Method of and circuitry for compensating offset voltages in a focusing and/or tracking circuit

A method for compensating offset voltages selectively in a focusing circuit that focuses a beam from a source of light on a recording medium and in a tracking circuit that positions a beam of light on data storage tracks on a recording medium. A beam of light reflected from the recording medium is directed onto a photodetector which has a plurality of photodiodes. The difference of the output voltages from the photodiodes is taken for generating an error signal which is stored as a reference during a first stage in open or closed state of the circuit selected. The selected circuit is then closed and a compensation parameter is supplied in a second stage to controls in the selected circuit. The compensation parameter is then varied until the error signal is substantially equal to the stored reference. The compensation of the offset voltages is independent of the tracking circuit, and the offset voltages in the focusing circuit can be compensated when the tracking circuit is closed.

BACKGROUND OF THE INVENTION 
The invention concerns a method of compensating offset voltages in a 
focusing circuit that focuses a beam from a source of light on a recorded 
medium and/or in a tracking circuit that positions the beam of light on 
the data-storage tracks on the recorded medium, whereby the beam is 
reflected from the medium onto a photodetector that consists of several 
photodiodes and whereby a focusing and/or tracking error signal is derived 
from the output voltages of the photodiodes by constructing the difference 
between them. 
In equipment for playing back data that can be read out of the data-storage 
tracks on a recorded medium with an optical pick-up, a beam of light is 
focused on the recorded medium by a focusing circuit and positioned on the 
data-storage tracks on the medium by a tracking circuit. The optical 
pick-up in such equipment--compact-disk players, optico-magnetic equipment 
for recording and playing back, equipment for recording and playing back 
DRAW disks, and videodisc players for example--includes a laser diode, 
several lenses, a prismatic beam divider, a refraction grating, and a 
photodetector. An optical pick-up of this type is described in Electronic 
Components and Applications 6, 4 (1984), pages 209 to 215. 
Lenses focus the beam of light emitted by the laser diode onto the compact 
disk, which relfects onto a photodetector. The data recorded on the disk 
and the actual value for the focusing circuit and for the tracking circuit 
are obtained from the signal leaving the photodetector. The aforesaid 
literature calls the actual value of the focusing circuit the focusing 
error and the actual value of the tracking circuit the radial-tracking 
error. 
The focusing circuit is controlled by a coil. An objective lens travels 
along an optical axis through the coil's magnetic field. The focusing 
circuit displaces the lens to ensure that the beam of light emerging from 
the laser diode is constantly focused on the compact disk. The tracking 
circuit, which is often called a radial drive mechanism, displaces the 
optical pick-up radially in relation to the disk, positioning the beam on 
the spiral data-storage tracks on the disk. 
The radial drive mechanism in some equipment consists of a 
coarse-adjustment mechanism and a fine-adjustment mechanism. The 
coarse-adjustment mechanism can for example be a spindle that radially 
displaces the entire optical pick-up--the laser diode, the lenses, the 
prismatic beam divider, the refraction grating, and the photodetector. The 
fine-adjustment mechanism radially tilts the beam of light, at a 
prescribed acute angle for example, advancing the beam slightly along the 
radius of the disk due to the tilting motion alone. 
FIG. 1 illustrates the photodetector PD in the optical pick-up of a 
compact-disk player wherein three laser beams L1, L2, and L3 are focused 
on a compact disk. A pick-up of this type is called a three-beam pick-up 
in the aforesaid reference. 
Middle beam L1 is the main beam, and beams L2 and L3 are beams of the +1st 
and -1st order generated from main beam L1 by a refraction grating. 
Photodetector PD consists of four square photodiodes A, B, C, and D 
arranged in a square. Diagonally opposite the large square comprising 
photodiodes A, B, C, and D are two other photodiodes E and F, which are 
also square. 
Main beam L1 is focused on photodiodes A, B, C, and D and generates a data 
signal HF=AS+BS+CS+DS and a focusing-error signal FE=(AS+CS)-(BS+DS). 
