BE903220A - Compact disc using digital demodulation of signal - uses synchronised reading from and writing to memory under control of synchronised clocks to maintain phase accuracy - Google Patents

Abstract

The data is gathered by a read head (101) which passes the signal to the read circuit comprising: a demodulator; a reproduction clock signal generator; a random access memory, with its write controller, to retian the demodulated signal; a clock signal to read information from the memory; a phase synchroniser for the two clock signals; a memory utilisation controller (110); and a clock frequency controller. - The signal from the read head is passed to a filter (106) and is then demodulated (108) to obtain the PCM signal, which is stored in a random access memory (111) under supervision of the memory controller (110) in association with a signal obtained by division (114) from the main clock. The memory data is used to control the VCO forming part of a phase locked loop circuit (117,118,119). The digital signal is applied to a digital-to-analog convertor (112) and then to a low pass filter (113) for application to the audio amplifier.

Description

       

  Disc playback system with operation

  
digital demodulation of information "

DISK PLAYING SYSTEM WITH OPERATION OF

DIGITAL DEMODULATION OF INFORMATION

HISTORY OF THE INVENTION

  
Subject of the invention

  
The present invention relates to a system for reading an information recording disc and, more particularly, to a system for reading a recording disc containing digitally processed information. The invention also relates to a system for use in a disc playback system for demodulating a PCM digital signal recorded on an information recording disc.

  
Description of the prior art

  
Among the digital signal recording discs, the so-called "audio-digital" disc or even the compact disc (generally called CD) is already known and makes it possible to process audio signals according to a predetermined digital modulation process and to record them on the disc as a train of pulses. Recently, a type of video disc has also been developed in which an audio signal, which is encrypted by a predetermined digital modulation method and converted into a pulse train signal, is superimposed on FM (frequency modulation) signals. representing a video signal and the audio signal. The recording is carried out using the recording signal thus obtained.

  
The details of this type of video disc (hereinafter referred to as LDD) are described, for example, in Japanese patent application No. [deg.] 58-45780.

  
The recording operation with this type of disc includes the processing of two channels of audio signals and two carrier signals of 2.3 MHz and 2.8 MHz, in the form of audio carrier signals, which are modulated in French. audio signals, respectively. In addition, the video signal undergoes frequency conversion such that the

  
  <EMI ID = 1.1>

  
base is at 8.1 MHz and the white point is at 9.3 MHz respectively.

  
The audio signals mentioned above are also encrypted and transformed into a pulse train signal using a modulation method such as the PCM (Pulse Code Modulation) method.

  
The pulse train signal takes the form of a signal suitable for recording and obtained by a process such as, for example, the Eight to Fourteen Modulation (EFM) process, while the spectrum of frequency of the audio signal is transformed into pulse train frequency constituents comprising pulses having a bandwidth within the range

  
  <EMI ID = 2.1>

  
In this expression, "T" represents the bit period of the PCM signal, 3T corresponds to 720 KHz and the maximum bandwidth 11T corresponds to 200 KHz.

  
The pulse train signal having this characteristic is superimposed on the main video carrier signal at a level less than 1/10 of that of the main carrier, then is cut and amplified near the zero crossing point. The pulse width modulation signal thus produced is used as the recording signal.

  
In CD (compact disc) playback systems, the PCM digital signal demodulation system is generally designed so that a playback clock signal is output at a fixed frequency and the rotation of the recording disc or 'synchronized in phase with respect to the read clock signal. In addition, the instability of the time base is eliminated, by reading the information which is temporarily stored in a memory in synchronism with the read clock signal, this information being stored in the memory in synchronism with a signal. registration clock synchronized with a reproduction clock signal.

  
On the other hand, in the case of the LDD mentioned above, the demodulation of the digital signal originating from the reproduction signal is carried out by virtue of the prior synchronization of the reproduction signal with respect to a reference clock signal for synchronization video. Therefore, if the data is stored in memory, in the case of a PCM demodulation system, using a different reference clock signal which is used for ordinary CD players, this will inevitably result in a time difference. between the reproducing video signal and the reproduced audio signal, due to a slight phase difference between the two reference signals.

  
In addition, if the read clock signal has a phase shift forward with respect to the write clock signal, the memory can be emptied. Conversely, if the write signal has a phase shift forward with respect to the read clock signal, the memory capacity may be overflowed.

  
On the other hand, the development of a compatible playback system, capable of reading both CDs and LDDs, gave rise to various difficulties, such as those mentioned above, which had to be resolved to allow the current use of a demodulation system, in order to process a CD output signal and to process an LDD output signal, in order to reduce costs.

  
In addition, the production of the above-mentioned recording clock signal, in accordance with the reproduction clock signal contained in the digital reproduction signal, is generally carried out using a PLL (Phase Locked Loop) circuit. = phase blocking circuit), comprising a voltage-controlled oscillator, to generate the registration clock signal and a phase comparator to generate a control voltage intended for the voltage-controlled oscillator, as a function of the phase difference between the reproduction clock signal and the recording clock signal.

  
However, during the transition time of a pin servo system to drive a pin motor between the unlocked state and the locked state, or during the time when a direct access instruction, such as a analysis, exploration or jump instruction is deleted, the period for blocking the PLL circuit tends

  
to extend because the reproduction clock signal is not stable before this period.

