JPS62121937A - Optical information signal reproducing device - Google Patents

Abstract

PURPOSE:To reproduce an information signal from a high density recording medium by offsetting a crosstalk components included in the information signal read by the 1st optical spot by a crosstalk component in reflected light caused by the 2nd and 3rd optical spots projected on adjacent recording traces. CONSTITUTION:Light from a light source 1 is inputted to a diffraction grating 3 through a collimate lens 2. Zero-order light and + or - first-order diffracted lights, that is, three fluxes in total, are emitted from the grating 3, and inputted to a convergence lens 5 through a beam splitter 4. Spots S1-S3 are image-formed on the signal surface of a recording medium 7, which rotates at a high speed in the direction of an arrow A and gives reflected lights from the spots S1-S3 to photodetectors P1-P3, respectively. Output signals from the photodetectors P1-P3 are processed in order to remove a crosstalk component arising in a reproduction signal and to obtain a signal for tracking control. The reproduction signal from which the influence of a crosstalk is removed and the tracking control signal are outputted to output terminals 8 and 9, and the information signal is reproduced from the high density recording medium.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention is directed to a recording medium having a signal surface on which a recording trace of an optically recorded information signal is formed. The present invention relates to an optical information signal reproducing device that projects a light spot of a diameter and reproduces an information signal based on reflected light from a signal surface.

(Prior Art) A first optical spot for signal reading is projected onto a signal surface of a recording medium having a signal surface on which a recording trace of an optically recorded information signal is formed;
On the straight line including the first light spot,
The second and third optical spoilers 1- for tracking are also projected at symmetrical positions with the optical spot as the center of symmetry.

Conventionally, various configurations have been used as an optical information signal reproducing device that is configured to obtain a desired information signal based on the reflected light generated on the signal surface of the recording medium by the first to third light spots described above. The format is known.

(Problems to be Solved by the Invention) By the way, in an optical information signal reproducing device, information signals are reproduced from a recording medium on which information signals are recorded at high recording density. The diameter of the light spot used to read the information signal from the recording medium is determined by the wavelength of the light used for reproduction and the numerical aperture (NA) of the condensing lens that focuses the light on the signal surface of the recording medium. ), so in order for an optical information signal reproducing device to read the information signal without causing crosstalk from adjacent recording traces on the signal surface of the recording medium, it is necessary to Recording traces are recorded and formed on the signal surface of the recording medium so that the recording trace interval is larger than the spot diameter of the light determined by the wavelength of the laser beam used in the signal reproducing device and the numerical aperture of the condensing lens. It is necessary to make such recording media symmetrical for reproduction.

If the wavelength of the laser beam used to read the information signal is 780 nm and the numerical aperture of the condensing lens is 0.5, as in the optical information signal reproducing device currently in use, recording is possible. The minimum diameter of the laser beam spot focused on the signal surface of the medium is 1.28 microns.

Therefore, in order to prevent crosstalk from occurring during reproduction by the optical information signal reproducing apparatus described above, current recording media that are reproduced by the optical information signal reproducing apparatus described above are The interval between recording traces on the signal plane is 1.6 microns, and the minimum bit length is 0.56 microns.
In the case of microns, NTSC signals for one hour can be recorded on one side of a recording medium with a diameter of 30 cm.

By the way, it is desired that the recording medium used for reproduction in an optical information signal reproduction device be capable of reproduction for a longer period of time, compared to the conventional recording medium described above. There is a demand for recording media with even larger recording capacities.

However, in the conventional optical information signal reproducing device, when the recording interval on the recording medium used for reproduction is reduced, the quality of the reproduced signal deteriorates due to an increase in crosstalk. It was impossible to put the recording medium into practical use.

(Means for Solving the Problems) The present invention provides a first method for reading signals on a signal surface of a recording medium having a signal surface on which a recording trace of an optically recorded information signal is formed. on a straight line including the first light spot,
The second and third optical spots for tracking are also projected at symmetrical positions with the first optical spot as the center of symmetry, and the recording medium is illuminated by the first to third optical spots. In an optical information signal reproducing device that is configured to obtain a desired information signal based on reflected light generated on a signal surface, the center of the first light spot described above is located at the Nth point in the sequentially arranged recording traces. When the center of the second light spot is located at the center of the recorded trace, the center of the second light spot is
The third light spot is located between the Nth recording trace and the (N-1)th recording trace, and the center of the third light spot is located between the Nth recording trace and the (N+1)th recording trace. The first to third light spots described above are projected onto the signal surface of the recording medium so that the light reflected from the signal surface of the recording medium by the first to third light spots described above is means for photoelectrically converting each of the reflected lights from the respective light spots by means of first to third photodetection bases that individually photoelectrically convert the reflected light from each of the light spots;
Means for aligning the positions of the output signals from each of the photodetectors on the time axis, and the output signals from the second photodetector and the third photodetector. An adder circuit that obtains a sum signal, the amplitude of the signal read from the (N-1)th and (N+1)th record trace in the output signal from the above-mentioned adder circuit, and the output signal of the first photodetector. a signal amplitude adjustment means for adjusting the amplitude of the output signal from the adding circuit so as to equalize the amplitude of the signal read from the (N-1)th and (N+1)th record traces; a subtraction circuit for obtaining a difference signal between the output signal from the photodetector No. 1 and the output signal from the signal amplitude adjustment means, and the optical system is configured to obtain a reproduced signal of the Nth recording trace from the subtraction circuit. information signal reproducing device, and
In the optical information signal reproducing device as described above, the second photodetector includes two photodetectors arranged on both sides of a line passing through the center of the reflected light of the second light spot and parallel to the recording trace. The third photodetector passes through the center of the reflected light of the third light spot,
In addition, by using a light-receiving element configured with two light-receiving elements arranged on both sides of a line parallel to the recording traces, the center of the first light spot is the center of the Nth recording trace among the recording traces lined up sequentially. When the center of the second light spot is located between the Nth recording trace and the (N-1)th recording trace, and the center of the third light spot is located at By the second light spot among the first to third light spots, which are arranged in a relative position such that they are located midway between the Nth recording trace and the (N+1)th recording trace. Of the two light receiving elements of the second photodetector that receives reflected light, the output signal from the light receiving element on the side where the reflected diffracted light from the (N-1)th recording trace shows a larger distribution. Of the two light-receiving elements of the third photodetector that receives the reflected light from the third light spot, the light-receiving element on the side where the reflected diffracted light from the Nth recording trace shows a larger distribution. and the sum signal of the output signal from the second. An optical information signal reproducing device comprising means for obtaining a difference signal between the sum signal of the output signals from the light receiving elements on the other side of the third photodetector, and the tracking control system is driven by the difference signal. , as well as projecting a first light spot for signal reading onto the signal surface of a recording medium having a signal surface on which a recording trace of an optically recorded information signal is formed, and The second and third light spots are also projected at symmetrical positions with the first light spot as the center of symmetry on the straight line including the first light spot, and In an optical information signal reproducing device in which a desired information signal is obtained based on reflected light generated from a signal surface of a recording medium by a light spot of When the center of the second light spot is located at the center of the Nth record trace in the lined up record traces, the center of the second light spot is located between the Nth record trace and the (N-1)th record trace. or on the (N-1)th record trace,
And the center' of the third light spot is the Nth recording trace (
The intermediate position with the (N+1)th record trace or the
) The above-mentioned first to third light spots are projected onto the signal surface of the recording medium so as to be located on the recording trace, and the recording medium by the above-described first to third light spots is means for photoelectrically converting the reflected light from the signal surface of the signal plane by first to third photodetectors that individually photoelectrically convert the reflected light from the respective optical spots, and outputs from each of the above-mentioned photodetectors; means for matching the positions of the signals on the time axis; an adding circuit for obtaining a sum signal of the output signal from the second photodetector and the output signal from the third photodetector; ( in the output signal from the adder circuit
The amplitudes of the signals read from the N-1)th and (N+1)th recording traces and the (in the output signal of the first photodetector)
a signal amplitude adjusting means for adjusting the amplitude of the output signal from the adding circuit so as to equalize the amplitude of the signals read from the N−1)th and (N+1)th recording traces; and a first light source. An optical information signal comprising a subtraction circuit that obtains a difference signal between the output signal from the detector and the output signal of the signal amplitude adjustment means, and the reproduction signal of the Nth recording line is obtained from the subtraction circuit. The present invention provides a playback device.

(Example) Hereinafter, specific contents of the optical information signal reproducing apparatus of the present invention will be explained in detail with reference to the accompanying drawings.

FIG. 1 is a perspective view showing an example of the configuration of a part of the optical information signal reproducing device of the present invention.
is a light source (for example, a semiconductor laser), and this light source 1 emits linearly polarized light from its minute light emitting part.

The light emitted from the light source 1 is made into parallel light by the collimating lens 2 and then applied to the diffraction grating 3, and the diffraction grating 3 emits three beams of zero-order light and ±1st-order diffracted light. Ru. The three beams mentioned above are connected to the beam splitter 4.
, and are applied to the focusing lens 5, thereby forming individual first to third spots Sl, S2 . The image is formed as S3. The recording medium 7 is rotating at a high speed at a predetermined rotational speed in the direction of arrow A in the figure.

The three small diameter light spots St, S2, and S mentioned above
The reflected light from 3 is returned to parallel light by a condensing lens 5, and then reflected laterally by a beam splitter 4,
The reflected light from the first light spot S1 is given to the first photodetector P1, and the reflected light from the second light spot S2 is given to the second photodetector P2. Further, the reflected light from the third light spot S3 described above is provided to a third photodetector P3.

