GB2098379A

GB2098379A - Circuit arrangement for a disk player for reproducing information prerecorded in the form of pits

Info

Publication number: GB2098379A
Application number: GB8213031A
Authority: GB
Original assignee: Victor Company of Japan Ltd; Nippon Victor KK
Current assignee: Victor Company of Japan Ltd; Nippon Victor KK
Priority date: 1981-05-12
Filing date: 1982-05-06
Publication date: 1982-11-17
Also published as: FR2506053A1; DE3217482A1; JPS57186216A; KR840000017A

Abstract

In a circuit arrangement for a disk player of the type arranged to reproduce information prerecorded in the form of pits, a rectangular wave signal is produced by processing the sinusoidal-like wave signal produced by a pickup 2 scanning the pits on spiral or coaxial tracks on a prerecorded disk 12. The average D.C. voltage of the rectangular wave is produced by a circuit 11 and fed back to an input of the circuit 10 which produces the rectangular wave signal so as to automatically change the threshold thereof. Since the threshold is changed in accordance with the output rectangular wave signal, data errors due to distorted sinusoidal signal caused by dull or blunt edged pits are effectively prevented, providing uniform and accurate reproduction of data throughout the entire disk. <IMAGE>

Description

SPECIFICATION Circuit arrangement for a disk player for reproducing information prerecorded in the form of pits BACKGROUND OF THE INVENTION This invention relates generally to a disk player used for reproducing various information from a disk in which the information has been prerecorded in the form of pits on spiral or coaxial tracks.

Recently, PCM recording techniques show its development where a signal such as an audio signal is converted into a PCM signal (or the PCM signal is further frequency modulated) to be directly recorded as an intermittence of pits onto a disk. With such a disk, since reproduction is effected with the rotational speed of the disk fixed irrespective of the reproducing position of the pickup device, the relative velocity between the reproducing stylus and the disk at an outer portion of the disk is greater than that at an inner portion of the same. Here, the outer and inner portions of a disk respectively mean a portion close to the periphery and a portion close to the center of the disk.Therefore, in order to reproduce the signal with the same condition throughout the entire of the disk, it is needed to make the length of pits in the direction of the rotation shorter for the inner portion (last 1 5 minutes or so in a disk having reproducing time length of 60 minutes per one side) than for the outer portion.

During manufacture of disks, so called dull or blunt-edge portions are apt to occur at the periphery of each pit due to a limit in beam radiation techniques. Therefore, it is difficult to form, with high precision, rows of pits each having a small length at the inner portion of the disk. The pits on spiral or coaxial tracks of an information recorded disk are reproduced by scanning the same with a stylus of a pickup so as to produce a sinusoidal wave signal. And then, the reproduced sinusoidal wave signal is waveform shaped to produce a rectangular wave signal.

'However, due to the above-mentioned dull edge of the pits positioned at the inner portion, the waveform of the reproduced sinusoidal signal corresponding to such pits are apt to be distorted, while the reproduced waveform corresponding to pits positioned at the outer portion does not suffer from distortion to result in normal sinusoidal waveform even if identical signals have been respectively recorded at the outer and inner portions. Like the above, the phenomenon that the reproduced waveform corresponding to a pit in the inner portion is distorted is caused by not only the reason described in the above but also by the fact that the reproduced signal corresponding to a pit in the inner portion lowers especially in a high frequency range.

In conventional reproducing apparatus, since no circuit for compensating for the distortion of the reproduced waveform corresponding to a pit positioned at the inner portion has been provided, waveform interference (carrier leak) occurs in the output of a demodulator due to the distortion in the waveform, and as a result of this, conventional reproducing apparatus has suffered from a drawback that signals cannot be accurately reproduced at the inner portion of a disk because of data errors caused by difficulty in reading reproduced data.

SUMMARY OF THE INVENTION The present invention has been developed in order to remove the above-described drawbacks inherent to the conventional circuit arrangement for disk players or disk reproducing apparatus.

