Optical disc recording apparatus and method

Abstract: An optical disc recording apparatus for recording information signals on an optical disc having an optical recording medium with a larger thermal time constant, such as an organic dyestuff base material, by irradiating the disc with a recording light modulated by signals produced by eight fourteen modulation of information signals, is disclosed. The apparatus includes a recording pulse forming unit for forming recording pulses from the eight fourteen modulated signals, which recording pulses are of pulse widths shorter than the eight fourteen modulated signals by a pulse width corresponding to the elongation from the regular length of a pit formed on the recording medium on the basis of the eight fourteen modulated signals, and an optical head supplied with the recording pulses formed by the pulse forming unit for producing the light which has been eight pulse width modulated by the recording pulses.

Description: This invention relates to an optical disc recording apparatus wherein base signals are recorded on an optical disc having a recording medium by a pulse width light modulated recording light. Compact discs (CDs) in which digital audio signals are recorded on a read-only optical disc are now in widespread use. An optical disc has also been evolved in which the recording layer is formed by an optical recording medium making use of the optical or magneto-optical signal recording and reproducing principle. A write-once optical disc (CD-WO) on which the information can be written only once or an overwritable magneto-optical disc (CD-MO) on which data can be rewritten, both compatible with the compact disc, are also finding practical use and application. The recording layer of the write-once optical disc (CD-WO) is formed by an optical recording medium, the state of which is changed by heat produced by the irradiated recording light beam; the recording medium is, for example, an organic dyestuff type optical recording medium. With the optical disc recording/reproducing apparatus for recording or reading data on or from a write-once disc or an overwritable disc, as the optical disc is rotated at a constant angular speed or a constant linear speed, the recording track of the optical disc is scanned by a light beam emitted by an optical head which incorporates a laser diode driven by a laser driving circuit for outputting the laser light for recording and/or reproducing the information, a photodetector for detecting the irradiating laser light reflected by the optical disc, and so forth, for recording and/or reproducing the digital data. It is known that, when the data are recorded on the optical disc of the optical disc recording and/or reproducing apparatus, the error rate depends in general on the power of the laser light used for writing the information, and that it is necessary to write the information at an optimum laser power. When recording the CD signals on the optical discs, such as CD-WO or CD-MO, it is also necessary to produce a playback output consistent with the CD standards for assuring compatibility of the CD-WO or CD-MO with respect to the established read-only compact disc. When baseband signals are recorded on an optical disc the recording layer of which has been formed by an optical recording medium whose state may be changed by heat produced by the irradiated recording light which has been pulse width modulated by the baseband signals, a recording delay may be caused as a result of thermal time constants of the optical recording medium, with the result that the pit length on the recording layer becomes longer than the regular length corresponding to the baseband signals. That is, as shown in figure 1 of the accompanying drawings, when the recording light having a width corresponding to the pulse width of the baseband signal is irradiated onto the disc, with the baseband signal being a CD signal represented by the EFM data (eight to fourteen modulated data) having pulse widths of from 3T to 11T, where T indicates a clock bit duration, the time delay τ B which elapses after the end of irradiation of the recording light until the end of formation of a pit Pa is longer than the time delay τ F which elapses between irradiation and the start of formation of the pit on the recording layer. The result is that the length L PA of the pit actually formed on the recording layer becomes longer than the correct length L PO of the regular pit P O corresponding to the pulse width of the baseband signals. Above all, with the optical disc having its recording layer formed by an optical recording medium having a significant thermal time constant, the recording pit pattern is drastically affected by the recording delay due to thermal time constants of the optical recording medium. Hence, with the CD-WO optical disc in which CD signals represented by the EFM data with pulse widths of from 3T to llT are recorded on the recording medium