FR2514969A1 - DEVICE FOR CODING A TRAIN OF CHARACTERS WITH INFORMATION COMPRESSION - Google Patents

Abstract

AN ORIGINAL CHARACTER TRAIN IN WHICH THE CHARACTERS APPEAR WITH RELATIVE FREQUENCIES WHICH VARY OVER TIME IS CODE IN AN ENCODER 14 INTO A TRAIN OF CODE WORDS BEFORE BEING TRANSMITTED BY A LINK 20 TO A DECODER 24 WHICH RETURNS THE ORIGINAL DATA TRAIN. THE CODE WORDS ARE DISTRIBUTED INTO THREE GROUPS OF WHICH THE WORDS CONSIST OF ONE, TWO AND THREE SUBWORDS RESPECTIVELY. THE ENCODER CONTINUALLY DETERMINES THE FREQUENCY OF APPEARANCE OF THE ORIGINAL CHARACTERS AND IT ENCODES THEM IN A WAY TO ASSIGN A CODEWORD TO A SINGLE SUBWORD WITH THE ORIGINAL CHARACTERS HAVING THE HIGHEST FREQUENCY OF APPEARANCE, WHILE THAT THE LESS FREQUENT ORIGINAL CHARACTERS ARE CODED WITH THREE SUBWORDS.

Description

DEVICE FOR CODING A TRAIN

  CHARACTERS WITH INFORMATION COMPRESSION

  The present invention relates to the coding of an original character train into a shorter code word train, and the decoding of code words.

  Different original characters may appear

  be with different frequencies in a character train

  original res The fact of coding in the form of shorter code words the original characters which appear more frequently makes it possible to reduce the length of the train of words

  The relative frequencies of occurrence of diffe-

  The original characters may change over time.

  The invention generally consists of a dis-

  positive intended to encode a stream of original characters in a stream of code words, each of the original characters having a frequency of appearance in the stream of characters

  origin which is likely to vary over time Cha-

  none of the code words belongs to a group among a set

  ble of groups of code words and the code words in the respective groups comprise different respective numbers of subwords The device comprises an encoder intended to encode each of the original characters in the form of one of the code words , the original characters appearing most frequently being coded in the form of code words having fewer subwords; and a group size selector to replace with new ones

  numbers the numbers of code words belonging respective-

  to some, but not all, of the groups and to recalculate

  the numbers of code words belonging to other groups

  pes, based on the new numbers, whereby a

  at least part of the codeword stream is shortened.

  Another aspect of the invention involves a control device 8 intended for counting the number of appearances, in the original character train, of each original character in a set comprising less than all of the characters. and a group size selector which reacts to the control device by

  changing the numbers of code words belonging respectively-

  groups, whereby at least part of the

  code word stream is shortened.

  Another aspect of the invention relates to a provision

  sitive in which the code words belong to a pre-

  mier, to a second or to a third group of code words, and the code words which belong to the first, second and third groups respectively comprise one, two and three subwords The device comprises an encoder intended to code a first set original characters in the form of code words belonging to the first group, a second set of original characters in the form of code words belonging to the second group, and a third set of the original characters in the form of words of

  code belonging to the third group, the original characters

  gine appearing most frequently being coded under the

  form of code words with a smaller number of sub-

  words; and a group size selector intended to load the numbers of code words belonging respectively to the first, second and third groups, depending on variations in the frequency of appearance, whereby a

  at least part of the codeword stream is shortened.

  In embodiments / advantageous, the selection

  group size is connected so as to replace the number of code words belonging to one of the groups with a new number, and to recalculate the numbers of code words belonging to the other groups, directly from

of the new number.

  Another aspect of the invention relates to an apparatus for

  reil comprising a device for determining the frequency of appearance rank, intended to assign to each of the original characters a rank corresponding to its relative frequency of appearance estimated current in the original character train; an encoder intended to encode in a word

  of code having a subword, according to a coded selection-

  born, each of the original characters, having a rank not exceeding

  not a first selected number, to be coded as

  me of a code word having two subwords each of the characters

  original tres having a rank greater than the first number and not exceeding a second number, and to be coded in the form of a code word having three subwords each of

  original characters having a rank higher than the second name-

  bre, the second number being fixed by the value of the first number; a control device intended to count the number of appearances in the original character string of each original character in a set that does not include all of the original characters; and a selector which operates under the control of the control device so as to update the first number to give it a new value, when the new value shortens at least part of the stream of code words and so as to

  recalculate the second number from the first number.

