GB2134356A

GB2134356A - Video disc player

Info

Publication number: GB2134356A
Application number: GB08318878A
Authority: GB
Inventors: Minoru Kosaka; Toshihiko Hosaka; Toru Akiyama
Original assignee: Universal Pioneer Corp
Current assignee: Universal Pioneer Corp
Priority date: 1979-10-12
Filing date: 1983-07-13
Publication date: 1984-08-08
Also published as: GB2061072A; GB2061072B; DE3038650A1; GB2134356B; GB8318878D0; DE3038650C2

Abstract

A video disc reproducing device has a tracking servo system having a pair of light sources, a pair of photo- electric transducer units and a differential amplifier which produces a tracking error signal. A switch is provided in the tracking servo system and is closed when the tracking error signal has a predetermined level when the signal level of the audio carrier exceeds a predetermined value.

Description

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SPECIFICATION Video disc player

5 The present invention relates to an apparatus for reading and reproducing the information recorded on a video disc of, for example, the type in which the information recorded thereon is to be read out in an optical fashion.

10 The present invention is particularly concerned with a tracking servo system for use in a video-disc reading and reproducing apparatus.

In a video disc presently in use for a video disc player, video and audio information is stored in a 15 multiplex form consisting of a video signal carrier which is frequently modulated with a standard NTSC (National Television System Committee of U.S.A.) video signal and two-channel audio signal carriers each of which is frequency modulated with an audio 20 signal. The video signal carrier is higher in frequency than the audio signal carriers and is superposed in the form of pulsewidth modulation on the audio signal carriers in the multiplex signal recorded on the video disc.

25 In the case of a video disc of the optically scanned type, the multiplex signal thus consisting of the frequency modulated video and audio signal carriers superposed on each other is stored as a series of depressed areas or "pits" formed in each or one 30 information-carrying face of the video disc and arranged in a spiral track or a number of concentric tracks about the center axis of the disc. The video and audio information thus stored is read out by optically scanning the individual pits along the spiral 35 track or each of the concentric tracks by means of a beam of laser light thereby detecting the lengths of and spacings between the scanned pits. During playback of such a video disc, the disc is driven for rotation about the center axis thereof usually at a 40 fixed velocity and the beam of the laser light is displaced radially of the disc by a tracking device forming part of the video disc player. The laser beam directed to the target track is reflected from the information-carrying face of the video disc or passed 45 through the disc. The beam of light thus reflected from or passed through the video disc is converted into an electric reproducing signal by means of a photoelectric transducer unit for further conversion into video and audio signals.

50 A pick-up device of the video disc player is moved radially of the information-carrying face of the disc by the aid of the tracking servo system under the control of a tracking error signal indicative of the location of the reading or detecting point of the 55 pick-up device with respect to the target track on the disc. The reading or detecting point corresponds to a scanning spot in the case of the optical video disc player. To control the tracking servo system reliably on the basis of such an error signal, it is important 60 that the servo loop of the tracking servo system be closed when or after the detecting point is moved close to the target track on the video disc. If, in the case of the optical video disc player, the servo loop is closed and the servo system is locked in before the 65 scanning spot of light is moved close to the target track, it may happen that the tracking mirror forming part of the optical pick-up system and operative to deflect the scanning beams in a radial direction of the video disc is abruptly initiated into motion to reach the targer track and thus overshoots the target track. In an extreme case, the tracking mirror may be caused to oscillate and disable the tracking servo system form being locked in. Such an event may be caused not only during scanning of a video disc but generally when the servo loop of the tracking servo system is to be closed from an open condition.

It is therefore, an object of the present invention to provide a video disc information reading and reproducing apparatus including an improved tracking servo system which can be locked in reliably and in a stable condition when the servo loop of the system is to be closed from an open condition.

According to the invention, there is provided a video-disc information reading and reproducing apparatus including a source of a reproducing signal read out from a video disc scanned by a scanning beam of light which is deflected radially of the video disc and a signal correcting device which is operative to compensate forthe attenuation of the signal level of high frequency components of said reproducing signal and which includes signal level detector means operative to produce an output signal variable with the signal level of a frequency modulated audio carrier component contained in the reproducing signal, and a tracking servo system for deflecting said scanning beam radially of said video disc, the tracking servo system comprising: error signal producing means operative to produce a tracking error signal which is continuously variable in magnitude with the distance between the center line of a target track on said video disc and a detecting point to be placed on said target track and which has polarities respectively corresponding to the directions in which the scanning beam is to be deflected radially of the video disc, comparing means for comparing the output signal from said signal level detector means with a reference signal having a predetermined level for producing an output signal when the former is higher in level than the latter, control signal producing means responsive to said tracking error signal and said output signal from said comparing means and operative to produce a control signal when said tracking error signal has a predetermined level in the presence of the output signal from the comparing means, and switch means provided in the servo loop of said tracking servo system and operative to close the servo loop in response to the control signal delivered from said control signal producing means.