Forward beam L2 is focused on photodiode E and rear beam L3 on photodiode 
F. Both of these outer beams L2 and L3 generate a tracking-error signal 
TE=ES-FS. AS, BS, CS, DS, ES, and FS are the photoelectric voltages 
emitted by photodiodes A, B, C, D, E, and F respectively. Since an 
astigmatic collimator lens is positioned in the path of the main beam L1 
in the optical pick-up, the beam will be circular when precisely focused 
on the large square that comprises photodiodes A, B, C, and D and will be 
elliptical when it is out of focus. 
FIG. 1a illustrates precise focus and precise tracking, which will be 
described hereinafter. Since the spot of light produced on the large 
square by main beam L1 is circular, focusing-error signal 
FE=(AS+CS)-(BS+DS)=0, and the zero tells the focusing circuit that the 
focus is precise. 
FIG. 1b illustrates imprecise focus deriving from the lens being too far 
from the compact disk. Focusing-error signal FE=(AS+CS)-(BS+DS)&lt;0, and the 
negative value tells the focusing circuit that the distance between the 
lens and the disk is too great. The controls in the focusing circuit 
accordingly displace the lens toward the disk until focusing-error signal 
FE becomes zero again. 
FIG. 1c illustrates the opposite type of imprecise focus deriving from the 
lens being too near the compact disk. Focusing-error signal is 
positive--FE=(AS+CS)-(BS+DS)&gt;0, and the positive value tells the focusing 
circuit that the lens is too near the disk. The controls accordingly 
displace the lens away from the disk until focusing-error signal FE 
becomes zero. 
How the tracking circuit operates will now be explained. 
The beams L1, L2, and L3 illustrated in FIGS. 1a, 1b, and 1c are precisely 
on the track, and tracking-error signal TE=ES-FS=0. 
FIG. 1d illustrates beams L1, L2, and L3 to the right of the track. 
Tracking-error signal is negative--TE=ES-FS &lt;0. The controls in the 
tracking circuit displace the optical pick-up to the left until 
tracking-error signal is TE becomes zero. 
In the opposite situation, shown in FIG. 1e, with the beams to the left of 
the track, the tracking-error signal becomes positive--TE=ES-FS&gt;0, and the 
controls in the tracking circuit displace the optical pick-up to the right 
until tracking-error signal TE becomes zero. 
The unobjectionable playback of data--whether audio and video in a 
videodisc player or audio alone in a compact-disk player--requires, in 
addition to precise focusing of the beam on the videodisc or compact disk, 
precise tracking over the disk. 
The variable amplifier in the focusing circuit is, however, like any other 
variable amplifier, affected by an offset voltage at a level that is both 
dependent on temperature and subject to long-term drift. The 
offset-voltage drift is caused, along with other parameters in an 
amplifier by the amplifier aging. 
Focusing-error signal FE=(AS+CS)-(BS+DS) is constructed in a differential 
amplifier. Since the differential amplifier is also affected by an offset 
voltage and since photodiodes A, B, C, and D emit are not ideal and will 
emit different voltages or currents at the same light density, the 
situation is another source of detrimental offset voltages. 
To prevent the playback of data from being detrimentally affected by offset 
voltages, as many offset voltages as possible must be compensated. 
Compensation, however, can be carried out only approximately by manually 
adjusted potentiometers because they cannot take changes in the offset 
voltages due to temperature fluctuations and the aging of specific 
components into account. 
SUMMARY OF THE INVENTION 
The object of the invention is accordingly to provide a method of 
compensating offset voltages in a focusing and/or tracking circuit such as 
to allow automatic compensation. 
This object can be attained in accordance with the invention in that, when 
the circuit is closed, the focusing-error or tracking-error signal is 
compared with a prescribed reference and in that a compensation parameter 
is supplied to the controls in the focusing and/or tracking circuit and 
varied until the focusing-error and/or tracking-error signal coincides 
with the reference. 
A second object can also be attained in accordance with the invention in 
that a compensation parameter is added to the focusing-error and/or 
tracking-error signal during an initial stage with the circuit either open 
or closed and with the photodiodes uniformly illuminated and is varied 
until the sum coincides with a reference, in that the focusing-error 
and/or tracking-error signal is compared with another reference during a 
second stage with the circuit closed, and in that another compensation 
parameter is supplied to the controls in the focusing and/or tracking 
circuit and is varied until the focusing-error and/or tracking-error 
signal coincides with the reference. 