  In preferred embodiments, the assembly

  not including all of the original characters

  takes the original characters for which the number of subwords in the associated code words would be changed by changes in the first number and the second number, and in particular the original characters having ranks equal to the first number, at first number decreased by one, and for each number between the second number minus the number of subwords in the alphabet of subwords,

  and the second number plus the number of subwords in alpha-

  subword bet, decreased by one; the control device includes a counter intended to count in direction

  increasing from a median value for each pair-

  tion of the original characters having ranks equal to the first

  mier number and the first number minus one, and count in descending direction for each appearance of the original characters having ranks equal to each number between the second number minus the number of subwords in the sub alphabet -words, and the second number plus the number of words in the alphabet of words, minus one

  unit, while the selector includes an adder of-

  to increase the first number by one when the

  counter reaches a higher value and decrease the pre-

  mier number by one when the counter reaches a lower value; the selected code is changed when the first number is updated to take a new value; and there is also a decoder intended to decode the code word stream to give the original character train, the decoder comprising a device

  determining the frequency of occurrence of deco-

  deur, intended to determine for each of the characters of origin in the alphabet of origin the rank which corresponds to its frequency of relative appearance estimated current, and the decoder directly determines from code words present in the train of code words which of the codes that can be selected is used

for coding.

  Another aspect of the invention relates to a provision

  sitive intended to encode an original character train into a code word train, each of the original characters having a frequency of appearance in the original character train which varies over time The device comprises a memory for rank intended to record the rank which corresponds to the current estimated relative frequency of appearance of each original character in the original character string, and a device for updating

  rank to swap the rank of each original character

  which appears in the original character string, with the rank of that of the original characters which has the rank immediately below, unless the original character

  who appears in the train has the lowest possible rank

  corn. Another aspect of the invention relates to a device which comprises: an encoder intended to encode in the form of a code word having a subword each of the original characters having a rank of not exceeding a first number selected, to be coded in the form of a code word having two subwords each of the original characters having a rank greater than the first number and not exceeding a second number, and to be coded in the form of a word code having three subwords each of the original characters having a rank greater than the second number, the second number being fixed by the value of the first number; a coding modifier for words with two subwords, intended to divide by 16 any code word having two subwords and whose last subword is zero, and to add the result to 15 plus the product of 15 by the first number, to give a revised codeword; and a coding modifier for code words with three subwords, to divide by 16 all

  code word with three subwords and the last of which

  word is zero, add the result to 3573 plus the second number and, if the resulting code word is less than 3856, subtract 16 from the resulting code word, for

give a revised codeword.

  The invention allows rapid and adaptive coding.

  effective of an original character string to give a compressed word string; the code changes to adapt to changes in the relative frequency of appearance of the original characters, in order to minimize the length of the

  code word train; code adaptations are made

  killed quickly and economically by changing only one independent variable, from which the other intervening numbers can be easily recalculated and the decoder can automatically adapt to changing codes, based on the code words received, without requiring the emission of special switching signals from

coder code to decoder.

  The invention will be better understood on reading

  the following description of an embodiment given

  by way of nonlimiting example The rest of the description

  refers to the accompanying drawings in which: -

  Figure 1 is a block diagram of a system

coding-decoding.

  Figure 2 is a table showing a

  characteristic coding sequence.

  Figure 3 is a block diagram of the

Figure 1.

  Figure 4 is a block diagram of the decoder

of figure 1.

  Figures 5 A to 5 D are flowcharts of the

coding process.

  Figures 6 A-6 D are flow diagrams of the pro-

decoding stops.

  Overall structure and operation In FIG. 1, the terminal 10 generates an original character train on line 12, in the form of voltages, currents, frequencies or other parameters representing binary elements. The encoder 14 receives the original characters on its access 16 and it codes them in a train of binary code words which are emitted by the access 18 on the link 20 The number of bits in the train of code words is generally less than the number of bits in the associated original character train La

  form of the code words makes it possible to carry out an independent decoding

  during to obtain the initial train of original characters

  born. The link 20 can fulfill various functions,

  including modulation and demodulation, communication

  tation, recording and detection / correction of transmission errors, but, in all cases, the link finally applies to access 22 of decoder 24 a sequence of code words identical to that which it received from the encoder 14 The decoder 24 decodes the code words to give a train of characters of identical origin to the train received on the access 16, and it applies them by the access 26 to the

  line 28 which transmits them to terminal 30.

