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The invention relates to an apparatus for reading a flat reflecting record
carrier on which information, for example video and/or audio information,
is stored in an optically readable structure, which apparatus comprises a
radiation source which supplies a read beam, an objective system for
focussing the read beam on the plane of the information structure, a
radiation-sensitive signal detection system for converting the read beam
which is modulated by the information structure into an electric signal,
and a focus detection system which is coupled to an electronic circuit for
controlling the position of the objective system relative to the
information structure. A focus detection system is to be understood to
mean an auxiliary system which consists of a multiplicity of optical
elements and radiation-sensitive detectors for determining a deviation
between the actual and the desired position of the plane of focussing of
the objective system.
The information structure may consist of a multitude of quasi-concentric
tracks, which together constitute one continuous spiral track. The read
apparatus is equipped so that only a small track portion of the size of
approximately the smallest detail in the information structure is imaged
on the signal detection system. The radiation paths between the radiation
source and the plane of a track portion to be read and between said plane
and the signal detection system, however, may be subject to variations.
Said variations may have different causes. First of all, it may happen
that the record carrier is not perfectly flat. Secondly, when the record
carrier is a foil, it may flutter as the carrier rotates. Furthermore, the
optical elements of the read apparatus may be susceptible to vibrations.
During the occurrence of such variations the signal detection system not
only receives radiation from a track portion to be read, but also
radiation from the surrounding area of said portion. As a result, the
modulation depth of the output signal of the signal detection system
decreases, while moreover cross-talk may occur, since the signal detection
system no longer receives radiation from one track only but also radiation
from adjacent tracks. Satisfactory signal detection is then no longer
possible owing to the reduced modulation depth and the cross-talk.
Therefore, it is of great significance that the deviation between the
actual and the desired position of the plane of the information structure
can be detected and the focussing of the objective system can be
corrected. For detecting said deviation said previous apparatus is
provided with a focussing detection system, in which a narrow auxiliary
beam (or focussing beam) is derived from the radiation source, which
auxiliary beam is eccentrically passed through the objective system. After
reflection at the information structure of the record carrier and a second
passage through the objective system, the focussing beam falls onto a
system of two radiation-sensitive focussing detectors which are separated
by a gap. Comparison of the radiation intensities which are incident on
the focussing detectors provides an indication of the degree of focussing.
To enable the focussing of the objective system to be controlled, said
system can be disposed in a loudspeaker coil. The objective system can
then be moved relative to the plane of the information structure by
applying a suitable control signal to the loudspeaker coil. Said control
signal is supplied by an electronic circuit to whose input terminals the
output signals of the focussing detectors are applied.
When inserting a record carrier in the read apparatus the objective must be
withdrawn so far that the record carrier cannot hit the objective. After
the read apparatus has been switched on, the objective must be moved
towards the record carrier until the distance between the objective and
the record carrier is the required value for optimum reading. When during
said "locking in" of the objective system the movement of said objective
system is determined by the signals supplied by the focussing detectors,
the objective system may not be braked sufficiently in due time at
decreasing distance between said system and the record carrier, so that it
collide with the record carrier.
The object of the present invention is to provide a read apparatus in which
steps have been taken to prevent the objective system from colliding with
the record carrier. The apparatus according to the invention is
characterized in that the electronic circuit includes first means for
superimposing an additional signal on the control signal supplied by the
focussing detection system during locking-in of the objective system, in
such a way that a resulting first control signal is obtained at the output
of the electronic circuit which causes the objective system to move
towards the record carrier, second means for limiting the resulting first
control signal to a specific value, and third means to accomplish that
when the objective system has approached the record carrier to
approximately the required distance, a second control signal of a sign
opposite to and an amplitude substantially greater than that of said
resulting first control signal is obtained at the output of the electronic
circuit, so that the objective system is subject to a high acceleration in
a direction away from the record carrier.
A preferred embodiment of an apparatus according to the invention is
further characterized in that the electronic circuit includes a
differential amplifier whose inputs are connected to the focussing
detectors and whose output is connected to a first input of an operational
amplifier to a second input of which an auxiliary voltage source is
connected via a switch, and a transistor output stage whose output circuit
includes a drive element for the objective system.