  
On the other hand, an oscillator for controlling the rotation of the recording disc is provided separately in some of the conventional CD players. The frequency and phase of the reproduction clock signal are also determined in accordance with the rotation of the recording disc. However, there may be frequency and phase errors of an oscillating signal coming from an oscillator of the aforementioned type and intended to control the rotation of the recording disc, with respect to the frequency and the phase of the signal. above-mentioned reading clock, generated by a reading clock signal oscillator as a result of temperature variations, etc.

   If the frequency of the read clock signal becomes lower than the frequency of the write signal, which is determined by the frequency of the play clock signal, the stored data will remain in memory due to the decrease in the frequency of the read clock signal. Therefore, it is necessary to provide a sufficiently large memory capacity. As a result, the memory capacity must be increased when the magnitude of the estimated error becomes greater.

  
Abstract of the invention

  
It is therefore an object of the present invention to solve the difficulty described above and to provide a reading system for reading information recorded on a recording disc in which the memory is always used in an appropriate manner. and which is capable of providing a digital signal free of instability from the time base, by virtue of the effective removal of the instability constituent included in the digital reproduction signal.

  
Another object of the present invention is to provide a reading system in which the time required for blocking the PLL circuit intended to generate a recording clock signal is shortened, so that the demodulation operation of the digital reproduction signal follows harmoniously from the other operating state.

  
Another object of the present invention is to provide a digital signal reproduction system

  
in which the instability of the time base of the reproduction signal can be eliminated, without increasing the memory capacity.

  
A reproduction system for reading information recorded on a recording disc and compliant

  
to the present invention comprises a demodulation device for demodulating a digital reproduction signal, a writing device for writing the demodulation signal using the demodulation device and for introducing it into memory, in synchronism with a signal d reproduction clock contained in the digital reproduction signal, a device generating a clock signal intended to generate a read clock signal for reading the information stored in the memory, a phase synchronization device for synchronizing the phase the output signal of the device generating the clock signal with the reproduction clock signal,

   a controller for controlling the memory usage factor and for producing signals corresponding to the memory usage factor and a controller for controlling the frequency of the read clock signal from the signal generating device clock.

  
According to another aspect of the present invention, there is provided a system for reproducing the information recorded on a recording disc and comprising a device for preventing the synchronization operation of the phase synchronization device and operable to synchronize in phase a read clock signal with a reproduction clock signal and to fix the frequency of the read clock signal at a constant value.

  
According to another aspect of the invention, a system is provided for reproducing the information recorded on a recording disc and comprising a device for generating a clock signal synchronized with a reproduction clock signal contained in a reproduction clock signal, this device comprising a voltage-controlled oscillator for generating the clock signal, a phase comparator for performing the phase comparison of the digital reproduction signal and the clock signal and for generating the control for the voltage-controlled oscillator and an inverter, for alternately applying said control voltage and a reference voltage of a predetermined level.

  
In accordance with yet another aspect of the present invention, there is provided a system for reproducing digital information, and comprising a demodulator for demodulating a digital reproduction signal, a writing device for writing the digital reproduction signal into a memory , in accordance with a reproduction clock signal contained in the digital re-production signal, a device generating a clock signal, intended to generate a clock signal for extracting the information recorded in the memory, and a device phase synchronization, to synchronize the phase of the output clock signal of the device generating a clock signal with the reproduction clock signal.

  
According to another aspect of the present invention, the system for demodulating digital information further comprises a device for transforming the output signal from the memory into analog signals.

  
Other objects and fields of application of the present invention will become more clearly apparent from the detailed description below. However, it is understood that the detailed description and the specific examples, with indication of the preferred embodiments of the present invention, are given solely by way of example, since different modifications or variants are possible, without departing from the invention. spirit and object of the present invention, as will become clear

  
  <EMI ID = 3.1>

  
detailed description below.

  
BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is a diagram representing the frequency spectrum of an RF signal read from a recording disc and on which a recording signal, which comprises a pulse signal, obtained by the PCM processing of an analog signal superimposed on the frequency modulated signals of a video signal and an audio signal is recorded. N denotes the level.

  
Figures 2A to 2C together constitute a block diagram, showing an embodiment of the system for reading the information recording disc.

  
Figure 3 is a circuit diagram showing the arrangement of the disc discrimination circuit used in the disc playback system of Figure 2. Figure 4 is a block diagram showing the arrangement of the signal detection circuit a reproduction clock used in the disc playback system of Figure 2, and Figure 5 is a block diagram of an embodiment of the system for demodulating the PCM digital signal recorded on a recording disc of the information.

  
Detailed description of preferred embodiments

  
Before describing in detail the preferred embodiments of the present invention, we will first examine FIG. 1, which represents a frequency spectrum of an RF (radio frequency) signal coming from a disk d recording on which the video signal and the audio signal are recorded, in accordance with the above-mentioned recording method.

  
In Figure 1, the letter A indicates the constituent of the encrypted audio signal, B indicates the constituent of the FM audio signal, C refers to the color information elements of the FM video signal and D refers to the information element of the FM video signal brightness.

  
Since the maximum dynamic range of the encrypted audio signal is around 90 dB, or above

  
by this value, the quality of the sound having undergone digital processing is greatly improved, compared with the audio signal recorded and reproduced using the conventional FM modulation method.

