Tracking servo system of recording disc information recording and reproducing apparatus

A tracking servo system of a video or audio disc player, wherein the control data signals extracted from the electric signals read out from an information-carrying face of a video or audio recording disc are inhibited from being delivered from the servo system when the servo loop forming part of the system is open.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to an apparatus for recording and 
reproducing the information carried on a recording disc such as, for 
example, a video or audio disc to be optically, electrostatically or 
otherwise scanned. Particularly, the present invention is concerned with a 
signal processing circuit in a recording-disc reading and reproducing 
apparatus. 
In a recording-disc information reading and reproducing apparatus of, for 
example, the optically or electrostatically scanning type, multiplex 
signals consisting of frequency modulated video and/or audio carriers 
superposed on each other are stored in the form of a series of depressed 
areas or "pits" formed in each or one information-carrying face of a video 
or audio disc. These depressed areas or pits are arranged in a spiral 
track or in a number of concentric tracks about the center axis of the 
disc. The video and/or audio information thus stored in the recording disc 
is read out by optically, electrostatically or otherwise scanning the 
individual pits along the spiral track or each of the concentric tracks. 
In the case of an optically scanning video or audio information reading 
and reproducing apparatus, for example, the video and/or audio information 
stored on a video or audio recording disc is read out by scanning the pits 
by means of a beam of laser light and thereby detecting the lengths of and 
spacings between the scanned pits. During playback of such a recording 
disc or during selection of desired pieces of information out of the 
information carried on the information-carrying face of the disc to be 
played back, the disc is driven for rotation about the center axis thereof 
and the beam of the laser light is displaced radially of the 
information-carrying face of the rotating disc. The laser beam thus 
directed onto a target track on the information-carrying face of the disc 
is reflected from the face or passed through the disc and the information 
picked up by the laser beam from the information-carrying face of the disc 
is converted into electric signals. These electric signals are further 
converted upon frequency demodulation into video and/or audio signals to 
be reproduced. 
The laser beam to read out the information recorded on an 
information-carrying face of a video or audio disc is transmitted from a 
laser emitter to a tracking mirror through a lens system and is reflected 
from the tracking mirror toward the information-carrying face of the disc 
to be played back or scanned by the beam. The tracking mirror is turnably 
mounted on a slider movable back and forth in a radial direction of the 
disc to be played back or scanned and is urged to stay in a predetermined 
neutral or home angular position about the axis of rotation thereof on the 
slicer. During scanning of the video or audio disc to be played back, the 
slider is driven to travel in such a direction with respect to the disc 
and, concurrently, the tracking mirror is driven to turn between two 
opposite limit angular positions from the neutral or home angular position 
thereof about the axis of rotation of the mirror on the slider. These 
motions of the slider and the tracking mirror are effected under the 
control of a tracking servo system. The tracking servo system is arranged 
so that the servo loop to control the oscillating motions of the tracking 
mirror is closed when or after the laser beam directed toward the target 
track of the disc is incident on a spot close to the target track. If the 
servo loop is closed and the servo system is locked in before the scanning 
spot of the 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 beam in a radial direction of the video 
or audio disc is abruptly initiated into motion to reach the target 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 
from 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. 
When the tracking mirror being turned about the axis of rotation thereof 
reaches one of the predetermined limit angular positions thereof, the 
tracking servo system controls the servo loop for the tracking mirror to 
open so as to allow the mirror to return to the neutral or home angular 
position thereof by the action of, for example, a return spring connected 
to or otherwise engaging the mirror. When the tracking mirror is thus 
returned to the neutral or home angular position thereof, the tracking 
servo system controls the servo loop for the mirror to close for a second 
time for enabling the mirror to repeat the oscillating motions. 
The signals which are read out from an information-carrying face of a video 
or audio disc contain not only the video and/or audio signals to be 
reproduced but various control data signals such as the address signals 
indicative of the prescribed addresses in the spiral track or each of the 
concentric tracks on the information-carrying face of the disc. If there 
control data signals are used for the control of the movement of the 
mirror-carrying slider while the servo loop for the tracking mirror is 
open, the slider may be controlled erroneously and may fail to correctly 
scan the track or tracks on the information-carrying face of the disc 
being scanned by the beam. 
The present invention contemplates provision of an improved tracking servo 
system which is free from such a problem. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a 
recording-disc reading and reproducing apparatus which includes a pick-up 
unit for producing a scanning spot on a recording disc and for deflecting 
said scanning spot radially of said recording-disc in response to a drive 
signal in order to make the scanning spot to trace a target track of said 
recording disc, and for producing an electric reproducing signal 
representative of a track signal recorded on said target track, said track 
signal containing an information signal to be reproduced and control data 
signals such as address signals, which is characterized by a tracking 
servo system including tracking error signal producing means operative to 
produce a tracking error signal continuously variable in magnitude with an 
amount of deviation, if any, of said scanning spot from said target track, 
and a pick-up unit actuating circuit for producing said drive signal in 
response to said tracking error signal; and a signal processing means 
including control data signal extracting means adapted to be operative for 
extracting said control data signal from said track signal only when said 
tracking servo system operates stably. 