The control signal producing means in the tracking servo system may comprise a comparator circuit operative to compare the tracking error signal with a reference signal having a predetermined level and produce an output signal when the former is higher in level than the latter, a differentiator circuit for differentiating the output signal from the comparator circuit with respect to time, and gating means for passing therethrough the output signal from the differentiator circuit in the presence of the output signal from the comparing means for thereby pro70

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ducing the aforesaid control signal from the control signal producing means.

As an alternative, the control signal producing means included in the tracking servo system may 5 comprise first and second comparator circuits concurrently responsive to the tracking error signal and operative to compare the tracking error signal with a reference signal having a predetermined level for producing output signals when the former is respec-10 tively higher and lower in level than the latter, a two-position switch shiftable between the first and second comparator circuits, a differentiator circuit having an input terminal selectively connected to the first and second comparator circuits across the two 15 position switch and operative to differentiate the output signal from selected one of the first and second comparator circuits with respect to time, and gating means for passing therethrough the output signal from the differentiator circuit in the presence 20 of the output signal from the above mentioned comparing means for thereby producing the control signal from the control signal producing means.

The error signal producing means may in the case of the optical video disc player, comprise means for 25 emitting two index beams of light toward a target track on the video disc for producing two focused spots located in predetermined relationship to the focused spot of the scanning beam incident on the video disc, the relationship being such that the 30 respective areas over which the focused spots of the two index beams incident on the video disc overlap the target track are substantially complementary to each other.

Embodiments of the present invention will now be 35 described with reference to the accompanying drawings in which like reference numerals designate similar or corresponding components and in which:-

Figure 1 is a graph showing a frequency spectrum of a reproducing signal optically read out from a 40 video disc;

Figure2 is a block diagram showing a representative example of a prior-art signal correcting device incorporated in an optical video disc player;

Figure 3 is a block diagram showing a preferred 45 signal correcting device;

Figure 4 is a block diagram showing a device for detecting the presence or absence of recorded information on an optically scanned video disc to be played back in an apparatus;

50 Figures 5A is a view schematically showing a basic concept in which the location of the scanning spot with respect to a target track of an optically scanned video disc is to be detected by the use of index beams of light for producing a tracking error signal 55 for use in a tracking error system according to the present invention for incorporation into an optical video disc player;

Figure 5B is a graph showing an example of the waveform of the tracking error signal produced in 60 the tracking servo system according to the present invention;

Figure 5C is a graph showing an example of the waveform of the audio carrier component extracted from the reproducing signal read outfrom an 65 optically scanned video disc in an apparatus according to the present invention;

Figure 6 is a block diagram showing the circuit arrangement of the tracking servo system according to the present invention; and 70 Figures 7A to 7G are graphs showing various waveforms appearing in the circuit arrangement shown in Figure 6; and

Figure 8 is a block diagram showing a modification of the tracking servo system illustrated in Figure 75 6.

As previously discussed briefly, video and audio information recorded on a video disc is stored in a multiplex form consisting of a video signal carrier which is frequency modulated with, for example, a 80 standard NTSC video signal and two-channel audio signal carriers each of which is frequency modulated with an audio signal. In this instance, the video signal carrier usually has a frequency sweep which ranges between 7.6 MHz corresponding to a black 85 peak and 9.3 MHz corresponding to a white peak, while the two-channel audio signal carriers have carrier frequencies at 2.3 MHZ and 2.8 MHz, respectively, each in a frequency band spanning between plus and minus 100 KHz from the carrier frequency, 90 as indicated in the frequency spectrum illustrated in Figure 1,the video signal carrier wave is represented by CV and the two-channel audio signal carrier waves are represented by Ca ad Ca', respectively.

During reproduction of the video and audio infor-95 mation thus optically, electrostatically or electro-magnetically recorded on a video disc, the signal levels of high frequency components of the reproducing signal read out from the video disc are subject to attenuation as the reading or detecting 100 point of the pick-up device approaches the central area of the disc. For the purpose of compensating for such attenuation of the high frequency components of the reproducing signal, it has been proposed to provide a signal correcting device in the reproduc-105 tion network of a video disc player. Figure 2

diagrammatically shows a representative example of the reproduction network including such a signal correcting device in a prior-art video disc player.