The second photodetector P2 described above has a second light spot S2.
It is composed of two light-receiving elements Pi and P2r, which are arranged on the surface side of a line that passes through the center of the reflected light from the recording trace and is parallel to the recording trace. two light-receiving elements P3g arranged on both sides of a line passing through the center of the reflected light from the light spot S3 and parallel to the recording trace;
It is composed of P3r.

Each output signal from the first to third photodetectors P1 to P3 described above is supplied to the circuit arrangement shown in FIG.

In the circuit arrangement shown in FIG. 2, the diameters of the first to third optical spots 81 to S3 projected onto the signal surface of the recording medium 7 are smaller than those formed on the signal surface of the recording medium 7. Signal processing for removing crosstalk components that occur in reproduced signals when information signals are read from recording traces on recording media with small recording trace intervals, and for obtaining signals for tracking control. The reproduction signal and the tracking control signal from which the influence of crosstalk has been removed are output to the respective output terminals 8 and 9.

Here, compared to the diameters of the first to third light spots 81 to S3 projected onto the signal surface of the recording medium 7, the distance between the recording marks formed on the signal surface of the recording medium 7 is When an information signal is read from a recording trace on a small recording medium by the first to third optical spots S1 to S3, the crosstalk component generated in the reproduced signal is generated in the circuit arrangement shown in the second figure. How this can be removed will be explained with reference also to FIGS. 3 to 6.

FIG. 3 shows recording traces T(n-1), T(n), and T(n+1) sequentially formed on the signal surface of the recording medium 7, and one light beam separated into three by the diffraction grating 3. The diameter of the first to third light spots 81 to S3, which have the same diameter and are projected onto each recording trace by the three light fluxes, is 1.28 microns as in the case of the conventional example described above. This corresponds to what is written assuming that the interval between successive recording marks formed on the signal surface of the recording medium 7 is 0.8 microns.

Now, as mentioned above, the light spots 81 to S3 imaged on the signal surface of the recording medium 7 have a Gaussian light intensity distribution in their cross section; The diameter of the light spot is 1/e" (where e is the base of the natural logarithm), as shown in Figure 4.
It is expressed by the diameter of the part that curves in. In addition, in Figure 4, D
a is an Airy disk, which is illustrated by a broken line for the first light spot S1 in FIG. Further, side lobes as illustrated in FIG. 4 occur around the light spot.

Since the energy distribution of the light spot projected onto the signal surface of the recording medium is as shown in Figure 4, the spot energy distribution of the light spot projected onto the signal surface of the recording medium is as shown in Figure 3. When the first to third optical spots are projected onto the signal surface of the recording medium, the side lobe at the customer's spot spreads beyond the adjacent recording trace, and the The first light spot S1 is detected by the first photodetector P1, which photoelectrically converts the light reflected by the first light spot, because the skirt part of its main lobe overlaps the pit of the adjacent recording trace. The output signal of
Naturally, the information signal from the adjacent recording trace is included as crosstalk. Note that the depth of the pits formed on the signal surface of the recording medium is set to be 174 wavelengths or less of the wavelength of the light projected thereon.

In the example shown in the third figure, since the second light spot Sl covers both the recording trace T(n) and the recording trace T(n-1), the light reflected by the second optical spot Sl is The photoelectrically converted signal contains approximately the same information signal of the recording trace T(n) and the information signal of the recording trace T(n-1) to be reproduced by the first light spot Sl. , and the third light spot S3 is the recording trace T(n)
and the recording trace T(n+1), therefore, the signal obtained by photoelectrically converting the light reflected by the third light spot S3 should be reproduced by the first light spot S1. Information signal of record trace T(n) and record trace T(
n+1) information signals are approximately equally included.

5(a) is a side sectional view at the El-El line position in FIG. 3, and FIG. 6(a) is a side sectional view at the E
FIG. 2 is a side sectional view at the 2-E2m position.

In FIG. 5(a), which corresponds to the state shown in FIG. 3, the center of the second light spot S2 is located between the recording traces T(n) and T(n-1), The diffracted light by the pits in the recording trace T(n) goes to the left in the figure, and the diffracted light by the pits in the recording trace T (n-1) goes to the right in the figure, so the reflected light ( Diffracted light) is shown in Figure 5 (
As shown in c), the light is highly distributed around the aperture AP of the condenser lens 5.

In FIG. 5(c), Y is the optical axis, and in the figure, the optical axis Y
The light on the left side of the optical axis Y is mainly modulated by the information signal of the recording trace T(n), and the light on the right side of the optical axis Y is mainly modulated by the information signal of the recording trace T(n-1). .

Next, as shown in FIG. 5(b), when the optical axis Y moves by X to the right in the figure compared to the case of FIG. Record trace T(n-1)
The modulation by the information signal is larger. (d) in Figure 5
) is the reflected light (diffraction light) corresponding to the case of (b) in Figure 5.
It is a figure showing the distribution state of.

The third light spot S3 shown in FIG.

As can be seen from FIG. 6(a), which shows a side sectional view at the E2-E2 line position in FIG.
Although it covers the pit of (n+1), the record trace T(n
), the state of reflection diffraction from the signal surface of the recording medium in this case is as shown in Figure 6 (
As shown in b), the distribution of reflected light on the side where the pits are present is reduced due to diffraction, and the distribution of reflected light on the right side shows a high distribution due to the absence of diffraction.

Of the light reflected by the second light spot S2, the signal received by the light receiving element P2Q of the photodetector P2 and photoelectrically converted contains many information signals of the recording trace T(n). In addition, the second light spot S mentioned above
Among the light reflected by 2, the signal received by the light receiving element P2r of the photodetector 11P2 and photoelectrically converted contains many information signals of the recording trace T (n-1).

Further, among the light reflected by the third light spot S3 described above, the signal received by the light receiving element P3r in the photodetector P3 and photoelectrically converted contains many information signals of the recording trace T(n). Also, the third light spot S mentioned above
3, the light receiving element P in the photodetector P3
There is a recording trace T in the signal received by 3Q and photoelectrically converted.
It contains many (n+1) information signals.

Next, as shown in FIG. 3, when the first to third optical spots having diameters larger than the recording trace interval are projected onto the signal surface of the recording medium, the crosstalk that occurs in the reproduced signal is 2. In order to facilitate understanding that the operation of the circuit arrangement shown in FIG.

This will be explained using mathematical formulas.

First, various symbols used in mathematical formulas will be explained (see Fig. 7).

U 1 (t)...signal recorded in record trace T(n)...signal U2(t)...signal recorded in record trace T(n-1)...record trace Signal τ recorded at T(n+1)...The distance between two adjacent light spots (for example, light spot S2 and light spot S1, or light spot S1 and light spot S3) in the direction of the recording trace. The time required for the recording medium to move, m...The signal component recorded in the recording trace T(n) in the reproduced signal of the first photodetector P1 and the signal component of the first photodetector P1 Ratio α of the signal component recorded other than the recording trace T(n) in the reproduced signal...Second. Ratio β at which the light-receiving elements in the third photodetectors P2 and P3 read signals from the recording trace on the light-receiving element side...The light-receiving elements in the second and third photodetectors P2 and P3 Ratio of reading signals from the recording trace on the opposite side of the element t... Time X... Tracking deviation (right side is positive) k...
Proportionality constant indicating the influence of output reduction due to tracking deviation. However, (k<1) If the signal obtained from the first photodetector pt in the circuit arrangement shown in the second figure is vl, the signal v1 is as follows.

V1=(1-k・I x I)Ul(t)+(1+k
x)m-U2(t)+(1-kx)m-03(t
) = (1-k・I x I)01(t) + m [
(12(t) +03(t) +(uz(t)−[3(
t))kx]----・-<1) The signal V2r output from the light-receiving element P2r in the second photodetector and the light-receiving element P2Q are
The signal V2Q output from
, as shown in equation (3), and furthermore, the signal V3 output from the light receiving element P3r in the third photodetector
r and the signal V3Q output from the light receiving element P3Q are:
They are expressed by the following equations (4) and (5), respectively.

V2r= (1+kx) a ・U2(t+ t)+
(1-kx)β・01 (at t+)・・・・・・(2) V2Q = (1+KX)β・U2(t+t)+(I
KX) α・Ul(t+τ)・・・・・・(3) V3 Q = (1−kz) a・U3(t −τ)
+ (1+kx)β・U1(−τ)・・・・・・(4
) V3r = (1-kx)β・U3(t-c)+(
1+kx) a ・Ul(−τ)...(5) As mentioned above, from the first to third photodetectors P1 to P3 shown in equations (1) to (5) The output signal of the recording medium 7
The first to third light spots 81 projected onto the signal plane of
- This is a signal obtained by photoelectrically converting the reflected light from S3, and the first to third optical spots 81 to S3 are on the signal surface of the recording medium 7 moving in the direction of arrow A in FIG. Since the projection is arranged in such a manner that it is arranged approximately along the moving direction of the recording medium 7, the first signal from the signal surface of the recording medium 7 is
The same information signal to be read by the light spot S1 of the second . Third light spot S″2°S3
The time point at which the information signal is read by is based on the time point at which the information signal is read by the first optical spot S1.

The second light spot S2 is earlier than the reference time point, and the third light spot S3 is later than the reference time point. Therefore, (2)~
Equation (5) contains an item for aligning the time axis of each signal with the time axis of the signal shown in equation (1).