  Another data train can run simultaneously-

  in the system in the opposite direction, and for this train, the decoder 24 operates as an encoder and the encoder 14

works as a decoder.

  Each original character is one of the 256 possible octects Each code word consists of 1, 2 or 3

  subwords (called fragments) of four bits each.

  In general, the first step in the coding process consists in ordering the 256 original characters in order of frequency of decreasing appearance in the original train. The coder keeps two tables, one of which is a list of ranks (of O to 255) in the order of the original characters, in which the i th item is the rank of

  second original character; and the second table is a read-

  te original characters (from O to 255) classified by rank, in

  which the i th item is the original character of rank i.

  When an original character of rank r appears in the original train, the two tables are updated so that the rank of this original character becomes r 1

  (if it is not already zero), and so that the character-

  original parent who had rank r 1 take row r.

  The two tables are therefore constantly updated to work

  reduce the relative frequencies of appearance of the characters

  original The rank (before updating the tables) of each

  that original character appearing in the original train

  determines the code word in the form of which it will be coded.

  To perform the encoding, the 256 characters

  possible origins are divided into three groups The characters

  original tres having ranks ranging from zero to a first number (N 4 E) are coded in the form of code words with a single fragment, those whose ranks range from N 4 E to a second number (N 8 E 1) are coded in the form of two-fragment code words, and those whose ranks range from N 8 E to a third number (NL) are coded in the form of three-fragment code words. The determination of N 4 E, N 8 E and NL is governed by the so-called Kraft equality:

N 4 E + (N 8 E N 4 E) + NL N 8 E 1

NS NS NS

  where NS is the number of different fragments dis-

  ponibles et NL is a selected number (here 267) which is at least as large as the number of possible original characters In the present case, each fragment contains 4 bits, which would normally allow to have 16 different fragments, but the fragment 0000 is specified as an illegal fragment, which gives NS 15, and the Kraft equality becomes:

N 4 E N 8 E N 4 E NL N 8 E

+ + 3

15 15

  When N 4 E is incremented by one (i.e. when one more original character is added to the coded group in the form of single-fragment code words), equality requires that NL N 8 E is incremented by 15 and N 8 EN 4 E is decremented by 14 Conversely, if N 4 E is decremented by one, NL N 8 E must 8 be decremented

  by 15 and N 8 E N 4 E must be increased by 14 Therefore

  quent, if the original character of rank N 4 E appears more often than the original characters of rank N 8 E 15 to N 8 E 1, N 4 E must be incremented by one (i.e. - saying that an additional original character must be added to the group which is coded in the form of code words with a single fragment); and N 4 E must be decremented by one if

  the original character of rank N 4 E i appears less frequent

  only the original characters of ranks N 8 E to

  N 8 E + 14, since this will shorten the coded stream.

  N 4 E and N 8 E are selected by counting from a median value (for example 64), in ascending direction for each appearance of the original characters of ranks N 4 E 1 and N 4 E and in decreasing direction for each appearance of the original characters of rows N 8 E-15 to N 8 E + 14. When this count exceeds a first threshold (for example 128), N 4 E is incremented; and when the count falls below a second threshold (for example 0), N 4 E is decremented Once N 4 E is determined, the value of N 8 E is recalculated

  in accordance with Kraft equality.

  The next step is to assign each character

  original mother to a codeword The process is represented in FIG. 2 which shows a tree having a number of levels (columns) 110 equal to the greatest number of fragments which constitute a codeword, that is to say three in this

  case The first level 115 includes branches 112 in nom-

  bre equal to the possible number of fragments (16 in this case) and each branch 112 of the first level represents either a possible 4-bit code word or a possible 4-bit prefix of an 8-bit or 12 code word bits Each branch 112 is designated by a number, not appearing in parentheses, which indicates in decimal form the number of the fragment represented by this branch, and any branch

  112 to which an original character can be assigned con-

  holds the decimal rank of this original character in parentheses The second level 116 comprises 16 branches 118 for each branch 112 whose fragment acts as a prefix Each branch 118 is similarly designated by a fragment number, not appearing in parentheses ,

  while the rank of the original character affected is indi-

  qué entre parentheses The third level 122 similarly includes 16 sheets 124 for each branch 118 whose fragment acts as a prefix Each original character assigned to a sheet 124 at the third level is coded in the form of a word with three fragments , including the fragments represented by the branch at the first level,

  the branch on the second level and the leaf on the third level.

