Patent Publication Number: US-4320511-A

Title: Method and device for conversion between a cyclic and a general code sequence by the use of dummy zero bit series

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method of carrying out conversion between a cyclic code sequence and a general code sequence. This invention relates also to an encoder and a decoder for carrying out the conversion. 
     In satellite communication of the known TDMA (time division multiple access) type, a cyclic code sequence is transmitted and received to improve the performance. It is therefore necessary to carry out conversion between a cyclic and a general code sequence, each consisting of a plurality of code blocks. More particularly, the general code sequence is encoded in a transmitter into the cyclic code by supplementing data bits of each code block of the general code sequence with a predetermined number of check bits that forms a cyclic code together with the data bits contained in that code block. The cyclic code sequence is decoded into the general code sequence in a receiver by removing the check bits of each code block. 
     Each cyclic or general code sequence as called herein has a variable duration determined by a burst that is known in the art. On the other hand, each code block is normally given a normal code block period. It is therefore necessary that each code sequence should comprise an additional code block of a variable additional code block period in addition to such normal code blocks. It is the practice to make the additional code block period shorter than the normal code block period. 
     A method of carrying out such a conversion is proposed in a Japanese technical paper, Densi Tusin Gakkai Tusin-hosiki Kenkyukai Siryo CS 77-66 (Papers for Communication System Study Committee of the Institute of Electronics and Communication System Engineers of Japan, No. 77-66), contributed by Keiichiro KOGA, Yutaka YASUDA, Takuro MURATANI, Yoshihiro IWADARE, and Katsuhiro NAKAMURA, under the title of &#34;TDMA Hosiki ni taisuru Ayamari Teisei Hugo no Tekiyo (Application of FEC Coding to TDMA Systems)&#34; and published in September, 1977. According to this paper, conversion between an additional code block of one of the cyclic and the general code sequences and a corresponding additional code block of the other of the sequences is carried out by preparing a dummy bit series for the data bits of the additional code block of the sequence that should be converted to the other sequence. However, the technical paper does not disclose how to discriminate the additional code block from the normal code blocks. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a method of carrying out a conversion between a cyclic and a general code sequence, which is capable of discriminating between normal code blocks and an additional code block. 
     It is another object of this invention to provide an encoder for carrying out the method mentioned above, which is simple in structure and is economical. 
     It is a further object of this invention to provide a decoder for carrying out the above-mentioned method, which is capable of readily decoding an additional code block even when the additional code block is variable in duration. 
     A method to which this invention is applicable is for carrying out a conversion between a cyclic code sequence and a general code sequence. The cyclic code sequence lasts a duration determined by a burst. Each of the cyclic and the general code sequences consists of a plurality of normal code blocks and an additional code block arranged in a predetermined order. Each of the normal code blocks of the cyclic code sequence consists of a normal cyclic code of a first predetermined number of data bits followed by a second predetermined number of check bits. The additional block of the cyclic code sequence consists of an additional cyclic code of a variable number of data bits followed by check bits. The variable number is less than the first predetermined number and dependent on the burst. The check bits of the additional cyclic code are equal in number to the second predetermined number. Each of the normal code blocks of the general code sequence is for data bits followed by a prescribed period. The additional code block of the general code sequences is also for data bits followed by a like prescribed period. The data bits in the normal and the additional code blocks of the general code sequence is equal in number to the first predetermined and the variable numbers, respectively. The prescribed period is for arranging a plurality of check bits, equal in number to the second predetermined number, but containing none of the last-mentioned check bits. 