A third object can also be attained in that the focusing-error and/or 
tracking-error signal is stored as a reference during an initial stage 
with the circuit either open or closed and with the photodiodes uniformly 
illuminated and in that a compensation parameter is supplied to the 
controls in the circuit during a second stage with the circuit closed and 
is varied until the focusing-error and/or tracking-error signal coincides 
with the stored reference.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The individual procedures in accordance with the invention will now be 
described with reference to the example of a focusing circuit and to the 
circuitry illustrated in the figures. 
A voltage +U is according to FIG. 2 at the interconnected cathodes of 
photodiodes A, B, C, and D. The interconnected anode of photodiodes A and 
C are connected to the adding input terminal and the interconnected anodes 
of photodiodes B and D to the subtracting input terminal of a differential 
amplifier DV. The output terminal of differential amplifier DV is 
connected by way of a resistor R1 to the input terminal of a variable 
amplifier RV and by way of another resistor R2 to the non-inverting input 
terminal of a comparator VL. The non-inverting input terminal of 
comparator VL is at reference potential UC by way of a capacitor C1. At 
the inverting input terminal of comparator VL is a reference voltage UR. 
The output terminal of comparator VL is connected to the input terminal E1 
of a microprocessor MP. The output terminal A1 of microprocessor MP is 
connected to the input terminal of a digital-to-analog converter DA1. The 
output terminal of digital-to-analog converter DA1 is connected to the 
output terminal of variable amplifier RV and to one terminal of controls 
SG at potential UV. The other terminal of controls SG, which are in the 
form of a coil, is at reference potential. 
The arrangements shown in FIGS. 2 to 6 are located in a housing G for 
example, the housing of a compact disc player, or in some other housing of 
the player, so that no light can strike the photodiodes A, B, C, and D. To 
achieve the condition that light does not reach the photodiodes, the light 
source L is switched off by a switch S, or the light reflected from the 
compact disc CD is screened or shut off by a diaphragm or shutter BL. 
Light from the light source L is focused on the compact disc CD, and this 
light is reflected from the compact disc onto the photodetector with the 
four photodiodes A, B, C, and D. 
To facilitate comprehension of the circuitry, let it be initially assumed 
that the focusing circuit is composed of ideal components that have no 
offset voltage. 
When the focus is precise, main beam L1 produces a circle on the four 
photodiodes A, B, C, and D as illustrated in FIG. 1a. Since each 
photodiode A, B, C, and D accordingly receives light of the same intensity 
and converts it into an electric current, equal voltages or currents will 
be emitted. Since variable amplifier RV is also assumed to be ideal, the 
voltage at its output terminal and hence at one terminal of controls SG 
will be zero. Controls SG, which are often called an actuator, accordingly 
displace the lens in the optical pick-up until the voltage at the output 
terminal of variable amplifier RV become zero. Assuming that the 
components are ideal, the focus will be precise because the voltage at the 
output terminal of differential amplifier DV is also zero. 
Let it now be assumed that variable amplifier RV is affected by an offset 
voltage but that differential amplifier DV and photodiodes A, B, C, and D 
are ideal. 
Controls SG will displace the lens until the voltage at the output terminal 
of variable amplifier RV becomes zero. Since variable amplifier RV is an 
actual component, the voltage entering and hence the voltage leaving the 
amplifier will differ from zero. In the position in which the lens is 
secured by the coil, main beam L1 will no longer be circular but, as 
illustrated in FIG. 1b or 1c, slightly elliptical, indicating that the 
focus is not precise. 
