Abstract:
An encoder for encoding a stream of data bits of a binary source signal into a stream of data bits of a binary channel signal, the bitstream of the source signal being divided into smaller n-bit source words (x 1 , x 2 ) which are converted by a logic circuit converter in the encoder into corresponding m-bit channel words, (y 1 , y 2 , y 3 ). The conversion of each n-bit source word is parity preserving (see Table I and FIG.  1 ). The relations hold that m&gt;n≧1, p≧1, and p can vary. Preferably, m=n+1. In order to comply with (d, k) runlength requirements, certain blocks of 2-bit source words are encoded into particular blocks of 3-bit channel words. A decoder is also disclosed for decoding a channel signal produced by the encoder.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a device for encoding a stream of databits of a binary source signal into a stream of databits of a binary channel signal, wherein the bitstream of the source signal is divided into n-bit source words, which device comprises converting means adapted to convert said source words into corresponding m-bit channel words, the converting means being adapted to convert a block of p consecutive n-bit source words into a corresponding block of p consecutive m-bit channel words, such that the conversion for each block of p consecutive n-bit source words is substantially parity preserving, where n, m and p are integers, m&gt;n≧1, p≧1, and where p can vary. The invention also relates to a recording device comprising the encoding device, for recording the channel signal on a record carrier, to the record carrier itself, to an encoding method, and to a device for decoding a stream of data bits of a binary channel signal obtained by means of the encoding device, so as to obtain a stream of databits of a binary source signal. 
     An encoding device mentioned in the foregoing is known from U.S. Pat. No. 5,477,222 (PHN 14448). The document discloses a device for encoding a stream of databits of a binary source signal into a stream of databits of a binary channel signal, satisfying a (1,8) runlength constraint. That means that, in a serial datastream of the channel signal at minimum one ‘zero’ and at maximum eight ‘zeroes’ are present between two consecutive ‘ones’ in the channel signal. It should be noted in this respect that normally an additional preceding step, such as a 1T precoding, is applied to the (1,8) constrained sequence, resulting in a runlength limited sequence with minimum runlength 2 and maximum runlength 9. 
     The known conversion is parity preserving. ‘Parity preserving’ means that the parity of the n-bit source words to be converted equal the parity (after modulo-2 addition) of the corresponding m-bit channel words into which they are converted. As a result, the n-to-m conversion device as claimed does not influence the polarity of the signal. 
     As the conversion is parity preserving, an efficient DC control can be applied, such as by inserting DC control bits in the datastream of the source words. 
     SUMMARY OF THE INVENTION 
     The invention has for its object to provide an improved device for encoding n-bit source words into corresponding m-bit channel words. 
     The device in accordance with the invention is characterized in that the converting means is adapted to convert the 8-bit bit sequence ‘00010001’ in the bitstream of the binary source signal into the 12-bit bit sequence ‘100010010010’ of the binary channel signal. 
     The invention is based on the recognition that encoding in accordance with the known encoding device may result in relatively long sequences comprising only the minimum transition runlength, leading to a deterioration of the bit detection carried out in a bitdetector in a receiver, following transmission and subsequent decoding of the channel signal in the receiver. In a channel signal satisfying a specific runlength constraint, such as (1,7) or (1,8), this means that relatively long sequences ‘. . . 0101010101. . . ’ occur, resulting in relatively long sequences ‘. . . 001100110011. . . ’ in the sequence after 1T precoding. 
     Encoding devices in accordance with the invention restrict the lengths of those sequences, so that an improved bitdetection in a receiver can be realized. 
     An encoding device in accordance with the invention can be used in combination with a bit-adder unit in which one bit is added to codewords of a certain length of the source signal. The signal so obtained can be applied to the encoding device of the present invention. The channel signal of the encoding device is applied to a 1T-precoder. The purpose of the bit-adder unit is to add a ‘0’- or a ‘1’-bit to the consecutive code words included in the input signal of the converter, so as to obtain a precoder output signal which is DC free, or includes a tracking pilot signal having a certain frequency. The precoder output signal is recorded on a record carrier. The adding of a ‘0’-bit in the input signal of the converter results in the polarity of the output signal of the 1T precoder remaining the same. The adding of a ‘1’-bit results in a polarity inversion in the output signal of the 1T precoder. The converter therefore influences the output signal of the 1T precoder such that the running digital sum value of the output signal of the 1T precoder can be controlled so as to have a desired pattern as a function of time. 
    