Referring to Figure 2, the reproduction network of 110 a known video disc player comprises a bandpass filter 10 (BPF) and a low-pass filter 12 (LPF). The bandpass and low-pass filters 10 and 12 have respective input terminals connected in parallel to a source (not shown) of a reproducing signal Sr. The 115 reproducing signal Sr picked up from a video disc is thus fed to the bandpass and low-pass filters 10 and 12 and is divided into two frequency modulated carrier components consisting of a video carrier component S\/ having frequencies higher than a 120 predetermined video cut-off frequency and an audio carrier component Sa having frequencies lower than a predetermined audio cut-off frequency. The audio cut-off frequency is lower than or equal to the video cut-off frequency. The video and audio carrier com-125 ponentsthus delivered from the bandpass and low-pass filters 10 and 12 are supplied to first and second demodulators 14 and 16, respectively, for extracting video and audio signals from the demodulated carrier components. Though not shown in the 130 drawings, the source of the reproducing signal Sris

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incorporated in an optical information pickup system including laser beam emitters and an optical lens/ mirror system, as is well known in the art, in the case of an optical video disc player.

5 The signal correcting device incorporated in the reproduction network thus constructed and arranged comprises an equalizer circuit 18 having a control terminal and connected between the source of the reproducing signal Srand the parallel com-10 bination of the bandpass and low-pass filters 10 and 12. The signal correcting device further comprises first and second signal level detectors 20 and 22 having input terminals connected to the output terminals of the bandpass and low-pass filters 10 15 and 12, respectively. The first and second signal level detectors 20 and 22 are thus operative to detect the signal levels of the video and audio carrier components Sv and Sa delivered from the bandpass and low-pass filters 10 and 12, respectively, and to 20 produce output signal which are variable with the signal levels thus detected. The output signals respectively delivered from the first and second level detectors 20 and 22 are supplied to a comparator circuit 24 having an output terminal connected to the 25 control terminal of the equalizer circuit 18. The comparator circuit 24 is operative to compare the respective output signals from the first and second signal level detectors 20 and 24 with each other and to control the equalizer circuit 18 in such a manner as 30 to produce an increased gain at high frequencies when the signal level of the signal level of the video signal component Sv is found to be decreasing as compared with the signal level of the audio signal components S a.

35 As pointed out previously, the signal correcting device of the above described nature has a drawback in that the equalizer circuit forming part of such a device cannot take up the unwanted variation in amplitude of the reproducing signal Sr as caused by 40 the fluctuations in the coefficients of reflection orthe dielectric constant of an information-carrying face of a video disc, the variation in the radiation output of the laser beam scanning the information carrying face of the disc, etc.

45 Figure 3 shows an arrangement to eliminate such a drawback of the signal correcting device incorporated in the reproduction network hereinbefore described with reference to Figure 2. In Figure 3, the reproduction network perse is shown constructed 50 similarly to that illustrated in Figure 2 and, thus, comprises bandpass and low-pass filters 10 and 12 and first and second demodulator circuits 14 and 16 having input terminals connected to the output terminals of the bandpass and low-pass filters 10 55 and 12, respectively. The bandpass and low-pass filters 10 and 12 provided in the embodiment of the present invention constitute first and second filter means adapted to pass therethrough currents having frequencies higher and lowerthan predeter-60 mined cut-off frequencies, respectively. The bandpass filter 10 constituting the first filter means may therefore be substituted by a high-pass filter (not shown). The bandpass and low-pass filters 10 and 12 have respective input terminals jointly connected to 65 a source (not shown) of a reproducing signal Sr through an equalizer circuit 18.

Also similarly to its counterpart in the reproduction network shown in Figure 2, the signal correcting device provided in the reproduction network illustrated in Figure 3 comprises first and second signal level detectors 20 and 22 having input terminals connected to the output terminals of the bandpass and low-pass filters 10 and 12, respectively. In the reproduction network shown in Figure 3, the first signal level detector 20 has an output terminal connected to the control terminal of the equalizer circuit 18 while the second signal level detector 22 has an output terminal connected to a control terminal of an automatic gain control circuit 26 (AGC) which is provided to make up for the inperfec-tions of the equalizer circuit 18.