To attain precise focus by compensating the offset voltage from variable 
amplifier RV, the voltage at the output terminal of differential amplifier 
DV is compared in comparator VL with reference voltage UR, which is zero 
in the present case. Microprocessor MP now varies the digital values at 
its output terminal A1, which digital-to-analog converter DA1 converts 
into an analog voltage and supplies to controls SG, until the comparator 
VL at the input terminal E1 of microprocessor MP indicates that the 
voltage at the output terminal of differential amplifier DV is zero. Since 
the lens is now focusing main beam L1 in the form of a circle on 
photodiodes A, B, C, and D, the focus is precise. Once comparator VL 
informs microprocessor MP that the focus is precise because the voltage at 
the output terminal of differential amplifier DV is zero, the 
microprocessor will retain the value at its output terminal A1, resulting 
in a constantly analog compensation voltage from digital-to-analog 
converter DA1 at controls SG. 
The equipment, a compact-disk player for example, is now ready to play. It 
is of particular advantage for the compensation to occur every time the 
player is turned on. 
If, now, the offset voltage is variable amplifier RV changes, as the result 
of temperature fluctuations or aging for example, the voltage at the 
output terminal of differential amplifier DV will also change and will no 
longer coincide with reference voltage UR. Since comparator VL so 
indicates to the microprocessor when the player is turned on, the 
microprocessor can readjust the offset-compensation voltage and ensure 
optimum compensation. 
An essential advantage of the circuitry illustrated in FIG. 2 is that the 
offset voltage in variable amplifier RV will be automatically compensated 
every time the compact-disk player is turned on. 
The circuitry illustrated in FIG. 3 and intended for carrying out the 
procedure according to the present invention, will now be described and 
its operation explained. 
The circuitry illustrated in FIG. 3 differs from the circuitry illustrated 
in FIG. 2 in that it includes another digital-to-analog converter DA2, the 
output terminal of which is connected to the output terminal of 
differential amplifier DV and the input terminal of which is connected to 
another output terminal A2 of microprocessor MP. 
It is assumed in conjunction with the circuitry illustrated in FIG. 3 that 
both variable amplifier RV and differential amplifier DV are affected by 
an offset voltage. Photodiodes A, B, C, and D are considered to be actual 
components, meaning they are not precise identical and can emit different 
voltages or currents subject to light of the same intensity. Accordingly, 
when the focus is precise and the lens is focusing main beam L1 in the 
form of a circle on photodiodes A, B, C, and D as illustrated in FIG. 1a, 
the voltage at the output terminal differential amplifier DV will not be 
zero as desired, but will be positive or negative, +a for example. 
During an initial stage with the focusing circuit either open or closed, 
photodiodes A, B, C, and D will be uniformly illuminated. This state can 
be easily attained by turning off the source of light and leaving the 
photodiodes in the dark. When the source of light is off, it makes no 
difference where the lens is or whether it is moving because no feedback 
can occur. 
In this state, with the source of light turned off, the voltage at the 
output terminal of differential amplifier DV is compared with reference 
voltage UR in comparator VL. 
The digital values emitted at the output terminal A2 of microprocessor MP 
are converted by digital-to-analog converter DA2 into an analog voltage. 
Microprocessor MP varies the digital values at its output terminal A2 
until comparator VL indicates that the analog voltage at the output 
terminal of digital-to-analog converter DA2 has compensated the voltage at 
the output terminal of differential amplifier DV. The digital value at the 
output terminal A2 of microprocessor MP at that instant is retained. The 
measure ensures that the voltage at the input terminal of variable 
amplifier RV will be zero when the focus is precise and main beam L1 is 
producing a circle on photodiodes A, B, C, and D as illustrated in FIG. 
1a. 
In the second stage of the procedure, the source of light is turned on and 
the method continues as described with reference to FIG. 2. 
Unfortunately there exists another offset parameter that has been ignored 
up to now. It is often called optical offset and derives from the opotical 
system in the pick-up. When the beam of light is precisely focused on the 
recorded medium, the beam will not, due to unavoidable defects in the 
optical components--the lenses, the prismatic beam divider, and the 
refraction grating--be circular, as it would be if those components were 
ideal, but will be slightly elliptical on photodiodes A, B, C, and D. When 
the beam is precisely focused on the recorded medium accordingly, the 
voltage at the output terminal of differential amplifier DV will not, in 
spite of the compensation, be zero as desired but will be positive of 
negative. 
How this offset voltage, deriving from optical offset, can be compensated 
by means of the circuitry illustrated in FIG. 4 will now be explained. 