     According to this invention, the method comprises the steps of establishing normal code block periods of the normal code blocks of a predetermined one of the cyclic and the general code sequences and an additional code block period of the additional code block of the predetermined code sequence, converting the normal code blocks of the predetermined code sequence to the normal code blocks of the other of the cyclic and the general code sequences, respectively, when the normal code block periods are measured, and converting the additional code block of the one code sequence to the additional code block of the other code sequence when the additional code block period is measured, the additional code block converting step being carried out by assuming that the data bits in the additional code block of the predetermined code sequence is preceded by a dummy zero bit series of a data length equal to a difference between the first predetermined number and the variable number. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of a converter according to a first embodiment of this invention, the converter being for use as an encoder; 
     FIG. 2 is a time chart for describing operation of the converter illustrated in FIG. 1; 
     FIG. 3 is a block diagram of a converter according to a modification of the first embodiment of this invention; 
     FIG. 4 is a block diagram of a converter according to a second embodiment of this invention, the converter being for use as a decoder; 
     FIG. 5 shows a format of a normal code block of a cyclic code sequence supplied to the converter illustrated in FIG. 4; 
     FIG. 6 shows a format of an additional code block of a cyclic code sequence supplied to the converter illustrated in FIG. 4; 
     FIG. 7 is a time chart for describing the operation of a converter according to the second embodiment; 
     FIG. 8 shows a block diagram of a converter according to a third embodiment of this invention, the converter being for use as a decoder; 
     FIG. 9 is a block diagram of a distributor of the converter depicted in FIG. 8; 
     FIG. 10 is a time chart for describing operation of the converter illustrated in FIG. 9; and 
     FIG. 11 is a time chart for describing operation of a converter according to a modification of the third embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a converter according to a first embodiment of this invention is for use as an encoder in combination with a multiplexer 21 (shown by a broken line) responsive to a plurality of code sequences for supplying the encoder with a general code sequence G. The encoder is for encoding the general code sequence G to a cyclic code sequence C so as to supply an output terminal 22 with the cyclic code sequence C. The cyclic code sequence C is phase matched to and synchronized with the general code sequence G. Each of the general and the cyclic code sequences G and C consists of a plurality of normal code blocks and an additional code block. The additional code block may be arranged after a succession of the normal code blocks and immediately follows a preceding normal code block among the normal code blocks of the succession. The normal and the additional code blocks of the general code sequence G are designated by GN and GA, respectively, while the normal and the additional code blocks of the cyclic code sequence C, by CN and CA, respectively. 
     The cyclic code sequence C is arranged in a single burst which may be variable in duration. Each of the normal code blocks CN consists of a normal cyclic code of a bit number n which may be, for example, 127. The normal cyclic code consists of a first predetermined number k of data bits and a second predetermined number n-k of check bits following the data bits. The first and the second predetermined numbers k and n-k may be 112 and 15, respectively. The additional code block CA is an additional cyclic code consisting of data bits of a variable number k&#39; and check bits, equal in number to n-k. The variable number k&#39; is less than the first predetermined number k. 
     Each of the normal code blocks GN of the general code sequence GN comprises data bits, k in number, followed by a prescribed period. The additional code block GA comprises data bits, k&#39; in number, followed by a like prescribed period. The prescribed period is for arranging a plurality of check bits, equal in number to the predetermined number. Neither data bits nor check bits are arranged in any of the prescribed periods. 
     The encoder is supplied with a series of timing pulses TM and a sequence of clock pulses CK from the multiplexer 21. Each of the timing pulses TM is synchronized with a last data bit of each of the normal and the additional code blocks GN and GA. The clock pulses CK are synchronized with the respective data bits and are produced even during the prescribed periods of the normal and the additional code blocks GN and GA. The last data bit is readily detected in the multiplexer 21 in a usual manner. The timing pulse series TM is supplied to a timing control circuit 24 to establish normal code block periods of the normal code blocks CN and an additional code block period of the additional code block CA. More specifically, the timing control circuit 24 is a counter responsive to each of the timing pulses TM for counting the clock pulses CK to a predetermined count equal to the second predetermined number n-k to produce a sequence of control pulses CP. Each of the control pulses CP defines the prescribed period and thereby defines also a data bit period of any one of the normal and the additional code blocks GN and GA. Each of the control pulses CP takes a high level &#34;H&#34; (FIG. 2) during the data bit period and a low level &#34;L&#34; during the prescribed period. The high and the low levels &#34;H&#34; and &#34;L&#34; will be referred to as a logic &#34;1&#34; and a logic &#34;0&#34; level hereinafter, respectively. 
     The general code sequence G is supplied to a check bit generator section 25 as an input code sequence together with the control pulse sequence CP. The check bit generator section 25 comprises a first AND gate 26, an Exclusive OR circuit 27, and a divider 28 for carrying out a predetermined division. As is well-known in the art, the divider 28 is a combination of a shift register and a plurality of adders (mod. 2), which combination is connected in accordance with a predetermined degree of a generator polynomial determined as a divisor of the predetermined division. Herein, it suffices to say that the predetermined degree is given by the second predetermined number n-k and that the shift register of the divider 28 has a plurality of stages, equal in number to the predetermined degree of the generator polynomial, and is successively shifted by the clock pulses CK. 