The invention will now be described with reference to the drawing, in
which:
FIG. 1 shows an embodiment of a previously proposed apparatus for optically
reading a record carrier,
FIG. 2 clarifies the principle of the invention,
FIG. 3, partly in block-schematic form, shows an electronic circuit
according to the invention for use in the apparatus of FIG. 1, and
FIG. 4 in detail shows an embodiment of such an electronic circuit.
In FIG. 1 the reference numeral 1 represents a round disc-shaped record
carrier. Information tracks 3 are disposed in the plane 2 of said record
carrier. An information track may consist of a multitude of areas
alternating with intermediate areas, while the information, for example a
colour television program, may be contained in the lengths of the areas
and the intermediate areas. The record carrier body is
radiation-transmitting and the information surface 2 radiation reflecting.
The record carrier is read with a radiation source 6, for example a laser,
which emits a narrow radiation beam 30. The beam enters the beam splitter
7 via the surface 8. The surface 9 of the beam splitter is partially
radiation-transmitting, so that a part of the beam 30 is transmitted as a
read beam 31, while an other part of the radiation beam 30 is reflected to
the surface 8.
The read beam 31 is focussed on the information surface 2 of the record
carrier 1 by an objective system 12, which is schematically represented by
a single lens. The auxiliary lens 10 ensures that the entire entrance
pupil of the objective system 12 is filled. The record carrier can be
rotated with the aid of a spindle which extends through an opening 4 in
the record carrier. The read beam 31 is then modulated in accordance with
the information structure which consecutively appears in the beam
cross-section. After reflection at the surface of the information
structure the modulated read beam traverses the objective system for a
second time and subsequently falls onto, for example, a dividing mirror
11. This mirror transmits a part of the modulated read beam to the
radiation-sensitive signal detector 16. The signal supplied by said
detector is applied to an electronic ciricuit 20 in which, in known
manner, an information signal S.sub.i can be formed, which can be rendered
visible and audible, for example with the aid of a conventional television
receiver 21.
When reading a record carrier whose optical information structure has very
small details, for example of the order of 1 1.mu. m, it is necessary that
small variations in the position of the information surface can be
detected so as to enable the focussing of the objective to be corrected.
For determining said variations use is made of the part of the beam 30
which is reflected at the surface 9 of the beam splitter 7. This part of
the beam 30, after total reflection at the surface 8 leaves the beam
splitter as a second radiation beam 32. The beam 32 functions as a
focussing beam and passes through the aperture of a diaphragm 15. The
focussing beam enters the objective system 12 at a comparatively large
distance from the optical axis of said objective system. After refraction
by the objective system the focussing beam impinges on the radiation
reflecting information surface 2. For the focussing beam the information
surface functions as a mirror which reflects the beam to the objective
system. The objective system then aims the focussing beam at an assembly
of two radiation-sensitive focussing detectors 18 and 19, which are spaced
apart by a narrow gap 17.
Then path of the focussing beam includes an auxiliary lens 14. Said
auxiliary lens focusses the beam 32 in the focal plane of the objective
12, so that the focossing beam emerges from the objective as a parallel
beam. Thus, a radiation spot of constant size is formed on the information
surface 2, so that the radiation spot in the plane of the focussing
detectors will also have a constant size independent of the distance
between the information surface 2 and the objective 12.
The focussing detectors are arranged so that in the case of a correct
position of the plane of the information structure, as is shown in FIG. 1,
the image of the diaphragm aperture, which image is formed with the aid of
the reflecting surface 2 and the objective system, is symmetrical relative
to the detectors 18 and 19. The focussing detectors then receive equal
radiation intensities. When the plane of the information structure is
lowered or raised, the distance between the location where the focussing
beam enters the objective system and the optical axis will decrease and
increase respectively. The focussing beam will then be refracted through a
smaller or greater angle respectively. As a result of this, the detector
18 will receive more radiation or less radiation respectively than the
detector 19.
The signals S.sub.18 and S.sub.19 from the focussing detectors are applied
to an electronic circuit 22 in which said signals are compared and a
control signal S.sub.r is derived. The signal S.sub.r is applied to a
device for moving the objective system, for example a loudspeaker coil 13,
in which the objective system is suspended. Instead of by a loudspeaker
coil the objective system can also be driven by other means, such as
piezo-electric, magnetostrictive, or electrostatic means, each time using
the signal S.sub.r. Further, the invention to be described hereinafter is
not limited to the use of the focus detection system of FIG. 1. Other
opto-electronic systems for deriving a focus signal, such as for example
that described in previous U.S. patent application Ser. No. 229,291, filed
Feb. 25, 1972, and now U.S. Pat. No. 3,833,769 may also be employed.