of significant thermal time constant, the centre line LS of the eye pattern of the playback output is not in a correct matching state as shown in figure 2, while a modulation factor of 60% or more cannot be achieved with the preservation of the asymmetry in accordance with the CD standards unless recording is conducted at an excessive power. On the other hand, the recording layer formed by the organic dyestuff base optical recording medium has a larger time constant and tends to be increased in push-pull on pit formation, such that, should recording be conducted at an excessive power, it becomes extremely difficult to maintain the push-pull (mean value) according to the CD standards within the range of from 0.04 to 0.07. EP-A-0,289,260 describes a device for producing pits on an optical disc, the device controlling the operation of a laser such that the starting point and duration of operation is adjusted according to the length of the pit required and the length of the preceding blank length whenever it is formed. It is therefore an object of the present invention to provide an optical disc recording apparatus according to which recording in accordance with the CD standards may be performed on an optical disc having a recording medium having a significant thermal time constant, such as an organic dyestuff base optical recording medium. The invention is specified in the appended claims. The preamble of the claims is based on EP-A-289 260. Thus, in accordance with the present invention, recording may be made on the optical disc having a recording medium with a significant time constant such as the organic dyestuff base optical recording medium meeting with the CD standards. The invention will be further described by way of non-limitative example with reference to the accompanying drawings in which:- Figure 1 shows a pit pattern formed on an optical disc by a conventional optical disc recording apparatus. Figure 2 shows an eye pattern of the playback output of the pit pattern shown in figure 1. Figure 3 is a block diagram showing the arrangement of a recording/reproducing system of an optical recording and/or reproducing apparatus to which the present invention is applied. Figure 4 is a block diagram showing a concrete construction of a recording pulse generator of the optical disc recording apparatus according to the present invention. Figure 5 is a time chart showing the operation of the recording pulse generator. Figure 6 shows a pit pattern formed on the optical disc by the optical disc recording apparatus according to the present invention. Referring to the drawings, an optical disc recording apparatus according to the present invention will be explained in detail. In figure 3, there is shown in a block diagram the construction of a recording/reproducing system when the present invention is applied to an optical disc recording/reproducing apparatus in which digital data is optically recorded and/or reproduced in accordance with the data format consistent with the CD standards. The overall construction of the optical disc recording/reproducing apparatus is first explained with reference to figure 3. With the optical disc recording/reproducing apparatus, shown in figure 3, digital data are optically recording and/or reproduced on or from a write-once optical disc 1 formed of an organic dye base optical recording medium having a significant thermal constant, in accordance with the data format consistent with the CD standards. The optical disc 1 is rotated at a predetermined rpm by means of a spindle motor 3 under control of a motor control unit 2. With the optical disc 1, the recording and reading of the digital data in accordance with the data format consistent with the CD standards is performed by an optical head 4 operating as optical information recording and reading means. The optical head 4 has enclosed therein a laser diode driven by a laser driving circuit 14 as later described to effect irradiation of an information recording and reading laser beam, a photodetector for detecting the laser light irradiated by the laser diode and reflected by the optical disc 1, or the like. The optical head 4 is moved as a whole along a radius of the disc 1. The optical head 4 is so designed that the recording track on the optical disc 1 is scanned by the laser light output from the laser diode whilst focusing servo and tracking servo are continuously applied by the operation of head servo circuit 6 driven and controlled by a system controller 5. The recording system of the optical disc recording/reproducing apparatus includes an encoder 12 for converting digital data D IN supplied via an input/output interface 11 and an input terminal 10 to be recorded into a record data string of a data format consistent with the CD standards, a recording pulse generator 13 for producing writing pulses corresponding to the record data string to the laser drive circuit 14, and the like. The laser diode of the