The automatic gain control circuit 26 is connected between the source of the reproducing signal Srand the input terminal of the equalizer circuit 18 and is constituted by a gain-controlled amplifier adapted to produce a gain which is variable in response to the signal imparted to the control terminal thereof. Controlled by the signal delivered from the second signal detector 22, the automatic gain control circuit 26 is thus operative to produce an output signal having a substantially fixed signal level corresponding to the signal level of the audio carrier component Sa irrespective of the variation, if any, in the signal level of the original reproducing signal Sr supplied to the gain control circuit 26. On the other hand, the signal which is supplied from the second signal level detector20to the control terminal of the equalizer circuit 18 varies the frequency characteristic of the gain produced in the equalizer circuit 18.

Since the audio carrier component Sa extracted from the signal thus delivered from the equalizer circuit 18 has relatively low frequencies, the signal level thereof remains substantially constant throughout the radius of an information-carrying face of a video disc. The signal level of the reproducing signal Sr supplied to the automatic gain control circuit 26 is regulated on the basis of such a constant signal level of the audio carrier component Sa. The reproducing signal supplied through the equalizer circuit 18 to the reproduction network of the network shown in Figure 3 has, for this reason, a constant amplitude notwithstanding the influences of the fluctuations in the coefficients of reflection of an information-carrying face of a video disc, the variation in the radiation output of the laser beam to scan the video disc, orthe like. The performance characteristic of the equalizer circuit 18 can be therefore controlled properly simply on the basis of the signal level of the video carrier component Si/so as to accurately compensate for the attenuation in the signal levels of the high frequency components of the reproducing signal Sr picked up from a central area of an information-carrying face of a video disc.

Turning to Figure 4, another apparatus is characterized by a device for detecting the presence or absence of recorded information on the scanned face of a video disc. Such a device is provided in combination with a signal correcting device similar to that incorporated in the apparatus of Figure 3. Thus, the apparatus shown in Figure 4 includes all

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the circuit components of the apparatus of Figure 3 and comprises filters 10 and 12, demodulators 14 and 16, an equalizer circuit 18, signal level detectors 20 and 22 and an automatic gain control circuit 26, all 5 of which are arranged similarly to their respective counterparts in th apparatus of Figure 3. The apparatus of Figure 4 is thus also based on the discovery that the audio carrier component Sa contained in the original reproducing signal Sr has relatively low 10 frequencies as compared with those of the video carrier component Sv and therefore has a substantially constant signal level throughout the radius of the scanned face of a video disc. The intent of the apparatus of Figure 4 is therefore to detect the 15 presence or absence of recorded information on the scanned face of a video disc through detection of the signal level of the audio carrier component Sa of the reproducing signal Sr so that the presence or absence of the recorded information orthe end of 20 the scanned track on an information-carrying face of a video disc can be detected accurately without respect to the radial location of the reading or detecting point on the disc.

For this purpose, the apparatus of Figure 4 further 25 comprises a two-input comparator circuit 28 having one input terminal connected to the output terminal of the second signal level detector 22. The comparator circuit 28 is thus constantly supplied with a signal variable with the signal level of the audio signal 30 component Sa delivered from the low-pass filter 12. The other input terminal of the comparator circuit 28 is connected to a source (not shown) of a reference signal which is herein assumed to be supplied in the form of a predetermined voltage Er approximating a 35 ground potential. Though not shown in detail, the comparator circuit 28 is constructed and arranged so that an output signal So is delivered therefrom when the voltage level of the signal supplied from the second signal level detector 22 is higher than the 40 predetermined reference voltage Er.

In the presence of audio information in the scanned track of a video disc, the signal delivered from the second signal level detector 22 is higher than the ground potential Er so that the comparator 45 circuit 28 produces the signal So at its output terminal. In the absence, however, of audio information in the scanned track or at the end of the scanned track on a information-carrying face of a video disc, there is no output signal delivered from the second 50 signal level detector 22. Under this condition, the comparator circuit 28 can not produce the output signal So. If there is no audio information in the presence of video information in the scanned track of a video disc, then audio carriers carrying no audio 55 signals are fed from the low-pass filter 12 and enable the second signal level detector 22 to deliver an output signal having a certain voltage level higher than the reference voltage Er which is close to the ground potential. Under this condition, the compara-60 tor circuit 28 also produces the signal So at its output terminal.