In the circuit layout shown in the second diagram, the above-mentioned (1) to (5)
), the output signal from the second photodetector P2 is delayed by 2τ by the delay circuit DLP2, and the output signal from the first photodetector P1 is delayed by 2τ. The output signal of is delayed by τ by the delay circuit DLPI.

In the state where the time # of the output signals from each photodetector Pi-P3 is matched as described above, when the calculation is performed between each signal, the equations (1) to (5) are expressed as follows. This means that there is no need to consider such time parameters.

In the circuit arrangement shown in the second diagram, the output signal V2 from the two light receiving elements P2Q and P2r in the second photodetector P2
fl, V2r are (V2r-
After the signal is calculated as V2O), it is subjected to a time delay of 2τ by the delay circuit DLp2, and is supplied to the adder circuit ADD and also to the envelope detector Detp2.

In addition, two light receiving elements P3 in the third photodetector P3
The output signals V3Q and V3r from Q and P3r are subtracted by the subtraction circuit 5.
After being calculated as (V3O-v3r) by UBp3, it is supplied to the adder circuit ADD and also to the envelope detector Detp3.

The output signals from the envelope detector Detp2 and the envelope detector Datp3 described above are supplied to the subtraction circuit 5UBt, and a tracking error signal is sent to the output terminal 9 from the subtraction circuit 5UBt.

Further, in the above-mentioned adder circuit ADD, the signal (V2r-V2O) supplied thereto and (V2O-V3
r) is performed to obtain a signal as shown in equation (6). That is, the output signal (V2
r-V2O)+(V:3Q-V3r)=(1+kx)(
α-β)02-(1-kx) (a-β)U1+(1-
kx) (α-β)03-(1+kx)(α-β)01=
[U2+U3+ (02-03)kxko (α-β
)201(α-β) (6) The signal component shown in the brackets in the above equation (6) is the signal shown in the above equation (1), i.e. , the first optical spot S reading the information signal from the Nth recording trace T(n) on the signal surface of the recording medium 7
In the output signal of the first photodetector P1, which photoelectrically converts the reflected light of 1, an adjacent recording trace T (n 1 )y (n
+ 1 ) is isomorphic to the signal component shown in brackets in equation (1), which occurs as a crosstalk component from .

Therefore, the adder circuit A as shown in the above equation (6)
The output signal from the DD is multiplied by a coefficient such as B=m/(α-β), which is the signal expressed by equation (1), that is, the output of the first photodetector P1. By subtracting it from the signal, a signal V as shown by the first-order equation (7) is obtained.

■= Mouth 1-l X l + 2 m] Ul
--(7) Equation (7) above indicates that the reproduced signal does not include information signals of adjacent recording traces, that is, the reproduced signal without crosstalk is obtained.

The calculation performed to obtain the signal shown by the above equation (7) is performed by the gain adjustment circuit JAG and the subtraction circuit SUB in the circuit arrangement shown in the second figure, and the gain adjustment circuit GAJ is performed by the addition circuit. B=m for the output signal from ADD, that is, the signal shown in equation (6)
/(α-β) and supplies it to the subtraction circuit SUB, and in the subtraction circuit SUB, the first
From the output signal from the photodetector P1, that is, the signal v1 shown in equation (1), the above-mentioned gain adjustment circuit GA
The signal obtained by subtracting the output signal of J, that is, the above (7)
As shown in the equation, a reproduced signal V containing no crosstalk is generated, and the reproduced signal V is sent to the output terminal 8.

Next, a description will be given of how to set the gain adjustment mode in the gain adjustment circuit GAJ described above. As illustrated in FIG. 8, two types of distinguishable signals fl and f2 are recorded alternately on successive recording traces on a recording medium, and the first
When the light spot S1 is following the record trace where the signal f2 is recorded, the switch SW in the circuit arrangement shown in the second figure is turned on, and the subtraction is applied to the bandpass filter BPF that extracts the signal of the frequency f1. When the signal output from the circuit SUB is supplied, if the coefficient B=m/(α-β) set in the gain adjustment circuit GAJ is inappropriate, the signal output from the subtraction circuit SUB may be Since the signal f1 of the adjacent recording trace is included as a crosstalk component, it is extracted by the bandpass filter BPF and detected by the amplitude detection circuit ADH.

The amplitude of the crosstalk component described above is (m-B), and the amplitude detection circuit ADE described above outputs a signal proportional to (m-B) and supplies it to the gain adjustment circuit GAJ.

The gain adjustment circuit GAJ performs an automatic control operation so that m=B, so that a signal without crosstalk is sent from the subtraction circuit SUB to the output terminal 8. Since the above-mentioned (m-B) signal has a polarity, the above-mentioned automatic control circuit operates accurately.

The gain adjusted as described above is not completely fixed, but is adjusted each time the recording medium is replaced. In other words, even though the interval between recording marks on G recording media is constant, the depth of the pits and the angle of the edge of the pits vary depending on the conditions at the time of mastering the master disc. This is because the crosstalk rate m often fluctuates over time, and therefore it is desirable to perform gain adjustment for each different recording medium. Note that the recording medium to be reproduced by the optical information signal reproducing apparatus of the present invention has an eighth part outside the original information signal reproduction area.
It is preferable to use a recording area provided with recording areas in which two types of gain adjustment signals are alternately recorded in successive recording traces as illustrated in the figure.

Now, as mentioned above, the depth of the pits formed on the signal surface of the recording medium is set to 1/4 or less of the wavelength of the light projected thereon, so that the 2nd and 3rd optical spots second and third photodetectors P2.
In each of the two light-receiving elements in P3, the signal read by the light-receiving element is such that the information signal of the recording trace on the side where the light-receiving element is arranged is larger than the information signal of the recording trace on the other side.

Therefore, the above-mentioned α and β have the relationship α-β>0, and the above-mentioned coefficient B = m / (α-β) does not become infinite, so the above-mentioned (1) to (7) ) and the operation of the circuit arrangement shown in the second diagram hold true. The ratio m of crosstalk to the main signal is 0 when the recording trace interval is 0.
．． 8 microns, the first beam projected onto the signal plane of the recording medium 7
~If the diameter of the third light spot is 1.28 microns,
Since it is small as expected to be approximately -20 dB,
The coefficient B=m/(α−β) described above does not take a large value.

Furthermore, the second. Third light spot S2. When the position of S3 is shifted from the middle position of the recording traces to the recording traces T(n-1) and recording traces T(n+1), the photodetector P2. Information signal U2. Since the size of U3 increases as in the case of (b) in FIG. 5 described above, the value of (α-β) becomes even larger and the coefficient
The value of -m/(α-β) can be made small, and therefore one circuit can be stabilized.

Photodetector P2. according to the above-mentioned formulas (2) to (5). In the description of the output signal from P3, it was described symmetrically using the same α and β, but the second. Third light spot S2. S
3 is arranged symmetrically with respect to the recording trace T (n), and the second . The above description is valid because the distribution of the reflected and diffracted light from the third light spots S2 and 33 is also symmetrical.

In a state in which the center of the first light spot S1 correctly coincides with the center of the recording trace T(n), the second light spot S1. Third light spot S2. The position of S3 is changed from the middle position of the recording trace to the recording trace T(n-1) and the recording trace T with the recording trace T(n) as the axis of symmetry.
If it is moved to the (n+1) side, α and β in equations (3) and (5) can be replaced with other coefficients γ and δ, and in this case, equation (6) % equation %() It changes to the above equation (8), but in this case, the coefficient B is
Crosstalk can be eliminated by setting = m/(α−δ). The output signal from the subtraction circuit SUB in this case is as follows.

V = [1-k l x l +2m(γ-β)/(
α−δ)]U1, but crosstalk is also removed in this case.

Next, the tracking error signal sent to the output terminal 9 in the circuit arrangement shown in the second figure will be explained. As described, in the second illustrated circuit arrangement, the second photodetector P
The output signals V:'Q, V2r from the two light receiving elements P2Q and P2r in 2 are subtracted by the subtraction circuit 5UBp24: Therefore, (
v2r-v2Q), and then the delay port DL
A time delay of 2τ is applied by ρ2, and the signal is supplied to the envelope detector Detp2, and is also supplied to the third photodetector P.
The output signals VLQ and V3r from the two light receiving elements P3Q and P3r in 3 are converted to (V3
Q V3r) After being calculated as in (7), it is supplied to the envelope detector Detp3, and the envelope detector Detp2 and the envelope detector Detp3 described above are
The output signal from the subtraction circuit 5UBt is supplied to the subtraction circuit 5UBt, and the tracking error signal is sent from the subtraction circuit 5UBt to the output terminal 9.
However, since the output signals from the envelope detector Detp2 and the envelope detector Detp3 described above have lost the individual pit information on the signal surface of the recording medium 7, these signals are This corresponds to the terms indicating recorded information in the above-mentioned formulas (2) to (5), that is, Ult, U2(t), and U3(t) replaced with 1.

Therefore, when the output signal of the envelope detector Detp3 is subtracted from the output signal of the envelope detector Detp2 by the subtraction circuit 5UBt, V2r −V2 Q
+ V3r −V312 = 4 (a−β)kx −
(9) A signal as shown in equation (9) above is obtained.

By the way, α and β have a relationship of α>β.

The above-mentioned equation (9) indicates a tracking error signal proportional to the tracking deviation X. Note that the envelope detector Detp2 and the envelope detector Det described above
A low pass filter may be used as p3. That is, the signal components of the tracking deviation are distributed below IKHz, and the information signals Ul(t), U2(t), and U3(t) recorded on the signal surface of the recording medium 7 are distributed at IKHz.
Because it is distributed over z.