  
The preferred embodiments of the system for reproducing the information recorded on a recording disc will be described with the aid of FIGS. 2A - 2C to 4, reproduced in the appendix.

  
FIGS. 2A - 2C represent a so-called "compatible" disc playback system which is capable of playing both CDs and LDDs, an ordinary video disc on which the video signal and the audio signal are recorded according to the modulation method frequency, respectively.

  
In FIG. 2A, since there are different disc rotation speeds for the digital audio disc (CD) and the video disc (LD and LDD), the reading system is provided with a first pin motor 1, for drive the LD and LDD and a second pin motor 2, to drive the CD. The first and second spindle motors 1 and 2 are chosen, alternately, according to the type of disc to be read and the passage from one to the other is effected by a rocking movement, using a switching mechanism, driven by a motor 3.

  
To detect the disc size of a disc

  
5 which must be read, three detectors 6, 7 and 8 are provided, arranged in this order, in the radial direction, from the center of the disc, at positions corresponding to the diameter of the disc having different dimensions. The dimension of

  
  <EMI ID = 4.1>

  
compact and 3 or 10 "(about 20 or 30 cm) in diameter for the video disc (LD and LDD). The output signals of the three detectors 6, 7 and 8 are applied to a circuit 9 to profile the shape of the wave and where it takes place. An output signal from circuit 9 to profile the shape of the wave is applied in turn to a disc discrimination circuit

  
10. The detectors 6, 7 and 8 are, for example, optical detectors. However, it is not necessary to limit oneself to this type of detector.

  
In the disc discrimination circuit 10, the type of disc to be read is detected as CD, LD or LDD, according to the detector output signal

  
6, 7 and 8.

  
FIG. 3 represents the practical arrangement of the disc discrimination circuit 10.

  
The size of the CD is the smallest (12 cm) and the detector 6 is located at the innermost location. Consequently, in the case of this disc discrimination circuit shown in FIG. 3, an output signal

  
of an AND gate circuit 78, which receives an output signal from the detector 6 and the inverted signals from the output signals from the detectors 7 and 8 supplied via the inverters 79 and 80, is used as detection information

  
from the CD. More particularly, when the detector 6 is activated and the other detectors 7 and 8 are triggered, the disc to be read is considered to be a CD disc. In the case of a video disc, the size of the disc is 8 "or 12 "(approximately 20 or 30 cm). In this case, the output signals from detectors 7 and 8 are applied to the AND gate circuits 70 and 71, via an OR gate 69. The other terminal of Entrance

  
of the AND gate circuit 71 is supplied by a synchronous frame detection signal, which is produced periodically when a synchronous frame signal is detected by a synchronous frame detection circuit 21, which will be described later. This synchronous frame detection signal is inverted by an inverter 72 and applied to the other input terminal of the AND gate circuit 70. With this arrangement, LDD detection information is obtained when the synchronous frame signal is applied, while the output signal is produced by the detectors 7 and 8. On the other hand, if the synchronous frame signal is not supplied, detection information LD is produced in the AND gate circuit 70.

  
If we still examine FIG. 2A, we see that this information, used for discriminating the disks, is used to cause an indicator 11, intended to indicate what is the type of disk and also serve as control signals for different switches. , which will be described below.

  
A read head 12, intended to read the information contained in the disc 5, is supported by a sliding base (not shown) which can slide in the radial direction on the disc 5. The sliding base is driven by a drive mechanism which includes a sliding motor, a reduction gear, etc. The information read from the record disc using

  
the read head 12 is applied to an RF amplifier 13 and then to a digital information demodulation system 14 of FIG. B, to an audio-analog demodulation system 15 and to a video demodulation system 16 shown in Figure 2C. The RF amplifier 13 has a wide bandwidth, varying from 5 KHz to 14 KHz, and it is able to amplify a PCM audio signal for reproduction, an FM audio signal for reproduction and a video signal for its own only.

  
In FIG. 2B, the digital information demodulation system 14 comprises an inverter 17, which is actuated as a function of the disc to be read. The inverter 17 is arranged on the contact A side, in the case of an LDD, and on the B contact side in the case of a CD, according to the information on the disc, provided by the disc discrimination circuit. 10. In other words, the system for processing the digital reproduction signal undergoes the appropriate switching depending on whether it is reading an LDD or playing a CD.

  
When playing a CD, the RF reproduction signal represents the PCM audio information and this PCM audio information is processed by MTF compensation.
(Modulation Transfer Function) in an equalizer circuit 18. On the other hand, in the case of reading an LDD, the PCM audio information which is contained in the RF reproduction signal, just like the frequency modulated audio information and the frequency modulated video information are applied to a

  
LPF (Low Pass Filter) 19 and then transmitted to a de-emphasis circuit 20. The PCM audio information, for example, takes the form of an EFM signal. However, recording is performed in such a way that the low frequency component of the PCM signal is enhanced, because the component of the digital signal is disturbed by the low frequency component of the FM video signal, if the digital signal itself even is superimposed on a frequency modulated video signal.