In FIG. 2 the signal S.sub.f which is obtained by comparison of the output
signals of the focussing detectors 18 and 19 is represented by the drawn
line 40 as a function of the distance of the objective system to the front
of the record carrier (the distance d in FIG. 1). The signal S.sub.f is
plotted in volts, the distance d in mm.
When the objective system is located at a distance greater than a from the
record carrier, a being for example 2.5 mm, the focussing beam falls
beyond the detectors 18 and 19, so that the focus detection system is
inoperative and the signal S.sub.f is zero. When the objective system has
approached the record carrier to within the distance a, the detector 19
will receive radiation from the focussing beam and the detector 10 will
not yet receive any radiation, so that the signal is positive. In the case
of a positive signal S.sub.f the objective is subject to an acceleration
in an upward direction, towards the record carrier. Apart from the small
voltage drop around the distance b, the signal S.sub.f remains positive
until the distance between the objective system and the record carrier has
the value c, which is for example 0.7 mm. The detectors 18 and 19 then
receive equal amounts of radiation from the focussing beam. At smaller
distances from the objective system to the record carrier the detector 18
receives a greater radiation intensity than the detector 19, and the
objective system is subject to a downward acceleration, away from the
record carrier.
During said "locking in" of the objective system two problems may occur.
The information structure, as is shown in FIG. 1, may be located at the
rear of a radiation-transmitting record carrier. This has the advantage
that the record carrier itself functions as a protective layer, thus
substantially eliminating the influence of dust particles which may settle
on the record carrier, or of scratches which may arise in the record
carrier on the read process. However, owing to said structure of the
record carrier the focussing beam may also be reflected by the front of
the record carrier, apart from by the information surface 2. Thus, if the
objective system is so far from the record carrier (the distance d in FIG.
2) that it is focussed on said front, the sign of the signal S.sub.f may
be reversed. In that case it is not unlikely that the focus control system
"views" said distance as the correct distance and controls in response to
said distance.
When the objective system has approached the record carrier to the distance
c, it will have a certain speed. The objective system must then be braked
rapidly. The positive maximum and the negative maximum of the signal
S.sub.f are of the same order of magnitude, so that the maximum negative
acceleration will also be of the same order of magnitude as the maximum
positive acceleration. If, as is shown in FIG. 2, the path of positive
acceleration has been longer than that of negative acceleration, the speed
of the objective at the distance c cannot be reduced to zero within said
distance, so that the objective system will collide with the record
carrier.
According to the invention the electronic ciricuit 22 features steps to
mitigate said problems. In the first place a positive bias is superimposed
on the voltage derived from the signals from the focussing detectors. In
FIG. 2 said bias is designated V.sub.B. In the circuit 22 the positive
electric current through the loudspeaker coil which exerts a force towards
the record carrier on the objective system, is limited so that within the
path from a to c the output signal S.sub.r of the circuit 22 will be as
represented by the dashed curve 41. The positive acceleration of the
objective system is consequently substantially limited. The signal S.sub.r
is now also positive for the distance b, so that there is no longer the
risk of the focus being controlled in response to the distance to the
front of the record carrier. The bias voltage V.sub.B should merely
compensate for the negative voltage at b, so that said bias can be very
small, for example 0.3 V in the present embodiment.
Secondly, when it is established that the objective system has approached
the record carrier to the distance e, the full negative voltage derived
from the detector signals S.sub.18 and S.sub.19 is transferred by the
circuit 22. Compare the dotted curve 42 in FIG. 2. As a result, the
objective system is strongly braked, so that it returns to the distance c.
The distance between c and e is for example only 2.mu. m, so that when
using the circuit according to the invention the objective system will
overshoot its nominal position by a very small distance only. The circuit
can establish that the distance e is reached in that the signal S.sub.f
supplied by the focussing detectors exhibits an appreciable decrease.