The signal component of the tracking deviation can be separated and extracted from the information signals Ul(t), U2(t), and U3(t) using a low-pass filter.

In a state where the center of the first light spot S1 coincides with the center of the recording trace T1, the second... Third light spot 32.
The position of S3 is from the middle position of the recording trace to the recording trace T(n−
1), when placed on the side approaching or moving away from the recording trace T(n+1), α and β in the above equations (3) and (5) are replaced with γ and δ, and the result is as follows. of(
10) 20(α-β)+(γ-δ)] kx as in the formula
(10) It becomes proportional to the tracking deviation.

Note that all the mathematical formulas used to explain the configuration and operation so far have been linear, but in the optical information signal reproducing device of the present invention, the effects of tracking deviation and crosstalk when tracking control operations are performed are small and linear. Therefore, the explanation using the above formula corresponds to the actual situation.

In an optical information signal reproducing device, focus control Qtl is also required to image a light spot on a recording medium. It is easy to obtain a focus error signal using the above method, and use the focus error signal to drive the condensing lens using the focus control technique described above to perform focus control.

In the optical information signal reproducing device of the present invention shown in the second diagram already described, a second photodetector P2 and a third photodetector P3 are provided.
However, in the optical information signal reproducing apparatus according to another embodiment of the present invention shown in FIG.
As the photodetector P3, one consisting of a single light receiving element is used.

In the optical information signal reproducing device shown in FIG.

The second photodetector P2 is provided at the position where the light receiving element P2r in the second illustrated embodiment is arranged.

Further, the third photodetector P3 is provided at the position where the light receiving element P3Q in the second illustrated embodiment is arranged.

As described above, various types of photodetectors each equipped with one photodetector for side spots are currently commercially available, so the optical information signal reproducing apparatus of the embodiment shown in FIG. The advantage is that it can be provided at a low price.

Now, in the optical information signal reproducing device shown in FIG.
Since the optical information signal reproducing device shown in the second diagram does not include the light-receiving element P2Q in the second photodetector P2 and the light-receiving element P3r in the third light-receiving element, the signal explained with respect to the circuit arrangement shown in the second diagram is There is no V2Q, V3r,
The above-mentioned equation (6) can be expressed as the following equation (6a).

V2r+V312= (U2+U3+(0203)k
x) (!+2U1β...(6a) Then, B'
By multiplying by a coefficient of =m/α and then subtracting it from the signal shown in equation (1), a signal V shown in equation (7a) below is obtained.

V= [1-k l X l-2mβ/(!] Ul-
(7a) The above equation (7a) showing the output signal from the subtraction circuit SUB in FIG. 9 includes crosstalk components U2. U3 is not included.

The coefficient of B'=m/α described above is a coefficient to be set in the gain adjustment circuit GAJ. By the way, as described, α〉
Since β and m < 1, the above (7a
2mβ/α of the third term in the equation ) is much smaller than 1, so the reproduced signal V will not deteriorate, and the same effect as the optical information signal reproducing device shown in the second diagram described above can be expected. .

The tracking error signal from the subtraction circuit 5UBt is V2r.
-V3 Q = 2 (a-β)kx--(9a), but compared to this and the tracking error signal obtained from the subtraction circuit 5UBt in the optical information signal reproducing device shown in the second figure, which has already been described. It is now 1/2 the size. However, since the signal of equation (9a) is proportional to the tracking error X, it goes without saying that it can be used for tracking control.

Next, other embodiments of the optical information signal reproducing apparatus of the present invention will be described with reference to FIG. 10 and subsequent sections. FIG. 10 shows a concentric or spiral shape on the signal plane, and
An information signal is reproduced using a recording medium in which grooves (V grooves) having a V-shaped cross section are closely provided, and recording traces of information signals are recorded and formed on each slope of vWl of the signal surface. FIG. 2 is a perspective view of a portion of an optical information signal reproducing device configured to perform the following steps.

As mentioned above, (1) A recording medium in which a recording trace of an information signal is formed on each slope of the groove, when optically reproducing the information signal of the recording trace by projecting a light spot onto the recording trace, It has attracted attention because it has the advantage of making it easy to obtain a reproduced signal with less crosstalk from adjacent recording traces, and the applicant company previously proposed in Japanese Patent Application No. 59-167989 that The present invention proposes an optical information signal reproducing device that can easily and reliably perform tracking control on recorded media.

The optical information signal reproducing device previously proposed by the applicant company uses a recording medium on which a V-groove is formed on the signal surface of the recording medium for reproducing information signals. Therefore, it is easy to reproduce the information signals of adjacent recording traces with little crosstalk, and the reproduction speed is approximately twice that of the case where a conventional general planar recording medium is used as the target of reproduction. The present invention is applied to this already proposed optical information signal reproducing device. This provides the advantage that crosstalk can be prevented even more reliably.

In FIG. 10, 1 is a light source (for example, a semiconductor laser), and this light source 1 emits linearly polarized light from its minute light emitting part. The light emitted from the light source 1 is made into parallel light by a collimating lens 2 and then passed through a diffraction grating 3.
The diffraction grating 3 emits three beams of zero-order light and ±1st-order diffracted light. The three beams described above pass through the beam splitter 4 and are applied to the focusing lens 5, thereby forming first to third light spots Sl of respective minute diameters on the signal surface of the recording medium (disc) 7V.
, S2. The image is formed as S3. The recording medium 7v is rotating at a high speed at a predetermined rotational speed in the direction of arrow A in the figure.

The aforementioned three small diameter light spots Sl, S2°S
The reflected light from 3 is returned to parallel light by a condensing lens 5, and then reflected laterally by a beam splitter 4,
The reflected light from the first light spot S1 described above is given to the first photodetector P1.

Further, the reflected light from the second light spot S2 mentioned above is given to the second photodetector P2, and the reflected light from the third light spot S3 mentioned above is given to the third photodetector P3. Given.

The first photodetector P1 described above has a first light spot S1.
It is composed of two light receiving elements PIQ and Plr arranged on both sides of a line that passes through the center of the reflected light from the recording trace and is parallel to the recording trace, and the second photodetector P2 mentioned above is Two light receiving elements P2Q are arranged on both sides of the gland, passing through the center of the reflected light from the second light spot S2 and parallel to the recording trace.

P2r, and the third photodetector is
It is composed of two light receiving elements P3Q and P3r arranged on both sides of a line that passes through the center of the reflected light from the third light spot S3 and is parallel to the recording trace.

Each output signal from the first to third photodetectors P1 to P3 described above is supplied to the circuit arrangement shown in FIG. 12. In the circuit arrangement shown in FIG. 12, the diameters of the first to third optical spots 81 to S3 projected onto the signal surface of the recording medium 7v are smaller than those formed on the signal surface of the recording medium 7v. Signal processing for removing crosstalk components that occur in a reproduced signal when an information signal is read from a recording trace on a recording medium where the interval between recording traces is small.

Signal processing is performed to obtain a signal for tracking control, and a reproduced signal from which the influence of crosstalk has been removed and a tracking control signal are output to output terminals 8 and 9.

The signal surface of the recording medium 7v has successive V-grooves in close contact with each other, and each slope of each V-groove having a groove width of W constitutes an individual recording trace. The width of the recorded trace is approximately W/2.

The diameters of the first to third optical spots 81 to S3 projected onto the signal surface of the recording medium 7v are larger than the width of the recording trace and smaller than the width of the groove. As shown in FIG. 11(a), the first light spot S1 is centered at the Nth recording trace T on the recording medium 7V.
(n), the first light spot S1 having a diameter larger than the width of the recording trace as described above is located between the Nth recording trace T(n) and (N-1 )th record trace T(n-1) and (N+1)th record trace T(n+
1), therefore, the reflected light from the signal surface by the first optical spot S1 described above is the Nth recording trace T(n) as shown in FIG. 11(b).
The reflected light E1 from the (N-1)th recording trace T(n-
1) and the (N+1)th recorded trace T(
reflected light E3 from n+1).

Since the reflected light E2 and the reflected light E3 have substantially opposite phases, they interfere with each other and are diffused over a wide range, and the reflected light E1 is distributed in the geometrical direction of reflection. . The above-mentioned reflected light E1 is the reflected light from the center of the light having a high light intensity at the center as shown in FIG. 4, so the intensity of the reflected light E1 is high.

Since the reflected light E1 from the signal surface of the recording medium 7v described above is modulated according to pits caused by information signals in the recording traces or changes in reflectance caused by the information signals in the recorded traces, this reflected light El is photoelectrically converted. By doing this, it is possible to obtain a reproduced signal corresponding to the information signal recorded on the signal surface of the recording medium 7v.

When the first light spot S1 is projected onto the N-th recording mark T(n) formed on one inclined surface of the V groove on the signal surface of the recording medium 7v, the reflected light El is condensed. The light is received by the light receiving element PIQ of the first photodetector P1 through the aperture of the lens 5 (the reflected light E1 from the first light spot S1 is received only by the light receiving element PIQ of the first photodetector P1).
The reason why the light is received is because the angle of the slope of the V groove formed on the signal surface of the recording medium 7v is set as such. Moreover, the light receiving element P112 of the first photodetector P1
The cross-sectional shape of the light received by the lens is similar to the side surfaces of a biconvex lens, as shown in FIG.

The first light spot St is the Nth recording trace T(
When the reflected light E1 is projected onto the recording mark formed on the inclined surface opposite to the inclined surface of the V-groove in which the
The light is received by the light receiving element Plr of the photodetector P1.

