Reproducing apparatus for reproducing plural types of optical discs

Abstract: A reproducing apparatus for reproducing plural types of optical discs comprising: a comparator (2) for digitizing a reproduced signal obtained by reproduction of a first optical disc which is to be reproduced under a first optical condition defined by a first reproducing reference under a second optical condition defined by a second reproducing reference applied to reproduction of a second optical disc having a recording density higher than that of the first optical disc, the comparing operation of the reproduced signal with a reference voltage different from that defined by the first reproducing reference; and a pulse length correction circuit (3-6) for correcting a pulse length of a digital signal outputted from the comparator such that the pulse length of the digital signal is equal to another pulse length which would be obtained by digitizing another reproduced signal using the reference voltage defined by the first reproducing reference, the another reproduce signal being obtained by reproducing the first optical disc in accordance with the first reproducing reference.

Description: This invention relates to a reproducing apparatus for plural types of optical discs whose recording densities are different from each other. When information is recorded on an information recording medium as a digital data, recording is carried out such that the digital signal indicative of the information is modulated as digital data by a specific modulation method selected from various types of modulating methods. In this respect, if the information recording medium is an optical disc, the situation is the same as that of the above-mentioned case. For, example, in an optical disc which is known as a compact disc (CD), recording and reproducing are carried out by modulation using a modulation method (EFM) such that eight data bits are enlarged to fourteen channel bits. In the CD for which the above-mentioned EFM is adopted, digital data is formed on the surface of the CD by a bit train of a modulated signal including nine types of bit lengths which are successively set between the modulated signal of bit length (pulse length or pulse width) 3T corresponding to the minimum interval Tmin between bits and a bit length of 11T corresponding to the maximum interval Tmax between bits. In the above-mentioned recording/reproducing method, digital data including nine types of bit lengths, which are successively set between the modulated signal of the shortest bit length 3T and a longest bit length of 11T, are recorded on the optical disc (CD) as an array of pits. Reproducing (reading out) the information recorded in accordance with the condition of pits arranged on the CD is carried out by projecting and focusing a reproducing light beam (read out light) having a specific wavelength 780nm) on a CD through a condenser lens (objective lens) with a specific numerical aperture (NA=0.45). When the recorded information from the above-mentioned CD is reproduced, the reproducing light is projected on the array of pits provided on the signal plane of the CD such that a portion of a beam waist of the reproducing light condensed by the condenser lens is positioned on a signal plane of the CD, i.e., the signal plane of the CD lies at a focus point of the condenser lens. In-this condition, a reproduced signal is obtained by photoelectric conversion of the reproduced light, reflected and diffracted from the signal plane of the CD in this condition. The above-mentioned reproduced signal, i.e., the reproduced signal obtained when light spot having a given diameter is focused onto the signal plane of the CD so positioned at the focus point of the condenser lens, is supplied to a comparator to which a voltage value which is about half of the amplitude of the reproduced signal, is applied as a comparator reference voltage (threshold voltage) in order to perform demodulation operation. This causes demodulation of digital data which comprises the bit train of a modulated signal including nine types of bit lengths which are successively set between the modulated signal of bit length (pulse length or pulse width) 3T corresponding to the minimum interval Tmin between bits and a bit length of 11T corresponding to the maximum interval Tmax between bits. Then, to demodulate, with high fidelity, the digital data recorded in the condition that various conditions specified in the specification of the CD are satisfied, it is necessary for demodulation that: as mentioned above, the reproducing light (read out light) having a specific wavelength (780nm) through a condenser lens (objective lens) with a specific numerical aperture (NA=0.45) is focused on a CD; and the reproducing light is projected and focused on the array of pits provided on the signal plane of the CD such that a portion of a beam waist of the reproducing light condensed by a condenser lens positions on a signal plane of the CD, in other words, the signal plane of the CD positions at a focus point of the condenser lens. In this condition, in order to demodulate, the reproduced signal obtained from a photodetector is applied to the comparator to which the voltage value which is about half of an amplitude of the reproduced signal is applied. For example, if the wavelength of the reproducing light or the numerical aperture of the condenser lens used in the reproduction apparatus deviates from the values specified in the specification of the CD or a voltage value of the reference voltage (threshold voltage) of the comparator used for demodulation of the reproduction signal deviates over the tolerance from the voltage value which is about half of the amplitude of the reproduce signal, it may be happen that the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set between the modulated