    
     The invention will be further described in the following figure description, in which 
     FIG. 1 shows a first embodiment of the encoding device, 
     FIG. 2 shows a second embodiment of the encoding device, 
     FIG. 3 shows a third embodiment of the encoding device, 
     FIG. 4 shows a fourth embodiment of the encoding device, 
     FIG. 5 shows the application of the device in an arrangement for inserting one bit on equidistant positions in the serial source signal, 
     FIG. 6 shows a first embodiment of the decoding device, and 
     FIG. 7 shows a second embodiment of the decoding device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an encoding device that is capable of converting 2-bit source words into 3-bit channel words. The device described here is in fact the device also disclosed in U.S. Pat. No. 5,477,222, with further modifications to realize the objectives of a minimization of the repeated minimum transition runlength. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The device has an input terminal  1 , for receiving a stream of databits of a binary source signal S. The terminal  1  is coupled to an input of a shift register  2  having eight cells X 1  to X 8 , in the present example, so as to receive eight consecutive source bits of the source signal S. The shift register  2  functions as a serial-parallel converter. The outputs of the cells are coupled to corresponding inputs i 1  to i 8 , respectively, of a logic circuit LC, for supplying the logic values (x 1 , . . . , x 8 ) of the source bits present in the cells. The logic circuit LC forms part of the conversion means CM. 
     The device further includes a second shift register  4  having twelve cells Y 1  to Y 12 . The logic circuit LC has twelve outputs o 1  to o 12 . Those outputs of the logic circuit LC are coupled to corresponding inputs of the twelve cells Y 1  to Y 12 , respectively, of the shift register  4 . An output  6  of the shift register  4  is coupled to an output terminal  8 . The shift register  4  functions as a parallel-serial converter, so as to obtain the binary channel signal C. 
     Further, a detector unit  10  is available for detecting specific sequences in the serial datastream of the source signal S. To that purpose, the outputs of the eight cells X 1  to X 8  of the shift register  2  are coupled to corresponding inputs, denoted  12 , of the detector unit  10 . In the present embodiment, the detector unit  10  has three outputs, denoted o 1 , o 2  and o 3 , for generating a first, second and third control signal, respectively. Those outputs are coupled to corresponding control signal inputs c 1 , c 2  and c 3 , respectively, of the logic circuit LC. 
     The functioning of the logic circuit LC in response to the control signals applied to its inputs c 1 , c 2  and c 3 , is as follows. 
     The logic circuit LC is capable of converting 2-bit source words SW into 3-bit channel words, such that the conversion for each 2-bit source word is parity preserving. That means that the number of ‘ones’ in the source word to be converted equals the number of ‘ones’ in the corresponding channel word, a modulo-2 addition on the ‘ones’ in the channel word being carried out. Or, otherwise said: if the number of ‘ones’ in the source word is even, the number of ‘ones’ in the channel word will be even. And: if the number of ‘ones’ in the source word is odd, the number of ‘ones’ in the channel word will be odd. 
     As an example, the converting means LC is adapted to convert the 2-bit source words SW into 3-bit channel words CW in accordance with the following table: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 source word (x 1 ,x 2 ) 
                   
                 channel word (y 1 ,y 2 ,y 3 ) 
                   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SW 1   
                 00 
                 CW 1   
                 101 
               
               
                   
                 SW 2   
                 01 
                 CW 2   
                 100 
               
               
                   
                 SW 3   
                 10 
                 CW 3   
                 001 
               
               
                   
                 SW 4   
                 11 
                 CW 4   
                 000 
               
               
                   
                   
               
             
          
         
       
     
     It should be noted here, that the first bit in the source word is applied first to the shift register  2  and that the first bit in the channel word is supplied first from the output  6  of the shift register  4 . 
     It should be further noted here, that the logic circuit LC converts 2-bit source words stored in the cells X 1 ,X 2  into 3-bit channel words and stores those channel words in the cells Y 1 , Y 2 , Y 3  of the shift register  4 , in response to an absence of any control signal at the control signal inputs c 1 ,c 2  and C 3 . Each conversion in this way, is followed by a shift over two positions to the left in the shift register  2 , and a shift over three positions to the left in the shift register  4 . The shift over two positions in the shift register  2  is required so as to make the shift register  2 , and thus the converter, ready for a subsequent conversion. The shift over three positions in the shift register  4  is required in order to output the 3-bit channel word generated. 
     The device of FIG. 1 can be used to generate a channel signal C in the form of a (d,k) sequence satisfying the d=1 constraint. That means that at least one ‘zero’ is present between two subsequent ‘ones’ in the serial datastream of the channel signal. That is, that a concatenation of two or more ‘ones’ in the channel signal is prohibited. 
     It might occur that the unmodified conversion, such as by means of the device of FIG. 1, of combinations of two subsequent 2-bit source words might violate the d=1 constraint. Those combinations are the combinations; ‘00 00’, which by unmodified conversion would lead to the two 3-bit channel words ‘101 101’; ‘00 01’, which by unmodified conversion would lead to the two 3-bit channel words ‘101 100’; ‘10 00’, which by unmodified conversion would lead to the two 3-bit channel words ‘001 101’ and ‘10 01’, which by unmodified conversion would lead to the two 3-bit channel words ‘001 100’. 
     The occurrence of such combinations should be detected so that a modified encoding of blocks of two 2-bit source words into blocks of two 3-bit channel words can take place. Therefore, the device of FIG. 1 is, in addition to the ‘normal’ encoding of 2-bit source words into 3-bit channel words, capable of detecting the above identified combinations, and is capable of realizing a modified encoding, such that the d=1 constraint in the channel signal is still satisfied. 
     Because of the fact that the outputs of the cells X 1  to X 4  of the shift register  2  are coupled to corresponding inputs of the detector unit  10 , this detector unit  10  is capable of detecting the position in the serial bitstream of the source signal where unmodified encoding of single 2-bit source words in the bitstream into corresponding single 3-bit channel words would lead to a violation of the d=1 constraint in the channel signal C, and are adapted to supply a control signal at its output o 1  in response to such detection. 
     More specifically, the detector unit  10  detects whether the cells X 1  to X 4  comprise one of the 4-bit sequences that are given in table II, and generates a first control signal at its output o 1 . As soon as the detector circuit  10  detects a combination of two 2-bit source words present in the four cell positions x 1 ,x 2 ,x 3 ,x 4 , which combination equals one of the combinations given in the left hand column of table II, the logic circuit LC converts the combination in accordance with the modified coding as given in table II: 
     