In order to produce signals to control the tracking and focus servo systems incorporated in an optical video disc player, two index beams of laser light are 65 radiated onto the information-carrying face of the video disc being scanned by a scanning laser beam. The three beams of laser light are focused at points located in predetermined relationship to each other on the information-carrying face of the video disc 70 being played back. Figure 54 shows an example of such a relationship among the respective focused spots of these three beams, wherein the focused spots of the index beams are denoted by B-! and B2 and the focused spot of the scanning beam is 75 denoted by B3. In the example herein shown, the three beams are assumed to be directed toward a target track T so that one of thefocussed spots Bt and B2 of the index beams overlaps the target track T over one half area of the spot and the other focused 80 spot overlaps the target track T over the other half area of the spot when the focused spot B3 of the scanning beam is correctly located on the target track T. When the focused spots B1f B2 and B3 of the index and scanning beams are thus located with 85 respect to the target track T, the signal level ofthe reproducing signal resulting from the light beam reflected from or passed through the focused spot B3 ofthe scanning beam peaks up. If, furthermore, a tracking error signal is produced through detection 90 ofthe difference between the levels ofthe signals produced from the light beams reflected from or passed through the focused spots B-] and B2 ofthe index beams, the error signal assumes a zero value when the focussed spots Bn and B2 are located as 95 illustrated in Figure 5A since the levels of such signals are substantially equalized under such a condition.

If the scanning beam is them moved radially ofthe video disc in one direction perpendicularto the 100 targer track T as indicated by arrow A in Figure 5A, the level of the tracking error signal will vary sinusoidally as indicated by curve Qa in Figure 5B as the scanning beam is moved from one ofthe parallel tracks or track portions to another. The level ofthe 105 sinusoidal wave Qa varies in proportion to the distance between the center point ofthe focused spot B3 ofthe scanning beam and the center line of the target track closest to the focused spot B3. On the other hand, the polarities ofthe signal indicated by 110 the sinusoidal wave Qa corresponds to the directions in which the focused spot B3 ofthe scanning beam is moving toward and away from the target track. Figure 5C shows an example of the waveform ofthe reproducing signal Sr read out from the target 115 track thus scanned.

In controlling the tracking servo system reliably on the basis of the tracking error signal Qa produced in the above described manner, it is important that the servo loop ofthe tracking servo system be closed 120 when the focused spot B3 ofthe scanning beam is moved close to thetargettrack. If the servo loop is closed when the focused spot B3 ofthe scanning beam is located far ahead of the target track T or, in other words, the tracking error signal Qa is at a 125 relatively high level, the tracking mirror to deflectthe scanning beam in a radial direction ofthe video disc is abruptly initiated into motion attempting to reach the target track T and may overshoot the target track. In an extreme case, the tracking mirror may be 130 caused to oscillate violently and disable the servo

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system from being locked in. This embodiment of the present invention is intended to provide an improved tracking servo system which can be locked in reliably in a stable condition when the servo loop 5 ofthe system is to be closed from an open condition.

Referring to Figure 6 ofthe drawings, the servo loop of such an improved servo system is assumed, by way of example, to be provided in combination with a signal correcting device comprising an 10 equalizer circuit 18, signal level detectors 20 and 22 and an automatic gain control circuit 26 (AGC).

These equalizer circuit 18, signal level detectors 20 and 22 and automatic gain control circuit 26 are constructed and arranged similarly to their respec-15 tive counterpart in the apparatus of Figure 3 in combination with bandpass and low-pass filters 10 and 12 and first and second demodulators 14 and 16 which are also arranged similarly to their respective counterparts in the apparatus of Figure 3. In Figure 6, 20 furthermore, the source ofthe reproducing signal Sr is shown constituted by a photoelectric transducer unit 29 which is adapted to electrically pick up the information read out by the scanning beam incident on a video disc.

25 Now, the servo loop of the tracking servo system provided in combination with such a signal correcting device comprises a pair of, first and second photoelectric transducer units 30 and 32. The transducer units 30 and 32 are responsive to the beams of 30 light reflected from or passed through the focussed spots Bt and B2, respectively, ofthe index laser beams and are operative to produce output signals St and S2, respectively. The signals S, and S2 have signal levels respectively proportional to the areas 35 over which the focussed spots B-| and B2 ofthe index laser beams overlap the target track T on the video disc being played back, as will be understood from the description previously made with reference to Figure 5 A.