signal of bit length (pulse length or pulse width) 3T corresponding to the minimum interval Tmin between bits and a bit length of 11T corresponding to the maximum interval Tmax between bits, cannot be demodulated correctly. Thus, for example, the modulated signal of 3T bit length corresponding the the shortest interval Tmin between bits, which is recorded on the CD may be demodulated as a modulated signal having a bit length other than a bit length of 3T. In the reproducing apparatus of the CD, as the modulated signal of the bit length of 3T corresponding to the shortest interval Tmin between bits should be demodulated as a demodulated signal of 3T bit length for example, in order to obtain the demodulated signal having the same bit length as that recorded on the CD. Thus, the reproducing light beam (read out light) having the specific wavelength (780nm) through a condenser lens with the specific numerical aperture (NA=0.45) is focused on a CD; a reproduced signal is obtained in the condition that a light spot with a given diameter is focused on the array of pits provided on the signal plane of the CD; and the reproduced signal is applied to the comparator to which a voltage value (threshold voltage) which is about half of an amplitude of the reproduced signal is applied in order to perform demodulation operation. Then, in order to correctly demodulate the modulated signal recorded on the optical disc on which information is recorded as the array of pits formed on the signal plane of the optical disc, as clearly described in the above description of the CD mentioned as an example, it is necessary that the diameter of spot of the reproducing light to be used on reproducing of the information signal recorded on the optical disc ranges in the specific values specified in correspondence with the size and form of the pit; and that the comparator reference voltage (threshold voltage) used for demodulating the reproduced signal is the voltage value of about half of the amplitude of the reproduced signal, etc. For the reasons mentioned above, as a reproducing apparatus for plural types of optical discs capable of correct demodulation of the information signal recorded on plural types of optical discs having different recording densities, the reproducing apparatus needs different structures in either or both of a light source of the reproducing light and the condenser lens used, that is, such reproducing apparatus requires different optical heads having light sources of the reproducing light capable of emitting light having different wavelengths and condenser lenses having different numerical apertures respectively. More specifically, because the size of the pit formed on the signal plane of the optical disc becomes smaller with the higher the recording density generally in the case that the recording density is increased but the rotational speed of the optical disc is remained constant, in the reproducing apparatus for obtaining the reproduced signal using diffraction phenomenon of the reproducing light reflected at pits by projecting and focusing the reproducing light onto the signal plane, the spot of the reproducing light having a diameter with a given relation according to an width of the pit in the radial direction of the optical disc to be reproduced is projected and focused onto the signal plane of the optical disc. Moreover, conditions of array of pits formed on the signal plane of the optical disc are different from each other in accordance with the recording density of the optical disc though recording/reproducing is performed for modulation of digital data by the same modulating method for example. In the case that optical discs having different recording densities are used and driven at different rotational speeds, the pit of an optical disc having a higher recording density has pits whose width is smaller than that of an optical disc having a lower recording density. As mentioned, conditions of pit arrays formed on the signal planes of optical discs are different from each other with recording densities of optical discs. Then, because the condition of the light diffraction at the pit portion is totally different if pits of differing sizes are read out with a spot having the same wavelength and the same diameter, it is clear that the reproduced signal output from the photodetectors are different from each other. In order to resolve the above-mentioned problem that the diffraction state of light produced by pits changes with change in relation between the width of the pit and the diameter of the spot used for reading out the pit, it may be considered that a cross-sectional area of the reproducing light on the signal plane of the optical disc is changed with change in condition of condensing of the reproducing light. However, wave front of the reproducing light at the signal plane of the optical disc is disturbed because a cross section of the reproducing light projected onto the signal plane of the optical disc is not the beam waist portion. Therefore, though cross section of the reproducing light at the signal plane of the optical disc is changed in accordance with the above-mentioned resolution method, that is, though the diameter of the reproducing light at the signal plane of the optical disc is formed to have the specific diameter, the reproduced signal cannot be reproduced correctly from the optical disc. Therefore, as a reproducing apparatus for producing plural types of optical discs capable of correct demodulation of the information signal recorded on plural optical discs having different recording densities, the reproducing apparatus needs different read heads in either or both of a light source of the reproducing light and the condenser lens used in the structure of the reproducing apparatus. However, because it is undesirable that in one reproducing