Method for recording digital data

Abstract: There is provided a digital data recording system in which digital data which is encoded by the RLL (Run Length Limited) encoding system in which code "1" does not continue is recorded onto a recording medium in accordance with an NRZL (Non Return to Zero) rule. In this system, the digital data is input, a width (T) of pulse corresponding to code "1" in the input digital data is converted into a pulse of a width (T′) larger than the pulse width, and the digital data of the converted pulse width is recorded onto the recording medium. A change amount of the pulse width is made different in accordance with the minimum number of codes "0" arranged between codes "1" and "1" in the digital data. With this system, the error rate of the digital data recorded on the recording medium can be reduced. In the case of the same error rate, the recording density can be improved.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a recording method when digital data is recorded onto a recording medium. Related Background Art As a system for encoding data to be recorded and recording, a system in which an input data series is converted into a Run Length Limited code (hereinafter, this code is referred to as "RLL code") series and recorded in accordance with the NRZL (NRZ) rule is known. The NRZL rule means a system in which, for instance, when the RLL coded data is as shown in Fig. 1A, this data is converted into a signal as shown in Fig. 1B. In the case of decoding, an interval between code 1 and code 1, that is, data between marks is decoded. A recording medium generally has a reproducing characteristic such that a response of a reproduction output decreases at a high frequency as shown in Fig. 2. Therefore, when a pulse width of recording signal is narrow, the response decreases. The RLL codes are generally expressed as d, k, m, and n codes. In this case, m denotes the number of bits constituting one block of the input data series and n indicates the number of bits of one block of the input data series after completion of the conversion in the corresponding RLL code. On the other hand, d represents the minimum number of continuous code bits "0" which is permitted in the RLL codes. Namely, when d ≧ 1, the code bit "1" does not continue. k indicates the maximum number of continuous code bits "0" which is permitted in the RLL code. In the RLL codes, the case where the values of d and k, particularly, the value of d is relatively large will now be considered. As an example, the case of d = 3 is shown in Fig. 3. In this case, when a light source for recording is flickered and recorded on an optical recording medium in accordance with the NRZL rule, particularly, in the case of a high density and a short flickering time of the light source, there could occur such a situation that an enough energy is not given to the recording medium and the optical recording is not satisfactorily performed (that is, the response lags). This is because the surface of the medium is changed by the heat, thereby recording. Fig. 4 shows examples of a change in reproduction output in the case where a width T of recording time is large and a reproduction output in the case where it is small. Therefore, in the conventional NRZL recording of the RLL codes, there are problems such that when the lighting time of the light source is short, the reproduction output of an enough high level cannot be obtained and it is difficult to reduce errors of information. In other words, there is a problem such that in the case of the same error rate, it is difficult to improve the recording density. Patent Abstracts of Japan, Vol 9, No 312 (P-411) 2035, 07.12.85, discloses an optical recording arrangement in which the pulsewidth of an original signal representing a write hole is widened in order to overcome the problem that due to the threshold level the write holes on an optical disc are reduced. Topical Meeting on Optical Data Storage, Stateline, Nevada, 11-13 March 1987, pages 106-110, Optical Society of America, Washington DC, USA; D G Howe et al "Digital Optical Recording - The Case for Coding", discloses a digital optical recording arrangement, utilising a Run Length Limited (RLL) modulation code in which the pulsewidth corresponding to code "1" in the digital data input is converted into a wider second pulsewidth in order to overcome the problem referred to earlier in connection with the arrangement disclosed in Patent Abstracts of Japan, Vol 9, No 312. According to the present invention a digital data recording method whereby digital data encoded by an encoding system in which code "1" does not occur twice without an intervening code "0" and run length of the code "0" is limited is recorded on a recording