  Each branch, twig or leaf indicated by a dotted line represents the illegal fragment 0000, so that only 15 different fragments are actually available at each level 110 The levels 110 of the tree in Figure 2 are designed to provide N 4 E branches available at the first level, to code N 4 E original characters in the form of code words with a single fragment, N 8 EN 4 E branches available 116 at the second level to code N 8 EN 4 E characters d origin in the form of two-fragment code words, and NL N 8 E sheets available 124 at the third level for coding the original characters

  remaining in the form of three-fragment words.

  Structure of the encoder As shown in Figure 3, the encoder has an input 16 on which it receives the character string

  original 8-bit, possibly at an irregular rate

re.

  The memory Ml, 42, contains 256 elements represented

  sensing the possible 256 original characters The content of the memory elements Ml, 42 is fixed at values

  between O and 255 The r value conserved

  seen in the i th element of the memory Ml, 42, represents

  the rank r of the i th original character, in frequency of appearance

  rition in the original character string For example, if the 10 th element of the memory Ml, 42, contains the value 128, the original character represented by the 10 th element is estimated to be the 128 th character, in order

  decreasing appearance, in the original train.

  The memory M 2, 44 contains 256 positions represented

  feeling the 256 possible frequency ranks of the original characters The value i kept in the rth element of M 2 represents the original character i which carries the rank r

  with regard to the frequency of occurrence By cons-

  quent, the memory Ml is a table of ranks in the order of the original characters, while the memory M 2 is a table of the original characters in the order of the rows If the lth ilth ith element of Ml is r, the r element of M 2 is i Il

  it is very convenient to initially set i to the i th elements

both Ml and M 2.

  The register N 4 E 46 contains the number N 4 E and the register N 8 E 48 contains the number N 8 E N 4 E and N 8 E are taken initially equal to non-negative whole numbers, from

  so that Kraft equality is satisfied, when we choose

  sit for NL the value 267, as follows

15 N 4 E + N 8 E222

  The CE 64 register, which can be initially set to any value (e.g. 64), contains the

  account on the basis of which the number N 4 E is changed.

  The arithmetic unit 60 performs the necessary calculations The code word stream which results from the coding is available on the output port 18 The register W 62

  contains fragments of each codeword before trans-

  mission to access 18 The registers Ri 54, R 2 56, Ai 52

  and A 258 are connected as shown Each.

  component of the encoder 14 is connected to the data bus 49, as well as to the data processing control device, as shown in FIG. 3, and it is controlled by the device 50 The components of the encoder 14 are produced

  in the classic form of a microprocessor and a memory.

  Encoder operation The results of each digital operation described

  below are rounded up to the nearest whole number.

  The following description refers to FIGS. 3,

  SA, 5 B, 5 C and 5 D When an original character byte is

  received on access 16, the control device 50 transmits it

  refers to the register Ai 52 The original character present in the register Al 52 then acts as an address pointer for the table Ml 42, which supplies the register Ri 54 with the rank r of the original character If the register Ri 54 does not contain an O (i.e. if the original character

  does not have the lowest rank, i.e. the frequency of appearance

  highest rition), the value of the rank (r) is transferred from the register Ri 54 to the register R 2 56 which is then decremented by 1 to take the value (r-1) The register R 2 56 then acts as a pointer address for table M 2 44, and the original character having the rank (r-1) is placed in the register A 2 58, which acts as an address pointer for the register MI 44 The number present in the register Ai 52 (that is to say the original character) is loaded in the position (r-1) of the table M 2 44 The register R 2 56 is then incremented by 1 to take the value r, and the number present in the register Al 52 is recorded in the position r of M 2 44 The position Al of the table Ml 42 is

  decremented, and position A 2 of table Ml 42 is incremented

lied. th

  If we consider that the i element of the table.

