Head control means for optical videodisc player

The invention relates to methods for controlling the starting and stopping of the reading of recording supports read in appropriate readers. The method according to the invention comprises providing a zone free from information at least at the beginning of the track of such a support. The reading head of the reader advances rapidly to that zone and, after having detected the presence thereof, continues its advance at a normal speed.

This invention relates to processes for controlling the starting and 
stopping of the reading of a recording support. The invention also relates 
to the recording supports and apparatus which enable this method to be 
carried out. 
It is known that information can be recorded onto a disc-form support on a 
spiral track with adjacent turns. In the discs commonly used for recording 
musical programs, this track is in the form of a groove produced by 
intaglio engraving which begins with a few blank turns having a wide pitch 
which enable a reading head to be brought rapidly into the zone comprising 
the program to be read, this reading head being provided with a needle 
which engages in the groove and which is used both for reading the 
recorded signals and for causing the head to move in a substantially 
radial direction relative to the disc. Towards the end of the track, the 
arm supporting the head moves through a position in which it releases a 
device which stops the reading operation. In order to ensure that this 
device does not operate during the recorded program, the recorded program 
finishes before the arm reaches the above-mentioned position so that the 
track terminates in a few blank turns having a wide pitch which enable the 
arm to move rapidly into the release position. 
It is also known that it is possible to record on a support of the same 
disc type informations corresponding to a frequency range very much wider 
than that used in musical programs, for example in the form of a video 
signal representing a television program. The apparatus and supports in 
which these techniques are applied are known by the name of "videodisc" 
and we shall be using this term throughout the rest of this text. 
One of these techniques comprises modulating a carrier wave with the 
signals to be reproduced and recording this carrier wave on a spiral track 
with adjacent turns carried by a disc in the form of lines of variable 
length and spacing. 
In order to read a disc of this type, the disc is rotated about its axis 
and a reading head is moved radially in relation to the disc. In this 
case, the reading head operates without material contact by detecting the 
disturbances caused by the track in a light beam passing through the disc 
or reflected at its surface. The head moves at a constant speed so that it 
advances by one turn of the spiral forming the track per revolution. A 
tracking servomechanism keeps the reading light beam on the track and 
compensates irregularities of any kind. Another servomechanism keeps the 
beam constantly focussed on the surface of the disc. Depending upon the 
particular design, this servomechanism may or may not require the presence 
of the track to engage. Squelch circuits enable the output of the 
demodulated signals to be cut when the demodulation is not correct 
(absence of the carrier wave for example) so as to prevent excessively 
noisy signals from being obtained. These servomechanisms and squelch 
circuits are auxiliary devices. 
The above-described method of controlling the starting and stopping of 
reading as used in discs for musical programs may be adapted to the 
reading of a videodisc without any guide grooves (which also comprises 
certain magnetic processes), but unfortunately this would impose severe 
stresses upon the tracking servomechanism and it would be necessary to use 
discs complying with strict standards in regard to the diameter of the 
beginning of track and end of track turns. 
In accordance with the present invention it is provided a process for 
controlling the starting and the stopping of the reading of a recording 
disc intended for recording informations onto a spiral track, said track 
having at least at its beginning a blank zone free from recorded 
informations, said disc being rotated in a reader comprising a reading 
head which operates without mechanical guiding in contact with said disc, 
said process comprising the successive steps of: 
moving rapidly said reading head from a rest position towards the centre of 
the disc; 
detecting said beginning of the track; 
decelerating said reading head to a normal advance speed under the control 
of the detecting of said beginning: 
detecting the end of said track; 
moving backward rapidly said reading head towards said rest position under 
the control of the detecting of said end.

FIG. 1 illustrates a section along a plane passing through its axis X.sub.1 
X.sub.2 and confined to the right-hand half of a videodisc 11 according to 
the invention. The turns of the recording track are diagrammatically 
represented by the serrations 16. The surface of this disc is divided into 
four concentric zones 12, 13, 14, 15. The zones 12 and 15 do not comprise 
any recording track. The recording track is inscribed at a constant pitch 
on the median zones 13 and 14. That portion of the track inscribed on the 
zone 13 is blank which, in the case of an optically read disc means that 
only the pure carrier wave is recorded in the form of lines of constant 
length and spacing which materialise the track and distinguish the zone 13 
from the zone 12. 