The tracking servo system thus constructed and arranged basically may 
further comprise a tracking servo loop for controlling the displacement of 
the scanning spot with respect to the recording disc, and switch means 
provided in the tracking servo loop and operative to be open in response 
to the output signal of the tripping means. In this instance, the tracking 
servo system may further comprise signal level detector means for 
detecting the signal levels of the signals produced by the detecting and 
transducing means and an output signal variable with the detected signal 
levels, comparing means for comparing the output signal from the signal 
level detector means with a reference signal having a predetermined level 
for producing an output signal when the former is higher in magnitude than 
the latter, and control signal producing means responsive to the tracking 
error signal and operative to produce a control signal when the tracking 
error signal is higher in magnitude than a reference signal having a 
predetermined level in the presence of the output signal from the 
comparing means, the switch means being operative to close in response to 
the control signal produced by the control signal producing means. The 
tripping means provided in the tracking system thus constructed and 
arranged may be connected between the control signal producing means and 
the switch means and between the switch means and the control data signal 
cut-off means and responsive to the control signal from the control signal 
producing means and to the signal passed through the switch means, the 
tripping means being operative to close the switch means in response to 
the control signal from the control signal producing means in the absence 
of the aforesaid actuating signal at the control terminal of the control 
data signal cut-off means and to cause the switch means to open in the 
presence of the actuating signal at the control terminal of the control 
data signal cut-off means regardless of the presence and absence of the 
control signal from the control signal producing means. 
As an alternative, the tripping means provided in the tracking servo system 
constructed and arranged basically as hereinbefore set forth may comprise 
an inhibitor network connected between the detecting and transducing means 
and the control data signal cut-off means and responsive to the signals 
delivered from the detecting and transducing means, the inhibitor network 
being operative to produce the aforesaid variable signal variable with the 
signals from the detecting and transducing means and to compare the 
variable signal with the first named reference signal for delivering an 
output signal as the aforesaid actuating signal to the control terminal of 
the control data signal cut-off means when the former is higher in 
magnitude than the latter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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 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 with each other on 
the information-carrying face of the video disc being played back. FIG. 1A 
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.sub.1 and B.sub.2 and the focused spot of the scanning beam 
is denoted by B.sub.3. In the example herein shown, the three beams are 
assumed to be directed toward a target track T so that one of the focused 
spots B.sub.1 and B.sub.2 of the index beams overlaps the target track T 
over one half area of the spot and the other focused spot overlaps the 
target track T over the other half area of the spot when the focused spot 
B.sub.3 of the scanning beam is correctly located on the target track T. 
When the focused spots B.sub.1, B.sub.2 and B.sub.3 of the index and 
scanning beams are thus located with respect to the target track T, the 
signal level of the reproducing signal Sr resulting from the light beam 
reflected from or passed through the focused spot B.sub.3 of the scanning 
beam peaks up. If, furthermore, a tracking error signal is produced 
through detection of the difference between the levels of the signals 
produced from the light beams reflected from or passed through the focused 
spots B.sub.1 and B.sub.2 of the index beams, the error signal assumes a 
zero value when the focused spots B.sub.1 and B.sub.2 are located as 
illustrated in FIG. 1A since the levels of such signals are substantially 
equalized under such condition. 
If the scanning beam is then displaced radially of the information-carrying 
face of the video disc in one direction perpendicular to the target track 
T as indicated by arrow A in FIG. 1A, the level of the tracking error 
signal will vary sinusoidally as indicated by curve Qa in FIG. 1B as the 
scanning beam is moved from one of the parallel tracks or track portions 
to another. The level of the sinusoidal wave Qa varies in proportion to 
the distance between the center point of the focused spot B.sub.3 of the 
scanning beam and the center line of the target track closest to the 
focused spot B.sub.3. On the other hand, the polarities of the signal 
indicated by the sinusoidal wave Qa corresponds to the directions in which 
the focused spot B.sub.3 of the scanning beam is moving toward and away 
from the target track. FIG. 1C shows an example of the waveform of the 
reproducing signal Sr read out from the target 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 of the tracking servo system be closed when 
the focused spot B.sub.3 of the scanning beam is moved close to the target 
track. If the servo loop is closed when the focused spot B.sub.3 of the 
scanning beam is located far ahead of the target track T or, in other 
words, the tracking error signal Qa is at a relatively high level, the 
tracking mirror adapted to deflect of displace the scanning beam in a 
radial direction of the information-carrying face of the 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 caused to oscillate violently and disable the servo system from 
being locked in. The embodiment of the present invention is intended to 
provided an improved tracking servo system which can be locked in reliably 
in a stable condition when the servo loop of the system is to be closed 
from an open position. 