In this way 1. Depending on when the first light spot S1 is projected on one inclined surface in V\fi of the signal surface of the recording medium 7v and when it is projected on the other inclined surface, the first light spot S1 The reflected light is photoelectrically converted by the light receiving element Pi of the first photodetector P1, or photoelectrically converted by the light receiving element Plr of the first photodetector PL. Then, the first light spot S1 is projected onto the light receiving element of the first photodetector P1 that is not the light receiving element to which the light reflected by the first light spot S1 is given as described above. Since the diffused light reflected from the adjacent recording trace is provided, the output signal thereof is unnecessary.

Therefore, among the two light receiving elements in the first photodetector P1, only the output signal from the light receiving element to which the light reflected by the first light spot S1 is given is extracted. Two light receiving elements Pin in the photodetector P1 of
, Plr are taken out via a changeover switch SWa and supplied to a delay circuit DLPI.

As mentioned above, in a recording medium in which a V-groove is formed on the signal surface, since the adjacent recording traces are formed on surfaces with different slopes, there is very little crosstalk from the adjacent recording traces. Although the signal can be obtained by the first photodetector P1, the reflected light E2. Since the diffused light due to the interference of E3 is received by the first photodetector P1, a slight crosstalk occurs in the output signal from the first photodetector P1.

Now, the second optical spot S2 projected onto the signal surface of the recording medium is -1, with its center centered on the Nth recording trace T(n) and the (N-1)th recording trace T(n-1). ), the reflected light from this second light spot S2 is, as shown in FIG. 11(c),
The second photodetector P is divided into two by the above-mentioned edge 10.
The light is equally incident on the two light receiving elements P2II and P2r of No.2.

The light incident on the light receiving element P2Q in the second photodetector P2 is mainly reflected light from the recording trace T (n), and therefore the reflected light contains the information signal Ul. Furthermore, the light incident on the light receiving element P2r in the second photodetector P2 is mainly caused by the recording trace T(n-1).
The reflected light contains the information signal U2.

Third optical spot S projected onto the signal surface of the recording medium
3 is the record trace T(n) whose center is the Nth, and (N+
1) Located at the bottom 11 of the boundary with the third recording trace T(n+1), the reflected light from the recording trace T(n) by the third optical spot S3 and the (N+1)th recording trace T(n+1 The reflected lights from ) cross each other as shown in FIG. 11(d).

The reflected light from the recording trace T(n) contains information of Ul, which is incident on the light receiving element P3Q in the third photodetector P3, and the reflected light from the recording trace T(n+1) contains the information of U.
3, which is incident on the light receiving element P3r in the third photodetector P3.

Two light-receiving elements P2r in the second photodetector P2 are represented using the same symbols as already described with reference to FIG.
P2fi (7) output signals V2r and VHI are as described above (
2) and (3), and two light receiving elements P3r in the third photodetector P3.

The output signals V3r and Vll of P2O are as described in (4) above,
The equations (5) are interchanged.

Therefore, the subtraction circuit 5UBp3 to which the output signals of the two light receiving elements P3Q and P3r of the third photodetector P3 in the circuit arrangement shown in FIG. If the connection polarity is reversed.

Therefore, the adder circuit A in the circuit arrangement shown in FIG.
The output signal from DD is V2r-V2Q+V3r-V2
O, and exactly the same result as in equation (6) is obtained. The output signal from the adder circuit ADD is sent to the gain adjustment circuit GA.
J and the subtraction circuit SUB undergo the same signal processing as in the previously described embodiments, and are sent to the output terminal 8 as an output signal without crosstalk. In an optical information signal reproducing device that reproduces a recording medium 7A whose signal surface is formed in a V-groove, an operation is performed such that crosstalk is not originally included in the output signal from the first photodetector P1. from,
The value of m mentioned above is small, and the second. Third photodetector P2. Regarding the light incident on P3, since the value of α described above is large, the value of the coefficient B=m/(α−β) described above is smaller than that in the case of the circuit layout shown in the second diagram, and almost no correction is required. It can even be said that it is not necessary.

Therefore, the circuit arrangement shown in FIG. 12 has the ability to eliminate crosstalk even if the recording trace interval is made much smaller than the diameter of the optical spot. In other words, if the technology for forming V-grooves on the signal surface of a recording medium and the technology for recording and forming recording traces of information signals on the inclined surface of the V-groove improve, high-density recording will be possible at a much higher level than the current level. Playback becomes easier.

(7) INVp2. INVp3 is a polarity inversion circuit, and this polarity inversion circuit INVP2. INVp3
In the V-groove formed on the signal surface of a recording medium, the polarity of the signal is reversed when reading the information signal from the recording traces formed on the sloped surface with opposite slope directions. The switching of the changeover switch SWa described above is performed synchronously.

When reading information signals from recording traces formed on inclined surfaces having opposite inclination directions in a V-groove formed on a signal surface of a recording medium, the polarity inverting circuit INVp2.
By inverting the polarity of the signal by INVp3, a tracking error signal of a constant polarity can be sent from the subtraction circuit 5UBt to the output terminal 9, so that the orientation of the inclined surface of the recording trace targeted for reproduction is alternated. Tracking control operations can be performed continuously even if the current changes.

In the circuit arrangement shown in Figure 12, an addition circuit is provided for the outputs of the two light receiving elements PIQ and Plr in the first photodetector P1, and a polarity inversion circuit INVp is provided.
If the circuit arrangement is such that the polarity of the signal is inverted only by 3, it will be the same as the circuit arrangement shown in the second diagram already mentioned, and the information signal from the recording medium without the V-groove formed on the signal surface will be Since it can also be reproduced, it is possible to reproduce information signals from both a recording medium with a V-groove formed on the signal surface and a recording medium without a V-groove formed on the signal surface. An optical information signal reproducing device capable of performing the operation can also be easily provided.

Each of the embodiments of the optical information signal reproducing device of the present invention described so far has a second light spot S2 and a third light spot among the first to third light spots 81 to S3 projected onto the signal surface of a recording medium. When the center of the first light spot S1 is located at the center of the Nth recording trace in the sequentially arranged recording traces, the center of the second light spot S3 is located at the center of the Nth recording trace in the sequentially arranged recording traces. and the center of the third light spot is located between the Nth and (N+1)th recorded traces. The first to third light spots described above are projected onto the signal surface of the recording medium so that the light reflected from the signal surface of the recording medium by the first to third light spots described above is The reflected light from each light spot is photoelectrically converted by the first to third photodetectors P1 to P3, and the output signals from each of the photodetectors P1 to P3 are converted to their respective time axes. In a state where the upper positions match, the first The crosstalk in the output signal of the photodetector P1 is canceled in the second embodiment. Since the third light spot is located between adjacent recording traces, the second. In order to be able to receive the reflected light from the third light spot, the configuration of the photodetector has become complicated.

13 and 15 show that the second light spot S2 and the third light spot S3 in the first to third light spots 81 to S3 projected onto the signal surface of the recording medium are the same as the first light spot When the center of the spot S1 is located at the center of the Nth recording trace in the sequentially arranged recording traces, the center of the second optical spot S2 is located on the (N-1)th recording trace. In addition, the center of the third light spot S3 is positioned on the (N+1)th recording trace, so that the first to third light spots 81 to S3 described above are
This shows an embodiment of an optical information signal reproducing device that allows the use of a generally commercially available 6-segment photodiode as a photodetector for receiving reflected light from a photodetector.

In FIG. 13, 1 is a light source (for example, a semiconductor laser), and this light source 1 emits linearly polarized light from its minute light emitting part. The light emitted from the light source 1 is made into parallel light by a collimating lens 2 and then passed through a diffraction grating 3.
The diffraction grating 3 emits three beams of zero-order light and ±1st-order diffracted light. The three beams described above pass through the beam splitter 4 and are applied to the focusing lens 5, whereby the signal surface of the recording medium (disc) 7, which is rotating at a high speed at a predetermined rotational speed in the direction of arrow A in the figure, is On the top, first to third light spots Sl each have an individual minute diameter,
The image is formed as S2,53.

FIG. 14 is a diagram showing the positional relationship between sequential recording traces formed on the signal surface of the recording medium 7 and the first to third light spots 5l-83 projected onto the signal surface, FIG. 14 shows the first to third signals projected onto the signal surface of the recording medium.
The second light spot S2 in the light spot 5l-33 of
and the third optical spot S3 means that when the center of the first optical spot S1 is located at the center of the Nth recording trace Tn among the recording traces arranged in sequence, the second optical spot S3
The center of the second light spot S3 is located on the (N-1)th recording trace T(n-1), and the center of the third optical spot S3 is located on the (N+1)th recording trace T(n-1). The current state is shown. Then, the second light spot S2 and the first light spot S2 are
The distance between the third light spot S3 and the first light spot S1 is equal to the distance between the third light spot S3 and the first light spot S1.

The three small diameter light spots 51 described above. S2゜S
The reflected light from 3 is returned to parallel light by a condensing lens 5, and then reflected laterally by a beam splitter 4, after which a focus error detection operation using an astigmatism method is performed. The light is incident on the cylindrical lens 6. The reflected light from the first light spot S1 that is emitted from the cylindrical lens 6 is given to the first photodetector P1 which is divided into four parts, and the light reflected from the above-described second light spot S1 is The reflected light from S2 is given to the second photodetector P2, and the reflected light from the third light spot S3 is given to the third photodetector P3.