  
As a result, a compensation process is carried out at the time of reproduction, so that the low-frequency component which is reinforced at the time of recording, is reduced by means of the de-emphasis circuit 20. Thanks to this arrangement, the signal-to-noise ratio of the digital signal can be improved, particularly with regard to low-frequency noise during recording and reproduction operations.

  
In addition, the switching operation, carried out using the inverter 17, can be replaced by a system arrangement such that the current supply of each signal processing system is subject to a command "on / off ".

  
The EFM signal passing through the inverter 17 is applied to an EFM demodulation circuit 22, passing through the synchronous frame detection circuit 21 and is also applied to a reproduction clock read signal circuit 23. When a reproduction clock signal is applied to the reproduction clock signal read circuit 23, the signal EFM is demodulated into digital signal PCM in the demodulation circuit EFM 22.

  
The demodulated digital signal PCM is then written into a memory 24, such as a RAM memory (raondom Access Memory = living memory), in accordance with the command coming from the memory controller 25. At this time,

  
the memory controller 25 performs the synchronization with a registration clock signal, which is

  
a divided signal from the reproducing clock signal and obtained by a divider 26.

  
The information stored in the memory 24 is read in synchronism with a read clock signal, obtained by dividing an oscillation signal of a VCO (Voltage Controlled Oscillator) 27 d ' a PLL circuit (Phase Locked Loop), by means of a divider 28.

  
This PLL circuit includes the VCO 27 mentioned above, a phase comparator (P / C) 31, which receives two dividing signals, that is to say a signal divided from the reproduction clock signal coming from a divider 29 and a divided signal of the oscillation signal of the VCO 27, coming from a divider 28. The PLL circuit further comprises an LPF (low-pass filter) 32, intended to receive an output signal from the phase comparator 31 and an inverter 33, making it possible to apply alternately the output voltage of the LPF 32 and a reference voltage Vref 1, supplied by a reference voltage source.

  
In this PLL circuit, the inverter 33 is operated as a function of the result of the discrimination obtained with the discrimination circuit 10. More particularly, the inverter is arranged on the side of the terminal "a" when reading a LDD, so as to provide the output signal from LPF 32 to VCO 27, while this inverter is arranged on the side of terminal "b" for playing a CD, so as to supply voltage Vref 1 to VCO 27 In this way, the read clock signal intended to read the information contained in the memory 24, is synchronized with the reproduction clock signal, thanks to the operation of the PLL circuit.

   As a result, when the CD is played, a loop switch 55, which will be described below, is activated and the output signal of the phase comparator 31 is applied, by an LPF 77, so as to cause the second pin motor 2, intended to drive the CD. As a result, the reproduction clock signal is synchronized in phase with a fixed clock signal, coming from VCO 27, the polarization of which is constant.

  
The digital signal, thus read in the memory 24, is transformed into an analog audio signal by means of a D / A converter 34 (digital / analog) and so as to transmit, by the LPF 35L and 35R, reproduction audio signals left and right, respectively.

  
With this arrangement, the memory utilization factor 24 is always controlled by the memory controller 25, which provides the information indicating in what state the memory 24 is, whether it is an overflow of its capacity or from the emptying of this memory, said information then being transmitted to a voltage generator 36. At the time of the reproduction of the LDD, the voltage generator 36 produces, in accordance with the information coming from the memory controller 25 and indicating the factor of use of the memory 24, a positive control signal in the event of the memory capacity being exceeded and a negative control signal in the event of the draining of the memory 24.

  
The output signal of the voltage generator 36 is superimposed on the output signal LPF 32 and is applied to the VCO
27 via the inverter 33. In this way, the frequency of the read clock signal is controlled.

  
As a result, the continuous control of the quantity of data stored in the memory 24 and the production of a positive or negative control voltage, depending on whether there is a shortage or an excess of processing capacity in the memory 24, allows you to operate the PLL circuit in order to accommodate this unusual state. Consequently, the memory 24 is always maintained in the normal operating state.

  
In FIG. 2C, the analog audio demodulation system 15 comprises BPFs (bandpass filters 37L and 37R, which only allow the components of audio carrier frequencies of 2.3 MHz and 2.8 MHz to pass through. BPF 37L and 37R output are applied to FM 38L and 38R demodulators when the input signals are processed by an FM demodulation process and are transmitted in turn as left and right reproduction audio signals via the de-emphasis circuits 39L and 39R.

  
In the video demodulation system 16, the RF reproduction signal is applied to a BPF and to the band eliminator circuit 40, which only processes the video information. In the BPF and the band eliminator circuit 40.1th signal component EFM, which is contained in the signal

  
RF reproduction in the case of LDD reproduction, and the audio carrier components of 2.3 MHz and 2.8 MHz are effectively eliminated. The information thus obtained is transmitted, in turn, to an FM demodulator 42 via a limiter 41, for frequency demodulation. The demodulated output signal from the FM demodulator 42 is in turn applied to a DOC (de-excitation compensator) 44, via an LPF 43, in order to compensate for the de-excitation.

   A DOS (de-excitation detector) 46 is provided for detecting de-excitation, in accordance with the RF reproduction signal, which is transmitted via an HPF (high-pass filter) 45, for example. DOC 44 comprises, by example, an analog switch which is triggered on reception of an output signal from DOS 46 and a holding capacitor, provided between an output terminal of the analog switch and a point on the circuit of the reference potential.