Referring to FIG. 2 of the drawings, the servo loop of such an improved 
servo system is shown comprising first, second and third photoelectric 
transducer units 10, 12 (PET) and 14 each adapted to convert luminous 
information into a corresponding electric signal. The first and second 
transducer units 10 and 12 are responsive to the beams of light reflected 
from or passed through the focused spots B.sub.1 and B.sub.2, 
respectively, of the index laser beams and are operative to produce output 
signals S.sub.1 and S.sub.2, respectively. The signal S.sub.1 and S.sub.2 
have signal levels respectively proportional to the areas over which the 
focused spot B.sub.1 and B.sub.2 of the index laser beam overlap the 
target track T on the video disc being played back, as can be understood 
from the description previously made with reference to FIG. 1A. On the 
other hand, the third photoelectric transducer unit 14 is adapted to pick 
up the information read out by the scanning beam incidental on the 
information-carrying face of the video disc. Thus, the third photoelectric 
transducer unit 14 produces a reproducing signal Sr containing the video 
and audio information to be reproduced. 
The output signals S.sub.1 and S.sub.2 delivered from the first and second 
photoelectric transducer units 10 and 12 are fed to subtractor means which 
is constituted by a differential amplifier 16 having two input terminals 
connected to the respective output terminals of the transducer units 10 
and 12. The differential amplifier 16 is adapted to process the signals 
S.sub.1 and S.sub.2 as, for example, the subtrahend and minuend, 
respectively, and thereby produce an output signal indicative of the 
difference between the levels of the signals S.sub.1 and S.sub.2. The 
output signal produced by the differential amplifier 16 is, thus, the 
above mentioned tracking error signal Qa and varies sinusoidally as shown 
in FIG. 1B as the scanning beam is displaced radially of the 
information-carrying face of the video disc in a direction indicated by 
the arrow A in FIG. 1A. 
The differential amplifier 16 has an output terminal connected through an 
equalizer circuit 18 (EQL) and across a switch 20 to an amplifier 24 (AMP) 
having an output terminal connected to a driver coil 26 for a tracking 
mirror 28. The tracking mirror 28 is pivotable about an axis fixed in the 
tracking unit of the optical video disc player. The switch 20 has a 
control terminal and is closed when the control terminal thereof is 
supplied with an actuating signal. 
The driver coil 26 for the tracking mirror 28 is grounded through a 
resistor 30. The voltage across the resistor 30 is constantly monitored by 
suitable monitoring means which is herein shown comprising a series 
combination of a window comparator 32 (COMP) and a monostable vibrator 34 
(MMV). The window comparator 32 is adapted to pass therethrough voltages 
which are higher and lower than predetermined positive and negative 
limits, respectively, having equal magnitudes. When the absolute value of 
the voltage across the resistor 30 becomes higher than the absolute value 
of such limits, the window comparator 32 produces an output signal such as 
a logic "1" signal and thereby triggers the monostable multivibrator 34. 
The absolute value of the positive and negative limits registered on the 
window comparator 32 is selected to correspond to the angle of a 
predetermined limit angular position of the tracking mirror 28 from a 
predetermined neutral or home angular position of the mirror about the 
axis of the mirror. The output terminal of the monostable multivibrator 34 
is connected to the reset terminal R of a set-reset flipflop circuit 36 
(FF). When triggered by the output signal from the window comparator 32, 
the monostable multivibrator 34 thus produces a single-shot signal and 
thereby resets the flipflop circuit 36. The set-reset flipflop circuit 36 
has a non-inverted signal output terminal Q and an inverted signal output 
terminal Q. The non-inverted signal output terminal Q of the flipflop 
circuit 36 is connected to the previously mentioned control terminal of 
the switch 20 forming part of the tracking servo loop. 
The tracking loop shown in FIG. 2 further comprises control signal 
producing means which includes first and second comparator circuits 38 and 
38' each having two, positive and negative input terminals. The positive 
input terminal of the first comparator circuit 38 and the negative input 
terminal of the second comparator circuit 38' are connected jointly to the 
output terminal of the differential amplifier 16, while the negative input 
terminal of the first comparator circuit 38 and the positive input 
terminal of the second comparator circuit 38' are connected to a source 
or sources of a reference signal having a predetermined voltage V.sub.1 as 
shown. Thus, the first comparator circuit 38 is operative to compare the 
level of the signal Qa with the predetermined voltage V.sub.1 and produce 
pulse signals Qb when the former is higher than the latter. On the other 
hand, the second comparator circuit 38' is operative to compare the level 
of the output signal Qa from the differential comparator 16 with the 
predetermined voltage V.sub.1 and produce pulse signals Qb' when the 
former is lower than the latter. 