apparatus, different read heads are provided for respective types of optical discs having different recording densities, it was considered that a reproducing apparatus used for an optical disc, having a specification that a diameter of a spot of the reproduced light projected on the signal plane of the optical disc in focus on reproducing is smallest was tried to be used for reproducing other type optical discs in common. However, it is obvious that a correctly demodulated signal cannot be obtained though the voltage value of about half of the amplitude of the reproduce signal as the comparator reference voltage (threshold voltage) on demodulation of the reproduced signal is used unless a spot of the reproducing light having a diameter with a given relation according to an width of the pit on the optical disc is projected onto the signal plane of the optical disc in the reproducing apparatus for obtaining a reproduced signal using diffraction phenomenon of reproducing light coming from pits by projecting the reproducing light onto the signal plane. Further, there is also a problem that it is possible that the comparator reference voltage (threshold voltage) cannot be set which is necessary for demodulation of the reproduced signal to obtain a correct demodulated signal. Therefore, a resolution to these problems is required. The above-mentioned point will be described more specifically. It is assumed that one of plural optical discs having different recording densities is a CD wherein reproducing light of 780 nm wavelength and a condenser lens of 0.45 numerical aperture as mentioned above are used. Also it is assumed that another optical disc capable of higher recording density than the CD is reproduced using light of 670 nm wavelength as reproducing light and a condenser lens of 0.50 numerical aperture. When a reproducing apparatus using the reproducing light of 670 nm wavelength and the condenser lens of 0.50 numerical aperture, i.e., a reproducing apparatus for reproducing the optical disc having a recording density which is higher than that of the CD reproduces a CD, the reproduced signal from the reproducing apparatus has an eye pattern shown in Fig. 5. Then, in the case that the reproduced signal as shown in Fig. 5 is supplied to the comparator to which the voltage value of about half of the amplitude of the reproduced signal as a comparator reference voltage (threshold voltage) for demodulation operation, that is, in the case that a comparator reference signal (threshold voltage) of the line S-S shown in Fig. 5 is used to demodulate, the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set to either one between the modulated signal of bit length (pulse length or pulse width) 3T corresponding to the minimum interval Tmin between bits and a bit length of 11T corresponding to the maximum interval Tmax between bits, cannot be demodulated correctly. In Fig. 5, a comparator reference voltage (threshold voltage) capable of correct demodulation of the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set to either one between modulated signal of bit length (pulse length or pulse width) 3T corresponding to the minimum interval Tmin between bits and a bit length of 11T corresponding to the maximum interval Tmax between bits is a voltage shown by the ling A-A in Fig. 5 when the reproduced signal is that one shown in Fig. 5. As clearly shown by the waveform of the reproduced signal in Fig. 5, a stable demodulated signal cannot be obtained at the line A-A. Therefore, the reproducing apparatus used for the optical disc defined by a specification such that a diameter of the reproducing light projected onto the signal plane of the optical disc in focus in the reproducing modes smallest cannot be used for other types of optical discs having different recording densities in common. "Large Scale Integrated Circuit System for Digital Audic. Disc Player" IEEE Transaction on Consumer Electronics VOL CE-28 No. 3 Aug 1982 pp 360-362 describes a chipset for a CD player. This includes an Automatic Data slice level control which adjusts the slice level against which the playback signal is compared so that the pulsed data signal is symmetric. US 4,887,041 describes a pulse width correction circuit for a transmission line that detects the duty ratio of the signal and modifies its correction accordingly. The present invention has been developed in order to alleviate the above-described drawbacks inherent to the conventional reproducing apparatus for reproducing plural types of optical discs. A reproducing apparatus adapted to reproduce data from an optical information recording disc, the apparatus comprising: means for reproducing a reproduced signal from said optical information recording disc; and digital signal generation means including: a comparator for generating a digital signal by comparing said reproduced signal with a comparator reference voltage; and a feedback circuit for providing a feedback signal as said comparator reference voltage; characterised in that said digital signal generation means further comprises: a pulse length correction circuit for correcting the pulse length of the output of said comparator to output a pulse length corrected digital signal, said pulse length correction circuit comprising a delay element supplied with the output of the comparator and an AND circuit supplied with the output of the comparator and the output of the delay element; and a low pass filter for low pass filtering said pulse length corrected digital signal output by said pulse length correction circuit to generate said feedback signal; and in that said pulse length corrected digital signal is output from said apparatus. A digital signal is generated by comparing the reproduced signal by the comparator to which the comparator reference voltage is applied, the comparator