FIG. 3 shows an electronic circuit according to the invention, partly in
block-schematic form. In said Figure the focusing detectors are designated
55 and 56 respectively. The output signals of said detectors are applied
to a dififerential amplifier 50. The output signal of the amplifier 50 is
fed to an input of an operational amplifier 51, which functions as a
summing device. The other input of the amplifier 51 is connected to a
voltage source 57, of for example -15 V, via a resistor R.sub.1, which
source supplies said bias voltage V.sub.B. The resistor R.sub.1 can be
connected to ground by a circuit S.sub.B. The signal from the amplifier 51
is applied to an output stage which comprises the transistors T.sub.1,
T.sub.2, T.sub.3, and T.sub.4. This output stage, which matches the high
impedances at the left to the low impedances at the right, is fed from a
voltage source, not shown, at the terminals 58 and 58'. The circuit of the
positive supply voltage includes a switch S.sub.c. The output circuit of
the output stage includes a switch S.sub.A before the loudspeaker coil
S.sub.p with which the objective system is moved. In response to a zero
level signal output from amplifier 51, the current flow through T.sub.1
and T.sub.2 will be equal, thereby making the current flow through
transistors T.sub.3 and T.sub.4 equal (providing switch S.sub.C is
closed). In response to a positive signal from amplifier 51 transistor
T.sub.1 will lower its conductivity, while transistor T.sub.2 will conduct
more heavily, thereby increasing the conduction through transistor T.sub.4
and decreasing the conduction through transistor T.sub.3, whereby the
current flows from ground through resistor R.sub.m, coil S.sub.p and
switch S.sub.A. In response to a negative current from amplifier 51
transistor T.sub.2 will lower its conductivity, transistor T.sub.1 will
increase its conductivity, resulting in an increase in conductivity of
transistor T.sub.3 and a decrease in conductivity of transistor T.sub.4.
This in turn causes current to flow from the positive source connected to
terminal 58 through switch S.sub.C, transistor T.sub.3, switch S.sub.A,
coil S.sub.p and resistor R.sub.m to ground. Thus in response to a
positive voltage from amplifier 51 current flows through coil S.sub.p in
one direction, while in response to a negative voltage from amplifier 51
current flows through coil S.sub.p in the opposite direction. When switch
S.sub.C is in the open position shown in FIG. 3 obviously no current can
flow through transistor T.sub.3. In response to the opening fo switch
S.sub.C current is therefore limited to a unidirectional flow through coil
S.sub.p. In addition, since current cannot flow into the collector of
transistor T.sub.2 from transistor T.sub.1, transistor T.sub.2 no longer
operates as an amplifier, but merely acts as a diode utilizing the
base-emitter path thereof. The effective amplification factor existing
between the output of amplifier 51 and the coil S.sub.p is thereby
decreased.
Before a record carrier is inserted in the read apparatus, the power supply
and the bias V.sub.B are applied first.
When fitting a record carrier in the read apparatus of FIG. 1 switches
S.sub.A, S.sub.B and S.sub.C are open. Next, the motor for driving the
spindle 3 and the laser are started, so that the record carrier, which may
be a thin disc or foil is allowed to straighten and the laser is allowed
to stabilize. Subsequently, the switch S.sub.A is closed.
During locking-in of the objective system the acceleration of the objective
system is initially determined by the bias V.sub.B. When the objective
system has approached the record carrier to a distance smaller than a,
said bias and the voltage V.sub.f, which is supplied by the amplifier 50,
together determine the acceleration. As the positive supply voltage for
the output stage is switched off, the positive current through the
loudspeaker coil cannot be supplied by the transistors T.sub.1, T.sub.2,
T.sub.3 and T.sub.4, but said current must flow via the operational
amplifier. Thus, said current can only have a limited value. At the
instant that the objective system has approached the record carrier to a
distance e, the voltage V.sub.f at the output of the amplifier will
suddenly drop very strongly. The influence of the bias then becomes
negligible, and there will be a large negative current through the
loudspeaker coil. Said current can flow because the negative supply
voltage is always available. The objective system is subject to a high
acceleration in downward direction and is rapidly moved to the distance c,
the required distance, from the record carrier. The switch S.sub.B can
then be closed, so that the bias is short-circuited. When furthermore
switch S.sub.C is also closed, the current through the coil S.sub.p can
only be determined by the output signal of the amplifier 50. Said signal,
which varies in accordance with the curve 40 in FIG. 2, which is steep
around point c, enables possible focussing errors to be corrected rapidly.
The switches S.sub.A, S.sub.B and S.sub.C, which in reality are electronic
switches, can be actuated by an electronic clock which in FIG. 3 is
schematically represented by 54. The time between the closure of the
switch S.sub.A and the closure of the switches S.sub.B and S.sub.C may for
example be 100 msecs. For reaching the distance e for example 40 msecs.
are required.