medium and the code "1" is converted into one pulse and recorded on the recording medium, said method comprising the steps of: inputting the digital data; converting a first pulsewidth (T), corresponding to code "1" in the digital data input in said inputting step, into a second pulsewidth larger than the first pulsewidth; and recording the digital data in which code "1" has the second pulsewidth on the recording medium; characterised in that in said converting step, the changed amount of said first pulsewidth (T) is varied in proportion to the number of codes "0" between adjacent codes "1" and "1" in said digital data. In the accompanying drawings, Figs. 1A and 1B are diagrams showing RLL codes in accordance with the NRZL rule; Fig. 2 is a diagram showing a general frequency characteristic of a reproduction output in the optical recording; Fig. 3 is a diagram showing an example of RLL codes; Fig. 4 is a diagram showing a reproduction output in the case where a width T of recording time is large and a reproduction output in the case where it is small; Fig. 5 is a block diagram showing a one shot multivibrator. Figs. 6A and 6B are diagrams showing a signal in the case where the time width of code 1 is converted; Fig. 7A and 7B are diagrams showing, for the purpose of reference a signal in the case where the time width of code 1 undergoes a fixed conversion; Fig. 8A is a diagram showing a signal which is recorded onto a recording medium; Fig. 8B is a diagram showing an analog reproduction signal of the signal recorded on the recording medium; Fig. 8C is a diagram showing a binarized signal of the analog reproduction signal shown in Figure 8B; Fig. 9 is a block diagram showing an arrangement of a circuit to convert the time width of code 1 in accordance with the present invention; Figs. 10A-10E are diagrams showing a signal in each section of the block diagram of Fig. 9; Fig. 11 is a block diagram showing an arrangement of the present invention; and Fig. 12 is a flowchart showing processes to convert the time width of code 1. Fig. 5 is a block diagram showing a code generator 1 for generating RLL codes and outputs the RLL codes corresponding to input data, a one shot multivibrator 2 serving as means for converting a time width T corresponding to code 1 in the RLL codes into a time width T′ longer than the time width T and outputting, and recording means 3 for recording information onto a recording medium in accordance with the code having the time width T′ converted. A magnetooptic disk apparatus for recording information onto a disk by irradiating a laser beam may be used as the recording means 3. Figs. 6A and 6B are explanatory diagrams of the above arrangement. It is now assumed that the time width of the code 1 bit in the RLL codes is T and the codes shown in Fig. 6A are output from the code generator 1. The one shot multivibrator 2 converts the time width T of code 1 in the RLL codes into T′ and outputs. In this case, the time width T′ is longer than the time width T. As will be seen from Fig. 4, the output of the code having the time width T′ is larger than the output of the code having the time width T. As shown in Fig. 6B, an interval between code 1 in Fig. 6A and next code 1 is equal to an interval between the leading edge of the time width T′ and the leading edge of the code of the next time width T′. Therefore, even if the time width of code 1 is changed by the multivibrator 2, the content of the information in the RLL codes does not change. Therefore, in the case of using an optical disk apparatus or a magnetooptic disk apparatus as the recording means, a sufficiently larger energy than that when a laser beam is irradiated onto the disk at the time width T is given to the disk, so that the recorded data of code 1 can be more precisely read out. Although the one shot multivibrator 2 has been shown in Fig. 5 as means for converting the time width T into the time width T′, it can be also converted by another means. For example, it is sufficient to construct in a manner such that when code 1 is input, the counting is started in response to the leading edge by using a counter for counting clock pulses and a high level signal is output until the pulses of the number corresponding to the time width T′ are counted. On the other hand, it is also possible to start the counting in response to a trailing edge of code 1 and to count the pulses of the number corresponding to only (T′ - T). Codes shown in Fig. 7A are similar to those shown in Fig. 6A. In the waveform shown in Fig. 7B, in the case of converting the time width T into T′ when d = 3, T′ = (d + 1) x T/2 As the time width T′ increases, code 1 can be easily discriminated but the width of code 0 is reduced. Therefore, there is a possibility