       
         
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 source words 
                 unmodified coding 
                 modified coding 
               
               
                   
               
             
             
               
                 00 00 
                 101 101 
                 100 010 
               
               
                 00 01 
                 101 100 
                 101 010 
               
               
                 10 00 
                 001 101 
                 000 010 
               
               
                 10 01 
                 001 100 
                 001 010 
               
               
                   
               
             
          
         
       
     
     As can be seen from the table, unmodified conversion of the single two 2-bit source words leads to a violation of the d=1 constraint, as two ‘ones’ occur at the boundary between the two channel words obtained. The logic circuit LC is therefore adapted to convert in a modified coding mode, the blocks of two 2-bit source words given in the left column of the above table into the blocks of two 3-bit channel words as given in the right column in the above table II. As can be seen, no violation of the d=1 constraint occurs anymore. Moreover, the modified encoding in the same way is parity preserving. Further, one of the two 2-bit source words, which is in the above table the second one, is encoded into a 3-bit channel word which is unequal to one of the four channel words of table I. The reason for this is that on the receiver side, a detection of this 3-bit channel word not belonging to the set of four 3-bit channel words of the table I is possible, so that a corresponding decoding, which is the inverse of the encoding as defined with reference to table II, can be realized. 
     The block of two 3-bit channel words obtained by means of the encoding in conformity with table II, is supplied by the logic circuit LC to its outputs o 1  to o 6 , which channel words are supplied to the six cells Y 1  to Y 6  of the shift register  4 . 
     It will further be clear that, a conversion of two 2-bit source words into two 3-bit channel words by the converter unit LC, is followed by a shift over four positions to the left in the shift register  2  and a shift over six positions to the left in the shift register  4 . The shift over four positions in the shift register  2  is required so as to make the shift register  2 , and thus the converter, ready for a subsequent conversion. The shift over six positions in the shift register  4  is required in order to output the two 3-bit channel words generated. 
     The k-constraint in a (d,k) sequence means that a concatenation of at most k ‘zeroes’ between two subsequent ‘ones’ in the channel signal are allowed. 
     It might occur that the unmodified conversion of three subsequent 2-bit source words might violate the k-constraint. 
     As an example: the sequence of source words ‘11 11 11’ would by unmodified conversion lead to the three 3-bit channel words ‘000 000 000’. If a (d,k) sequence should be obtained where k equals 6, 7 or 8, such combination of three 3-bit channel words should not occur. 
     Another example is the sequence of source words ‘11 11 10’ which by unmodified conversion would lead to the three 3-bit channel words ‘000 000 001’. This combination of three 3-bit channel words does not satisfy a k=6 or k=7 constraint. Moreover, this combination of three 3-bit channel words can follow a previous channel word that ends with a ‘0’, so that it might lead to a violation of a k=8 constraint. Further, the combination ends with a ‘1’, so that it might lead to a violation of the d=1 constraint, if the combination is followed by a 3-bit channel word that starts with a ‘1’. An equivalent reasoning is valid for the sequence of source words ‘01 11 11’. 
     A further example is the sequence of source words ‘01 11 10’ which by unmodified conversion would lead to the three 3-bit channel words ‘100 000 001’. This combination can, in the same way as given above, lead to a violation of the d=1 constraint. 
     The occurrence of such combinations should be detected so that a modified encoding can take place. Therefore, the device of FIG. 1 is, in addition to the ‘normal’ encoding of 2-bit source words into 3-bit channel words, as well as the modified encoding in accordance with table II, capable of detecting the above identified combinations, and is capable of realizing a modified encoding, such that the k-constraint in the channel signal is still satisfied. 
     Because of the fact that the outputs of the cells X 1  to X 6  of the shift register  2  are coupled to corresponding inputs of the detector unit  10 , this detector unit  10  is capable of detecting the position in the serial bitstream of the source signal where unmodified encoding would lead to a violation of the k-constraint in the channel signal C, and are adapted to supply a control signal at its output o 2  in response to such detection. 
     More specifically, the detector unit  10  detects whether the cells X 1  to X 6  comprise one of the 6-bit sequences that are given in table III, and generates a second control signal at its output o 2 . 
     As soon as the detector circuit  10  detects a combination of three 2-bit source words present in the six cell positions x 1 ,x 2 ,x 3 ,x 4 ,x 5 ,x 6 , which combination equals one of the combinations given in the left hand column of table III, the logic circuit LC converts the combination in accordance with the modified coding as given in table III: 
     