     During each of the data bit periods of the normal code blocks GN, the first AND gate 26 is enabled by the control pulse sequence CP. As a result, the data bits of each normal code blocks GN are supplied for use as a dividend of the predetermined division through the Exclusive OR circuit 27 to the divider 28. More particularly, the divider 28 starts carrying out the division from an initial state when a first one of the data bits of each normal code blocks GN is supplied to the divider 28. It is assumed that all stages of the shift register of the divider 28 take the logic &#34;0&#34; level in the initial state. The divider 28 continually carries out the division during the data bit period of each normal code blocks GN to produce a series of intermediate residue pulses specifying an intermediate residue of the division. The intermediate residue pulse series is successively fed back to the Exclusive OR circuit 27 during each data bit period of each normal code block GN. The Exclusive OR circuit 27 acts as a part of the divider 28 in a usual manner to carry out the division. When the data bit period of each normal code block lapses into the prescribed period following that data bit period, the first AND gate 26 disabled by the clock pulse CK. The divider 28 stops the division with a final residue of the division kept in the respective stages of the shift register. The final residue is successively produced as a first set of check bits in synchronism with the clock pulses CK during the prescribed period. 
     When the data bits of the additional code block GA are supplied to the divider 28, the division is also started from the initial state, as is the case with the normal code blocks. The divider 28 is kept in the initial state unless the data bits are supplied thereto. As described before, the variable number k&#39; of the additional code block GA is less than the first predetermined number k of each normal code block. From these facts, it is possible to assume that the data bits of the additional code block GA is preceded by a zero bit series of a data length equal to a difference between the first predetermined number k and the variable number k&#39;. Therefore, the divider 28 is capable of also producing a second set of check bits next following the data bits of the additional code block GA. 
     The encoder further comprises second and third AND gates 32 and 33, an inverter 34, and an OR gate 35 to combine the general code sequence G and the first and the second sets of check bits into the cyclic code sequence C. More particularly, the second AND gate 32 is enabled by the control pulse CP during each data bit period. The data bits of each of the normal and the additional code blocks GN and GA are, therefore, supplied through the second AND gate 32 and the OR gate 35 to the output terminal 22. On the other hand, the third AND gate 33, which is supplied with the intermediate residue pulse series during each data bit period, is disabled by the control pulse CP supplied through the inverter 34. As a result, the intermediate residue pulse series is intercepted by the third AND gate 33. 
     While the control pulse CP takes the logic &#34;0&#34; level, the third AND gate 33 is enabled with the second AND gate 32 disabled. The first and the second sets of check bits are supplied through the third AND gate 33 and the OR gate 35 to the output terminal 22 during the prescribed periods of the respective normal code blocks GN and the additional code block GA, respectively. As a result, the first set of check bits is combined with the data bits of each normal code block. It has been proved that such a combination forms a cyclic code, as described in &#34;Error-Correcting Codes,&#34; page 206, authored by W. W. Peterson and published 1972 by The MIT Press, Massachusetts. Likewise, the second set of the check bits is arranged just after the data bit set of additional code block GA. It is readily understood that such an arrangement also forms a cyclic code under the above-mentioned assumption. 
     Instead of a series of the timing pulses TM synchronized with the last bits of the respective code blocks GN and GA, it is possible to use a pair of timing pulses which are phase matched to a beginning and an end of the burst. The timing pulse pair is shown by dashed lines in FIG. 2. In this case, the timing control circuit 24 is a combination of a first and a second counter for counting the clock pulses CK to counts k and n-k, respectively. 
     Referring to FIG. 3, an encoder according to a modification of the first embodiment of this invention comprises similar parts designated by like reference numerals except that the first AND gate 26 is removed and that the cyclic code sequence C is supplied as the input code sequence to the Exclusive OR circuit 27, instead of the general code sequence G. For convenience of further description, it may be mentioned here that the input code sequence comprises a plurality of data bit sets of a first kind and a data bit set of a second kind and that the data bits of the first-kind sets and the second-kind set correspond to the data bits in the normal and the additional code blocks of either of the cyclic and the general code sequences, respectively. 