The sum signal of the output signals from the four light-receiving elements in the first photodetector P1 described above becomes the reproduction information signal, and the output signal of the light-receiving elements located diagonally in the first photodetector PI It is well known that the difference signal between the two output signals becomes the focus signal, and the difference signal between the output signals of the light receiving elements located on both sides of the recording medium in the direction in which the recording trace extends becomes the tracking error signal. Ru.

Also, the second. Third photodetector P2. P3 is configured such that the reflected light from the second light spot S2 is received by the second photodetector, P2, and the reflected light from the third photodetector P3 is received by the photodetector P3 of node 3. There is no need to do this, so here is the second step. Third photodetector P2. There is no problem even if the positional accuracy of P3 is not high.

The first optical spot S1 described above mainly reproduces the information signal of the recording trace Tn, the second optical spot S2 mainly reproduces the information signal of the recording trace T(n+1), and the third optical spot S3 mainly reproduces the information signal of the recording trace T(n+1). To reproduce the information signal of the recording trace T(n-1), each output signal from each photodetector P1 to P3 is supplied to the circuit arrangement shown in FIG. In the circuit arrangement shown in FIG. 15, the first to third light spots 8 projected onto the signal surface of the recording medium 7 are
A crosstalk component that occurs in a reproduced signal when an information signal is read from a recording trace on a recording medium where the interval between recording traces formed on the signal surface of the recording medium 7 is small compared to the diameters of the recording medium 7. Signal processing is performed to remove the effects of crosstalk, and a reproduced signal from which the influence of crosstalk has been removed is output to the output terminal 8.

Here, compared to the diameters of the first to third light spots 51-33 projected onto the signal surface of the recording medium 7, the distance between the recording marks formed on the signal surface of the recording medium 7 is How is the crosstalk component generated in the reproduced signal when the information signal is read from the recording trace on a small recording medium by the first to third optical spots 81 to S3 in the circuit arrangement shown in Figure 15? The explanation will be made with reference to FIG. 14 as to whether or not it can be removed.

FIG. 14 shows recording traces T(n-1), T(n), and T(n+
The diameters of the first to third optical spots 81 to S3 having the same diameter, respectively, projected onto 1) are 1°28 microns, as in the case of the conventional example described above, and the recording This corresponds to what is written assuming that the interval between the aforementioned sequential recording marks formed on the signal surface of the medium 7 is 0.8 microns.

Now, as mentioned above, the light spots 81 to S3 that are imaged on the signal surface of the recording medium 7 have a Gaussian light intensity distribution in their cross section; As mentioned above, the diameter of the light spot is expressed by the diameter of the part where the light intensity becomes 1/e'' (where e is the base of the natural logarithm) as shown in Figure 4. In addition, side lobes as illustrated in FIG. 4 are generated around this.

Since the energy distribution of the light spot projected onto the signal surface of the recording medium 7 is as shown in FIG.
As shown in FIG. 14, when the first to third optical spots 5t-53 having a diameter larger than the recording trace interval are projected onto the signal surface of the recording medium 7, the customer's spot The side lobe extends to the adjacent recording trace, and the first optical spot S1 for reading the information signal has a skirt portion of its main lobe that overlaps the pit of the adjacent recording trace. Therefore, the output signal from the first photodetector P1 that photoelectrically converts the light reflected by the first light spot S1 naturally contains the information signal of the adjacent recording trace as crosstalk. It has become.

Note that the depth of the pits formed on the signal surface of the recording medium 7 is set to be 1/4 or less of the wavelength of the light projected thereon.

In the example shown in Figure 14, the second light spot S2 covers both the recording trace T(n) and the recording trace T (n-1,), so the light reflected by the second optical spot S2 described above There is a first light spot Sl in the photoelectrically converted signal.
includes an information signal of a recording trace T(n) and an information signal of a recording trace T(n-1) to be reproduced by
Furthermore, since the third light spot S3 covers both the recording trace T(n) and the recording trace T(n+1), the signal obtained by photoelectrically converting the light reflected by the third optical spot S3 includes: The information signal of the recording trace T(n) to be reproduced by the first light spot S1 and the recording trace T(n+
1) information signal.

Therefore, the signal photoelectrically converted by the photodetector P2 that received the reflected light from the second light spot S2 includes the information signal of the recording trace T(n), and also contains the information signal of the recording trace T(n). The signal photoelectrically converted by the photodetector P3 which receives the light reflected by the spot S3 includes an information signal of the recording trace T(n).

Next, as shown in FIG. 14, when the first to third optical spots 81 to S3 having a diameter larger than the recording trace interval are projected onto the signal surface of the recording medium 7, In order to facilitate understanding that crosstalk can be effectively eliminated by the operation of the circuit arrangement shown in FIG. 15, a mathematical formula will be used.

First, the symbols used in the formulas are explained as follows.

Ul(t)...Signal recorded in record trace T(n)...Signal U2(t)...Signal recorded in record trace T (n-1)...Record trace T Signal m recorded at (n+1)...Signal component recorded at recording trace T(n) in the reproduced signal of the first photodetector P1 and the reproduced signal of the first photodetector P1 Record trace T(n) inside
The ratio ζ to the signal components recorded other than the second (
Or the ratio t of the signal detected from the recording trace Tn to the signal detected by the third) photodetector P2 (or P3) from the recording trace T(n-1) (or T(n+1))... Time X ...Tracking deviation (right side is positive) k...
A proportionality constant indicating the influence of output reduction due to tracking deviation. However, (k<1) Note that the difference between two adjacent optical spots (for example, optical spot S2 and optical spot Sl, or optical spot S1 and optical spot S3) The distance in the direction of the recording trace between recording medium 7
The time delay τ that occurs in the signal corresponding to the time τ required for the signal to move is also included in the equation, but the time delay is determined by the delay circuit DLpl shown in FIG.
Since the signal can be treated as a zero state due to the delay of the signal by DLp2, the time delay is omitted in the following formula. Obtained from the first photodetector 'P1 in the circuit arrangement shown in Figure 15. If the signal is vl, the signal v1 is Vl
= (1-k・1xl)01(t)+m[U2(t)+
υ3(t)+ (03(t)−02(t))k−xl・
-411), but this equation (11) is the same as that shown by the already mentioned equation (1).

Further, a signal v2 output from the second photodetector P2,
The signal v3 output from the third photodetector P3 is expressed by the following equations (12) and (13), respectively.

V 2 = (1-kx)・U2(t) + (1-k
x)ζ-ut(t)-(12)U3 = (1+kx)・
U3(t)+(1+kx)ζ-ut(t) ・(13
) As described above, the output signals from the first to third photodetectors P1 to P3 shown by equations (11) to (13) are the first to third photodetectors P1 to P3 projected onto the signal surface of the recording medium 7. This is a signal obtained by photoelectrically converting the reflected light from the third optical spots 81 to S3, and the first to third optical spots 81 to S3 are recorded as moving in the direction of arrow A in FIG. The information to be read from the signal surface of the recording medium 7 by the first light spot S1 is projected onto the signal surface of the medium 7 in a manner that is arranged approximately along the moving direction of the recording medium 7. An information signal identical to the second . third light spot S2,
The time point at which the information signal is read by S3 is earlier in the second light spot S2 than the reference time point when the information signal is read by the first light spot S1; Spot S3 is delayed by τ compared to the reference time point, but in the circuit arrangement shown in Figure 15, the time axes of each signal shown in equations (11) to (13) above are aligned. As a means for this purpose, the output signal from the second photodetector P2 is outputted by a delay circuit DLP2.
At the same time, the output signal from the first photodetector P1 is delayed by τ by the delay circuit DLPI.

In the circuit arrangement shown in Figure 15, the output signal v2 of the second photodetector P2 is supplied to the adder circuit ADD after being subjected to a time delay of 2 by the delay circuit DLp2, and is supplied to the adder circuit ADD. The output signal v3 from the photodetector P3 is supplied to an adder circuit ADD.

The above-mentioned adder circuit ADD performs the addition of the signals v2 and U3 supplied thereto as described above, and adds the signals v2 and U3 supplied to it as described above.
From D, the signal V2+V3 is supplied to the gain adjustment circuit GAJ via a bypass filter HPF provided as necessary. When the gain of the 31m gain circuit GAJ is set to m, the output signal from the gain adjustment circuit GAJ is expressed by the following equation (14).

m(V2+V3)=m[U2+03+(U3-
02)k-X]+2ζmU1(1-kX)-...(1
4) Then, the signal represented by the above equation (14) is supplied to the subtraction circuit SUB, and the subtraction circuit SUB uses the above-mentioned first signal.
By subtracting the signal shown in the above equation (14) from the output signal from the photodetector P1, that is, the signal v1 shown in equation (11), as shown in the following equation (15), A reproduced signal V containing no crosstalk is generated, and the reproduced signal V is sent to the output terminal 8.

V=V1-m(V2+V3) =(1+klxl-2lζI m)[1-・-・・(1
5) In addition, the bypass filter HPF in FIG.
In the case where the width of the bits is expanded when the recording wavelength is short, the width of the pits recorded on the recording medium 7 is widened (such as the recording medium described in Japanese Patent Application No. 1514/1983). ), crosstalk increases in high-frequency signals, so
This is to increase the amount of crosstalk correction for high frequency signals. In addition, the switch SW shown in FIG.
, band pass wave filter BPF, amplitude detection circuit ADE, etc. are as described for the circuit arrangement shown in FIG. 2, that is, the gain ga in the gain adjustment circuit GAJ
This is used to set the mode.