  Ml 42 contains the original character rank i, and if we consider that the r th element of the table M 2 44 contains the original character of rank r, the preceding operation has the effect of placing the rank of the character of origin entering the register R? 54, to swap the rank of this character

  of origin with the rank of the original character of immediate rank

  much lower, and to update accordingly

the tables MI 42 and M 2 43.

  The content of the register R? 54 (i.e. the rank r of the incoming original character) is compared with the contents of register N 4 E 46 and register N 8 E 48, and the value of CE is changed as follows If r is equal to N 4 E or N 4 E 1, CE is incremented If r is less than N 8 E + 15 and

  greater than or equal to N 8 E 15, CE is decremented.

  Based on the results of the previous comparisons

  between the contents of registers Ri 54, N 4 E 46 and N 8 E 48, the following actions are performed: If the content of R? is less than N 4 E (which indicates that the incoming original character is included in the group to be coded as single fragment words), RI 54 is incremented by 1 and its content is transferred to the register W 62 (i.e. the original character is coded as a code word equal to a number greater than one unit than its rank; this is shown in Figure 2, in which N 4 E 10 and the original character of rank O is coded in the form of fragment 1, while the character

  original rank 1 is coded as fragment 2).

  If the content of the register Rl is at least as

  greater than N 4 E but less than N 8 E (i.e. the characteristic

  tère of incoming origin must be coded in the form of a word with two fragments), the quantity (N 4 E) 15 + 17 + (N 8 EN 4 E 1) is added to the content of the register Rl 54 and it is transferred to register W 62 (to become the potential two-fragment codeword) If the 4 low-weight bits of the result are equal to O (i.e. they represent the illegal fragment (0000)) , the quantity w + 15 (N 4 E) + 15 is calculated and the result is transferred 1-6 to the register W 62 (to become the code word for two

final fragments).

  If the content of the register Rl is at least as

  greater than N 8 E (i.e. the code word will have a long

  three fragments), the quantity 3829 is added to the content of the register Rl 54 and the result is placed in the register W 62 If the 4 bits of light weight of the result are not equal to O (i.e. the third fragment is not illegal), the content of the register W 62 is divided by 16 and the result is added to 3573 + N 8 E If this result is less than 3856, the quantity is subtracted from it

  16 The result is transferred to register W 62.

  The effect of the preceding coding operation is to load one, two or three fragments different from zero into the register W 62 as a code word, depending on the rank of the incoming source word. The zero fragment is not written by permanently in the W register 62 in a position of least significant weight as the position of a non-zero fragment The code word is then transmitted from the output port 18, starting with the non-zero fragment

with the highest weight.

  Finally, if CE has reached the upper threshold value,

  re (SUP) or lower (INF), it must be restored to the median value (MED) and N 4 E and N 8 E must be changed to adjust the number of original characters coded in the form of words of one fragment, two fragment and three fragment code to minimize the length of the coded train This

  is accomplished in the following manner.

  If CE SUP and if N 8 E is greater than 15, CE is

  restored to MED, N 4 E is incremented by 1 and N 8 E is decreased

lied to 15.

  If CE = INF and N 4 E is not zero, CE is

  restored to MED, N 4 E is decremented by 1 and N 8 E is incremented

tee of 15.

  SUP is the upper threshold mentioned previously and INF is the lower threshold mentioned previously SUP is greater than MED which is itself greater than INF, and these values define the speed with which the coding is changed, when the relative frequencies of appearance of the characters in the original train change For 8-bit original characters, use the following values

  vantes: SUP = 128, MED, 64 and INF 0.

  When CE takes a high value, N 4 E is increased

  mented, so more original characters are encoded as single fragments, and that's

  the opposite occurs when CE takes a low value.

  This has the effect of minimizing the length of the codeword train when the relative frequency of appearance of the

original characters change.

  At this point, the encoder 14 awaits the appearance of the

  next original character, and the process repeats.