It is, therefore, an object of the present invention to provide a circuit arrangement for a disk player which is capable of reproducing uniformly and correctly the data or information prerecorded in a disk throughout the entire areas of the disk.

According to a feature of the present invention the threshold voltage of a waveform shaping circuit, which is responsive to picked up signals from a disk, is made variable in accordance with the average D.C. voltage at the output terminal of the waveform shaping circuit. In order to change the threshold or reference voltage, an integrator having a predetermined time constant is employed.

In accordance with the present invention there is provided a circuit arrangement for a disk player of the type arranged to reproduce information prerecorded in the disk in the form of pits, comprising: first means responsive to a reproduced signal from the disk for producing a rectangular wave signal on the basis of a threshold; second means for producing a signal indicative of the average D.C. voltage of the rectangular wave signal; and third means for feeding back the signal from the second means to the first means in such a manner that the threshold is changed in accordance with the average D.C. voltage.

BRIEF DESCRIPTION OF THE DRAWINGS The object and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Figs. 1 A and 1 D are waveform charts useful for understanding the operation of the circuit arrangement according to the present invention; Fig. 2 is a schematic block diagram of a first embodiment of the circuit arrangement according to the present invention; and Fig. 3 is a schematic block diagram of a second embodiment of the circuit arrangement according to the present invention.

The same or corresponding elements and parts are designated at like reference numerals throughout the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior to describing the embodiments of the present invention, the above described drawback inherent to the conventional circuit arrangement will be discussed for a better understanding of the present invention with reference to Figs. 1 A through 1D.

Figs. 1 A and 1 C show waveforms of reproduced signals "a" and "c" developed at a pickup which scans spiral or coaxial tracks of a disk in which information has been prerecorded in the form of pits. When the pickup scans pits at the outer portion, which is close to the periphery, of a disk, the waveform of the reproduced signal "a" is normal as shown in Fig. 1 A. Namely, the waveform of such a reproduced signal "a" is very close to a sinusoidal wave. On the other hand, when the pickup scans pits at the inner portion, which is closed to the center of the disk, the waveform of the reproduced signal "c" suffers from distortion as shown in Fig. 1 C because of dull or blunt-edged pits.As a result, when the reproduced signal "c" is processed to produce a rectangular wave signal "d" the width of each rectangular wave or pulse is undesirably narrowed due to such a distorted reproduced signal "c" as shown in Fig. 1 D. 1 B shows a normal rectangular wave signal "b" derived from the sinusoidal wave signal "a" of Fig. 1 A for the comparison with the signal "d" of Fig. 1 D.

Fig. 2 shows a circuit diagram of a first embodiment of a circuit arrangement according to the present invention. A turntable 20 for supporting and rotating a disk 1, which will be described later, is driven by a drive mechanism 16 so that the turntable 20 rotates at å predetermined rotational speed. A pickup having a reproducing stylus 2 is arranged to move linearly in a radial direction of the turntable 20 so that the stylus 2 moves toward the center of the disk from the peripheral portion thereof or vice versa.

Namely, a linear drive mechanism 18 is associated with the pickup to drive the pickup. The abovedescribed arrangement is substantially the same as that of conventional disk players.

This embodiment has a feature in that the clipping level of a limitter 4 is made variable in accordance with its output because a buffer amplifier 5, an integrator 6 having a resistor R and a capacitor C, and a gain amplifier 7 are connected between the output and input of the limitter 4 so that the output of the limitter 4 is negatively fed back to the input thereof.

The disk 1 is an audio disk in which an audio signal has been recorded in the form of intermittence of pits by converting the audio signal into a PCM signal where the PCM signal is further frequency modulated. When reproducing from the outer portion of the disk 1 by means of the reproducing stylus 2 of the pickup, the intermittence of the pits will be derived as the variation of electrostatic capacitance across the output terminals of the pickup, and thus a signal corresponding to this will be waveform shaped by an equalizer 3 so that the reproduced signal will be taken as a sinusoidal signal "a" shown in Fig.