optical head 4 is pulse-driven in a pulse-width-modulated fashion by recording pulses corresponding to the record data string by the laser drive circuit 14 for recording the digital data D IN on the recording track of the optical disc 1 as the record data string having a data format consistent with the CD standards. The reproducing system of the optical disc reproducing apparatus includes a reading signal processor 22 to which information readout signals obtained at the optical disc 1 as the detection output of the photodetector of the optical head 4 are supplied by means of an amplifier 21. The reading signal processor 22 performs a decoding operation corresponding to the encoding operation by the encoder 12 of the recording system on the information readout signals supplied from the optical head 4, to reproduce play or address data corresponding to the recorded contents on the disc 1. The play data reproduced at the reading signal processor 22 are filtered in a suitable manner by a digital filter 23 and thence supplied to a digital to analog converter 24. The converter 24 causes the play data to be converted into corresponding analog signals which are output as reproduced audio signals at output terminal 25. The address data or the like, reproduced by the readout signal processor 22, are supplied to the system controller 5. To this system controller 5, there are connected an input operating section 7 for setting operational modes of the optical disc recording and/or reproducing apparatus and a random access memory 8 for storage of a variety of control data or the like. The system controller 5 controls the operations of the head servo section 6, recording pulse generator 13, laser driving circuit 14 and the readout signal processor 22 under the operational mode preset by the input operating section 7. The CD signal obtained by conversion at the encoder 12 of digital data D IN supplied at the input terminal 10 via input output interface 11, which CD signal is represented by a record data string having the data format consistent with the CD standards, that is the EFM data having pulse widths of from 3T to 11T, T designating the clock bit interval, is directly used at the recording pulse generator 13 as the recording pulses for pulse width modulation of the laser diode of the optical head 4 by the laser driving circuit 14 to produce the recording laser light which is then irradiated on the optical disc 1. In this manner, a recording pulse having a pulse width shorter by a pulse width corresponding to the elongation from the regular length of the pit formed on the disc 1 is produced and used for recording. Referring to figure 4 showing the concrete construction of the recording pulse generator 13, the generator 13 includes a 6 bit output shift register 13A, to a data input terminal of which the above mentioned EFM data are supplied, and a 7 bit input encoder 13C to which 6 bit output data Q a , Q b , Q c , Q d , Q e and Q f from the shift register 13A are directly input as the first to sixth bit input data Q A , Q B , Q C , Q D , Q E and Q F and to which the second bit output data Q b of the shift register 13A is input as the seventh bit input Q G by means of a delay circuit 13B. The shift register 13A has its data input terminal supplied with EFM data from the encoder 12 and its clock input terminal supplied with bit clocks for the EFM data. As shown in figure 5, the shift register 13A outputs 6 bit output data Q a , Q b , Q c , Q d , Q e and Q f which are the EFM data sequentially shifted each by one clock bit period T by bit clocks. The delay circuit 13B affords a delay of the order of 20 to 24 nsec to the second bit output data Q b of the shift register 13A. The delay output data DLQ b , obtained by means of the delay circuit 13B, is supplied as the seventh bit input Q G to the 7 bit input encoder 13C. The encoder 13C is constituted by a programmable logic array for performing a conversion operation Q OUT = Q B ·Q C (Q B ·Q C ·Q D ·Q E ·Q F ) + Q G ·Q C ·(Q B ·Q C ·Q D ·Q E ·Q F ) = Q b .Q c .(Q b .Q c .Q d .Q e .Q f ) + DLQ b ·Q c (Q b ·Q c ·Q d ·Q e ·Q f ) By such conversion, the encoder 13c processes the EFM data having pulse widths of from 4T to 11T to generate recording pulses Q OUT with pulse widths of (n-1)T from the EFM data with pulse widths of nT, while generating a recording pulse Q OUT with a pulse width of (2+DL)T from the EFM data with a minimum transition distance T MIN equal to 3T. The laser driving circuit 14 performs a pulse width modulation on the laser diode of the optical head 4 by a recording pulse Q OUT generated by the recording pulse generator 13. In this manner, the recording laser light is produced by pulse width modulation of the laser diode of the optical head 4 and irradiated on the optical disc 1 to form pits having lengths corresponding to pulse widths of the recording pulses Q OUT on the optical disc 1 for recording. Tests have been conducted for generating