  Once each original character has been processed, the coder 14 contains tables (Ml and M 2) which represent the frequency ranks of all the characters

  possible origins, numbers (N 4 E and N 8 E) which determine

  the original character groups which will be coded

225 14969

  in the form of one, two and three fragment code words, a CE number which measures the relative interest of changing N 4 E and

  N 8 E, and a program for definite coding of the character

  original parent received following, in accordance with its rank and with the values of N 4 E and N 8 E. Structure of the decoder

  As shown in FIG. 4, the decoder 24 comprises

  carries an input port 22, on which it receives a stream of code words, each of them comprising one, two or three 4-bit fragments, an output port 26 on which it supplies the original character stream decoded, and an arithmetic unit 78 for performing calculations The table M 3 82 corresponds to the table M 2 of the coder, the registers N 4 D 76 and N 8 D 77 correspond to the registers N 4 E and N 8 E of the coder, the register CD 80 corresponds to the register CE of the encoder and register S 74 contains a value which indicates

  if the fragment treated is the first, the second or the third

  sth fragment of a code word The components of the decoder

  24 are connected to data bus 79, and they are controlled

  dice by the data processing control device The components of the decoder 24 are produced under the

  classic form of a microprocessor and memory.

Decoder operation

  Before operation, the table M 3 is initialized

  set to be identical to the M 2 table of the coder and the registers N 4 D, N 8 D and CD are positioned at the same values as the corresponding registers of the coder The register S is initially positioned at zero in preparation for the processing of the first fragment of the first code word

entering.

  The following description refers to FIGS. 4,

  6 A, 6 B, 6 C, and 6 D When a fragment is received on the access

  22, the control device 70 transfers it to the register

  tre R 3 72 and it performs the following actions, depending on the value which is recorded in the register S 74 The value present in the register S indicates whether the fragment being processed is a first fragment, a second

  fragment or a third fragment of a code word.

  If the register S is at zero, which indicates the first

  mier fragment in a code word, the content of the register R 3 is compared with the value present in the register N 4 D 76 If the content of the register R 3 is less than or equal to N 4 D (i.e. that the code word has a length of a single fragment), the register R 3 is decremented to obtain the rank of the original character; and otherwise the content of the register R 3 is transferred to the register S. If the content of the register S is greater than O and less than 16, which indicates the second fragment in a code word, the content of the register S 74 is multiplied by 16 and the result is added to the content of R 3 If the

(N 8 D N 4 D)

  content of R 3 is less than 16 N 4 D + 17 + 15 the quantity 15 N 4 D + 15 is subtracted from R 3, the result is multiplied by 16 and it is brought back into the register R 3 If the content of R 3 is less than l 15 N 4 D + 17 + (N 8 DN 4 D)

(N 8 D -N 4 D) 15 +

* N 8 Dl, the quantity l 15 N 4 D + 17 + 1-5 is subtracted

  te of the content of R 3 and the register S is set to zero, while

  that otherwise the content of the register R 3 is approx.

  yé to register S. If the content of register S 74 is greater than or equal to 16; which indicates the third fragment of a code word, the content of this register is multiplied by 16 and the result is added to the content of R 3 If the content of R 3 is greater than 3840, the number 16 is added to it, and if the resulting value of R 3 is less than

  3829 + N 8 D, the quantity 3573 + N 8 D is subtracted from the count-

  naked of R 3, the result is multiplied by 16 and this result is recorded in the register R 3 The quantity 3829 is then subtracted from the content of the register-R 3 and the register

S is set to zero.

  When these operations are finished, if the value present in S is equal to 0, which indicates that a new

  code word will be received, the value present in the register

251 4969

  tre CD must be updated to conform to the value of the CE register, so that the coder and the decoder work with the same codes This is accomplished by the control device 70 which performs the following actions: The content of the register R 3 (which is the rank of the original character corresponding to the code word which has just been decoded) is compared with the contents of the registers N 4 D and N 8 D If R 3 = N 4 D 1 or R 3 = N 4 D, the register CD 80 is incremented If the content of the register R 3 is greater than or equal to N 8 D 15 and less than N 8 D + 15, the register CD

is decremented.

  The control device 70 then places, on the outlet access 26, the content of the position R 3 of the table M 3 82 (which is the original character of rank R 3) and, if R 3 is greater than zero (i.e. if the original character

  is not the lowest ranking), the contents of the posi-

  R 3 and R 3 1 in table M 3 are exchanged, which effectively swaps the ranks of the original character which

  has just been treated and the original character of immediate rank

  diatly lower, which makes the M 3 table correspond to

table M 2 44 of encoder 14.