       
         
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                 source words 
                 unmodified coding 
                 modified coding 
               
               
                   
               
             
             
               
                 11 11 11 
                 000 000 000 
                 000 010 010 
               
               
                 11 11 10 
                 000 000 001 
                 001 010 010 
               
               
                 01 11 10 
                 100 000 001 
                 101 010 010 
               
               
                 01 11 11 
                 100 000 000 
                 100 010 010 
               
               
                   
               
             
          
         
       
     
     The logic circuit LC converts in the second modification of the coding mode, the blocks of three 2-bit source words given in the left column of the above table III into the blocks of three 3-bit channel words as given in the right column in the above table. By realizing the modified encoding as per table III, a channel signal has been obtained which satisfies the k=8 constraint. Moreover, the modified encoding in the same way is parity preserving. This means in the present situation that, if the number of ‘ones’ in the combination of three 2-bit source words is odd (even), the number of ‘ones’ in the combination of the three 3-bit channel words obtained is odd (even). Further, two of the three 2-bit source words, which is in the above table the second one and the third one, is encoded into a 3-bit channel word which is unequal to one of the four channel words of table I. The reason for this is that on the receiver side, a detection of these two consecutive 3-bit channel words not belonging to the set of four 3-bit channel words of the table I is possible, so that a corresponding decoding, which is the inverse of the encoding as defined with reference to table III, can be realized. 
     The block of three 3-bit channel words obtained by means of the encoding in conformity with table III, is supplied by the logic circuit LC to its outputs o 1  to o 9 , which channel words are supplied to the nine cells Y 1  to Y 9  of the shift register  4 . 
     It will further be clear that, a conversion of three 2-bit source words into three 3-bit channel words by the converter unit LC, is followed by a shift over six positions to the left in the shift register  2  and a shift over nine positions to the left in the shift register  4 . The shift over six positions in the shift register  2  is required so as to make the shift register  2 , and thus the converter, ready for a subsequent conversion. The shift over nine positions in the shift register  4  is required in order to output the three 3-bit channel words generated. 
     A further requirement for encoding the source signal is that the repeated minimum transition runlength in the channel signal should be limited. The repeated minimum transition runlength is defined as the length of the sequence of subsequent transitions between ‘0’ and ‘1’, or: a sequence ‘. . . 01010101010. . . ’, in the case where the d-constraint equals 1. As an example, the bit sequence ‘00 01 00 01’ results, after modified conversion using table II, in the bit sequence ‘101 010 101 010’. In the same way, the bit sequence ‘10 01 00 01’ results, after modified conversion using table II, in the bit sequence ‘001 010 101 010’. Such sequences deteriorate the bit detection in a receiver. Restriction of the length of 01-sequences is thus required. 
     Because of the fact that the outputs of the cells X 1  to X 8  of the shift register  2  are coupled to corresponding inputs of the detector unit  10 , this detector unit  10  is capable of detecting the position in the serial bitstream of the source signal where unmodified encoding would lead to a violation of the requirement that repeated minimum transition runlength is limited and is adapted to supply a control signal at its output o 3  in response to such detection. 
     More specifically, the detector unit  10  detects whether the cells X 1  to X 8  comprise one of the 8-bit sequences that are given in table IV, and generates a third control signal at its output o 3 . 
     As soon as the detector circuit  10  detects a combination of four 2-bit source words present in the eight cell positions x 1 ,x 2 ,x 3 ,x 4 ,x 5 ,x 6 ,x 7 ,x 8 , which combination equals one of the combinations given in the left hand column of table IV, the logic circuit LC converts the combination in accordance with the modified coding as given in table IV into a 12-bit bitsequence as given in the right column of this table IV. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                 8-bit bitsequence 
                 12-bit bitsequence 
               
               
                   
                 in source signal 
                 in channel signal 
               
               
                   
                   
               
             
             
               
                   
                 00 01 00 01 
                 100 010 010 010 
               
               
                   
                 10 01 00 01 
                 000 010 010 010 
               
               
                   
                   
               
             
          
         
       