40 The output signals St and S2 thus delivered from the first and second photoelectric transducer units 30 and 32 are fed to subtractor means which is shown constituted by a differential amplifier 34 having two input terminals connected to the respec-45 tive output terminals of the transducer units 30 and 32. The differential amplifier 34 is adapted to process the signals St and S2 as, for example, the subtrahend and minuend, respectively, and thereby produce an output signal indicative ofthe difference between 50 the levels ofthe signals St and S2. The output signal produced by the differential amplifier 34 is, thus, the above mentioned tracking error signal Qa and varies sinusoidally as shown in Figure 7 A as the scanning beam is displaced radially of the information-55 carrying face ofthe video disc in a direction indicated by the arrow A in Figure 5A. The differential amplifier 34 has an output terminal connected through an equalizer circuit 36 and across a switch 38 to an amplifier 40 having an output terminal 60 connected to a driver coil 42 for a tracking mirror 44. The tracking mirror 44 is pivotable about an axis fixed in the tracking unit ofthe optical video disc player. The tracking loop shown in Figure 6 further comprises first and second comparing means which 65 are constituted by first and second comparator circuits 46 and 46' each having two, positive and negative input terminals. The positive input terminal ofthe first comparator circuit 46 and the negative input terminal ofthe second comparator circuit 46' are connected jointly to the output terminal ofthe differential amplifier 34, while the negative input terminal of the first comparator circuit 46 and the positive input terminal ofthe second comparator circuit 46' are connected to a source or sources of a predetermined reference voltage V-i as shown. Thus, the first comparator circuit 46 is operative to compare the level ofthe signal Q a with the reference voltage V, and produce pulse signals Qb when the former is higher than the latter. If, in this instance, the reference voltage V-i is selected at the level of a ground potential as indicated in Figure 7A, then the waveform ofthe pulse signals Q b appearing at the output terminal ofthe first comparator circuit 46 becomes such that is shown in Figure 7B. On the other hand, the second comparator circuit 46' is operative to compare the level ofthe output signal Qa from the differential comparator 34 with the predetermined reference voltage and produce pulse signals Qb' when the former is lower than the latter.

The respective output terminals of the first and second comparator circuits 46 and 46' thus arranged are connected across a two-position switch 48 and through a digital low-pass filter 50 (LPF) to the input terminal of a differentiator circuit 52. The digital low-pass filter 50 may be realized by using a retrigerrable monostable multivibrator. The two-position switch 48 controlled to selectively provide connection between the first comparator circuit 46 and the low-pass filter 50 when the scanning beam is moved in the direction ofthe arrow A shown in Figure 5A, or connection between the second comparator circuit 46' and the low-pass filter 50 when the scanning beam is moved in the opposite direction indicated by A' in Figure 5A. The pulse signals Qb or Qb' delivered respectively from the first or second comparator circuit 46 or 46' and passed through the two-position switch 48 and the low-pass filter 50 is differentiated with respect to time in the differentiator circuit 52. Thus, the differentiator circuit 52 produces impulse signals Qc indicated in Figure 7C when the two-position switch 48 is in a position providing connection from the first comparator circuit 46 to the low-pass filter 50 as shown in Figure 6. The digital low-passfilter 50 is provided for the purpose of enabling the tracking servo system to be locked in at low frequencies of the tracking error signal Qa because the servo system might be disabled from being locked in when the tracking error signal Qa occurs at excessively high frequencies. When the scanning or deflecting rate ofthe detecting point is increased, the cut-off frequency of the low-pass filter 50 is also increased with the increase in the deflecting rate. For this purpose, the low-pass filter 50 has a cut-off frequency control terminal connected to a scanning rate shifter 54 so that the cut-off frequency ofthe filter is variable with the scanning rate selected by the scanning rate shifter 54.

The impulse signal Qc delivered from the differen70

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tiator circuit 52 are fed to one input terminal of a two-input logic "AND" gate circuit 56. The other input terminal ofthe "AND" gate circuit 56 is connected to the output terminal of third comparing 5 means which is shown constituted by a comparator circuit 58 having positive and negative input terminals. The negative input terminal ofthe comparator circuit 58 is connected to a source of a predetermined reference voltage V2, while the positive input 10 terminal ofthe comparator circuit 58 is connected to the second signal detector 22 ofthe previously described signal correcting device incorporated in the reproduction network as shown.

In the embodiment illustrated in Figure 6, the 15 second signal level detector 22 ofthe signal correcting device has a first output terminal connected to the control terminal ofthe automatic gain control circuit 26 and a second input terminal connected to the positive input terminal ofthe above mentioned 20 comparator circuit 58. Though not shown in the drawings, furthermore the second signal level detector 22 ofthe signal correcting device is largely composed of an envelope detector for detecting the envelope ofthe frequency modulated audio signal 25 carrier Sa delivered from the low-pass filter 12 and a smoother circuit for converting the output signal of the envelope detector into a signal for controlling the automatic gain control circuit 26. The output terminal ofthe smoother circuit constitutes the above 30 mentioned first input terminal ofthe signal level detector 22 and the output terminal ofthe envelope detector constitutes the above mentioned second output terminal ofthe signal level detector 22. To the positive terminal ofthe comparator circuit 58 is thus 35 impressed a signal Qd indicative ofthe envelope of the frequency modulated audio signal carrier Sa. An example of the waveform of such a signal Qc/is indicated in Figure 7D.