  The controller 70 then compares the value of CD with thresholds SUP and INF identical to those

  used by encoder 14, and it performs the following actions

  vantes: If CD = SUP and N 8 D is greater than 15, CD is

  positioned at MED, N 4 D is incremented by 1 and N 8 D is decreased

lied to 15.

  If CD = INF and N 4 D is greater than 0, CD is posi-

  tioned at MED, N 4 D is decremented by 1 and N 8 D is incremented

of 15.

  These operations have the effect of internally updating the values CD, N 4 D and N 8 D so that they correspond exactly to the respective values CE, N 4 E and

N 8 E in encoder 14.

  Finally, the original decoded character present on

14969

  output access 26 is applied to line 28 The control device 70 waits for the appearance of the following code word on

  input access 22, then the process repeats.

  The decoder 24 therefore decodes each code word received to give the corresponding original character, using the reverse process to that of the coder, based on the decoding parameters N 4 D and N 8 D and on a list M 3 of original characters, ordered by rank N 4 D and N 8 D are changed when a CD value exceeds the SUP and INF thresholds,

  programmed in advance, which are the same as those programmed

  més in the encoder 14 CD is updated to be identical to the value CE M 3 is updated to indicate the ranks

  currents of the original characters, by an identi-

  than the one used in the encoder 14 No information

  mation other than the original character set must

  be transmitted from coder 14 to decoder 24 to coordinate

  give their respective activities, from the fact that information

  coding required is implicitly contained in

the code words themselves.

  It goes without saying that many modifications can

  can be made to the device described and shown, without departing from the scope of the invention For example, the code words can comprise a group of code words

  made up of more than three fragments The points of separation

  ration between groups of code words are then determined

  born by more than two separation values, in accordance with Kraft inequality, and several of these values are then

  independent variables, while the remaining values

  your are determined after you set these independent values.

251 4969

Claims (9)