1 A. The signal "a" is then fed to a limitter 4 to be clipped with a clipping point corresponding to a level 1, shown in Fig. 1 A so that unnecessary noises or the like are removed thus, a rectangular wave "b" shown in Fig. 1 B will be derived from the output terminal of the limitter 4. The rectangular wave signal "b" is fed to a demodulator 8, and demodulation is effeted in a well known conventional manner so that a train of pulses constituting a digital signal will be derived from an output terminal 9, and then the digital signal will be converted into an original audio signna! after being converted into a PCM signal by an unshown circuit. The above-described operation is the same as in conventional reproducing apparatus.

In addition to the above-described conventional operation, the integrator 6 responsive to the rectangular wave output signal from the limitter 4 is used to produce an average D.C. voltage .of the rectangular wave output signal from the limitter. Namely, the output signal of the limitter 4 is negatively fed back via the buffer amplifier 5, the integrator 6 and the gain amplifier 7 to the input of the limitter, where the time constant of the integrator 6 is set to a sufficiently great value as will be described hereinlater, and therefore, it can be regarded that the clipping level 11 of the limitter 4 does not substantially change because the D.C. level obtained as the result of conversion in the integrator 6 does not change greatly when pits at the outer portion (first 45 minutes in a disk having reproducing time length of 60 minutes per one side) are reproduced.

Accordingly, in an interval for reproducing the outer portion for the first 45 minutes from the beginning of reproduction in a disk having a reproducing time length of 60 minutes, reproduction is effected in the same manner as in the conventional apparatus.

When entering into a period of the last 15 minutes or so in reproduction, the output signal from the equalizer 3 will be such that the width of each positive going pulses becomes narrow as indicated by the signal "c" in Fig. 1 C, while the width of each negative going pulses becomes wide. Therefore, if no negative feedback path were provided to the limitter 4, the signal "c" shown in Fig. 1 C would become a signal "d" as shown in Fig. 1 D after being clipped with a clipping level 12.

However, the signal "d" is fed via the buffer amplifier 1 5 to the integrator 6 to be integrated therein so that an average D.C. voltage (see level 13 shown in Fig. 1 D) of the rectangular wave signal will be derived. This voltage is then amplified by the gain amplifier 7 to be fed to an element which changes the clipping level of the limitter 4.In the above, the time constant of the integrator 6 is set to a sufficiently great value so that it is possible to detect only the variation in the average D.C. voltage of the rectangular waves respectively obtained when the reproducing stylus reproduces the outer portion and the inner portion respectively, while it is impossible to detect the variation in the average D.C. voltage due to frequency deviation of the FM signal "b" at the outer portion (generally speaking, this variation is very small such that it is smaller than the above mentioned variation in the average D.C. voltage at the outer and inner portions).

With the output from the gain amplifier 7, the clipping level for the signal "c" from the equalizer 3 is lowered from the level 12 to another level 14, so that the signal "c" is substantially clipped at the level 14 causing the limitter 4 to produce an output signal "b" having a waveform of Fig. 1 B.

As a result, even if distortion occurs in the reproducing waveform from the equalizer 3 due to pits at the inner portion, it is possible to obtain an output signal having no distortion in the same manner as the reproducing waveform corresponding to the pits at the outer portion.

Therefore, it is possible to reproduce correctly data throughout the entire disk without suffering from data errors as in the conventional apparatus.

Fig. 3 shows a circuit diagram of a second embodiment of the present invention apparatus.

The second embodiment is directed to a circuit arrangement using a comparator for waveform shaping rather than a limitter. This embodiment has a feature in that the reference voltage of a comparator 4 is made variable in accordance with its output because a buffer amplifier 5, an integrator 11 having a resistor R' and a capacitor C', and a gain amplifier 7 are connected between the output and input of the comparator 10 so that the output of the comparator 10 is negatively fed back to its input.