  
When de-energization occurs, the level of the output signal from LPF 43, immediately before the output signal from DOS 46 is produced, is maintained and applied to the circuit of the next phase. The dropout compensation is done this way. The output signal of this DOC 44 is transmitted as a video output signal.

  
The DOC 44 output signal is also applied to a synchronous horizontal separator circuit 47, allowing the horizontal synchronization signal to be separated

  
and then pass it on. The horizontal synchronization signal is applied to the phase comparators 48 and 49 when a phase difference is detected between the horizontal synchronization signal and a reference signal supplied by a reference signal generator 50. The signal

  
output of the phase comparator 48 is applied to the negative input terminal of a conductor 51 and an output signal of the comparator 49 is applied to the conductor 52, via an equalizing amplifier 52, on the other input terminal of this conductor. The output signal of the conductor 51 is applied to an equalizing amplifier 53 and, subsequently, to a drive device 54, controlling

  
the operation of the first drive motor 1, in order to rotate the LD and the LDD. These circuit parts constitute a pin servo motor system.

  
The output signal of the equalizing amplifier 52 is applied to the loop switch 55, to an inverter 56 and to a drive device 57, so

  
to drive an incorporated control device (not shown) of the read head 12. When this control device is operated, a light point, intended to read the information, moves in a tangential direction on the recording track of the disc . This part constitutes a servo-tangential system. The control device may consist, for example, of a tangential mirror which displaces the position of the light point on the disc in the tangential direction of the recording track, by virtue of its own angular displacement. On the other hand, it is also possible to use an arrangement in which the lens can be moved in a direction perpendicular to the axis of light, so that the light point for reading information moves in the direction tangential on the recording track.

  
Loop switch 55 is engaged in response to a spindle lock signal, which is generated by a spindle lock detection circuit 58, when the synchronization of the spindle servo system is almost complete. , at the start of reproduction, the spindle motor 1 is started, so as to rotate as a function of the output signal of the phase comparator 48 and a coarse adjustment of the axis is carried out time (spindle servo control). When the blocking of the spindle servo is almost complete, thanks to this operation, the loop switch 55 is engaged and makes

  
  <EMI ID = 5.1>

  
of the output signal of the phase comparator 49. In this way, a fine control of the time axis is obtained (tangential servo). Thanks to this operation, it becomes possible to eliminate any residual unstable constituent, thanks to the operation of the tangential servo system, which cannot be eliminated by the operation of the pin servo system.

  
However, even in the tangential servo system, the mechanical system of the mechanism driving the control device cannot follow the high frequency of the residual instability component. As a result, it is not possible to completely eliminate this instability. Consequently, in the system of digital demodulation of the aforementioned information 14, the cut-off frequency of the LPF 32 of the PLL circuit, intended to generate the read clock signal, is set at a frequency level lower than

  
the highest frequency of the frequency range of the above-mentioned tangential servo loop, so that the high-frequency component of the residual instability is eliminated.

  
In this way, the residual component of instability is completely removed. Preferably, if the cut-off frequency of the LPF 32 is set to a frequency lower than an eccentric frequency (30 - 8 Hz for the LDD) an instability constituent, originating from the eccentricity of the disc, can be completely eliminated.

  
In the description above, the pin servo system and the tangential servo system are operated in accordance with the horizontal synchronism signal, however, it should be noted that these systems can also be operated based on a carrier signal. color signal of 3.58 MHz, which is contained in the FM reproduction video signal.

  
The servo systems of the video disc reproduction time (LD, LDD), are described above.

  
In the case of the reproduction of a CD, the spindle servo is performed in accordance with the output signal from the phase comparator 31 of the above-mentioned digital information demodulation system 14. More particularly, the output signal of the phase comparator 31 is applied to the loop switch 55, which closes when the CD is played and to the drive device 60, so as to drive the second spindle motor 2 .

  
In conventional systems, the aforementioned tangential servo operation is not performed during the reproduction of the CD. However, since the second spindle motor

  
2 cannot adequately follow a high frequency specific to the component of the output signal of the phase comparator 31, the tangential servo control is also adopted in the system according to the present invention. In this tangential servo control system, a high frequency component of the time axis error signal, recorded by the HPF 61, is applied to the inverter
56 and to the drive device 57, so as to actuate the control device of the read head 12. The reverser 56 is actuated according to the result of the detection of the disc discrimination circuit 10 and the movable contact is arranged on contact "a" in the case of reproduction of an LDD, or of an LD, and is arranged on contact "b" in the case of reproduction of a CD.

  
Furthermore, in the aforementioned example, the tangential servo system, at the time of the reproduction of the CD, is actuated as a function of the divided signal originating from the reproduction clock signal produced in the circuit for reading the reproduction clock signal. 23. However, it is possible to arrange the system so that the tangential servo system is actuated according to the divided signal from the synchronized frame signal detected in the synchronized frame signal detection circuit 21, because the signal of synchronized frame is synchronized with the reproduction clock signal.

  
An output signal from the spindle lock detection circuit 58 is applied to an inverter 62 and then to an input terminal of an OR gate 63, so as to constitute a signal indicating the state of the servo system of spindle (blocked or not state). At the other input terminal of the OR gate 63 is applied a read information signal, generated at the time of a jump detection read instruction, etc. The OR gate 63 also receives the output signal of the LDD information, produced by the disc discrimination circuit 10. An output signal from the OR gate

  
  <EMI ID = 6.1>

  
tion 23, via a command control circuit 64.