reference voltage being is different from that determined by a first reproducing reference, the reproduced signal being obtained by reproducing a first optical information recording medium disc, which is to be reproduced under the first reproducing reference under a second optical reproducing condition determined by a second reproducing reference which is applied to reproduction of a second optical information recording medium disc having a recording density higher than that of the first optical information recording medium disc. A pulse length of the digital signal generated as mentioned above is so corrected to have a regular pulse length of the digital signal by the pulse length correction circuit because the pulse length of the reproduce signal reproduced under the first reproducing reference mentioned above is different from another pulse length which would be obtained by digitizing the regular reproduced signal using the comparator reference voltage determined by the first reproducing reference. The object and features of the present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: Fig. 1 is an embodiment of the reproducing apparatus for reproducing plural types of optical information recording discs of the invention; and Fig. 2 shows the eye pattern of the reproduced signal input to the circuit arrangements of Figs. 1. The same or corresponding elements or parts are designated as like references throughout the drawings. Recording of digital data in an optical disc for which EFM (Eight to Fourteen Modulation) is adapted as a modulation method is carried out by forming of pit arrays according to a modulated signal including nine types of bit lengths which are set successively to either one between the modulated signal of bit length (pulse length or pulse width) 3T corresponding to the minimum interval Tmin between bits and a bit length of 11T corresponding to the maximum interval Tmax between bits, wherein T is a cell length of channel bit. When the above-mentioned optical disc is a CD, reproducing (reading out) of the recorded information on the CD is carried out using an optical system comprising a condenser lens (objective lens) of 0.45 numerical aperture for reproducing light of 780 nm wavelength. A reproduced signal obtained by the optical system or optical head (not shown) including a photodetector (not shown) is applied to a comparator to which a comparator reference voltage (threshold voltage) of about half of an amplitude of the reproduced signal is applied. This enables correct reproducing of the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set to either one between the modulated signal of bit length (pulse length or pulse width) 3T of the minimum interval and a bit length of 11T of the maximum interval. In addition to this, when the reproduction of the CD is carried out by the reproducing apparatus so built up that a light spot projected and focused onto a signal plane of an optical disc has a diameter smaller than that projected onto the signal plane of the CD in order to be suited for reproduction of an optical disc with a recording density higher than that of the above-mentioned CD, the digital data comprising the bit train of the modulated signal including nine types of bit lengths which are successively set to either one between the modulated signal of bit length (pulse length or pulse width) 3T of the minimum interval and a bit length of 11T of the maximum interval cannot be correctly reproduced though the reproduced signal is applied to the comparator to which the comparator reference voltage (threshold voltage) of about half of an amplitude of the reproduced signal is applied as mentioned above. The reproducing apparatus for reproducing plural types of optical information recording medium discs of the invention has a structure such that: reproducing operation can be carried out in different optical reproducing conditions from a specific optical reproduction condition specified for reproducing an optical disc to be reproduced by the optical head; and that signal processing is so carried out that a correct digital signal is demodulated though reproduction is made in the different optical reproducing condition from a specific optical reproduction ccondition specified for reproducing the optical disc to be reproduced. This is similar to the above-mentioned case that a CD is reproduced by a reproducing apparatus having structure such that: a light spot projected onto a signal plane of an optical disc (not shown), whose diameter is smaller than that projected onto the signal plane of the CD. Hereinbelow will be described an embodiment of the reproducing apparatus for reproducing plural types of optical information recording disc of the invention shown in Fig 1. In Fig. 1, the-reproduced signal supplied from the optical head (not shown)is inputted to the non-inverting input terminal of the comparator 2 with dc components cut off by a capacitor 15. The non-inverting input terminal of the above-mentioned comparator 2 is also supplied with a given voltage through a resistor 16. The given constant voltage supplied to the input terminal 14 is a constant de voltage of 2.5 volts if a digital signal outputted from the comparator 2 is of a logic low level of 0 volt or a logic high level of 5 volts. The inverting input terminal of the comparator 2 is supplied with an output of a low-pass filter 5. A comparing output signal Sa outputted from the above-mentioned comparator 2 is supplied to the AND circuit 3 and to a delay element 18. The output signal Sb from the above-mentioned delay element 18 is sent to the AND circuit 3. An output signal Sc from the AND circuit 3 is sent to the output terminal 7 and to the low-pass filter 5 mentioned above. The low-pass filter 5 supplies low-frequency component of the output signal Sc of the above-mentioned AND