     During each of the data bit periods of the general code sequence G, the data bits in each of the normal and the additional code blocks GN and GA are supplied through the second AND gate 32 and the OR gate 35 to the output terminal 22 and are fed back to the Exclusive OR circuit 27 as the first-kind sets and the second-kind set, respectively. This operation is equivalent to that of FIG. 1. During each prescribed period, the Exclusive OR circuit 27 is supplied with each of the check bits directly and through the third AND gate 33 and the OR gate 35. In other words, the same input signals are given the Exclusive OR circuit 27. Therefore, the Exclusive OR circuit 27 continuously gives the divider 28 logic &#34;0&#34;level during the prescribed period. The encoder illustrated in FIG. 3 is thus equivalent to that of FIG. 1. 
     Referring to FIG. 4, a converter according to a second embodiment of this invention is for use as a decoder in combination with a demultiplexer 40 supplied with a cyclic code sequence C which is produced either by an encoder illustrated hereinabove or by a conventional encoder. The demultiplexer 40 is capable of detecting the beginning and the end of each burst to produce a duration signal comprising first and second pulses CP 1  and CP 2 . The first and the second pulses CP 1  and CP 2  are timed relative to the beginning and the end of each burst, respectively, to specify the duration of the cyclic code sequence C. The normal and the additional cyclic code blocks CN and CA will be simply named the normal and the additional code blocks, respectively, unless the simple naming is confusing. 
     Turning to FIGS. 5 and 6 for a short while, the additional code block CA will be compared with any one of the normal code blocks CN. In FIG. 5, the n-bit normal cyclic code consists of first through k-th data bits a(1) to a(k) and first through (n-k)-th check bits c(1) to c(n-k) arranged in succession in synchronism with the clock pulses CK. The data and the check bits are first through n-th bits of the cyclic code. In other words, the first data bit a(1), the first through the (n-k-1)-th check bits c(1) to c(n-k-1), and the (n-k)-th check bit c(n-k) are a leading bit, (k+1)-th through (n-1)-th bits, and an n-th or last bit of the cyclic code. 
     In FIG. 6, the additional code block CA consists of an additional cyclic code. With an encoder illustrated with reference to FIG. 1 or 3, the data bits of a general code sequence G are encoded into the additional cyclic code by assuming that a dummy zero bit series of a data length equal to the difference between the first and the second predetermined numbers (k-k&#39;) precedes the data bits under consideration. It is therefore possible on describing a decoder according to this invention to deem the additional cyclic code to be equivalent to a normal cyclic code from which the first through the (k-k&#39;)-th data bits a(1) to a(k-k&#39;) are omitted. Data bits of the additional code sequence are now numbered from (k-k&#39;+1) to k. For convenience of further description, let that one of the normal code blocks CN which immediately precedes additional code block CA be called a preceding code block. 
     Referring back to FIG. 4 and referring also to FIG. 7, let the cyclic code sequence C be a sequence of m code blocks, where m represents an integer greater than two. The preceding and the additional code blocks are an (m-1)-th and an m-th code block. 
     The decoder comprises a timing circuit 41 responsive to the first and the second pulses CP 1  and CP 2  for timing the normal and the additional code block periods to produce a sequence of first control pulses CO 1  and a second control pulse CO 2 . The first control pulses CO 1  are timed relative to the respective normal code block periods of the cyclic code sequence C. As depicted in FIG. 7 for the (m-1)-th code block with a solid line, the first control pulse CO 1  appears at an instant a prescribed interval of time T 0  after the leading bit of the related code block. This applies to the remaining normal and the additional code blocks. The first control pulses CO 1  for such code blocks are illustrated by dashed lines. The second control pulse CO 2  appears at an instant a first predetermined duration T 1  after the second pulse CP 2 . It is possible to measure the &#34;prescribed interval&#34; from the n-th or the last bit of each code block in harmony with the definition of the first predetermined duration T 1 . 
     The decoder comprises a processing circuit 42, which will later be described in detail. The processing circuit 42 monitors the data and the check bits of each of the m code blocks of the cyclic code sequence C. If an error is found in one of the code blocks, the processing circuit 42 generates an error correcting pattern a certain interval of time after the n-th or last bit of that code block. It is possible to select a common access time T a  for generation of such error correcting patterns. 
     The prescribed interval T 0  and the first predetermined duration T 1  are selected in consideration of the access time T a . For example, the prescribed interval T 0  is equal in duration to a sum of the normal block period and the access time T a . The first predetermined interval T 1  may be equal to the access time T a . 