  
FIG. 4 represents a practical arrangement of the circuit 23, for the detection of the reproduction clock signal.

  
In FIG. 4, the reproduction signal EFM is compared in phase with the oscillation signal of a VCO (voltage-controlled oscillator) 66, by a phase comparator
65, in which a phase difference signal is created.

  
The phase difference signal is then applied to VCO 66, via an LPF 67 and an inverter 68. These circuit elements make it possible to constitute the PLL circuit, to generate the clock signal of reproduction.

  
Normally, the inverter 68 is placed on the contact "a" and supplies the output signal from the LPF 66 to the VCO

  
68. When the control signal SC is emitted by the control circuit of the aforementioned command 64, the inverter is arranged on the contact "b" and supplies a predetermined reference voltage Vref2, produced by a predetermined source of reference voltage at VCO 66. In other words, when the servo spindle system is not in the locked state or when the light point for reading the information moves through several sections of the recording track, in accordance with a read instruction such as analysis, detection or jump, the reference voltage Vref 2 is applied to VCO 66, so as to fix the oscillation frequency at a frequency value close to the frequency of the reproduction clock signal .

  
As a result, the blocking of the reproduction clock signal is accelerated after the blocking of the spindle servo system or the deletion of the read instruction.

  
  <EMI ID = 7.1>

  
audio output part a pair of 73L output terminals and
73R, which are the left and right terminals of the analog audio output signals, as well as a pair of output terminals 74L and 74R, which constitute the left and right terminals of the digital audio output signals.

  
The output terminals 73L and 73R receive the audio output signals from the analog audio demodulation system. These audio output signals are also applied to the output terminals 74L and 74R via an inverter 75, when the LD is reproduced. The inverter 75 is arranged, for example, on the side of the normal terminals "a", in the case of the reproduction of an LD and is arranged on the side of the contacts "b", in the case of the reproduction of an LDD and of a CD, as a function of the output signal of the disc discrimination signal produced by the disc discrimination circuit 10.

  
The audio output signals from the digital information demodulation system 14 are applied to the output terminals 74L and 74R, via the left and right mode inverters 76L and 75R and the inverters
75L and 75R.

  
With this arrangement, normal audio signals are transmitted by terminals 73L and 73R in the case of LD reproduction and high quality audio signals (digitally processed) are transmitted by output terminals 74L and 74R, when reproduction of LDD and CD. In addition, when reproducing the LDD, normal audio signals are also transmitted through the output terminals 73L and 73R.

  
The mode changeover switches 76L and 76R are provided for changing the output mode of the audio signal in the digital audio system, to the analog phase. More particularly, when the audio output signals from the digital audio demodulation system 14 are stereophonic signals, just use the output mode described above. However, if the audio output signals are in multiple voice mode, the left channel is in Japanese and the right channel is in a foreign language, for example English. This is why, if the mode changeover inverters that can be operated independently 76L and 76R are used, the signals to be transmitted by the output terminals 74L and 74R are chosen from three modes, that is to say: Japanese ; the foreign language; Japanese and the foreign language.

   The operation of the selection inverters

  
76L and 76R mode is performed based on the control information from an operating element (not shown).

  
As mode changeover switches 76L and 76R, a pair of relays can be used which can be actuated independently of each other. In general, it is sufficient to use a relay provided with a movable contact and a pair of fixed contacts to modify the signals. However, in this embodiment of the present invention, relays are used provided with an additional movable contact and two additional fixed contacts. More particularly, in the example of relay 76 of channel L, relay 76 comprises

  
  <EMI ID = 8.1>

  
res of fixed contacts S12, S13 and S22, S23, arranged in correspondence with the movable contacts S11 and S12. Among these fixed contacts, two fixed contacts, S12 and S23, which are furthest from each other, are used as input terminals for the two input signals (audio signals of the left and right channels) and one (S11) of the movable contacts is used as an output terminal. With this arrangement, two gaps are created between the left signal conductors

  
and right and effectively avoids crosstalk between the left and right signals. In addition, it should be noted that crosstalk can be prevented even more effectively if additional contacts and, therefore, additional gaps are provided.

  
Furthermore, even if this is not shown in the figure, the system is equipped with a servo focusing system, which controls the position of the read head 12 relative to the disc 5, in a direction perpendicular to the surface. of the disc and. a servo tracing system for controlling the position of the read head 12 in the radial direction relative to the disc. Therefore, it is desired - &#65533; table to switch the error signal processing systems according to whether it is the reproduction of the video disc
(LDD and LD) or digital audio disc reproduction
(CD). Thanks to this arrangement, one can get a good servo control regardless of the disc type.

  
On the other hand, the digital signal to be recorded in the CD or the LDD can be an encrypted image information signal or control information for controlling a computer other than the audio information including the signal.

  
From the above, it will be understood that the reproduction system of a recording disc, specific to the present invention, is arranged so that the memory utilization factor is always under control and that the frequency of the read clock signal is controlled as a function of the memory usage factor.