In Fig. 3, the reference 12 is an audio disk in which an audio signal has been recorded in the form of intermittence of pits by converting the audio signal into a PCM signal where the PCM signal is further digitally modulated, such as PE, MFM, ZM or the like. When reproducing prerecorded data from the disk 12 by means of a reproducing stylus 2, a signal corresponding to the intermittence of the pits will be derived from an equalizer 3. This signal will be processed so that its unnecessary high frequency components are removed by a low pass filter 1 3. As a result an output signal is obtained at the output of the low pass filter 13, where the output signal has the waveform of the signal "a" shown in Fig. 1A. This signal is then compared with a reference voltage applied to a comparator 10 to be converted into a rectangular wave signal "b" shown in Fig. 1 B.And then the rectangular wave signal will be decoded by a decoder 14 to be taken out from an output terminal 9. In this case, since the time constant of the integrator 1 1 has been set to a sufficiently large value in the same manner as in the first - embodiment, the reference voltage of the comparator 10 is constant when reproducing the outer portion of the disk 12 so that reproduction is effected in the same manner as in the conventional apparatus.

When entering into a reproducing period of the last 1 5 minutes or so, distortion occurs in the output signal from the high pass filter 13 as shown in the signal "c" of Fig., 1 C in the same manner as described in connection with the first embodiment. Thus, the output of the comparator 1 0 would assume a waveform "d" of Fig. 1 D if no feedback were effected. However, since the reference voltage of the comparator is lowered with the detection of the D.C. level by the integrator 1 1 in the same manner as in the first embodiment, a signal such as the signal "b" of Fig.

1 B will be derived from the comparator 10. As a result, it is possible to uniformly and correctly reproduce data from the entire of a disk without suffering from data errors which occur in the conventional apparatus.

As described in the above, in the disk player or information recording medium reproducing apparatus according to the present invention, a circuit is provided between the output and input of another circuit which obtains a rectangular wave signal, where the former circuit produces an average D.C. voltage of the output of the latter circuit to feed back this voltage to the input terminal of the latter circuit so as to change the threshold level in accordance with the output of the latter circuit. Therefore, even if distortion occurs in a sinusoidal wave signal obtained when reproduced signals from the inner portion of an audio disk, which has been PCM recorded, it is possible to obtain a correct rectangular wave signal by changing the threshold. Accordingly, the present invention has an advantage that it is possible to obtain a correctly reproduced signal because it is possible to uniformly and correctly obtain demodulated signals or decoded signals from the entire of a disk without suffering from data errors.

The above-described embodiments are just examples of the present invention, and therefore, it will be apparent for those skilled in the art that many modifications and variations may be made without departing from the spirit of the present invention.

Claims

1. A circuit arrangement for a disk player of the type arranged to reproduce information prerecorded in said disk in the form of pits, comprising: (a) first means responsive to a reproduced signal from said disk for producing a rectangular wave signal on the basis of a threshold; (b) second means for producing a signal indicative of the average D.C. voltage of said rectangular wave signal; and (c) third means for feeding back said signal from said second means to said first means in such a manner that said threshold is changed in accordance with said average D.C. voltage.

2. A circuit arrangement as claimed in Claim 1, wherein said first means comprises a limitter.

3. A circuit arrangement as claimed in Claim 1, wherein said first means comprises a comparator.

4. A circuit arrangement as claimed in Claim 1, wherein said second means comprises an integrator having a predetermined time constant.

5. A circuit arrangement as claimed in Claim 4, further comprising a buffer amplifier between the output terminal of said first means and the input terminal of said integrator.

6. A circuit arrangement as claimed in Claim 4, further comprising a gain amplifier between the output terminal of said integrator and an input terminal of said first means, from which input terminal said threshold is applied thereto.