The respective output terminals of the first and second comparator circuits 
38 and 38' thus arranged are connected across a two-position switch 40 and 
through a digital low-pass filter 42 (LPF) to the inputer terminal of a 
differentiator circuit 44 (DIF). The two-position switch 40 is controlled 
to selectively provide connection between the first comparator circuit 38 
and the low-pass filter 42 when the scanning beam is displaced in the 
direction of the arrow A shown in FIG. 1A, or connection between the 
second comparator circuit 38' and the low-pass filter 42 when the scanning 
beam is displaced in the opposite direction indicated by A' in FIG. 1A. 
The pulse signals Qb or Qb' delivered respectively from the first or 
second comparator circuit 38 or 38' and passed through the two-position 
switch 40 and the low-pass filter 42 is differentiated with respect to 
time in the differentiator circuit 44. Thus, the differentiator circuit 44 
produces impulse signals Qc when the two-position switch 40 is in a 
position providing connection from the first comparator circuit 38 to the 
low-pass filter 42 as shown in FIG. 2. The digital low-pass filter 42 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 
rate is increased, the cut-off frequency of the low-pass filter 42 is also 
increased with the increase in the scanning rate which, in this instance, 
is defined as the velocity at which the scanning beam directed onto the 
information-carrying face of the video disc is displaced radially of the 
particular face of the disc. For this purpose, the low-pass filter 42 has 
a cut-off frequency control terminal connected to a scanning rate shifter 
46 (SRS) so that the cut-off frequency of the filter is variable with the 
scanning rate selected by the scanning rate shifter 46. 
The impulse signals Qc delivered from the differentiator circuit 44 are fed 
to one input terminal of a two-input logic "AND" gate circuit 48. The 
other input terminal of the "AND" gate circuit 48 is connected to the 
output terminal of comparing means which is constituted by a comparator 
circuit 50 having positive and negative input terminals. The negative 
input terminal of the comparator circuit 50 is connected to source of a 
reference signal having a predetermined voltage V.sub.2, while the 
positive input terminal of the comparator circuit 50 is connected to 
signal level detector means adapted to detect the signal level of the 
reproducing signal Sr and to produce an output signal Qd variable with the 
detected signal level. In the arrangement shown in FIG. 2, such detector 
means is shown comprising an envelope detector circuit 52 (ENV. DET) 
operative to detect the level of the envelope of the frequency modulated 
reproducing signal Sr of, particularly, the envelope of the audio carrier 
component of the reproducing signal Sr and produces as the above mentioned 
signal Qd a signal variable with the detected level of the envelope. To 
the positive terminal of the detector circuit 52 is thus impressed the 
signal Qd indicative of the level of the envelope of the frequency 
modulated reproducing signal Sr or the audio carrier component thereof. 
The comparator circuit 50 is adapted to compare the level of the output 
signal Qd from the envelope detector 52 with the predetermined voltage 
V.sub.2 and produce a series of positive pulse signals Qe when the former 
is higher than the latter. The pulse signals Qe are fed to one input 
terminal of the logic "AND" gate circuit 48 so that the impulse signals Qc 
supplied from the differentiator circuit 44 are selectively passed through 
the "AND" gate circuit 48 as indicative at Qf in FIG. 2 in the presence of 
the pulse signals Qe from the comparator circuit 50. The output terminal 
of the "AND" gate circuit 48 is connected to the set terminal S of the 
flipflop circuit 36, the non-inverted signal output terminal Q of which is 
connected to the control terminal of the switch 20 of the tracking servo 
loop as previously noted. 
The leading and trailing edges of the pulse signals Qb supplied from the 
first comparator circuit 38 correspond to the zero value of the tracking 
error signal Qa produced when the scanning beam is displaced in the 
direction of the arrow A in FIG. 1A with respect to the 
information-carrying face of the video disc being played back. More 
specifically, each of the trailing edges of the pulse signals Qb indicates 
that the focused spot B.sub.3 of the scanning beam is correctly located on 
each of the parallel tracks portions shown in FIG. 1A while each of the 
leading edges of the pulse signals Qb indicates that the focused spot 
B.sub.3 of the scanning beam is located centrally between every adjacent 
two of the parallel tracks or track portions. Supplied with such pulse 
signals Qb, the differentiator circuit 44 produces alternately positive 
and negative impulse signals Qc which are positive in response to the 
trailing edges of the pulse signals Qb and negative in response to the 
leadihng edges of the pulse signals Qb. 