The comparator circuit 58 is adapted to compare 40 the level ofthe output signal Qd from the signal level detector 22 with the reference voltage V2 and produce a series of positive pulse signals Qe when the former is higherthan the latter as will be seen from Figures 7D and 7E. The pulse signals Qa are fed 45 to one input terminal ofthe logic "AND" gate circuit 56 so that the impulse signals Qc supplied from the differentiator circuit 52 are selectively passed through the "AND" gate circuit 56 as indicated by Qf in Figure 7F in the presence of the pulse signals Qe 50 from the comparator circuit 58. The output terminal ofthe "AND" gate circuit 56 is connected to a control terminal ofthe switch 38.

The leading and trailing edges ofthe pulse signals Qjb supplied from the first comparator circuit 46 55 correspond to the zero value of the tracking error signal Qa produced when the scanning beam is deflected in the direction ofthe arrow A in Figure 5A. More specifically, each of the trailing edges ofthe pulse signals Qb indicates that the focused spot B3 of 60 the scanning beam is correctly located on each ofthe tracks or track portions shown in Figure 5A while each ofthe leading edges ofthe pulse signals Q b indicates that the focussed spo t B3 ofthe scanning beam is located centrally between every adjacent 65 two of the tracks or track portions. Supplied with such pulse signals Sb, the differentiator circuit 52 produces alternately positive and negative impulse signals Qc which are positive in response to the trailing edges ofthe pulse signals Q b and which are 70 negative in response to the leading edges ofthe pulse signals Qb.

On the other hand, the pulse signals Qe supplied from the comparator circuit 58 indicate that the frequency modulated audio signal carrier Sa is at 75 peak levels or close to the peak levels. Such pulse signals Qe are fed as gate signals to the "AND" gate circuit 56 so that only the positive ones ofthe impulse signals Qc are passed through the "AND" gate circuit 56. Thus, each ofthe impulse signals Q f 80 delivered from the "AND" gate circuit 56 occurs when the amount of tracking error is minimum and concurrently the audio signal carrier Sa is at or close to a peak value. The switch 38 is closed at such a timing so that the tracking servo system can be 85 locked in accurately and reliably.

It may be mentioned that the tracking servo system can be locked in not only in the presence of an audio signal in the original reproducing signal Sr but also in the absence of an audio signal in the 90 reproducing signal Sr but also in the absence of an audio signal in the reproducing signal Sr provided the reproducing signal Sr contains only video information. This is because of the fact that the reproducing signal Sr containing video information contains 95 an audio carrier although the signal may not contain audio information.

When the scanning beam is deflected to move in a direction indicated by the arrow A' in Figure 5A with respect to the tracks or track portions on a video disc, 100 the two-position switch 48 is shifted to a position providing connection from the second comparator circuit 46' to the low-pass filter 50. Under such a condition, the tracking error signal delivered from the differential amplifier 34 takes a waveform shown 105 in Figure7G so that the pulse signals Q b' produced by the second comparator circuit 46' appear in a waveform similar to the waveform ofthe pulse signals Qb shown in Figure 7B. The circuit components subsequent to the comparator circuit 46' thus 110 operate similarly to those connected to the first comparator circuit 46.

Although it has been assumed that the servo loop ofthe embodiment of Figure 6 is closed during scanning of the tracks on a video disc, the servo loop 115 is controlled in a similar manner when the loop is to be closed during other operational conditions;

While, furthermore, it has been assumed that the tracking servo system embodying the present invention is provided in combination with the signal 120 correcting device including the automatic gain control circuit 26, viz., forming part ofthe apparatus Figure 3, such a system can be provided in combination with a prior-art signal correcting device ofthe nature shown in Figure 2, viz., a signal correcting 125 device which is devoid ofthe automatic gain control circuit. Figure 8 shows such an embodiment ofthe present invention.