     
     The modified conversion as per table IV is again parity preserving. 
     It should be noted that the bitstream of the channel words is in NRZI (non-return to zero-inverse) notation, which means that a ‘one’ results in a transition in the write current for recording the channel signal on a magnetic record carrier. 
     In the foregoing, the situations where a modified encoding is required are detected by the detector unit  10  from the. source words. It should however be noted that detection could be carried out on the channel words generated. Reference is made in this respect to FIG.  2   b  in U.S. Pat. No. 5,477,222. 
     FIG. 2 shows another embodiment of the encoding device in which detection of the situations where a modified encoding is required is done based on the channel words generated by an unmodified coding as per table I. 
     The device of FIG. 2 includes a detector  10 ′ having 12 inputs for receiving the four subsequent 3-bit channel words obtained by means of an unmodified encoding as per table I in the circuit LC′. The detector  10 ′ detects whether the two subsequent 3-bit channel words at the outputs o 1  to o 6  of the circuit LC′, obtained using the unmodified coding, equal one of the four 6-bit sequences given in the middle column under ‘unmodified coding’ of table II. If so, the detector  10 ′ issues a switching signal at its output  12  and an address signal AD at its output  12 ′. The switching signal is applied to a switching signal input  45  of the shift register  4 ′. The address signal AD is applied to an address signal input  46  of a ROM  47 . The detector  10 ′ generates one of four possible address signals AD 1  to AD 4 , in response to the detection of a corresponding one of the four 6-bit sequences in the middle column of table II. As an example, the address signal AD 1  is generated when the detector  10 ′ detects the sequence ‘101101’ and generates the address signal AD 4  upon detection of the 6-bit sequence ‘001100’. The ROM  47  has the 6-bit sequences shown in the right column of table II stored. Upon the receipt of the address signal AD 1 , the ROM supplies the 6-bit sequence ‘100 010’ at its outputs o 1  to o 6 , and upon the receipt of the address signal AD 2 , the ROM supplies the 6-bit sequence ‘101 010’ at those outputs. Upon the receipt of the address signal AD 3 , the ROM supplies the 6-bit sequence ‘000 010’ at those outputs, and upon the receipt of the address signal AD 4 , the ROM supplies the 6-bit sequence ‘001 010’ at those outputs. Each memory location of the shift register  4 ′ has now two inputs, one of them being coupled with a corresponding output of the logic circuit LC′, the other being coupled to a corresponding output of the ROM  47 . In response to the switching signal applied to the input  45 , the shift register accepts the information supplied to its lower inputs and shifts its contents over six positions to the left. As a result, a modified 6-bit sequence is supplied by the shift register  4 ′ to the output  8 . 
     The detector  10 ′ also detects whether the three subsequent 3-bit channel words at the outputs o 1  to o 9  of the circuit LC′, obtained using the unmodified coding, equal one of the four 9-bit sequences given in the middle column under ‘unmodified coding’ of table III. If so, the detector  10 ′ issues a switching signal at its output  12  and an address signal AD at its output  12 ′. The detector  10 ′ generates one of four possible address signals AD 5  to AD 8 , in response to the detection of a corresponding one of the four 9-bit sequences in the middle column of table III. As an example, the address signal AD 5  is generated when the detector  10 ′ detects the sequence ‘000 000 000’ and generates the address signal AD 8  upon detection of the 9-bit sequence ‘100 000 000’. The ROM  47  has the 9-bit sequences shown in the right column of table III stored. Upon the receipt of the address signal AD 5 , the ROM supplies the 9-bit sequence ‘000 010 010’ at its outputs o 1  to o 9 , and upon the receipt of the address signal AD 6 , the ROM supplies the 9-bit sequence ‘001 010 010’ at those outputs. Upon the receipt of the address signal AD 7 , the ROM supplies the 9-bit sequence ‘101 010 010’ at those outputs, and upon the receipt of the address signal AD 8 , the ROM supplies the 9-bit sequence ‘100 010 010’ at those outputs. 
     In response to the switching signal applied to the input  45 , the shift register accepts the information supplied to its lower inputs and shifts its contents over nine positions to the left. As a result, a modified 9-bit sequence is supplied by the shift register  4 ′ to the output  8 . 
     The detector  10 ′ further detects whether the four subsequent 3-bit channel words at the outputs o 1  to o 12  of the circuit LC′, obtained using the unmodified coding, equal one of the following two 12-bit sequences: ‘101 010 101 010’ or ‘001 010 101 010’. If so, the detector  10 ′ issues a switching signal at its output  12  and an address signal AD at its output  12 ′. The detector  10 ′ generates one of two possible address signals AD 9  and AD 10 , respectively, in response to the detection of a corresponding one of the two 12-bit sequences given above. As an example, the address signal AD 9  is generated when the detector  10 ′ detects the sequence ‘101 010 101 010’ and generates the address signal AD 10  upon detection of the 12-bit sequence ‘001 010 010 101’. The ROM  47  has the 12-bit sequences shown in the right column of table IV stored. Upon the receipt of the address signal AD 9 , the ROM supplies the 12-bit sequence ‘100 010 010 010’ at its outputs o 1  to o 12 , and upon the receipt of the address signal AD 10 , the ROM supplies the 12-bit sequence ‘000 010 010 010’ at those outputs. 
     In response to the switching signal applied to the input  45 , the shift register accepts the information supplied to its lower inputs and shifts its contents over twelve positions to the left. As a result, a modified 12-bit sequence is supplied by the shift register  4 ′ to the output  8 . 
     In the normal situation, when none of the constraints is violated, unmodified conversion is carried out in accordance with table I, and the switching signal is absent so that the shift register accepts the bits supplied by the logic circuit LC′ via the upper inputs of the shift register  4 ′. 
     It has been said previously that other conversion rules for converting single 2-bit source words into single 3-bit channel words are possible. Those conversion rules are given in the following three tables. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                 source word (x 1 ,x 2 ) 
                   
                 channel word (y 1 ,y 2 ,y 3 ) 
                   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SW 1   
                 00 
                 CW 1   
                 101 
               