On the other hand, the pulse signals Qe supplied from the comparator 
circuit 50 indicate that the frequency modulated reproducing signal Sr or 
the audio carrier component thereof is at peak levels or close to the peak 
levels. Such pulse signals Qe are fed as gate signals to the "AND" gate 
circuit 48 so that only the positive ones of the impulse signals Qc are 
passed through the "AND" gate circuit 48. Thus, each of the impulse 
signals Qf delivered from the "AND" gate circuit 48 occurs when the amount 
of tracking error is minimum and concurrently the level of the reproducing 
signal Sr or the audio carrier component thereof is at or close to a peak 
value. The switch 20 is closed at such a timing that the tracking servo 
system can be 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 reproducing signal Sr 
provided the reproducing signal Sr contains video information. This is 
because of the fact that the reproducing signal Sr containing video 
information contains an audio carrier although the signal may not contain 
audio information. 
When, on the other hand, the voltage across the resistor 30 connected to 
the driver coil 26 for the tracking mirror 28 is higher than a 
predetermined upper limit or lower than a predetermined lower limit which 
is equal in magnitude to the upper limit, the window comparator 32 
triggers the monostable multivibrator 34 and thereby causes the flipflop 
circuit 36 to be reset. The switch 20 of the servo loop for the tracking 
mirror 28 is now made open and prevents the tracking mirror 28 from being 
turned beyond its predetermined limit angular position about the axis of 
rotation thereof. The time constant of the monostable vibrator 34 is thus 
selected in consideration of the period of time for which the tracking 
mirror 28 is forced to restore its proper angular position by the action 
of the resilient biasing means (not shown) provided in association 
therewith after the switch 20 is made open. 
When the scanning beam is displaced in a direction indicated by the arrow 
A' in FIG. 1A with respect to the parallel tracks or track portions on a 
video disc, the two-position switch 40 is shifted to a position providing 
connection from the second comparator circuit 38' to the low-pass filter 
42 therethrough. Under such conditions, the tracking error signal 
delivered from the differential amplifier 16 takes a waveform so that the 
pulse signals Qb' produced by the second comparator circuit 38' appear in 
a waveform similar to the waveform of the pulse signals Qb shown in FIG. 
1B. The circuit components subsequent to the comparator circuit 38' thus 
operate similarly to those connected to the first comparator circuit 38. 
Although it has been assumed that the tracking servo loop of the embodiment 
of FIG. 2 is closed during scanning of the tracks on a video disc, the 
servo loop is controlled in a similar manner when the loop is to be closed 
during other operational conditions. 
In the tracking servo system shown in FIG. 2, there is further provided a 
control data signal extracting network 54 for extracting control data 
signals out of the output signals delivered from the third photoelectric 
transducer unit 14. As is well known in the art, the pieces of 
information, which the scanning beam traces in a a track on the 
information-carrying face of a video disc include not only the video and 
audio signals to be reproduced but control data signals such as the 
address signals which are representative of the addresses in the track 
being traced. These control data signals appear during each of prescribed 
discrete periods of cycle during which the video and audio signals as well 
as the synchronizing signals concomitant therewith in the waveform of the 
signals delivered from the phototranducer unit 14 are blanked out. Such 
control data signals are used primarily as the basic control signals for 
the tracking servo system, especially for the control of the linear 
movement of the mirror carrying slider (not shown) driven to move under 
the control of the tracking servo system. 
In the tracking servo system shown in FIG. 2, the control data signal 
extracting network 54 for extracting such signals is shown comprising a 
frequency demodulator circuit 56 (DEMOD) having an input terminal which is 
connected to the output terminal of the third phototransducer unit 14 and 
which is adapted to frequency demodulate the output signals delivered from 
the transducer unit 14. The frequency demodulator circuit 56 is operative 
to produce multiplex signals containing the video and audio signals to be 
reproduced and the above mentioned control data signals. These control 
data signals are fed to a clamping circuit 58 (CLAMP) and a period 
detector circuit 60 (PERIOD DET) which are commonly connected in parallel 
to the output terminal of the frequency demodulator circuit 56. The 
clamping circuit 58 is adapted to add a fixed or adjustable bias to the 
waveform of the signals delivered from the frequency demodulator circuit 
56. On the other hand, the period detector circuit 60 is adapted to detect 
the prescribed discrete periods of cycle during which the control data 
signals such as the above mentioned address signals contained in the 
output signals from the frequency demodulator circuit 56 appear and to 
thereby produce pulse signals each lasting for a period of time 
corresponding to each of such discrete periods of cycle. The clamping 
circuit 58 and the period detector circuit 60 thus operative have 
respective output terminals connected to a gate circuit 62 (GATE). The 
gate circuit 62 has an input terminal connected to the clamping circuit 58 
and a trigger terminal connected to the period detector circuit 60 and is, 
thus, enabled to pass therethrough the output signals from the clamping 
circuit 58 only in the presence of signals appearing at the output 
terminal of the period detector circuit 60. Only those control data 
signals are address signals which are contained in the output signals from 
the frequency demodulator circuit 56 are in this manner passed through the 
gate circuit 62. The clamping circuit 58 is provided simply for the 
purpose of upgrading the quality of the control data signals to be used as 
the basic control signals for controlling the linear movement of the 
tracking-mirror carrying slider and, for this reason, may be dispensed 
with if desired. In this instance, the output terminal of the frequency 
demodulator circuit 56 is connected directly to the input terminal of the 
gate circuit 62 as well as to the input terminal of the period detector 
circuit 60. 