In FIG. 14, there are recording traces T(n-1) on both sides of the central recording trace Tn that the first light spot S1 should follow.

The second light spot S2 and the third light spot S3 on T(n+1) are projected in an fl manner such that their centers are located closer to the central recording trace, but the recording trace described above is The second light spot S2 and the third light spot S3 located on T(n-1) and T(n+1) are centered on the recording traces T(n-1) and T(n+1). The second and third photodetectors P2. may be projected in such a manner that they are located at the center. P
The output signals V2' and V3' output from 3 are expressed by the following equations (16) and (17), respectively.

V2j=(1-k l x l )・U2(t)+(1
+kx)m-01(t)+(1-kx)m・υn-2(
t)--(16)V3'=(1-k I x I)
・U3(t)+(1-kx)m・U1(t)+(1+k
x) m-Un+2(t) --(17) Furthermore, (16)
, Un-2(t), Un+2(t
) are the record traces T (n-2) and T (n+2), respectively.
) shows the signal recorded in

The signal V shown in equations (16) and (17) above
2' and V3' are added by the adder circuit ADD as in the previous case, and the signal V2'+V output from the adder circuit ADD is
3' is a bypass filter HPF provided as necessary.
The output signal from the gain adjustment circuit GAJ is then subtracted from the output signal from the first photodetector P1 in the subtraction circuit SUB, so that the output signal from the subtraction circuit SUB is as follows. As shown in equation (18), a reproduced signal V' containing almost no crosstalk is generated, and the reproduced signal V' is sent to the output terminal 8.

V'=V1-m(V2'+V3') :(1-klxl)υ1+ 2 m k l x l
03 (18) Since m and lxl in the above-mentioned equation (18) are minute amounts, it is as if there is no crosstalk of the signal υ3.

Note that the term m8 is also omitted because it is very small.

By the way, as shown in FIG. 14, there are recording traces T on both sides of the central recording trace Tn that the first light spot S1 should follow.
(n − 1), T(n + 1), the second light spot S2 and the third light spot S3 are projected in such a manner that their centers are located closer to the central recording trace. If so, the second. Third photodetector P2. Output signal from P3 V2. The difference signal of V3 becomes a tracking error signal.

That is, the signal V2. expressed by the above-mentioned equations (12) and (13). The signals obtained by removing the signal components of 01, 02, and 03 in V3a using the envelope detection circuit are U2" and U3".
Then, U2”=(1-k x)+ζ(1-k x)
...(19)■3'''=(1+kX)+ζ(1+
k x)...(20) Above (19), (2
0) Since the signal shown by the formula is obtained, the above signal v2
If you get a signal with a difference of ``,■3''''.

A signal indicating the magnitude and direction of the tracking deviation, that is, a tracking error signal St, is obtained as shown in equation (21) below.

5t=V2"-U3"=-2kx(1+ζ)・-・-・
(21) (Effects) As is clear from the detailed explanation above, the optical information signal reproducing device of the present invention includes a signal surface on which a recording trace of an optically recorded information signal is formed. A first light spot for signal reading is projected onto the signal surface of the recording medium, and a symmetrical light spot with the first light spot as the center of symmetry is projected on the straight line including the first light spot. The second one for tracking the position. A third light spot is also projected, and a desired information signal is obtained based on the reflected light generated on the signal surface of the recording medium by the first to third light spots. In the optical information signal reproducing device, when the center of the first light spot is located at the center of the Nth record among the records arranged in sequence, the center of the second light spot is Located between the Nth recording trace and the (N-1)th recording trace, and the center of the third light spot is located between the Nth recording trace and the (N+1)th recording trace. The above-mentioned first to third
A light spot is projected onto the signal surface of the recording medium, and the light reflected from the signal surface of the recording medium by the first to third light spots is individually photoelectrically converted for each light reflected from each of the light spots. means for performing photoelectric conversion using the first to third photodetectors; and means for aligning the positions of the output signals from the respective photodetectors on the time axis.

an addition circuit that obtains a sum signal of the output signal from the second photodetector and the output signal from the third photodetector; The amplitude of the signal read from the (N+1)th record trace,
The amplitude of the output signal from the adder circuit is adjusted so that the amplitudes of the signals read from the (N-1)th and (N+1)th recording traces in the output signal of the first photodetector are equalized. It consists of a signal amplitude adjustment means to adjust, and a subtraction circuit that obtains a difference signal between the output signal from the first photodetector and the output signal of the signal amplitude adjustment means, and the Nth record trace from the subtraction circuit. In an optical information signal reproducing device configured to obtain a reproduced signal of , consists of two light-receiving elements placed on both sides of a line parallel to the recording trace,
Furthermore, the third photodetector is one that is composed of two light-receiving elements placed on both sides of a line that passes through the center of the reflected light of the third light spot and is parallel to the recording trace. When the center of the first light spot is located at the center of the Nth record among the records arranged in sequence, the center of the second light spot is located at the center of the Nth record and (N -1
)th recording trace, and the center of the third light spot is located between the Nth recording trace and the (N+1)th recording trace. Among the two light-receiving elements of the second photodetector that receives the light reflected by the second light spot in the first to third light spots arranged in the following manner, the (N-1)th recording Of the two light-receiving elements, the output signal of the light-receiving element Kana on the side where the reflected and diffracted light from the trace shows a larger distribution, and the third photodetector that receives the reflected light from the third light spot,
The sum signal of the output signal from the light-receiving element on the side where the reflected light from the Nth recording trace shows a larger distribution, and the second signal as described above. Optical information signal regeneration in which means is provided to obtain a difference signal between the sum signal of the output signals from the light receiving elements on the other side in the third photodetector, and the tracking control system is driven by the difference signal. A first light spot for signal reading is projected onto the device and the signal surface of a recording medium having a signal surface on which a recording trace of an optically recorded information signal is formed, and the first light spot described above is The second and third light spots are also projected at symmetrical positions with the first light spot as the center of symmetry on the straight line including the first light spot, and In an optical information signal reproducing device that is configured to obtain a required information signal based on reflected light generated on a signal surface of a recording medium by a light spot of
The center of the light spot of
When it is located at the center of the second record trace,
The center of the light spot is located at a position between the Nth recording trace and the (N-1)th recording trace or on the (N-1)th recording trace, and the center of the third optical spot The above-mentioned first to third light spots are arranged so that the light spot is located at an intermediate position between the Nth recording trace and the (N+1)th recording trace or on the (N+1)th recording trace. The first to third beams are projected onto the signal surface of the recording medium and photoelectrically convert the reflected light from the signal surface of the recording medium by the first to third optical spots individually for each reflected beam from the respective optical spots. means for photoelectric conversion by the third photodetector, means for matching the positions on the time axis of the output signals from each of the above-mentioned photodetectors, and an output from the above-mentioned second photodetector. an adding circuit that obtains a sum signal of the signal and the output signal from the third photodetector, and reading the (N-1)th and (N+1)th record traces in the output signal from the adding circuit; The adding circuit described above is configured to equalize the amplitude of the signal read from the (N-1)th and (N+1)th recording traces in the output signal of the first photodetector. and a subtraction circuit that obtains a difference signal between the output signal from the first photodetector and the output signal of the signal amplitude adjustment means described above, This is because it is an optical information signal reproducing device that obtains a reproduction signal of the Nth recording trace from a circuit.

In the optical information signal reproducing device of the present invention, information signals are reproduced from a recording medium on which recording is performed at a high density such that the interval between recording traces is smaller than the diameter of a light spot used to read information signals from recording traces. At this time, the crosstalk component contained in the information signal read by the first light spot is transferred to the second light spot projected onto the adjacent recording trace. The crosstalk component contained in the light reflected by the third light spot cancels out the crosstalk component, making it possible to easily reproduce an information signal free of crosstalk. It is possible to reproduce information signals from a recording medium on which information is recorded at a higher density than in the second embodiment described above. By arranging the third optical spot closer to the first optical spot than the center of the recording traces on both sides, information signals of other recording traces unnecessary for the crosstalk cancellation operation are transferred to the second optical spot. An information signal recorded in a recording trace that is prevented from being read by the third light spot, thereby making the crosstalk canceling operation by the first to third light spots the object of reproduction.

Since this is performed only with the information signals recorded in the recording traces on both sides of the recording trace, the reproduced signal from which crosstalk has been removed does not contain any other extra signal components, and furthermore, it is difficult to detect crosstalk components. The optical spot used for detecting the tracking error signal can be the same as the optical spot for detecting the tracking error signal, and furthermore, the crosstalk cancellation operation is not affected by the presence of tracking errors, so the best In addition, the crosstalk cancellation operation is not adversely affected by the tracking deviation that occurs when the recording medium is tilted in the width direction of the recording trace. Since there is no need for means for compensating for the influence of the inclination of the recording medium in the width direction, it is possible to provide an optical information signal reproducing apparatus at a low cost.

Further, in the embodiment shown in FIGS. 13 and 15, the second. To provide a device at a low cost since a photodetector having a single configuration can be used for each of the third light spots, simplifying assembly and adjustment, and allowing the use of general commercially available photodetectors. It also has the advantage of being able to.