     The processing circuit 42 comprises a first counter (not shown in FIG. 4). Energized the prescribed interval T 0  after each first control pulse CO 1 , the counter begins to count clock pulses synchronized with the data and the check bits of the cyclic code sequence C. The count varies from 1 to n as illustrated in FIG. 7 at PR 1 . A second counter is energized by the second control pulse CO 2 . The count varies as indicated at PR 2 . 
     By the use of the counts PR 1  and PR 2 , the processing circuit 42 produces an error indicating signal OS. Let an error be found at a certain bit in each of the (m-1)-th and the m-th code block. As the error correcting pattern for the (m-1)-th code block, the processing circuit 42 produces a first processed signal PC 1  at an instant at which the count PR 1  indicates the bit where the error is found. Similarly, a second processed signal PC 2  is produced for the m-th code block. The error indicating signal OS is given by such processed signals. 
     More particularly, the (m-1)-th code block is processed during a first processing period T f  that follows the first control pulse CO 1  for the (m-1)-th code block. The m-th code block is processed during a second processing period T s  that follows the second control pulse CO 2  and is equal in duration to the first processing period T f . The second processing period T s  consists of a first partial period, as shown by a dotted line, and a second partial period continuously following the first partial period, as shown by a solid line. The first partial period is for processing the zero bit series as illustrated with reference to FIG. 2. The second partial period is for processing the m-th code block and follows the first processing period T f  for the (m-1)-th code block. 
     The first processed signal PC 1  appears the first processing period while the second processed signal PC 2  appears the second partial period. This means that the m-th or additional cyclic code block is processed by assuming that the additional cyclic code block is preceded by the dummy zero bit series. 
     In addition, the processing circuit 42 produces a gating pulse GT for removing the check bits from each code block after detecting either of the normal and the additional code blocks CN and CA. 
     The decoder comprises a data buffer 43 for successively memorizing one code block of the cyclic code sequence at a time as a memorized code block to sequentially produce the memorized code blocks. Each of the memorized code blocks is supplied to an Exclusive OR circuit 44 in synchronism with the timing of the error indicating signal OS. The Exclusive OR circuit 44 combines the memorized normal and the memorized additional code blocks with the first and the second processed signals, respectively, to produce a combined cyclic code sequence. At the Exclusive OR circuit 44, an error bit of each of the memorized code blocks is inverted into a correct bit by each of the first and the second processed signals PC 1  and PC 2 . Therefore, the combined cyclic code sequence is produced as an error-free cyclic code sequence. A gate circuit 45 removes check bits from the error-free cyclic code sequence in response to the gating pulse GT supplied from the processing circuit 42 to produce the general code sequence G. In other words, the gate circuit 45 serves to convert the error-free cyclic code sequence into the general code sequence G. The gating pulse GT may be supplied from any other devices except the processing circuit 42. 
     Referring to FIG. 8, a converter according to a third embodiment of this invention is for use as a decoder in combination with the demultiplexer (not shown in this figure) and comprises similar parts designated by like reference numerals. The timing circuit 41 comprises first and second counters 46 and 47 responsive to the first and the second pulses CP 1  and CP 2  for producing the first and the second control pulses CO 1  and CO 2 , respectively. Responsive to the first pulse CP 1 , the first counter 46 produces also a sequence of third control pulses CO 3 , each of which is timed relative to the beginning of each of the normal and the additional code block periods in the cyclic code sequence. Supplied with the second pulse CP 2 , the second counter 47 produces a fourth control pulse CO 4  timed relative to the second pulse CO 2 . By way of example, the third and the fourth control pulses CO 3  and CO 4  are advanced in phase relative to the first and the second pulses CO 1  and CO 2  by the preselected interval T 0  and the first predetermined period T 1 , respectively. Both of the third and the fourth control pulses CO 3  and CO 4  are sent to the processing circuit 42. 
     The processing circuit 42 comprises a distributor 51. In this connection, it may be pointed out here that the variable duration of the additional code block CA may be shorter than the access time T a . Operation of the processing circuit 42 becomes a little different according as the variable duration is shorter and not shorter than the access time T a . 