In accordance with the present invention, the control data signal 
extracting network 54 is connected to a control data signal cut-off 
network 64 which, in the embodiment of FIG. 2, comprises a gate circuit 66 
(GATE) having an input terminal connected to the output terminal of the 
above mentioned gate circuit 62 and a control terminal connected to the 
inverted signal output terminal Q of the previously described flipflop 
circuit 36, as shown. The gate circuit 66 constituting the signal cut-off 
network 64 is, thus, enabled to pass therethrough the output signals from 
the gate circuit 62 only in the presence of a pulse signal appearing at 
the inverted signal output terminal Q of the flipflop circuit 36. 
As will have been understood from the previous description, a signal of, 
for example, a logic "1" value appears at the non-inverted signal output 
terminal Q of the flipflop circuit 36 and holds the switch 20 of the 
tracking servo loop closed in the presence of a logic "1" signal appearing 
at the set terminal S of the flipflop circuit 36. In this instance, a 
signal of, for example, a logic "0" value appears at the inverted signal 
output terminal Q of the flipflop circuit 36. The gate circuit 66 is 
assumed, by way of example, to be constructed and arranged in such a 
manner as to remain operative in the presence of the logic "0" signal at 
the control terminal thereof. The control data signal cut-off network 64 
provided in the tracking servo system shown in FIG. 2 is, in this fashion, 
held in a condition passing therethrough the control data or address 
signals passed through the gate circuit 62 of the control data extracting 
network 54 when the switch 20 in the tracking servo loop is kept closed. 
When, on the other hand, the monostable multivibrator 34 is triggered to 
deliver an output signal of, for example, a logic "1" value to the reset 
terminal R of the flipflop circuit 36, then the flipflop circuit 36 is 
cleared to produce a logic "0" output signal at the non-inverted signal 
output terminal Q and a logic "1" output signal at the inverted signal 
output terminal Q thereof. The logic "0" signal appearing at the 
non-inverted signal output terminal Q of the flipflop circuit 36 is 
impressed on the control terminal of the switch 20 and thereby causes the 
switch 20 to open. The logic "1" signal appearing at the inverted signal 
output terminal Q of the flipflop circuit 36 is appled to the control 
terminal of the gate circuit 66 and causes the gate circuit 66 to 
interrupt passage therethrough of the control data signals passed through 
the gate circuit 62 of the control data signal extracting network 54. The 
control data signals delivered from the signal extracting network 54 are 
thus cut off by the gate circuit 66 when the switch 20 in the tracking 
servo loop is held open. 
If desired, the embodiment of the tracking servo system hereinbefore 
described with reference to FIG. 2 may be modified in such a manner that 
the non-inverted and inverted signal output terminals Q and Q of the 
flipflop circuit 36 are connected respectively to the control terminal of 
the gate circuit 66 of the control data signal cut-off network 64 and to 
the control terminal of the switch 20 in the tracking servo loop. For this 
purpose, the switch 20 is arranged to close and open in response to 
signals of, for example, logic "0" and "1" values, respectively, applied 
to the control terminal thereof and, likewise, the gate circuit 66 is 
arranged to be operative and inoperative in response to signals of, for 
example, logic values "1" and "0" values, respectively, applied to the 
control terminal thereof. If, furthermore, the switch 20 is arranged to be 
open and the gate circuit 66 is arranged to be inoperative when both of 
the signals appearing at the control terminal of the switch 20 and the 
control terminal of the gate circuit 66 are of, for example, either a 
logic "1" value or a logic "0" value, then only one of the non-inverted 
and inverted signal output terminals Q and Q of the flipflop circuit 36 
may be connected to these control terminals of the switch 20 and the gate 
circuit 66. 