   Consequently, memory is always used under normal conditions and there is the advantage that, in the case of a direct read instruction, the state of use of the memory is optimized immediately after the completion of the instruction. and that the state of suppression of sensitivity of the output signal can be quickly removed and that after the removal of the lack of sensitivity, the component of instability contained in the digital reproduction signal is removed with certainty, while l digital information is obtained without any instability component.

  
On the other hand, and in accordance with the present invention, the read clock signal for reading the information stored in the memory is synchronized with the predetermined reference signal at the time of CD reproduction and with the clock signal of reproduction at the time of LDD reproduction. Thanks to this feature, the digital signal demodulation system is commonly used to reduce the cost of the compatible disc player and to effectively remove the jitter component included in the digital reproduction signal.

  
Furthermore, when it is a question of reading an LDD, the system is arranged so that the control voltage of the oscillator controlled by voltage of the PLL circuit intended to generate the recording clock signal is chosen at alternately between the output signal of the phase comparator and the predetermined voltage which is equal to a voltage making it possible to operate the voltage-controlled oscillator, in order to produce the recording clock signal having the appropriate frequency.

  
Consequently, the period required for blocking the PLL circuit is shortened, by arbitrarily choosing the predetermined voltage when the drive of the disc drive is in the unlocked state or at the time of the direct read instruction. This results in a smooth transition to the digital signal demodulation process.

  
An embodiment of the reproduction signal demodulation system, in accordance with the present invention, will be described with reference to FIG. 5 of the accompanying drawings.

  
In Figure 5, the construction of the video disc playback system is shown in part of the figure, to the left of the partially dotted line. As can be seen, a read signal from a disc D and obtained with a read head 101 is demodulated in a video demodulator 102 when a reproduction video signal is generated. This reproduction signal then gives rise to a reproduction synchronization signal, separated in a separator 103 and whose phase is compared with a reference signal coming from a signal generator 104 in a phase comparator 105.

  
A comparator output signal from comparator 105 is used in turn as a servo signal to control the drive of a spindle motor, so that the rotation of the recording disc D is controlled.

  
The read output signal from the read head is also applied to an LPF (low pass filter) 106 and a PCM audio signal is thereby obtained. This PCM audio signal takes the form of the EFM signal, for example,

  
and is sent in turn as an output signal from the system, via a de-emphasis circuit 107.

  
This output signal is applied, in turn, to

  
an EFM demodulation circuit 108 and a clock circuit 109, and is demodulated to obtain a digital PCM signal as a function of the reproduction clock signal.

  
This demodulation signal is recorded, in turn, in a living memory (RAM) 111, via a RAM controller 110, according to an output signal from a divider 114 of the signal reproduction clock.

  
The signal from RAM 111 is read by means of a divider output signal from divider 120, used to divide the VCO output signal
(voltage controlled oscillator) 119, contained in a PLL circuit (face locked loop). This PLL circuit consists of a PD (phase discriminator) 117, intended to receive the output signals of a divider 115 of the reproduction output signal and of a divider 16, of the VCO 119, an LPF (filter low pass) 18, to receive the PD output signal
117 and VCO 119 mentioned above.

  
With this arrangement, the read clock signal is synchronized in phase with the reproduction clock signal.

  
The digital read signal is then applied to a D / A converter 112 (digital / analog) and is then transmitted to an LPF 113, where it is transformed into an audio signal and is supplied in the form of an audio reproduction signal.

  
Also in this embodiment, the digital information can be any information relating to the control data used for controlling a reproduction system.

  
According to the demodulation system thus arranged, according to the present invention, the clock signal read in the RAM memory 111 is synchronized with the reproduction clock signal existing at the time of the recording of the data in the RAM memory. 111. Consequently, one should not fear the appearance of a frequency difference between these two clock signals and one should not fear either the accumulation of residual data in the RAM memory 111. It follows that 'There is no need to increase the capacity of RAM 111.

  
Furthermore, since any constituent of instability

  
is eliminated from the read clock signal thanks to the range limiting operation obtained with the loop filter 118 of the PLL circuit, a high quality reproduction signal can be obtained.

  
In addition, if the parts of the circuit situated to the right of the partially dotted line 30 are arranged with a demodulation circuit adapter for the digital audio system provided therein, the flexibility of use of the system can become excellent. For example, it may be sufficient to connect the input terminal of the adapter to the digital audio output terminal of the video disc playback system.

  
More particularly, in addition to the possibility of reproducing a video disc and a digital audio disc, it becomes possible to play a video disc carrying a recording signal, consisting of pulse train signals obtained by predetermined digital modulation. of an audio signal and superimposed on the FM signals of the video signal and the audio signal.

  
On the other hand, it should be noted that the digital information demodulation system, specific to the present invention, can be used in so-called "compatible" disc playback systems, which are initially designed for the playback of different types of information discs.