In the embodiment illustrated in Figure 8, the second signal level detector 22 forming part ofthe 130 signal correcting device is constituted by an en

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velope detector circuit adapted to detect the envelope of the frequency modulated audeo signal carrier Sa delivered from the preceding low-pass filter 12. The output terminal of such an envelope 5 detector circuit constitutes the output terminal ofthe signal level detector 22 perse and is connected solely to the positive input terminal ofthe comparator circuit 58.

Claims

10 CLAIMS

1. A video-disc information reading and reproducing apparatus including a source of a reproducing signal read out from a video disc scanned by a

15 scanning beam of light which is deflected radially of the video disc and a signal correcting device which is operative to compensate for the attentuation ofthe signal level of high frequency components of said reproducing signal and which includes signal level 20 detector means operative to produce an output signal variable with the signal level of a frequency modulated audio carrier component contained in the reproducing signal, and a tracking servo system for deflecting said scanning beam radially of said video 25 disc, the tracking servo system comprising: error signal producing means operative to produce a tracking error signal which is continuously variable in magnitude with the distance between the center line of a target track on said video disc and a 30 detecting point to be placed on said target track and which has polarities respectively corresponding to the directions in which the scanning beam is to be deflected radially ofthe video disc, comparing means for comparing the output signal from said 35 signal level detector means with a reference signal having a predetermined level for producing an output signal when the former is higher in level than the latter, control signal producing means responsive to said tracking error signal and said output 40 signal from said comparing means and operative to produce a control signal when said tracking error signal has a predetermined level in the presence of the output signal from the comparing means, and switch means provided in the servo loop of said 45 tracking servo system and operative to close the servo loop in response to the control signal delivered from said control signal producing means.

2. Apparatus as claimed in claim 1, in which said control signal producing means comprises first and

50 second comparator circuits concurrently responsive to said tracking error signal and operative to compare the tracking error signal with a reference signal having a predetermined level close to ground potential for producing output signals when the former is 55 respectively higher and lower in level than the latter, a two-position switch shiftable between said first and second comparator circuits, a differentiator circuit having an input terminal selectively connected to said first and second comparator circuits 60 across said two-position switch and operative to differentiate the output signal from selected one of the first and second comparator circuits with respect to time, and gating means for passing therethrough the output signal from said differentiator circuit in 65 the presence ofthe output signal from said comparing means for producing said control signal from said control signal producing means.

3. An apparatus as claimed in claim 2, in which said control signal producing means further comprises a low-pass filter connected between said two-position switch and said differentiator circuit.

4. An apparatus as claimed in claim 3, in which said control signal producing means further comprises means for varying the cut-off frequency of said low-pass filter in accordance with the rate at which said detecting point is scanned radially of said video disc.

5. An apparatus as claimed in claim 1 or claim 2, in which said error signal producing means comprises means for emitting two index beams of light toward a target track on said video disc for producing two focused spots located in predetermined relationship to the focused spot ofthe scanning beam incident on said video disc, said relationship being such that the respective areas over which the focused spots ofthe index beams incident on the video disc overlap the target track are substantially complementary to each other.

6. An apparatus as claimed in claim 5, in which said error signal producing means further comprise two photoelectric transducer means which are respectively operative to produce output signals continuously variable with said areas, and subtractor means for producing as said tracking error signal an output signal continuously variable with the difference between the respective levels ofthe output signals from said transducer means.

New claims or amendments to claims filed on 20/2/84

Superseded claims 1 New or amended claims:-

1. A video-disc information reading and reproducing apparatus including a source of a reproducing signal, the signal being produced from a video disc by a scanning beam of light which is deflected radially ofthe video disc, and a signal correcting device which is operative to compensate for the attentuation ofthe signal level of high frequency components of said reproducing signal and which includes signal level detector means operative to produce an output signal variable with the signal level of a frequency modulated audio carrier component contained in the reproducing signal, and a tracking servo system for deflecting said scanning beam radially of said video disc, the tracking servo system comprising: error signal producing means operative to produce a tracking error signal which is continuously variable in magnitude with the distance between the center line of a target track on said video disc and a detecting point to be placed on said target track and which has polarities respectively corresponding to the directions in which the scanning beam is to be deflected radially ofthe video disc, comparing means for comparing the output signal from said signal level detector means with a reference signal having a predetermined level for producing an output signal when the former is higher in level than the latter, control signal produc-

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ing means responsive to said tracking error signal and said output signal from said comparing means and operative to produce a control signal when said tracking error signal differs from a predetermined 5 level in the presence ofthe output signal from the comparing means, and switch means provided in the servo loop of said tracking servo system and operative to close the servo loop in response to the control signal delivered from said control signal 10 producing means.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1984.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.