such that the number of errors increases in the portion corresponding to code 0 due to the interference between codes. Namely, in the case where data shown in Fig. 8A was recorded, when this recorded data is read out of a recording medium, it shows such an analog waveform as shown in Fig. 8B. When the width of code 0 decreases, the edge slope portion of code 1 and the edge slope portion of the analog waveform corresponding to code 1 overlap, so that there occurs a case where the portion which should inherently be 0 is erroneously discriminated as 1 (Fig. 8C). In general, in one RLL code, the density of code 1 becomes maximum in the case where d codes 0 are arranged between code 1 and the next code 1. In this case, since the width of code 0 becomes minimum, a data error most easily occurs. Therefore, in such a case, by making the widths of codes 1 and 0 equal (duty factor 50%), the error rates of codes 1 and 0 can be equally reduced. Accordingly, it is desirable to set the time width T′ as shown in the equation (1). In the above embodiment, the duty factor has been set to 50% only when d codes 0 are arranged between code 1 and the next code 1. However, if the duty factor is always set to 50% irrespective of the number of codes 0, the error rates of codes 1 and 0 can be always reduced. A method in this case will now be described hereinbelow. Fig. 9 shows a practical embodiment of a time width converting method of the invention. Fig. 10 is a waveform diagram in each section of the circuit of Fig. 9. Reference numeral 11 denotes an input terminal of RLL encoded recording codes. For example, a recording code waveform a shown in Fig. 10A is input to the input terminal 11. Reference numeral 18 denotes a clock generator of a bit rate which is twice the bit rate of the recording code. For example, a crystal oscillator is used as the clock generator 18. Fig. 10B shows a waveform diagram of a signal which is generated from the clock generator 18. Reference numeral 19 denotes a 1/2 frequency divider consisting of a flip-flop circuit for frequency dividing an output of the clock generator 18 into the frequency of 1/2. Fig. 10C shows a waveform diagram thereof. Reference numeral 12 denotes a well-known counter. The recording code waveform is input to an enable terminal of this counter. When a low level signal is input to the enable terminal, it is counted. An output c of the frequency divider 19 is input to a count input terminal of the counter 12. An AND circuit 17 gets the AND of the recording code waveform a and the output c of the frequency divider 19. An output waveform of the AND circuit 17 is shown in Fig. 10D. A latch circuit 13 latches output data of the counter 12 in response to an output d of the AND circuit 17. Reference numeral 14 denotes a ROM to decide the time width T′. An output of the latch circuit 13 is input as an address to the ROM 14 and the data corresponding to T′ is output. In this case, assuming that the number of input data is n, n+1 is output as an output data. A latch circuit 15 is similar to the latch circuit 13 and latches an output of the ROM 14. Reference numeral 16 denotes a well-known parallel/serial (P/S) converter consisting of a shift register or the like which uses an output of the clock generator 18 as a read clock of serial data. The P/S converter 16 has an output terminal 20. In the foregoing arrangement, the counter 12 starts to count a clock c in response to the trailing edge of "1" of the input recording code. The count value of the counter 12 is latched when the signal d is generated from the AND circuit 17. The latched value coincides with the number of codes 0. In order to equalize the time widths of codes 1 and 0, assuming that the number of codes 0 is n, it is sufficient to set T′ as follows. T′ = n + 1 2 ·T Therefore, the value (n+1) which is obtained by adding "1" to the latched value is soon output from the ROM 14. The value (n+1) is immediately latched by the latch 15. An output of the latch circuit 15 is converted into a serial data train by the P/S converter 16. The clock to determine the reading rate of the serial output data is the output b of the clock generator 18 and its frequency is twice as high as that of the clock c of the counter 12. Therefore, the time width of the serial data read out of the P/S converter 16 is (n+1)/2. This means that this time width is compressed into 1/2 of that of the data rate of the input recording code. Therefore, as shown in Fig. 10E, a time width pulse T′ of 1/2 of the interval between the input recording codes "1" and "1" can be generated as an output of the output terminal 20. On the other hand, in an arrangement shown in Fig. 11, the duty factor 50% can be also obtained by executing a program shown in a flowchart