circuit 3 to the inverting input terminal of the comparator 2. Hereinbelow will be described structure and operation of the circuit arrangement shown in Fig. 1 on the condition that the reproduced signal supplied to the non-inverting input terminal of the comparator 2 from the input terminal 1 through the capacitor 15 is a signal whose eye pattern is shown in Fig. 2, that is, on the condition that, when the signal recorded on the CD is a modulated signal of 3T pulse length and 3T-bar pulse length modulated signal which are continuous in the time base, the original consecutive signal of modulated signal of 3T pulse length above-mentioned and the modulated signal of pulse length of 3T-bar is changed into the reproduced signal shown in Fig. 2, that is, its pulse length is changed into lengths of the reproduced signal around a signal level (S-S in Fig. 2) of about half of the amplitude of the reproduced signal, namely, an interval of S1 to S2 (4.2T) and an interval of S2 to S3 (1.8T). At the instance that the circuit arrangement shown in Fig. 1 starts its operation, though the comparator reference voltage supplied to the inverting input terminal of the comparator 2 from the low-pass filter 5 is not determined, the comparator 2 supplies the comparing output signals Sa obtained as a comparing result with respect to the reproduced signal shown in Fig. 2 with reference to the comparator reference voltage supplied from the low-pass filter 5 as that instance and sends it to the pulse length correction circuit comprising the AND circuit 3 and the delay element 18. The above-mentioned AND circuit 3 outputs a signal Sc of logic-and between the comparing output signal Sa outputted from the comparator 2 and the signal Sb outputted from the delay element 18. In the embodiment shown in Fig. 1, the dc components of the output signal Sc outputted at the output terminal 7 from the AND circuit 3 in the pulse length correction circuit is supplied to the inverting input terminal of the comparator 2 as a comparing reference voltage. Then, if the delay time of the delay element 18 in the pulse length correction circuit is set to for example, 1T, the comparator reference voltage supplied to the non-inverting input of the comparator 2 mentioned above is automatically controlled so that the output comparing signal outputted from the above-mentioned comparator 2 has always the shortest up-going pulse of pulse length 4T and the shortest down-going pulse of pulse length 2T by operation of a loop stating from the comparator 2, through the pulse length correction circuit comprising the delay element 18 and AND circuit 3, the low-pass filter 5, and returning to the comparator 2. The pulse length correction circuit comprising the 1T delay element 18 and the AND circuit 3 converts the up-going pulse of pulse length 4T and down-going pulse of pulse length 2T-bar into the up-going pulse of pulse length 3T and down-going pulse of pulse length 3T-bar and sends them to the output terminal 7. It is a matter of course that a signal whose pulse length is other than 3T can be obtained in a good condition as an output signal. The circuit arrangement shown in Fig. 1 includes a feedback circuit respectively. The detailed description about design in gain characteristic and phase characteristic, etc., of the feedback circuit is omitted because they are generally known. In the embodiment of the reproducing apparatus for reproducing plural types of the optical information recording medium discs of the invention described with reference to Fig 1 the demodulated signal outputted from the comparator, whose pulse length is not a regular is corrected (converted) to another demodulation signal of the regular pulse length by the pulse length correction circuit In the embodiment of the reproducing apparatus for reproducing plural types of the optical information recording medium discs of the invention of Fig. 1, the demodulated signal 1 irregular pulse outputted from the comparator 2 is corrected (converted) into another demodulation signal having the regular pulse length by the pulse length correction circuit and then the converted data is applied to a digital demodulation table and then, the final demodulated digital data is obtained from the digital demodulation conversion table. As described in detail above, the reproducing apparatus for reproducing plural types of the optical information recording medium discs of the invention provides a good demodulation signal readily as follows: A digital signal is generated by comparing the reproduced signal by comparator to which a comparator reference voltage is applied, the comparator reference voltage being is different from that determined by a first reproducing reference, the reproduced signal being obtained by reproducing an optical information recording medium disc, having lower recording density, to be reproduced under the first reproducing reference under a second optical reproducing condition which is applied to reproduction of another optical information recording medium disc having a recording density higher than the optical information recording medium disc which is to be reproduced under the first reproducing reference. A pulse length of a digital signal generated as mentioned above is so corrected to have a regular pulse length of the digital signal by the pulse length correction means because the pulse length of the reproduce signal reproduced under the first reproducing reference mentioned above is different from another pulse length which would be obtained by digitizing the regular reproduced signal using the comparator reference voltage determined by the first reproducing reference. Moreover, in the case that the digital signal is obtained by application of the reproduced signal obtained by reproduction of an optical disc having a lower recording density under a