[Brief explanation of drawings]

FIGS. 1, 10, and 13 are perspective views showing the schematic configuration of a part of the optical information signal reproducing apparatus of the present invention, respectively. FIGS. 2, 9, 12, and 15 are
3 and 14, respectively, are block diagrams of the circuit arrangement for signal processing in the optical information signal reproducing device of the present invention.
The figure is a diagram for explaining the relative arrangement of recording traces and the first to third light spots, Figure 4 is a diagram for explaining the intensity distribution of the light spots, Figures 5, 6, and 11 are FIG. 7 is an explanatory diagram of the reflected and diffracted light of the optical spot projected onto the signal surface of the recording medium, FIG. 7 is an explanatory diagram of the optical spot and crosstalk, and FIG. 8 is a diagram showing an example of the recording pattern of the gain adjustment signal. . DESCRIPTION OF SYMBOLS 1... Light source (for example, semiconductor laser), 2... Collimating lens, 3... Diffraction grating, 4... Beam splitter. 5... Focusing lens, 7,7v... Recording medium, (disk), S L, S 2. S 3...1st to 3rd
optical spot, Pl...first photodetector, P2...
・Second photodetector, P3...Third photodetector, 8, 9
...output terminal, DLpL, DLp2...delay circuit,
SUB, 5UBt, 5UBp2, 5UBp3... Subtraction circuit, De t p2. De t p3-envelope detector, ADD...addition circuit, GAJ...gain adjustment circuit, SW...switch, ADH...amplitude detection circuit, SW a-changeover switch, INVp2. INV
p3...polarity inversion circuit,

Claims (1)

[Claims] 1. Projecting a first light spot for signal reading onto a signal surface of a recording medium having a signal surface on which a record trace of an optically recorded information signal is formed. At the same time, second and third optical spots for tracking are also projected at symmetrical positions with the first optical spot as the center of symmetry on the straight line including the first optical spot, and , an optical information signal reproducing device configured to obtain a desired information signal based on reflected light generated on the signal surface of a recording medium by the above-described first to third light spots; When the center of the light spot is located at the center of the Nth record among the records arranged in sequence, the center of the second light spot is located at the center of the Nth record and the (N-1)th record. and the center of the third light spot is located between the Nth recording trace and (N+
1) Project the above-mentioned first to third light spots onto the signal surface of the recording medium so that the above-mentioned first to third light spots are located in the middle of the second recording trace, and means for photoelectrically converting the reflected light from the signal surface of the recording medium by means of first to third photodetectors that individually photoelectrically convert the reflected light from the respective optical spots; and each of the above-mentioned photodetectors. and an addition circuit for obtaining a sum signal of the output signal from the second photodetector and the output signal from the third photodetector. , the amplitudes of the signals read from the (N-1)th and (N+1)th recording traces in the output signal from the adder circuit, and the amplitudes of the signals read from the (N-1)th and (N+1)th traces in the output signal from the first photodetector. )-th and (N+1)-th recording traces; An optical information signal reproducing device 2 comprising a subtracting circuit for obtaining a difference signal between the output signal of the output signal and the output signal of the signal amplitude adjusting means described above, and obtaining a reproduced signal of the Nth recording trace from the subtracting circuit. , the second photodetector is composed of two light-receiving elements disposed on both sides of a line passing through the center of the reflected light of the second light spot and parallel to the recording trace; The third photodetector used was one consisting of two light-receiving elements placed on both sides of a line that passed through the center of the reflected light of the third light spot and was parallel to the recording trace. In the optical information signal reproducing device 3 according to claim 1, when the center of the first light spot is located at the center of the Nth recording trace in the sequentially arranged recording traces, The center of the second light spot is the Nth recording trace (N-1
)th recording trace, and the center of the third light spot is located between the Nth recording trace and the (N+1)th recording trace. The signal given to the addition circuit from the second photodetector that receives the light reflected by the second light spot in the first to third light spots arranged in the (N-1)th recording relationship is The reflected diffracted light from the trace is made into an output signal from the light-receiving element on the side showing a larger distribution, and is also given to the addition circuit from the third photodetector that receives the reflected light from the third light spot. Claim 1 includes a circuit arrangement such that the signal is an output signal from the light receiving element on the side where the reflected and diffracted light from the (N+1)th recording trace shows a larger distribution. In the optical information signal reproducing device 4 described above, when the center of the first optical spot is located at the center of the Nth recording trace among the recording traces arranged in sequence, the center of the second optical spot is Nth record trace and (N-1
)th recording trace, and the center of the third light spot is located between the Nth recording trace and the (N+1)th recording trace. Among the two light-receiving elements in the second photodetector that receives the light reflected by the second light spot in the first to third light spots arranged in the same manner, the reflection from the Nth recording trace Among the two light receiving elements in the third photodetector that receives the output signal from the light receiving element on the side where the diffracted light shows a larger distribution and the reflected light from the third light spot, the Nth recording A patent claim comprising an arithmetic circuit such that the output signal from the light receiving element on the side where the reflected and diffracted light from the trace shows a larger distribution is added to the output signal from the first photodetector. The optical information signal reproducing device 5 according to item 1 is a device for reading a signal on a signal surface of a recording medium having a signal surface on which a recording trace of an optically recorded information signal is formed. In addition to projecting the first light spot, second and third light spots for tracking are placed at symmetrical positions with the first light spot as the center of symmetry on a straight line including the first light spot. The optical information signal reproduction method is configured such that a desired information signal is obtained based on the reflected light generated on the signal surface of the recording medium by the above-mentioned first to third light spots. In the apparatus, when the center of the first light spot is located at the center of the Nth record among the records arranged in sequence, the center of the second light spot is located at the center of the Nth record. It is located between the recorded trace and the (N-1)th recorded trace, and the center of the third light spot is between the Nth recorded trace and (N+
1) Project the above-mentioned first to third light spots onto the signal surface of the recording medium so that the above-mentioned first to third light spots are located in the middle of the second recording trace, and means for photoelectrically converting the reflected light from the signal surface of the recording medium by means of first to third photodetectors that individually photoelectrically convert the reflected light from the respective optical spots; and each of the above-mentioned photodetectors. and an addition circuit for obtaining a sum signal of the output signal from the second photodetector and the output signal from the third photodetector. , the amplitudes of the signals read from the (N-1)th and (N+1)th recording traces in the output signal from the adder circuit, and the amplitudes of the signals read from the (N-1)th and (N+1)th traces in the output signal from the first photodetector. )-th and (N+1)-th recording traces; and a subtraction circuit for obtaining a difference signal between the output signal of the signal amplitude adjusting means and the output signal of the signal amplitude adjustment means, the optical information signal reproducing device comprising: a subtraction circuit for obtaining a difference signal between the output signal of the signal amplitude adjusting means and the output signal of the signal amplitude adjustment means, and obtaining a reproduction signal of the Nth recording trace from the subtraction circuit. , second
The photodetector consists of two light-receiving elements arranged on both sides of a line passing through the center of the reflected light of the second light spot and parallel to the recording trace, and a third light-receiving element. The photodetector used is one consisting of two light-receiving elements placed on both sides of a line that passes through the center of the reflected light of the third light spot and is parallel to the recording trace. When the center of the light spot is located at the center of the Nth record among the records arranged in sequence, the center of the second light spot is located at the center of the Nth record and the (N-1)th record. a relative positioning relationship such that the center of the third light spot is located midway between the Nth recording trace and the (N+1)th recording trace; Among the two light-receiving elements of the second photodetector that receives the reflected light from the second light spot in the first to third light spots, the light from the (N-1)th recording trace is The third light receiving element receives the output signal from the light receiving element on the side where the reflected diffraction light shows a larger distribution and the third light receiving element receives the reflected light from the third light spot.
Of the two light-receiving elements of the photodetector, the sum signal of the output signal from the light-receiving element on the side where the reflected and diffracted light from the Nth recording trace shows a larger distribution, and the second, Third
an optical information signal reproducing device 6, which is provided with means for obtaining a difference signal between the sum signal of the output signals from the light receiving elements on the other side of the photodetector, and drives a tracking control system by the difference signal; A first light spot for signal reading is projected onto a signal surface of a recording medium having a signal surface on which a recording trace of an optically recorded information signal is formed, and the first light spot described above is On the straight line including the spot, a second light spot is placed at a symmetrical position with the first light spot as the center of symmetry.
A third light spot is also projected, and a desired information signal is obtained based on the reflected light generated on the signal surface of the recording medium by the first to third light spots. In the optical information signal reproducing device, when the center of the first light spot is located at the center of the Nth record among the records arranged in sequence, the center of the second light spot is Nth record trace and (N-1
)th recording trace or on the (N-1)th recording trace, and the center of the third light spot is between the Nth recording trace and the (N+1)th recording trace. The above-mentioned first to third light spots are projected onto the signal surface of the recording medium so that the above-mentioned first to third light spots are located on the intermediate position or the (N+1)th recording trace, and the above-mentioned first to third light spots are means for photoelectrically converting the light reflected from the signal surface of the recording medium by the light spot by the first to third photodetectors individually photoelectrically converting the light reflected from the respective light spots; means for aligning the positions of the output signals from the photodetectors on the time axis, and a sum signal of the output signals from the second photodetector and the output signals from the third photodetector; the amplitudes of the signals read from the (N-1)th and (N+1)th recording traces in the output signal from the adder circuit, and the (in the output signal of the first photodetector) so as to equalize the amplitudes of the signals read from the N-1)th and (N+1)th recording traces,
It consists of a signal amplitude adjustment means for adjusting the amplitude of the output signal from the above-mentioned addition circuit, and a subtraction circuit that obtains a difference signal between the output signal from the first photodetector and the output signal of the above-mentioned signal amplitude adjustment means. , an optical information signal reproducing device configured to obtain a reproduced signal of the Nth recording trace from the subtraction circuit.