a table is employed, in which the code words having greater word lengths than the average word length are described by a common identifier and a succeeding PCM code. This is designated with "TAB having ESC" in FIG. 1.

In the illustrated example, the Huffman codes are selected according to the maximum of the sections, the codes being respectively available for 2,4,8 values, etc. Without this separation, the code for more than 32 values would have to be applied to the entire spectrum so that the required bit number for the block would be distinctly higher.

The points of separation and the code-table number for each section have to be transmitted as supplementary information.

The selection of Huffman codes for each section may occur, in particular, such that spectral coefficients having the same number are combined from at least two successive blocks to form a pair, respectively a n-tupel, and are encoded together by giving a code word.

Another embodiment of the invention provides an advantageous method of decoding the previously mentioned Huffman codes. For this purpose, the tree yielded in making the code is reproduced. Due to the condition that no code word may be the beginning of another code word, there is only one possible path from the "root" to the corresponding code word. In order to arrive at the code word, starting at the beginning, a bit of the code word is employed in each case to determine the path in the event of branching in the tree. The practical realization occurs by means of a table of pairs of addresses which are worked down always starting with the first pair. The first value of the pair contains the address of the next branch, which in the case of a "0" is to be selected in the value to be decoded, the second value the address of the branch in the case of a "1". Each address is marked as an address. If a value of the table is arrived at without this marking, a code word has been reached. In this event the value of the table corresponds to the value to be decoded. The succeeding bit to be decoded is consequently the first bit of the succeeding code word. Beginning with this another run through the table starting from the first pair of addresses commences.

In the following section, an example herefor shall be made more apparent:

______________________________________ Value Huffman Code Code Tree ______________________________________ 0 1 2 3 4 5 6 7 00 01 100 101 1100 1101 1110 1111 ##STR1## ______________________________________ Decoding Table ______________________________________ &0 &1 &2 &3 &4 &5 &6 &7 &1/&4 &2/&3 0/-- 1/-- &5/&8 &6/&7 2/-- 3/-- &8 &9 &10 &11 &12 &13 &14 &9/&12 &10/&11 4/-- 5/-- &13/&14 6/-- 7/-- ______________________________________

The character & stands for an address.

______________________________________ Decoding Example: 1101 = 5 ______________________________________ &0 1 ##STR2## &4 &4 1 ##STR3## &8 &8 0 ##STR4## &9 &9 1 ##STR5## &11 &11 ##STR6## No address ##STR7## dec. value = 5 ______________________________________

In the event that a Huffman code of pairs of values was compiled, the corresponding second value can be accommodated at the second, in the above example, unoccupied site of the table. Accordingly, this process can also be utilized for decoding Huffman codes, which encode more than two values together.

In the case of encoding, in which the beginning of a code word is only determined by the end of the preceding code word (as by way of illustration is the case with the Huffman code), a transmission error will lead to a propagation of the error.

A solution to this is provided by an embodiment of the present invention. This solution may, of course, also be utilized independently of other features: in order to do this a portion of the code words are first arranged in a raster, the length of which, by way of illustration, is larger or the same length as the longest code word. Thus no error