               
                   
                 SW 2   
                 01 
                 CW 2   
                 001 
               
               
                   
                 SW 3   
                 10 
                 CW 3   
                 100 
               
               
                   
                 SW 4   
                 11 
                 CW 4   
                 000 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE V 
               
               
                   
                   
               
               
                   
                 source word (x 1 ,x 2 ) 
                   
                 channel word (y 1 ,y 2 ,y 3 ) 
                   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SW 1   
                 00 
                 CW 1   
                 000 
               
               
                   
                 SW 2   
                 01 
                 CW 2   
                 100 
               
               
                   
                 SW 3   
                 10 
                 CW 3   
                 001 
               
               
                   
                 SW 4   
                 11 
                 CW 4   
                 101 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE VI 
               
               
                   
                   
               
               
                   
                 source word (x 1 ,x 2 ) 
                   
                 channel word (y 1 ,y 2 ,y 3 ) 
                   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 SW 1   
                 00 
                 CW 1   
                 000 
               
               
                   
                 SW 2   
                 01 
                 CW 2   
                 001 
               
               
                   
                 SW 3   
                 10 
                 CW 3   
                 100 
               
               
                   
                 SW 4   
                 11 
                 CW 4   
                 101 
               
               
                   
                   
               
             
          
         
       
     
     It is evident that extensions of those conversion rules for encoding blocks of two or three 2-bit source words into blocks of two or three 3-bit channel words can be obtained using the teachings given above. 
     FIG. 3 shows a modification of the device of FIG.  1 . The device of FIG. 3 is capable of generating a channel signal with a lower k-constraint, more specifically a k-constraint k=7. The modification lies in the addition of the a further shift register  70  having three storage locations  70 . 1 ,  70 . 2  and  70 . 3 , which has an input  72  coupled to the output  6  of the shift register  4 , and outputs  74 . 1 ,  74 . 2  and  74 . 3 , being the outputs of the storage locations  70 . 1 ,  70 . 2  and  70 . 3 , respectively. Further, a detector  76  is present having inputs  78 . 1 ,  78 . 2  and  78 . 3 , coupled to the outputs  74 . 1 ,  74 . 2  and  74 . 3 , respectively, of the shift register  70 , and having an output  82 . The output  74 . 1  of the shift register  70  is also coupled to a control input c 5  of the conversion circuit LC m , and the output  82  of the detector  76  is coupled to a control input c 4  of the conversion circuit LC m . Further, the detector  10  in FIG. 1 is slightly modified into the detector  80  in FIG.  3 . 
     As far as the conversion of single 2-bit source words into 3-bit channel words, the conversion of two 2-bit source words into two 3-bit channel words, and the conversion of three 2-bit source words into three 3-bit channel words is concerned, this conversion is the same as explained above with reference to the device of FIG.  1 . 
     The modification lies specifically in the conversion of 8-bit bitsequences of a specific form occurring in the source signal. The detector  80  is, in addition to detecting the 4-bit bitsequences of table II and the 6-bit bitsequences of table III (as explained with reference to the embodiment of FIG.  1 ), capable of detecting 8-bit bitsequences given in the table VII that follows below. In response to such detection, the detector  80  generates a control signal at its output o 3 , which control signal is applied to the control input c 3  of the logic circuit LC m . 
     The shift register  70  has the last three bits of the channel signal generated up to now stored in its storage locations  70 . 1 ,  70 . 2  and  70 . 3 , where the storage location  70 . 1  has the last bit of the channel signal generated up to now stored in it. This last bit of the channel signal, which is either a ‘0’ or a ‘1’ bit, is used as a further control signal for the logic circuit LC m , and is supplied to the logic circuit LC m  via its control input c 4 . Further, the detector  76  is adapted to generate a control signal at its output  82 , when the shift register  70  has the 3-bit bitsequence ‘010’ stored in its storage locations  70 . 1 ,  70 . 2  and  70 . 3 . This control signal is supplied to the logic circuit LC m  via its control input c 5 . 
     In accordance with the table VII, the logic circuit LC m  converts the 8-bit bitsequence ‘00 01 00 01’ into its corresponding 12-bit bitsequence ‘100 010 010 010’, in response to the control signal applied to its control input c 3 , and irrespective of the control signals appearing at its inputs c 4  and c 5 . Further, the logic circuit LC m  converts the 8-bit bitsequence ‘10 01 00 01’ into the 12-bit bitsequence ‘100 000 010 010’, in response to the control signal at its input c 5  being equal to ‘0’, and converts the said 8-bit bitsequence into the 12-bit bitsequence ‘000 010 010 010’, in response to the control signal being equal to ‘1’. When the 8-bit bitsequence ‘11 10 00 00’ appears at the inputs i 1  to i 8  of the logic circuit, a control signal is generated by the detector  80  at its output o 3 . Further, when the contents of the shift register  70  equals the 3-bit bitsequence ‘010’, a control signal is generated by the detector  76 . In response to both control signals, the logic circuit LC m  converts this 8-bit bitsequence into the 12-bit bitsequence ‘000 001 010 010’, as shown in table VII. When the 8-bit bitsequence ‘11 10 00 10’ appears at the inputs i 1  to i 8  of the logic circuit, a control signal is generated by the detector  80  at its output o 3 . Further, when the contents of the shift register  70  equals the 3-bit bitsequence ‘010’, a control signal is generated by the detector  76 . In response to both control signals, the logic circuit LC m  converts this 8-bit bitsequence into the 12-bit bitsequence ‘100 001 010 010’, as shown in table VII. When the 8-bit bitsequence ‘11 10 00 01’ appears at the inputs i 1  to i 8  of the logic circuit, a control signal is generated by the detector  80  at its output o 3 . Further, when the contents of the shift register  70  equals the 3-bit bitsequence ‘010’, a control signal is generated by the detector  76 . In response to both control signals, the logic circuit LC m  converts this 8-bit bitsequence into the 12-bit bitsequence ‘001 010 010 010’, as shown in table VII. When the 8-bit bitsequence ‘11 10 00 11’ appears at the inputs i 1  to i 8  of the logic circuit, a control signal is generated by the detector  80  at its output o 3 . Further, when the contents of the shift register  70  equals the 3-bit bitsequence ‘010’, a control signal is generated by the detector  76 . In response to both control signals, the logic circuit LC m  converts this 8-bit bitsequence into the 12-bit bitsequence ‘101 010 010 010’, as shown in table VII. 
     