This circuitry differs from the circuitry illustrated in FIG. 3 in that 
reference voltage UR cannot be modified. For this purpose one output 
terminal A3 of microprocessor MP is connected to the control input 
terminal of the source of reference voltage UR, which can for example be a 
digital-to-analog converter. 
A constant value is selected for reference voltage UR during the first and 
second stages described with reference to FIG. 3. In addition to the first 
two stages of the procedure, there is another stage that compensates the 
aforesaid optical offset and occurs while the compact-disk player is being 
manufactured. 
The test disk is inserted in the player. The digital values detected at the 
output terminals A1 and A2 of microprocessor MP during the first two 
stages are retained unmodified during the third stage. The source of light 
is turned on and focused precisely on the recorded medium, the test disk. 
Precise focus is determined by way of the test disk in that the jitter in 
the high-frequency signal is at a minimum when the focus is precise. The 
focus can, however, also be verified with a microscope for example. It is 
now decided how much the reference voltage UR will have to be modified to 
compensate the optical offset as well in that the offset voltage in 
differential amplifier DV has already been compensated during the first 
stage and the that in variable amplifier RV during the second stage. 
Reference voltage UR is accordingly varied until the jitter in the 
high-frequency signal is minimum, at which time the beam of light is 
precisely focused on the test disk. The accordingly obtained value of 
reference voltage UR is set and the player is ready to play. 
When the offset voltage in differential amplifier DV or in variable 
amplifier RV changes during later operation, it can be compenstated every 
time the equipment is turned on by the first and second stages of the 
procedure. 
The circuitry illustrated in FIG. 5 and intended for carrying out the 
method according to the present invention, will now be described and its 
operation explained. 
It differs from the circuitry illustrated in FIG. 4 in that it lacks 
digital-to-analog converter DA2. 
During an initial stage of the procedure, uniform illumination of 
photodiodes A, B, C, and D is ensured, as with the circuitry illustrated 
in FIG. 3, by turning off the source of light. In this state, which 
corresponds to that of precise focus, when main beam L1 produces a circle 
on photodiodes A, B, C, and D, microprocessor MP will vary reference 
voltage UR until comparator VL1 tells it that the reference voltage 
conincides with the voltage at the output terminal of differential 
amplifier DV, +a for example. 
To compensate the aforesaid optical offset as well, the first stage of the 
procedure is modified as for the circuitry illustrated in FIG. 3. With the 
focusing circuit open or closed, the beam of light is precisely focused on 
the compact disk during the manufacturing stage and the focus is verified 
with a microscope or by determining the minimum jitter in the 
high-frequency signal with a test disk. Microprocessor MP will now vary 
reference voltage UR until comparator VL1 tells it that the voltage 
coincides with the voltage at the output terminal of differential 
amplifier DV, +b for example. 
The second stage of the procedure is identical to that described with 
reference to the previous circuitry and occurs with the source of light 
on. 
Without additional compensation, controls SG would maintain the lens in a 
position in which the voltage at the output terminal of variable amplifier 
RV is zero. The voltage at the input terminal of variable amplifier RV, 
however, is generally not +a or +b as desired in this case and as it would 
be if the focus were precise, but will be positive or negative depending 
on the size of the offset voltage in variable amplifier RV. Microprocessor 
MP will accordingly vary the digital value that occurs at its output 
terminal A1 and is converted by digital-to-analog converter DA1 into an 
analog voltage supplied to controls SG until comparator VL tells it that 
the voltage at the output terminal of differential amplifier DV coincides 
with reference voltage UR, which is +a or +b in the numerical example in 
question. The digital value at the output terminal A1 of microprocessor MP 
at that time is retained, with digital-to-analog converter DA1 ensuring 
that the correct compensation voltage will always occur at controls SG. 
The compact-disk player is now ready to play. 
What is particularly advantageous in this circuitry is that both stages are 
carried out every time the player is turned on. 
Manual compensation with a potentiometer while the equipment is being 
assembled is no longer necessary, nor must it be repeated in the event of 
temperature fluctuation or aging of the components because the offset 
voltages are automatically compensated every time the player is turned on.