     Turning temporarily to FIG. 9, the distributor 51 comprises a reference pulse generator 52 responsive to each of the third control pulses CO 3  for producing a reference pulse RE at an instant delayed by a preselected period relative to each third control pulse CO 3 . The preselected period is equal to the first predetermined period T 1  or the access time T a . It is possible to use the first control pulses CO 1  as the reference pulses RE, without using the reference pulse generator 52. The reference pulses RE and the fourth control pulses CO 4  are sent to a difference detecting circuit 53. The difference detecting circuit 53 is energized by the fourth control pulse CO 4  and produces a holding signal HD that lasts a duration between the fourth control pulse CO 4  and the next following reference pulse RE. Briefly, the holding signal HD is produced only when the m-th code block is shorter than the access time T a . Use of the holding signal HD will be described later. The difference detecting circuit 53 may be a combination of a flip-flop and a gate circuit. 
     A phase comparator 54 compares the fourth control pulse CO 4  with the reference pulses RE to produce a first level signal until the fourth control pulse CO 4  and a second level signal following the fourth control pulse CO 4  and lasting until the next following reference pulse appears. The distributor 51 produces the third control pulses CO 3  as a sequence of first shift pulses SH 1 , as long as the third control pulse sequence CO 3  lasts. On the other hand, the fourth and the reference pulses CO 4  and RE are supplied to a selecting gate circuit 55 together with either of the first and the second level signals. The selecting gate circuit 55 selects the fourth control pulse CO 4  when the first level signal appears and, otherwise, the reference pulse RE to produce a second shift pulse SH 2 . The selected fourth control pulse CO 4  is representative of the fact that the m-th or additional code block is not shorter than the access time T a . The selected reference pulse RE is representative of the fact that the m-th code block is shorter than the access time T a . 
     The processing circuit 42 comprises a syndrome generator 56 supplied with the cyclic code sequence C, a buffer 57 connected to the syndrome generator 56, and a read-only memory 58 connected to the buffer 57. The bit pattern of each normal code block CN will now be called a first block bit pattern. The bit pattern of the additional code block CA will be called a second block bit pattern. The first block bit pattern varies in compliance with a combination of the data bits, k in number, and the check bits, (n-k) in number. The second block bit pattern is dependent on a combination of the data bits, k&#39; in number, and the check bits, again (n-k) in number. 
     Referring now to FIG. 10 in addition to FIG. 8, the syndrome generator 56 is responsive to one of the normal and the additional code blocks CN and CA at a time and generates syndromes SY 1  of a first-kind and a second-kind syndrome SY 2  for the normal and the additional code blocks CN and CA, respectively. The first-kind syndromes SY 1  thus correspond to the respective normal code blocks CN. Each syndrome SY 1  or SY 2  has a prescribed number of bits, such as fifteen bits, and held in the syndrome generator 56 during presence of the holding signal HD, as shown by a dashed line at SY, and then moved to the buffer 57. Each of the first-kind syndromes SY 1  has a first syndrome bit pattern peculiar to the first block bit pattern of the corresponding normal code block. The second-kind syndrome SY 2  has a second syndrome bit pattern peculiar to a combination of the zero bit series and the second block bit pattern. 
     The first-kind syndromes SY 1  except that corresponding to the (m-1)-th code block are successively supplied from the buffer 57 to the read-only memory 58 in response to the first shift pulses SH 1 . The first-kind syndrome SY 1  for the (m-1)-th code block is transferred to the read-only memory 58 in response to the second shift pulse SH 2 . The first-kind syndromes SY 1  serve as first-kind address signals AD 1  for the read-only memory 58. Likewise the second-kind syndrome SY 2  serves as a second-kind address signal AD 2 . The syndrome generator 56 and the buffer 57 thus serve as a syndrome section responsive to the cyclic code sequence C, the holding signal HD, and the first and second shift pulses SH 1  and SH 2  for producing the first-kind and the second-kind syndromes SY 1  and SY 2  for the normal and the additional code blocks CN and CA, respectively. 
     The first-kind and the second-kind syndromes SY 1  and SY 2  are given the ROM 58 as the first-kind and the second-kind address signals AD 1  and AD 2  for specifying addresses of the ROM. The ROM 58 stores an error correcting pattern CR corresponding to each of the first-kind and the second-kind syndromes to produce the error correcting pattern CR at the end of the access time after each syndrome is supplied from the buffer section 57. The error correcting pattern CR is for correcting each code block and consists of correcting pattern bits of a preselected number n equal to the sum of the first and the second predetermined numbers. Each of the pattern bits is placed with each bit position numbered in order. 