of Fig. 12. A CPU 30 has therein an ROM in which the program is stored, an RAM, a counter, and the like. The CPU 30 receives the RLL encoded recording code (step S1) and detects code 1 (step S2). When code 1 is detected, the counter in the CPU 30 is made operative to start counting (step S3). The counting operation is continued until the next code 1 is detected (steps S4 and S5). The counter counts the pulses in the CPU. When the next code 1 is detected, the count value of the counter at this time is fetched and the counter is reset, thereby obtaining a time width T₁ between codes 1 and 1 (step S6). To output pulses of the time width T′, a level "1" signal is output and the counting is started (step S7). The counting operation is continued until the time of T₁/2 elapses (steps S8 and S9). When the clocks of the number corresponding to T₁/2 are counted, the output of the level "1" signal is stopped and the counter is reset and the counting operation is stopped (step S9). Thus, the recording code of a duty factor 50% is obtained. The foregoing processes are repeated until the conversion of all of the input recording codes is completed (step S10). As mentioned above, in the system for recording the RLL codes in accordance with the NRZL rule, the recording error rate can be certainly reduced. In other words, there is an advantage such that with the same error rate, the recording density can be certainly improved. Table 1 shows an example of conversion into the RLL codes. This example relates to the case where d = 5 and n = 6. The left side in the table denotes the original data and the right side indicates the RLL code.

Claims: A digital data recording method whereby digital data, encoded by an encoding system in which code "1" does not occur twice without an intervening code "0" and run length of the code "0" is limited, is recorded on a recording medium and the code "1" is converted into one pulse and recorded on the recording medium, said method comprising the steps of: inputting the digital data; converting a first pulsewidth (T), corresponding to code "1" in the digital data input in said inputting step, into a second pulsewidth (T1) larger than the first pulsewidth; and recording the digital data in which code "1" has the second pulsewidth on the recording medium; characterised in that: in said converting step, the changed amount of said first pulsewidth (T) is varied in proportion to the number of codes "0" between adjacent codes "1" and "1" in said digital data. A digital data recording method according to claim 1 characterised in that said converting step converts the first pulsewidth (T) into approximately one-half of an interval (T₁) between the leading edges of pulses corresponding to consecutive codes "1". A method as claimed in claim 1 or 2, characterised in that said encoding system is a Run Length Limited encoding system. A method as claimed in claim 3, characterised in that said recording step, the Run Length Limited encoded digital data is recorded in accordance with an NRZ (Non Return to Zero) rule. A method according to any one of claims 1-4, characterised in that said recording step, said digital data is recorded by irradiating a light onto said recoding medium. A method as claimed in claim 5, characterised in that said recording step, the light is irradiated by pulses of the number of said codes "1". A digital data recording apparatus for recording on a recording medium digital data in which code "1" does not occur twice without an intervening code "0" and run length of the code "0" is limited, said apparatus comprising input means for inputting the digital data; code conversion means for converting a first pulsewidth (T), corresponding to code "1" in the digital data input in said inputting step, into a second pulsewidth (T1) larger than the first pulsewidth; and means for recording the digital data as a pulse on the recording medium in which code "1" has the second pulsewidth on the recording medium; characterised in that: said code conversion means is adapted to effect a conversion in which the changed amount of said first pulsewidth (T) is varied in proportion to the number of codes "0" between adjacent codes "1" and "1" in said digital data. A recording medium carrying digital data encoded by an encoding system in which code "1" does not occur twice without an intervening code "0" and the run length of the code "0" is limited, characterised in that each encoded "1" to be read from the medium has a pulsewidth (T1) which changes in proportion to the number of codes "0" recorded on the medium between said each code "1" and an adjacent code "1".

Inventors: SATO HIDEAKI, SATO, HIDEAKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B20
IPC codes: H03M7, G11B7, G11B20