With the focussing control described so far, the objective system is driven
with a constant force during locking-in of said system, so that the
objective system moves towards the record carrier with a constant
acceleration. The lock-in range, i.e. the distance over which the speed of
the objective system must be reduced to zero, can be substantially reduced
according to the invention when care is taken that the objective system
moves with a constant speed. For this use can be made of a back-EMF
(electromotive force) which is induced in the coil of the objective system
as soon as the objective system is put into motion. FIG. 3 schematically
shows a control circuit for driving the objective system with a constant
speed.
The loudspeaker coil S.sub.p forms part of a bridge circuit. The different
bridge arms are formed by the ohmic resistance (R.sub.L) of the coil, the
measuring resistance Rm and the two resistors R.sub.2 and R.sub.3. The
ratio R.sub.2 /R.sub.3 is selected to equal the ratio R.sub.L /Rm. When
the objective system starts moving, the bridge circuit is unbalanced. The
voltage between the points 60 and 61 is applied to an input of the
operational amplifiier 51 via an amplifier 52. By including in the
feedback path an element 53 which conducts in one direction only, it is
assured that feedback is provided for only one polarity of the current
through the coil. As a result, the speed of the objective system is
maintained constant in one direction only (towards the record carrier).
The feedback system has a certain open-loop gain. The magnitude of said
gain determines the degree of constancy of the speed of the objective
system. The feedback, which acts as a damping, exists only when the
objective system moves towards the record carrier. From the instant that
the objective system has reached the minimum distance to the record
carrier and the sign of thhe speed is reversed, the feedback is
discontinued. FIG. 4 shows an embodiment of the electronic circuit
according to the invention to the invention in detail. Only elements which
are of significance for the principle of the invention will be discussed.
The signal V.sub.f which is obtained from the focussing detection system is
amplified by an operational amplifier 62. The operational amplifier is
followed by a switch which consists of two diodes (D.sub.1 and D.sub.2)
and two transistors (T.sub.12 and T.sub.13), which switch can handle both
positive and negatiive voltages. The switching voltage of said switch is
applied to point A and is derived from a shift register, not shown. The
switch is followed by the output stage, whose output circuit includes the
coil S.sub.p with which the objective system is driven. This coil is
connected in series with the resistance R.sub.m. The voltage across said
resistance is fed back to that input of the operational amplifier to which
also the auxiliary voltage V.sub.B is applied. Owing to the feedback the
current through the coil S.sub.p is always proportional to the input
voltage. The coil S.sub.p and the resistance R.sub.m are two branches of a
bridge circuit whose other brancehes are constituted by the resistors
R.sub.30 and R.sub.31 is connected to the emitter of a transistor
T.sub.19, and that of the coil and the resister R.sub.m to the emitter of
a transistor T.sub.20.
When the bridge circuit is balanced there will be specific collector
current in the transistor T.sub.19. Said collector current causes such a
voltage drop across the resistor R.sub.13 that transistor T.sub.11 is cut
off. When the objective system begins to move, the collector current
through the transistor T.sub.19 will vary.
Before the objective system starts to lock in, the auxiliary voltage
V.sub.B is available at point B. The voltage at point A is then so high
that the switch is not yet turned on. After some time such a voltage is
applied to point A that the switch is turned on. The auxiliary voltage
V.sub.B remains available. Across the coil S.sub.p a negative voltage will
then appear, so that a force is exerted on the objective system in a
direction towards the record carrier. Owing to the movement of the
objective system the collector current through the transistor T.sub.19
changes in such a way that the transistor T.sub.11 is turned on. The
voltage at the input of the operational amplifier to which the voltage
V.sub.f is applied increases so that the voltage across the coil for
driving the objective system decreases. The control circuit will adopt a
balanced condition in which the objective system has a constant speed. As
the transistor T.sub.11 can only supply a current from a positive supply,
the speed of the objective system can be maintained constant in one
direction only. When the objective system moves away from the record
carrier the transistor T.sub.11 will cut off. After a time which suffices
to allow the objective system to assume the setting for a correct
focussing, the auxiliary voltage V.sub.b is disconnected. The transistor
T.sub.11 can then no longer be turned on and the feedback is no longer
present.
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