     The error correcting pattern OR is supplied at a time to a comparator buffer 59 for retaining the error correcting pattern CR with the bit positions numbered in order. From FIG. 10, it is readily understood that the error correcting pattern CR is supplied from the ROM 58 to the comparator buffer 59 at the end of the access time T a  after each syndrome is retained at the buffer section 57. 
     The processing circuit 42 comprises a first block counter 61 energized by the first control pulse CO 1  for counting the clock pulses. The count of the first block counter 61 varies from 1 to n, as shown in FIG. 10 at PR 1 , and is produced as a sequence of first addresses each of which specifies each bit positions of the error correcting pattern retained in the comparator buffer 59. The counts PR 1  appear each of the first processing periods T f . Energized by the second control pulse CO 2 , a second block counter 62 also counts the clock pulses from 1 to n, as shown in FIG. 10 at PR 2 . The counts PR 2  of the second block counter 62 appear the second processing period T s  and are produced as a sequence of second addresses, each of which also specifies each bit positions of the error correcting pattern CR retained in the comparator buffer 59. As shown in FIG. 10, the second block counter 62 is put into operation between the first and the (k-k&#39;)-th addresses simultaneously with the first block counter. 
     The first and the second addresses are sent to a switching circuit 63. The switching circuit 63 selects the first addresses during the first processing period T f  and the second addresses only during the second partial period following the first partial period to produce a sequence of selected addresses, as shown in FIG. 10 at SL. The switching circuit also produces the gating pulse GT for removing the check bits of each code block by the supervising each of the first and the second addresses in the usual manner. 
     The selected address sequence SL is supplied to a position adjusting circuit 64 together with the retained error correcting pattern. The position adjusting circuit 64 adjusts each bit position of the error correcting pattern CR to each of the selected address SL to produce the first and the second processed signals PC 1  and PC 2   as the output sequence OS. Thus, the bit positions of the error correcting pattern CR are produced in accordance with the selected address sequence SL. More particularly, the error correcting pattern for the additional code block is successively produced from the (k-k&#39;+1)-th address to the n-th address, as shown in FIG. 10. This means that the additional code block is processed by assuming that the additional code block is preceded by the dummy zero bit series between the first and the (k-k&#39;)-th addresses. The position adjusting circuit 64 may be a comparator. 
     Like in FIG. 4, the output sequence OS is supplied to the Exclusive OR circuit 44 to correct the error in the cyclic code sequence C supplied through the data buffer 43 and to produce the error-free cyclic code sequence EC. 
     A single burst has so far been described in the second and third embodiments. However, the single burst is usually followed by a succeeding burst with a spacing left therebetween. It is preferable that the spacing is as short as possible. With the third embodiments having the single buffer 57, the minimum spacing should be equal in duration to a difference between the access time T a  and the additional cyclic code period (k&#39;+n-k), as readily understood from FIG. 10. 
     Referring to FIGS. 8 and 11, a converter according to a modification of the third embodiment of this invention comprises a buffer 57 having a first and a second registers 57a and 57b, as shown by a dashed line. Signals and syndromes equivalent to those shown in FIG. 10 are designated by like reference symbols. The burst precedes the succeeding burst which has a first normal code block similar to each of the normal code blocks of the preceding burst. 
     The syndrome for the (m-1)-th code block of the preceding burst is simultaneously moved from the syndrome generator 56 to both of the first and the second registers 57a and 57b and is kept in the first and the second registers 57a and 57b, as shown by AD a  and AD b , respectively. The next following syndrome for the m-th block is moved to the first register 57a immediately after the next following syndrome is produced at the syndrome generator 56. On the other hand, the second register 57b keeps the syndrome for the (m-1)-th code block during the access time T a  and, thereafter, keeps the next following syndrome. For this purpose, the holding signal HD is used together with the first and the second shift signals SH 1  and SH 2  and is not supplied to the syndrome generator 56. Thus, the next following syndrome for the m-th code block is moved from the syndrome generator 56 to the second part 57b before the access time T a . It is possible to shorten the spacing between two adjacent bursts. 
     While a few embodiments of this invention have so far been described, it is now readily possible for those skilled in the art to put this invention into practice in various manners. For example, a plurality of the bit errors may be corrected by the use of the syndrome generator and the read-only memory. In addition, the processing circuit may be provided with more than two registers to make the spacing shorter.