       
         
               
               
               
             
           
               
                 TABLE VII 
               
               
                   
               
               
                 8-bit bitsequence 
                 last bit(s) in 
                 12-bit bitsequence 
               
               
                 in source signal 
                 channel signal 
                 in channel signal 
               
               
                   
               
             
             
               
                 00 01 00 01 
                 don&#39;t care 
                 100 010 010 010 
               
               
                 10 01 00 01 
                  0 
                 100 010 010 010 
               
               
                 10 01 00 01 
                  1 
                 000 010 010 010 
               
               
                 11 10 00 00 
                 010 
                 000 001 010 010 
               
               
                 11 10 00 10 
                 010 
                 100 001 010 010 
               
               
                 11 10 00 01 
                 010 
                 001 010 010 010 
               
               
                 11 10 00 11 
                 010 
                 101 010 010 010 
               
               
                   
               
             
          
         
       
     
     With this conversion, the channel signal satisfies k=7 and limits the repeated minimum transition runlength to 6. 
     FIG. 4 shows a modification of the embodiment of FIG. 2, modified so as to enable the conversion in accordance with the table VII above. The embodiment of FIG. 2 is modified into the embodiment of FIG. 4 by adding the shift register  70  and the detector unit  76 , which supply their control signals to control inputs c 4  and c 5  of the ROM  47 ′. No further description of the embodiment will be given, as with the above description of the embodiment of FIG. 2, with the explanation of the modified encoding of a number of 8-bit bitsequences in the source signal, it is clear for the skilled man to develop an enabling construction for the embodiment of FIG. 4, without employing any inventive activity. 
     As has been said previously, the devices described above are very suitable for including in the encoding arrangement where one bit is inserted after each q bits in a serial datastream in order to realize a polarity conversion, or not. Such an encoding arrangement is schematically shown in FIG. 5, where the encoder  40  is followed by the encoding device in accordance with the present invention  41 , and a 1T-precoder  42 , well known in the art. The output signal of the 1T-precoder  42  is applied to a control signal generator  43 , which generates the control signal for the converter  40 , so as to control whether a ‘0’ or a ‘1’ is inserted in the serial datastream applied to the converter  40 . The encoding device  41  can be inserted between the converter  40  and the 1T-precoder  42  without any modification, as the encoder  41  does not influence the polarity of the signal generated by the converter  40 . By means of the arrangement shown in FIG. 5 it is possible to embed a tracking tone of a certain frequency in the serial datastream, or keep the DC content of the datastream to zero. Further, when the encoding device  41  is adapted to generate a (d,k) sequence as explained above, it causes the output signal of the arrangement of FIG. 4 to be a (d,k) RLL output signal. Embodiments of the converter  40  are given in Bell System Technical Journal, Vol 53, No. 6, pp. 1103-1106. 
     The output signal of the 1T precoder  42  is supplied to a write unit  21  for writing the signal in a track on a record carrier  23 . The record carrier  23  can be a magnetic record carrier in longitudinal or disk form. The record carrier could also be an optical record carrier, such as an optical disk  23 ′. The write unit  21  comprises a write head  25 , which is a magnetic write head, when recording the signal on a magnetic record carrier or an optical write head, when recording the signal on an optical record carrier. 