FIG. 2 diagrammatically illustrates one example of embodiment of a 
videodisc reader using the method according to the invention. This reader 
comprises a rotation motor 210, a reading head 211, a radial advance motor 
212, a reading box 213, a detector 214, bistable trigger circuits 215 and 
216, an AND-gate 217 and a delay circuit 218. We shall denote the signals 
in accordance with a classical positive convention, in which the active 
signals are denoted by the number 1 and the non-active signals by the 
number 0, irrespective of their shape and level. The bistable circuits 215 
and 216 obey the logic S = 1.fwdarw.Q = 1 and Q = 0 and R = 1.fwdarw.Q = 0 
and Q = 1. In addition, the bistable circuit 216 changes state when a 
signal applied to its input H passes from 0 to 1 and remains in its 
initial state when this same signal passes from 1 to 0. 
The reader being assumed to be in operation, a starting signal ST applied 
fugitively to the input S of the bistable circuit 216 positions it in the 
state Q = 1 Q = 0. Since in that case no signal is read on the disc, the 
carrier-absent signal P, which will be described hereinafter, is and 
remains at 1. Accordingly, it does not act on the bistable circuit 216 
because it is applied to its input H. By contrast, it positions the 
bistable circuit 215 in the state Q = 1 Q = 0 because it is applied to its 
input R. 
Under these conditions, the rotation motor 210 receives a starting signal M 
emanating from the output Q of the bistable circuit 216. It is thus 
started and rotates the disc 11 at a suitable speed. 
The radial advance motor 212 receives a rapid advance signal V.sub.1 from 
the AND-gate 217, this gate combining the signals V (output Q of the 
bistable circuit 215) and M which are thus both at 1. The motor 212 then 
rapidly moves the reading head 211 towards the centre of the disc. The 
reading head transmits a reading signal L which is processed in the 
reading box 213 in the same manner as in an ordinary videodisc reader. As 
long as the head 211 reads either the exterior of the disc or the zone 12 
where there is no track, this signal only contains noise and the squelch 
circuits contained in the box 213 cut the information signals at the 
outputs of this box under the effect of a damping signal E which is the 
signal V retarded by the circuit 218. Since this delay is constant, it has 
no effect because the signal V is at 1 even before the beginning of the 
sequence. 
However, a rough signal A arising out of the amplification of the signal L 
is permanently delivered by the box 213 to the detector 214. The detector 
214 detects the presence or absence of the carrier in this signal A, for 
example by means of a filter followed by a threshold circuit, and 
accordingly delivers either the carrier-absent signals P or the 
carrier-present signals P. 
When the reading head begins to scan the zone 13 of the disc, a signal at 
the frequency of the carrier appears in the signal L and hence in the 
signal A because, although the radial advance speed of the head is high, 
it is still low by comparison with the speed of the track because the disc 
rotates at a very high speed which currently is of the order of 1500 
revolutions per minute. In this way, the head progresses by a relative 
oblique movement in relation to the track and intersects these tracks at a 
small angle which enables it to read sufficiently long sections of track 
to obtain a signal at the frequency of the carrier. The detector 214 thus 
cancels the signal P and delivers the signal P which, when applied to the 
input S of the bistable circuit 215, positions it in the state Q = 1 Q = 
0. The bistable circuit 216 remains positioned in the same state because 
the signal at its input H passes from 1 to 0 which does not cause it to 
change state. 
The signal V.sub.1 is thus cancelled because one of the signals (V) applied 
to the gate 217 passes to 0. The radial advance motor 212 thus receives a 
normal advance signal V.sub.3 emanating from the output Q of the bistable 
circuit 215. Under the control of this signal V.sub.3, the motor 212 slows 
down until it makes the head 211 advance at a normal reading speed. This 
same signal V.sub.3 is applied to the reading head in which it controls 
scanning of the focussing and tracking servomechanisms. It is possible to 
use certain types of focussing servomechanisms which are able to lock onto 
the light signal supplied by the surface of the disc where there is no 
track. In this case, they are left to operate on their own without control 
of their engagement. 
Since the signal V is cancelled, the signal E is also cancelled after the 
delay determined by the circuit 218 which controls stoppage of the squelch 
circuits contained in the box 213. This box may then deliver the 
information contained in the signal L. 