recording pulses Q OUT from the EFM data having the pulse widths of from 4T to 11T so that the recording pulses Q OUT are unanimously shorter by one clock bit distance than the corresponding EFM data, and recording the recording pulses thus produced on the optical disc formed by the organic dyestuff base optical recording medium. It has been shown that, as a result of pit elongation due to the thermal time constants of the optical recording medium, pits having the regular pit lengths of from 4T to 11T can be formed, as shown in figure 6. As regards the EFM data having the minimum transition distance T MIN =3T, it has been shown that a pit having a minimum pit distance of 3T consistent with the CD standards can be formed by forming a recording pulse Q OUT having a pulse width of (2+DL)T which is longer, by about 20 to 40 nsec, than the pulse width which is shorter by one bit distance T than 3T, and using the thus produced recording pulse for recording. With the above described optical recording and/or reproducing apparatus, a satisfactory recording with a duty cycle close to 50% can be made with stability on the optical disc 1 formed by the organic dyestuff base optical recording medium, such that a pit can be formed in accordance with the CD standards with a push pull value of not more than 0.07% and a modulation factor of not less than 60% under maintenance of asymmetry. The readout laser beam having the same beam spot diameter as that of the recording laser beam is irradiated by the optical head 4 on the optical disc 1 for reading out the information to reproduce the data at the readout signal processor 22 at approximately the same error rate as that of the conventional CD dedicated to signal reproduction.

Claims: An optical disc recording apparatus for recording information data by irradiating a recording medium of an optical disc (1) with light pulse-width modulated by eight to fourteen modulated (EFM) data signals, the recorded signals being subjected to elongation due to a recording time constant characteristic of the recording medium, said apparatus comprising: recording pulse generating means (13) for generating recording pulses from said eight to fourteen modulated information data signals; and an optical head (4) supplied with the recording pulses generated by said pulse generating means (13) and producing the recording light which has been light pulse-width modulated by said recording pulses, said recording pulses have pulse-widths shorter than the pulse-widths (L po ) of the corresponding eight to fourteen modulated information data signals recorded on the recording medium by a first amount, corresponding to the elongation of the un-elongated length of the corresponding recorded signal which would have been formed on the recording medium by light modulated with said eight to fourteen modulated data signals in the absence of said elongation; characterised in that: said recording pulse generating means generates pulses having pulse-widths of (n-1)T, for 4≤n≤11 and (2+DL)T for n=3, where T is a clock interval corresponding to said first amount and DL*T is a predetermined time duration such that said recording pulses corresponding to the minimum transition distance of said eight to fourteen modulated information data are longer by said predetermined time duration than the pulse width which is shorter by said first amount. A method for recording information data onto an optical disc by irradiating a recording medium of such an optical disc (1) with light pulse-width modulated by eight to fourteen modulated (EFM) data signals, the recorded signals being subjected to elongation due to a recording time constant characteristic of the recording medium, said method comprising: generating recording pulses from said eight to fourteen modulated information data signals; supplying said recording pulses to an optical head for producing recording light which is modulated by said recording pulses, wherein said recording pulses have pulse-widths shorter than the pulse-widths (L po ) of the corresponding eight to fourteen modulated information data signals recorded to the elongation of the un-elongated length of the corresponding recording signals which would have been formed on the recording medium by light modulated with said eight to fourteen modulated data signals in the absence of said elongation; characterised by: generating pulses have pulse widths of (n-1)T for 4≤n≤11 and (2+DL)T for n=3, where T is a clock interval corresponding to said first amount and DL*T is a predetermined time duration such that said recording pulses corresponding to the minimum transition distance of said eight to fourteen modulated information data are longer by said predetermined time duration than the pulse width which is shorter by said first amount.

Inventors: IIMURA SHINICHIRO, OGAWA HIROSHI, IIMURA, SHINICHIRO, OGAWA, HIROSHI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: G11B7
IPC codes: G11B7, G11B20