     FIG. 6 shows an embodiment of a decoding device for decoding the serial datastream obtained by the encoding device of FIG. 1 or  2 , so as to obtain a binary source signal. The decoding device has an input terminal  50  for receiving the channel signal, which input terminal  50  is coupled to an input  56  of a shift register  51 , comprising twelve cells Y 1  to Y 12 . The shift register  51  functions as a serial-parallel converter so that blocks of four 3-bit channel words are applied to inputs i 1  to i 12  of a logic circuit  52 . The logic circuit  52  comprises the four tables I, II, III and IV. Outputs o 1  to o 8  of the logic circuit  52  are coupled to inputs of cells X 1  to X 8  of a shift register  54 , which has an output  57  coupled to an output terminal  55 . A detector circuit  53  is present having inputs i 1  to i 9 , schematically indicated by the reference number  60 , coupled to outputs of cells Y 4  to Y 12  respectively of the shift register  51 , and outputs o 1 , o 2  and o 3  coupled to control inputs c 1 , c 2  and c 3 , respectively, of the logic circuit  52 . The detector circuit  53  is capable of 
     (a) detecting a ‘010 010 010’ bit pattern in the cells Y 4  to Y 12  of the shift register  51 , 
     (b) detecting a bit pattern ‘010 010’ in the cells Y 4  to Y 9  of the shift register  51 , whilst the bits in the cells Y 10 , Y 11  and Y 12  are unequal to ‘010’, and 
     (c) detecting a ‘010’ bit pattern in the cells Y 4 , Y 5  and Y 6 , whilst the bits in the bitcells Y 7 , Y 8  and Y 9  are unequal to ‘010’. 
     Upon detection of the ‘010 010 010’ bitpattern, the detector circuit  53  generates a control signal on its output o 1 , upon detection of the ‘010 010’ bit pattern in the cells Y 4  to Y 9 , the detector circuit  53  generates a control signal on its output o 2 , upon detection of the ‘010’ pattern in the cells Y 4  to Y 6 , the detector circuit  53  generates a control signal at its output  0   3 , whilst, when there is no ‘010’ bit pattern in the cells Y 4  to Y 12 , it generates no control signal on its outputs. 
     In the absence of the control signals, the logic circuit  52  converts the 3-bit channel word stored in the cells Y 1 , Y 2  and Y 3  into its corresponding 2-bit source word, as per the conversion table I, and supplies the 2-bit source word to the cells X 1  and X 2 . In the presence of the control signal at the input c 3 , the logic circuit  52  converts the block of two 3-bit channel words stored in the cells Y 1  to Y 6  into a block of two 2-bit source words, as per the conversion table II, and supplies the two 2-bit source words to the cells X 1  to X 4 . In the presence of the control signal at the input c 2 , the logic circuit  52  converts the block of three 3-bit channel words stored in the cells Y 1  to Y 9  into a block of three 2-bit source words, as per the conversion table III, and supplies the three 2-bit source words to the cells X 1  to X 6 . In the presence of the control signal at the input c 1 , the logic circuit  52  converts the block of four 3-bit channel words stored in the cells Y 1  to Y 12  into a block of four 2-bit source words, as per the conversion table IV, and supplies the four 2-bit source words to the cells X 1  to X 8 . 
     In this way, the serial datastream of the channel signal is converted into the serial datastream of the source signal. 
     The encoded information supplied to the input  50  could have been obtained from reproducing the information from a record carrier, such as a magnetic record carrier  23  or an optical record carrier  23 ′. The device in FIG. 6 thereto comprises a read unit  62  for reading the information from a track on the record carrier, where the unit  62  comprises a read head  64  for reading the information from said track. 
     FIG. 7 shows an embodiment of the decoder for decoding the channel signal generated by the encoders of the FIGS. 3 or  4 . From the table VII, it is clear that the specific 12-bit bitsequences in the channel signal that should be decoded in accordance with the table VII cannot all be detected by the sequence ‘010 010 010’. Therefore, the detector  53 ′ requires inputting of the 12-bit bitsequence in order to identify all the seven 12-bit bitsequences included in the table VII. 
     Whilst the invention has been described with reference to preferred embodiments thereof, it is to be understood that these are not limitative examples. Thus, various modifications may become apparent to those skilled in the art, without departing from the scope of the invention, as defined in the claims. As an example, the decoding device of FIG. 6 could be modified into a device in which the detector  53  detects the various modified decoding situations from the decoded information, instead of from the encoded information, as disclosed in FIG.  6 . 
     Further, the invention lies in each and every novel feature or combination of features.