second reproducing condition which is applied to reproduction of-another optical disc having a recording density higher than that of the above-mentioned optical information recording medium disc which is to be reproduced under the first reproducing reference, a digital signal of regular pulse length is outputted from the comparator by feeding back the digital signal outputted at the comparator through the pulse length correction circuit and the feedback circuit as a comparator reference voltage of the comparator so that a pulse length of the reproduced signal obtained by reproducing under the first reproducing reference of the optical disc to be reproduced under the first reproducing reference is the same as a pulse length of the digital signal obtained by digitizing using the comparator reference voltage determined by the above-mentioned first reproducing reference. Further, a digital signal is generated by comparing the reproduced signal with a comparator reference voltage which is different from that determined by a first reproducing reference, the reproduced signal being obtained by reproducing an optical information recording medium disc, having lower recording density, to be reproduced under the first reproducing reference under a second optical reproducing condition which is applied to reproduction of another optical information recording medium disc having a recording density higher than the optical information recording medium disc which is to be reproduced under the first reproducing reference. Moreover, a digital signal is generated by comparing the reproduced signal by comparator to which a comparator reference voltage is applied, the comparator reference voltage being is different from that determined by a first reproducing reference, the reproduced signal being obtained by reproducing an optical information recording medium disc, having lower recording density, to be reproduced under the first reproducing reference under a second optical reproducing condition which is applied to reproduction of another optical information recording medium disc having a recording density higher than the optical information recording medium disc which is to be reproduced under the first reproducing reference. Therefore, formerly, in a conventional reproducing apparatus for correctly reproducing and demodulating the information signal recorded in a plural different optical discs whose recording densities are different from each other, the reproducing apparatus requires different sets of one or both of a light source of a reproducing light and a condenser lens used in the reproducing apparatus. That is, different structures where light sources emitting different wavelength light and different condenser lens having different numerical apertures were required. On the other hand, in this invention, there are no requirements mentioned above. Thus, in the reproducing apparatus used for reproducing recorded information on each of optical discs whose recording densities are different from each other, this invention enables a reproducing apparatus used for an optical disc for which a diameter of a spot for reproducing light projected on the optical disc in focus on reproducing mode is specified to be used also for reproduction of other optical disc to be reproduced in the reproducing apparatus. Therefore, according to this invention, all problems of-the prior art can be suitably resolved.

Claims: A reproducing apparatus adapted to reproduce data from an optical information recording disc, the apparatus comprising: means for reproducing a reproduced signal from said optical information recording disc; and digital signal generation means including: a comparator (2) for generating a digital signal by comparing said reproduced signal with a comparator reference voltage; and a feedback circuit for providing a feedback signal as said comparator reference voltage; characterised in that said digital signal generation means further comprises: a pulse length correction circuit (18, 3) for correcting the pulse length of the output of said comparator (2) to output a pulse length corrected digital signal, said pulse length correction circuit comprising a delay element (18) supplied with the output of the comparator (2) and an AND circuit supplied (3) with the output of the comparator (2) and the output of the delay element (18); and a low pass filter (5) for low pass filtering said pulse length corrected digital signal output by said pulse length correction circuit to generate said feedback signal; and in that said pulse length corrected digital signal is output from said apparatus. A reproducing apparatus according to claim 1 wherein said feedback circuit is operative to generate said comparator reference voltage so as to generate said digital signal such that the pulse length of an up-doing pulse is made longer and the pulse length of a down-going pulse is made shorter by an interval of N, where N is a natural number more than one, channel bits or to generate the digital signal such that the pulse length of said up-going pulse is made shorter and the pulse length of said down-going pulse is made longer by said interval of N channel bits; and said pulse length correction means is operative to correct a pulse length of a digital signal output from said digital signal generation means by said interval of N, where N is a natural number more than one, channel bits. A reproducing apparatus according to claim 1 wherein said means for reproducing uses, to reproduce data from said disc, light of wavelength less than about 780 nm projected onto said disc by a lens of numerical aperture greater than about 0.45.

Inventors: SHIKUNAMI JUICHI, SHIKUNAMI, JUICHI
Applicants: VICTOR COMPANY OF JAPAN, VICTOR COMPANY OF JAPAN, LIMITED
Application year: 1991
Main classification: G11B20
IPC codes: G11B7, G11B20