    

Claims (12)

Claims 1. System for reproducing the information recorded on a recording disc comprising a read head for reading the information contained in the recording disc and comprising: a demodulation device for demodulating a digital reproduction signal, obtained from the read output signal of said read head and producing a demodulation signal; means for generating a reproduction clock signal to create a reproduction clock signal from said digital reproduction signal; a memory for storing said demodulation signal from said demodulation device; a writing device, for writing said demodulation output signal from the demodulation device, into a memory according to the reproduction clock signal contained in said digital reproduction signal; a device for creating a clock signal, intended to produce a read clock signal allowing to read the information stored in said memory; a phase synchronization device, intended to perform phase synchronization between said reading clock signal originating from said clock signal creator device and from the reproduction clock signal; a control device, intended to control the state of use of said memory and to produce output signals corresponding to a utilization factor of said memory; and a control device, for controlling the frequency of said output clock signal from said device creating a clock signal, as a function of said output signals from said control device. 2. A recording disc reproduction system designed for playing a recording disc of a first type, in which a frequency modulation signal of an audio signal, a frequency modulation signal of a video signal and a predetermined digital signal are recorded after superimposing on each other, and a recording disc of a second type, in which a predetermined information is recorded, after having been initially processed by a predetermined process of digital modulation and subsequently converted into a pulse train for recording, comprising: a read head for reading information contained in a recording disc and producing a read output signal; a demodulation device for demodulating a digital reproduction signal from the read output signal and in accordance with a predetermined demodulation process and for generating a demodulation signal; a device for generating a reproduction clock signal, intended to create a reproduction clock signal originating from said digital reproduction signal. a memory for storing said demodulation signal from the demodulation device; a recording device, for writing said demodulation signal into the memory, in synchronism with the reproduction clock signal contained in said digital reproduction signal; a device for generating a clock signal, designed to create a read clock signal making it possible to read the information contained in said memory; a phase synchronization device for synchronizing in phase said read clock signal from said read clock signal with said reproduction clock signal; and a device for preventing phase synchronization by means of said phase synchronization device and for setting the frequency of said read clock signal in the event of reproduction of the recording disc of the second type. 3. System for reproducing the information recorded on the recording disc, and comprising a read head for reading the information contained in the disc and comprising: a demodulation device for demodulating a digital reproduction signal originating from a read output signal emitted by the read head and producing a demodulation signal; a first clock signal generating device for creating a clock signal synchronized with the reproduction clock signal component contained in the digital reproduction signal; a memory for storing the demodulation signal; a writing device for writing said demodulation signal into a memory, as a function of the writing clock signal produced by the division of said first clock signal; a second device generating a clock signal, intended to create a read clock signal making it possible to read the information contained in said memory; and a phase synchronization device, intended to synchronize in phase said reading clock signal coming from said second device for creating a clock signal with said recording clock signal, wherein said first device generating a clock signal comprises a voltage-controlled oscillator, making it possible to generate said first clock signal, a phase comparator for comparing the phases of said digital reproduction signal and of said first clock signal and for producing a control voltage the level of which is equal to the phase difference between said digital reproduction signal and said first clock signal and an inverter, making it possible to apply said control voltage alternately and a voltage of <EMI ID = 9.1> born of reference voltage to said voltage controlled oscillator. 4. Reproductive system according to the. claim 3, and further comprising a control device for controlling the drive of said disc and being operable in the unlocked state, wherein said inverter makes it possible to select said reference voltage when said control device is put into operation in the unlocked state. The reproduction system according to claim 3, and further comprising a system control device making it possible to control the reproduction operation of the system and in which said inverter selects said reference voltage when a direct read instruction is supplied by said device. system control. 6. Digital information demodulation system for demodulating a reproduction signal, comprising: a demodulation device, making it possible to demodulate said digital reproduction information and to produce a demodulation signal; a memory for storing said demodulation signal; a writing device, making it possible to write the demodulation signal into the memory, as a function of the reproduction clock signal; a device generating a reproduction clock signal and making it possible to create a reproduction clock signal coming from said digital reproduction information; a device for generating a clock signal, making it possible to create a read clock signal for reading the information contained in said memory; and a phase synchronization device, allowing the synchronization in phase of said read clock signal coming from said device generating a clock signal and from said reproduction clock signal, in which said output signal coming from said memory is derived in analog form. 7. A digital information demodulation apparatus according to claim 6, wherein said phase synchronization device is provided, in its control loop, with a loop filter having a limited bandwidth. 8. A digital information demodulation apparatus according to claim 6, wherein the digital reproduction signal is a digital information signal from a reproduction signal of a video reproduction system. 9. Digital information demodulation system, allowing demodulation of a reproduction signal and comprising: a demodulation device for demodulating said digital reproduction information and for producing a demodulation signal; a memory for storing said demodulation signal; a writing device, for writing the demodulation signal into the memory, according to a reproduction clock signal; means for generating a reproduction clock signal, for generating a reproduction clock signal from said digital reproduction information; a device for generating a clock signal, making it possible to create a read clock signal enabling the information stored in said memory to be read; and a phase synchronization device, making it possible to ensure the phase synchronization of said read clock signal originating from said device creating a clock signal with said reproduction clock signal, from which the stored information is read in said memory, in synchronism with said read clock signal. 10. Digital information demodulation system # according to claim 9, wherein said phase synchronization device comprises a loop filter having a limited bandwidth. 11. The digital information demodulation system of claim 9, wherein said digital reproduction signal is a digital information signal from a reproduction signal. emitted by the reproductive system. 12. Digital information demodulation device according to claim 9, in which said digital reproduction signal is a digital information signal reproduced from the digital signal reproduction system.