To ensure that this sequence is correctly completed, it is necessary for 
the delay introduced by the circuit 218 to be longer than the longest of 
the delays attributable to the deceleration time of the motor 212 from the 
rapid advance speed to the normal reading speed and to the engagement time 
of the servomechanisms, which determines the minimum delay to be applied 
by the circuit 218. It is also necessary for the reading time of the zone 
13 of the disc under these conditions to be longer than this delay in 
order not to prevent the output of the first information signals recorded 
on the zone 14 of the disc, which fixes the number of turns contained in 
this zone 13 in dependence upon the delay determined in the manner already 
described. 
When, after having read the zone 14 of the disc, the reading head begins to 
read the zone 15, the carrier signals disappear from the signals L and A, 
which results in cancellation of the signal P and transmission of the 
signal P. This signal P positions the bistable circuit 215 in the state Q 
= 0 Q = 1 and causes the bistable circuit 216 to pass to the state Q = 0 Q 
= 1 because the signal at its input H passes from 0 to 1. 
The signal E becomes active again and controls the operation of the squelch 
circuits in the box 213. The delay introduced by the circuit 218 may be 
avoided by using for this circuit an arrangement which reacts differently 
to the descent and ascent of the signal E. 
The signal M is cancelled and the motor 210 stops although the signal 
V.sub.1 remains zero because one of the signals (M) applied to the gate 
217 has passed to 0. 
The motor 212 receives a return signal VR emanating from the output Q of 
the bistable circuit 216. This signal VR controls backward return of the 
head 211 and a mechanism inside the motor 212 stops the head when it is 
back in its rest position. It is pointed out that, since the motor 210 has 
been stopped, the reading head is no longer able to read the track during 
its return, so that the signal P remains active which prevents any 
relaxation phenomenon that would result in another partial reading. The 
reading cycle is thus terminated. 
The deceleration time of the motor 212 and the engagement time of the 
servomechanisms of the reading head are defined with fairly considerable 
latitude by reason in particular of the considerable difference between 
the rapid approach and normal reading speeds. As a result, it is necessary 
to reserve a relatively large information-free zone 13 on the disc 11 to 
cover the most pessimistic case. It is thus of advantage, if it is 
possible to distinguish the modulated carrier from the pure carrier, to 
make the reading head pass through the zone 13 at a speed which is higher 
than the normal speed (twice as high for example), but distinctly lower 
than the rapid approach speed. Under these conditions, when the presence 
of the information signals is detected, the radial advance motor of the 
head is switched to its normal speed and the squelch circuits are 
neutralised at that moment without introducing any delay, the deceleration 
of the approach motor being quasi-immediate under these conditions. It is 
thus possible to extend the track beyond the end of the program recorded 
by a few turns comprising only the recording of the pure carrier and 
situated in the zone 15 of the disc. By detecting the disappearance of the 
modulation signals from these turns, the stop sequence is controlled. 
In cases where the recorded information is in the form of a videosignal 
representing a television program, it is easy to detect the appearance of 
this signal because a signal of this type comprises synchronisation pulses 
which are numerous and characteristic, which enables them to be 
immediately identified. 
FIG. 3 diagrammatically illustrates one example of embodiment of a 
videodisc reader using this variant of the process according to the 
invention. This motor comprises a rotation motor 310, a reading head 311, 
a radial advance motor 312, a reading box 313, a detector 314, bistable 
circuits 315, 316, 320 and AND-gates 317 and 319. The conventions on the 
signals and bistable circuits are the same as in the description of the 
reader shown in FIG. 2. 
The reader being assumed to be in operation, a starting signal applied 
fugitively to the input S of the bistable circuit 316 positions it in the 
state Q = 1 Q = 0. The information-absent signal SY is and remains at 1. 
Accordingly, it does not act on the bistable circuit 316 because it is 
applied to its input H. Since it is also applied to the input R of the 
bistable circuit 320, it positions the bistable circuit 320 in the state Q 
= 1 Q = 0. The carrier-absent signal P is and remains at 1. Accordingly, 
it positions the bistable circuit 315 in the state Q = 1 Q = 0 because it 
is applied to its input R. 
Under these conditions, the rotation motor 310 receives a starting signal M 
emanating from the output Q of the bistable circuit 316. It is thus 
started and rotates the disc 11 at the required speed. 