  1 Device intended to code a character train   original tres in a codeword train, each of the original characters having a frequency of occurrence in the original character train which is likely to vary over time, each of the codewords belonging to a group among several groups of code words, and   the code words of the respective groups including names-   respective bres different from sub-words; characterized in that it comprises: an encoder (14) intended to encode each of the original characters in the form of one of the code words,   the original characters that appear most frequently   ment being coded in the form of code words having less than sub-words; and a group size selector for replacing the numbers of code words belonging to respectively less than all of the numbers with new numbers.   groups, and to recalculate the numbers of code words   relating to the other groups, on the basis of the new numbers, whereby at least part of the codeword train is shortened.   2 Device intended to code a character train   original parents in a string of code words, each of   original characters from an original alphabet, each   any of the original characters in the original alphabet having a frequency of occurrence in the character string   origin which is likely to vary over time, each   one of the code words belonging to a group from among several groups of code words, and the code words which are in these groups comprising respective different numbers of subwords; characterized in that it comprises: an encoder (14) intended to encode each of the original characters for   give one of the code words, the original characters appear   occurring most frequently being coded in the form of code words having fewer subwords; a control device intended to count the number of appearances, in the original character string, of each original character   in a set comprising less than the totality of the   original mothers; and a group-size selector which operates under the control of the   way to change the numbers of code word belonging res-   pectively to groups, thanks to which a part at least   of the codeword train is shortened.   3 Device intended to code a character train   original res in a string of code words, each of the characters   original parents with a frequency of occurrence in the original typeface which is likely to vary   in time, each of the code words belonging to a pre-   mier, a second or a third group of code words, and the code words in the first, second and third   groups comprising one, two and three sub-   words, characterized in that it comprises: an encoder (14) for   intended to encode a first set of original characters in the form of code words belonging to the first group, a second set of original characters in the form of code words belonging to the second group, and a third set of characters original in the form of words from   code belonging to the third group, the original characters   gine which appear most frequently being coded in the form of code words having fewer subwords; and a group size selector intended to change the number of code words belonging respectively to the former,   second and third groups, dependent on varia-   frequency of occurrence, whereby part   at least the codeword train is shortened.   or 3, 4 Device according to claim 2 characterized in that the group size selector is connected so as to replace with a new number the number of words of   code belonging to one of the groups, and to recalculate the names-   code words belonging to other groups, in pro-   yielding directly from the new number.   Device for coding a character train   original res in a string of code words, each of the characters   original terms from an original alphabet, each of the original characters in the original alphabet having a frequency of occurrence in the original character string which is likely to vary over time, each code words belonging to a first, a second or a third   group of code words, and the code words in these groups   pes comprising one, two or three subwords respectively   of the same length, each of the subwords coming from an alpha-   subword bet; characterized in that it comprises: a   matching frequency rank determination device   tion, intended to assign to each of the original characters a rank which corresponds to its frequency of relative appearance estimated current in the train of original characters; an encoder (14) for encoding in the form of a code word having a subword, in accordance with a selectable code, each of the original characters having a rank   which does not exceed a first number which can be selected   tioned, to code in the form of a code word having two subwords each of the original characters having a rank greater than the first number and not exceeding a second number, and to code in the form of a word code having three subwords each of the original characters having a rank greater than the second number, this second number being fixed by the value of the first number; a control device for counting the number of appearances, in the original character string, of each original character in a set comprising less than all of the original characters; and a selector which operates under the control of the control device by updating the first number to give it a new value, when this new value shortens at least part of the train of code words, and by recalculating the second number from of the first number. 6 Device according to claim 5, characterized in that said set includes the original characters for which the number of subwords in the associated code words would be changed by changes in the first number and the second number.   7 Device according to claim 6, characterized in that said set includes the original characters having ranks equal to the first number, to the first number decreased by one, and to each number between the second number minus the number of sub -words in the alphabet   of subwords, and the second number plus the number of sub-   words in the alphabet of subwords, minus one ".   6 or 7, 8 Device according to claim 5, characterized in that the control device comprises a counter (64) intended to count from a median value, in ascending direction at each appearance of the original characters having ranks equal to the first number and the first number   decreased by one, and decreasing with each pairing   tion of the original characters having ranks equal to each number between the second number minus the number of subwords in the alphabet of subwords, and the second number plus the number of subwords in the alphabet of subwords, decreased by one; and in that the selector comprises an adder intended to increase the first number by one unit when the counter reaches a higher value and to decrease the first number by one unit when the counter reaches a lower value.   9 Device according to claim 5, characterized in that the code which can be selected is changed   when the first number is updated and takes a new- the value. 231 'any of the res-   Device according to "indications 5 to 9, characterized in that it further comprises a decoder (24) intended to decode the train of code words to give the original character train, this decoder comprising a device for determining the frequency of appearance of decoder, intended to determine for each of the original characters in the alphabet of origin, the rank which corresponds to its frequency of relative appearance estimated current; and in that the decoder (24) directly determines from the code words present in the code word train which of the selectable codes was used for coding.   11 Device intended to code a character train   original tres in a train of code words, each of the original characters having a frequency of appearance in the original character train which varies over time, characterized in that it comprises: a rank memory   (Ml) intended to record the rank corresponding to the fre-   current estimated relative occurrence of each original character in the original character string and a rank update device for swapping the rank of each original character that appears in the original character string origin with the rank of the original character of rank immediately below, unless the original character which appears in the train has the most low rank possible.   12 Device intended to code a character train   original terms in a string of code words, each of the original characters from an original alphabet, each of the original characters in the original alphabet   having a frequency of appearance in the character train   original res which varies over time, these original characters   gine being ordered according to their frequency of pairing   current relative tion, each of the code words belonging to a first, a second or a third group of these words   code, and the code words in those groups including res-   pectively one, two or three subwords of the same length, each of the subwords belonging to a subword alphabet, characterized in that it comprises: an encoder (14) intended for   code in the form of a code word with a subword cha-   cun of the original characters having a rank which does not exceed a first number which can be selected, to be coded in the form of a code word having two subwords each of   original characters having a rank higher than the first name-   bre and not exceeding a second number, and to be coded in the form of a code word having three subwords each of   original characters having a rank higher than the second name-   bre, this second number being fixed by the value of the first number; a coding modifier for code words with two subwords, intended to divide by 16 any code word having two subwords and whose last subword is zero, and to add the result to 15 plus the product of 15 by the first number, to give a revised code word; and a coding modifier for code words with three subwords, intended to divide by 16 any code word having three subwords and whose last subword is zero, to add the result to 3573 plus the second number and, if the resulting code word is less than 3856, subtract 16 from the resulting code word, to give a word of revised code.