In a video disc player including a tracking servo system of the general 
nature hereinbefore described with reference to FIG. 2, the tracking 
mirror 28 driven to turn under the control of the tracking servo loop 
including the switch 20 is constantly urged to return to its neutral or 
home angular position about the axis of rotation thereof by means of a 
return spring (not shown) connnected to or otherwise engaging the tracking 
mirror 28. It therefore sometimes happens that the spring-loaded tracking 
mirror 28 is disabled from accurately following the variation in the 
current flowing through the driver coil 26 for the mirror 28 when the 
mirror 28 is turning close to one of the opposite limit angular positions 
thereof or is on the point of reaching such an angular position about the 
axis of rotation of the mirror 28. When this occurs, the tracking mirror 
28 tends to be brought out of control of the tracking servo loop and may 
therefore behave without respect to or at least with less respect to the 
current supplied to the driver coil 26. Such a tendency is pronounced when 
the positive and negative limits predetermined for the window comparator 
32 are selected at a large absolute value. If the slider carrying the 
tracking mirror 28 is controlled to move on the basis of the control data 
or address singals delivered from the control data signal extracting 
network 54 under such a condition, the slider is operated to move 
regardless of the actual angular position of the mirror 28 with respect to 
the target track on the information-carrying face of the video disc being 
scanned. 
In the embodiment of FIG. 2, this problem results essentially from the fact 
that the gate circuit 66 of the control data signal cut-off network 64 is 
made inoperative by tripping means which is composed of the combination of 
the mirror driver coil 26, window comparator 32, monostable multivibrator 
34 and flipflop circuit 36 and which is thus responsive to the signal 
passed through the switch 20 and accordingly the voltage developed across 
the driver coil 26 for the tracking mirror 28. Since the signal passed 
through the switch 20 is variable with the output signal Qa from the 
differential amplifier 16, the above described problem encountered in the 
embodiment of FIG. 2 can be solved through modification of the tripping 
means in such a manner that the modified tripping means is responsive to a 
signal which is variable with the output signal Qa from the differential 
amplifier 16 in a manner essentially different from the manner in which 
the signal passed through the switch 20 is variable with the signal Qa. 
FIG. 3 shows an embodiment of the tracking servo system having such 
modified tripping means incorporated therein. 
Referring to FIG. 3, the modified tripping means is provided in combination 
with control data signal extracting and cut-off networks which are 
constructed and arranged similarly to their respective counterparts in the 
embodiment of FIG. 2 and which are accordingly also designated by 
reference numerals 54 and 64, respectively. 
The tripping means in the embodiment of FIG. 3 is adapted to operate when 
the tracking mirror 28 is brought or liable to be brought out of control 
of the tracking servo system and comprises a gate inhibitor network 68 
having an input terminal connected to the output terminal of the 
differential amplifier 16 and an output terminal connected to the control 
terminal of the gate circuit 66 constituting the control data signal 
cut-off network 64. 
As previously described in detail, the differential amplifier 16 is 
operative to produce an output signal Qa which is representative of the 
difference between the levels of the output signals S.sub.1 and S.sub.2 
delivered from the first and second photoelectric transducer units 10 and 
12, respectively. The output signal Qa delivered from the differential 
amplifier 16 is, thus, indicative of an amount of deviation of the 
scanning beam from a target track on the information-carrying face of the 
video disc being scanned and varies in sinusoidal form as indicated in the 
graph of FIG. 1B as the scanning beam is displaced radially of the 
information-carrying face of the disc. 
The gate inhibitor network 68 is constructed and arranged to be operative, 
in effect, for comparing such an amount of deviation with a predetermined 
value. This predetermined value may correspond to such an angle of 
rotation of the tracking mirror 28 that is smaller than the angle between 
the neutral or home angular position and each of the above mentioned 
predetermined limit angular positions of the mirror 28 about the axis of 
rotation thereof. Upon detection of an amount of deviation larger than 
such a predetermined value, the gate inhibitor circuit 68 produces a pulse 
signal having a logic "1" value by way of example and a predetermined time 
duration and supplies the signal to the control terminal of the gate 
circuit 66, thereby inhibiting the gate circuit 66 from passing 
therethrough the control data or address signals passed through the gate 
circuit 62 of the control data signal extracting network 54. 
FIG. 4 shows diagrammatically an example of the circuit arrangement of the 
gate inhibitor network 68 to achieve these functions. In FIG. 4, the gate 
inhibitor network 68 is shown, comprising a series combination of a 
capacitor 70, a rectifier circuit 72, a smoother circuit 74, a comparator 
circuit 76 (COMP) and a monostable multivibrator 78 (MMV). The capacitor 
70 is connected between the output terminal of the differential amplifier 
16 and the input terminal of the rectifier circuit 72 and is provided for 
alternating-current filtering or direct-current absorbing purposes. The 
capacitor 70 is, thus, operative to suppress the direct-current component 
possibly contained in the waveform of the output signals Qa delivered from 
the differential amplifier 16. The signals Qa having a waveform 
alternating as shown in the graph of FIG. 1B at frequencies approximating 
4 to 5 KHz for example, are passed through the rectifier circuit 72 and 
are thereafter fed to the smoother circuit 74 for being rendered into a 
substantially smooth waveform having, for example, a positive polarity. 