The radial advance motor 312 receives a rapid advance signal V.sub.1 
emanating from the AND-gate 317 which combines the advance signal V 
(output Q of the bistable circuit 315) and the signal M, these signals 
thus both being equal to 1. The motor 312 the rapidly moves the reading 
head 211 towards the centre of the disc. This head transmits a reading 
signal L which is processed in the reading box 313 in the same manner as 
in an ordinary videodisc reader. As long as the head 311 does not begin to 
read the zone 13 of the disc 11, this signal only contains noise and the 
squelch circuits in the box 313 function under the control of the damping 
signal E emanating from the output Q of the bistable circuit 320. 
However, the box 313 permanently delivers a rough signal A emanating from 
the amplification of the signal L and an information signal B emanating 
from the processing of this signal L. The detector 314 detects the 
presence or absence of the carrier in the signal A in the same way as the 
detector 214 in FIG. 2 and, accordingly, delivers the carrier-absent 
signals P or the carrier-present signals P. 
When the reading head begins to read the zone 13 of the disc 11, the 
detector 314 cancels the signal P and delivers the signal P which, when 
applied to the input S of the bistable circuit 315, positions it in the 
state Q = 1 Q = 0. The signal V.sub.1 is thus cancelled because one of the 
signals (V) applied to the gate 317 passes to 0. The AND-gate 319 then 
combines the signals M and the tracking signals C (output Q of the 
bistable circuit 315) and E. Since these three signals are in the state 1, 
the gate 319 delivers a semi-rapid advance signal V.sub.2 to the motor 
312. 
The signal C simultaneously controls the scanning of the focussing and 
tracking servomechanisms of the head 311 or, optionally, of the tracking 
servomechanism only. 
The motor 312 decelerates until it makes the head 311 advance at a rapid 
reading speed and the zone 13 of the disc is rapidly read. Large sections 
of track are effectively read because the tracking servomechanism corrects 
the excess advance imparted by the motor 312 and, when this servomechanism 
comes to a stop, it disengages to re-engage on a turn situated further 
away. 
When the reading head begins to read the zone 14 of the track, the detector 
314 detects the presence of modulation signals in the information signal 
B, for example by detecting line synchronisation signals by one of the 
separation techniques used in all television receivers. The detector 314 
thus cancels the signal SY and delivers the signal SY which, when applied 
to the input S of the bistable circuit 320, positions it in the state Q = 
1 Q = 0. Since the signal SY is applied to the input H of the bistable 
circuit 310 and passes from 1 to 0, the bistable circuit 316 does not 
change state. 
The signal E is then cancelled and the squelch circuits in the box 312 
cease operation. Since the signal E is applied to the gate 319, the gate 
cancels the signal V.sub.2. By contrast, the normal advance signal V.sub.3 
emanating from the output Q of the bistable circuit 320 is applied to the 
motor 312 which decelerates extremely quickly to reach its normal speed, 
the tracking servomechanism compensating the speed differential during the 
deceleration time which prevents any loss of information after the squelch 
circuits have opened. 
As already mentioned, an additional zone comprising a few turns of the 
track recorded without modulation as in the zone 13 is reserved in this 
variant at the beginning of the zone 15 of the disc and hence at the end 
of the track. When the reading head begins to read these turns, the 
detector 314 detects the absence of modulation and thus cancels the signal 
SY whilst redelivering the signal SY which positions the bistable circuit 
320 in the state Q = 0 Q = 1. The signal V.sub.3 is thus cancelled. On 
this occasion, the signal SY passes from the state 0 to the state 1 and 
thus causes the bistable circuit 316 to pass into the state Q = 1 Q = 0. 
The signal E is thus transmitted and actuates the squelch circuits in the 
box 313. By contrast, the signal M is cancelled and the motor 310 stops. 
Since the gates 317 and 319 receive the signal M, the signals V.sub.1 and 
V.sub.2 remain at 0. By contrast, the motor 312 receives a return signal 
VR emanating from the output Q of the bistable circuit 316. This signal VR 
controls the return of the head 311 and a mechanism inside the motor 312 
stops the head once it is back in its rest position. 
Since the disc no longer rotates during this return movement, the signal L 
is zero so that the detector 314 cancels the signal P and delivers the 
signal P which positions the bistable circuit 315 in the state Q = 1 Q = 
0. Thus, the servomechanisms are disconnected again and the reading cycle 
is thus terminated. 
The operation of a reader of the type described above is thus entirely 
automatic except for the manual action required for controlling startup. 
The following documents have been quoted in the French prosecution: 
French application FR 2 181 036 (RCA) 
German application DTOS 2 103 503 (Licentia).