The comparator circuit 76 has two input terminals, one of which is 
connected to the output terminal of the smoother circuit 74 and the other 
of which is connected to a source of a suitable reference signal in the 
form of a predetermined voltage V.sub.3. This predetermined voltage 
V.sub.3 is herein assumed, by way of example, to be of a value which the 
voltage of the output signal from the smoother circuit 74 will assume when 
the amount of deviation of the scanning beam from a target track on the 
information-carrying face of the video disc being scanned by the beam 
corresponds to such an angle of rotation of the tracking mirror 28 that is 
smaller than the angle between the neutral or home angular position and 
each of the previously mentioned predetermined limit angular positions of 
the mirror 28. 
The comparator circuit 76 is operative to produce an output signal of, for 
example, a logic "1" value when the voltage of the signal delivered 
thereto from the smoother circuit 74 is higher than the predetermined 
voltage V.sub.3. The signal thus delivered from the comparator circuit 76 
is fed to the input terminal of the monostable multivibrator 78 which is 
adapted to produce a pulse signal having a predetermined duration in 
response to the signal from the comparator circuit 76. The output terminal 
of the monostable multivibrator 78 is connected to the control terminal of 
the gate circuit 66 of the control data signal cutoff network 64 shown in 
FIG. 3. The pulse signal produced at the output terminal of the monostable 
multivibrator 78 is therefore fed as the output signal of the gate 
inhibitor network 68 to the control terminal of the gate circuit 66 and 
maintains the gate circuit 66 inoperative for a period of time dictated by 
the pulsewidth of the output signal from the monostable multivibrator 78. 
The gate circuit 66 is in this fashion made inoperative and as a 
consequence the flow of the control data signals delivered from the 
control data signal extracting network 54 is blocked by the gate circuit 
66 when the tracking mirror 28 is brought or being brought out of control 
of the tracking servo system. If desired, the monostable multivibrator 78 
may be dispensed with so that the output signal from the comparator 
circuit 76 is fed directly to the gate circuit 66. 
It has been described that the gate circuit 66 constituting the control 
data signal cut-off network 64 in each of the embodiments of the present 
invention is rendered inoperative either by the tripping means composed of 
the window comparator 32, monostable multivibrator 34 and flipflop circuit 
36 as in the embodiment of FIG. 2 or the tripping means constituted by the 
gate inhibitor network 68 as in the embodiment of FIG. 3. It will be, 
however, apparent that, if desired, arrangements may be made so that the 
gate circuit 66 is made inoperative selectively by the tripping means of 
the embodiment of FIG. 2 and the tripping means of the embodiment of FIG. 
3. In order to realize such arrangements, a logic "OR" gate circuit may be 
provided which has its input terminals respectively connected to the 
inverted signal output terminal Q of the set-reset flipflop circuit 36 as 
in the embodiment of FIG. 2 or to the output terminal of the gate 
inhibitor network 68 as in the embodiment of FIG. 3 and its output 
terminal connected to the control terminal of the gate circuit 66. 
It will also be apparent that, while the embodiments of the present 
invention has been described on the assumption that the tracking servo 
system proposed by the present invention forms part of a video disc player 
of the optically recording and reading type, the gist of the present 
invention is applicable not only to such a type of video disc player but 
to any other types of video or audio disc players of, for example, the 
electrostatically recording and reading type. The term "scanning spot" as 
referred to in the appending claims and elsewhere in the foregoing 
description regarding the general aspects of the present invention should 
therefore be understood to mean not only a spot at which a beam of light 
is incident on an information-carrying face of a disc but a spot at which 
a pickup element of another type such as an electrode stylus of an 
electrostatic reading device is directed onto an information-carrying face 
of an audio or video disc. 
It will further be understood that the gate circuit 66 forming part of the 
control data signal cut-off network 64 in each of the embodiments shown in 
FIGS. 2 and 3 may be constituted, if desired, as part of the gate circuit 
62 of the control data signal extracting network 54. In this instance, the 
gate circuit 62 of the signal extracting network 54 may be of the type 
having an inhibitor terminal and thus adapted to cut off the passage of a 
signal between the input and output terminals thereof in the presence of a 
signal at the inhibitor terminal. The inhibitor terminal of the gate 
circuit of this type is connected to the inverted output terminal Q of the 
flipflop circuit 36 in the embodiment of FIG. 2 or to the output terminal 
of the gate inhibitor network 68 in the embodiment of FIG. 3. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of 
tracking servo systems differing from the types described above. 
While the invention has been illustrated and described as embodied in a 
recording and reproducing apparatus, it is not intended to be limited to 
the details shown, since various modifications and structural changes may 
be made without departing in any way from the spirit of the present 
invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.