Optical disc player for optically reproducing recorded information on an optical information disc into an RF signal

In an apparatus for optically reproducing the recorded information, a circuit to reproduce an RF signal is provided. At least a pair of photo sensors are arranged adjacently in a direction parallel with a tangential line of a recording track of an information recording disc. A light beam is irradiated onto the recording surface of the disc and the reflected light from the recording surface is received by these photo sensor. The phases of both output signals of the photo sensors are made coincident by the phase shift means. The resultant output signals having the same phase are added thereby to form the RF signal. For this purpose, the phase difference between both output signals of the photo sensors is detected and either one of phases of the output signals is shifted in accordance with the phase difference detecting output.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for optically reproducing the 
recorded information and, more particularly, to a circuit for reproducing 
an RF (high frequency) signal in the apparatus for optically reproducing 
the recorded information. 
In the apparatus for optically reproducing the recorded information, the RF 
signal, a focussing error signal and a tracking error signal are produced 
on the basis of the reflected light obtained by irradiating a light beam 
onto the recording surface of an information recording disc (hereinafter, 
simply referred to as a disc). On the other hand, it has been well known 
that in order to produce the RF signal and the focussing error signal by a 
common photo sensing device, what is called a four-split type photo 
sensing device is used. Namely, the photo sensing surface of this device 
is divided into four surfaces by a split line in the direction which is 
parallel with the tangential line of the recording track and by another 
split line perpendicular to the former split line. 
In the RF signal producing circuit using such a four-split type photo 
sensing device, the light beam for reproduction is focussed onto the 
recording surface of the disc so as to irradiate as a light beam spot 
thereto. The reflected light from the recording surface based on the 
irradiated light beam is led to the four split surfaces of the photo 
sensing device. The outputs of the surfaces locating on the same side 
regarding the split line perpendicular to the tangential direction of the 
track are respectively added to obtain two addition outputs A and B. 
Further, the addition outputs A and B are added thereby to produce the RF 
signal. 
In the RF signal producing circuit, when the depth d of pit which forms the 
recording track is smaller than .lambda./4 (.lambda. denotes a wavelength 
of the light beam), the addition outputs A and B have the same waveform 
and there is a constant time difference (phase difference) between them. 
Therefore, the leading and trailing edges of the addition output (the RF 
signal) of the outputs A and B are dull. The amplitude of the output 
waveform when the length of pit is short is smaller by a predetermined 
level than the amplitude of the output waveform when the length is long. 
Namely, the attenuated amount of the high frequency characteristic which 
is caused due to the addition appears in the RF signal. 
To prevent such a situation, the same applicant as the present invention 
has already proposed a circuit such that phase adjusting means for making 
the phases of both addition outputs A and B coincident is provided, the 
addition outputs A and B are transmitted through the phase adjusting 
means, the resultant addition outputs A and B whose phases are coincident 
are added to form the RF signal, thereby making the leading and trailing 
edges of the RF signal steep and enabling the reduction of the level of 
the RF signal to be suppressed (refer to Japanese Utility Model Laid-Open 
No. 60-23932). 
However, such a situation occurs only when the depth d of pit is smaller 
than .lambda./4. When d=.lambda./4, no phase difference occurs between 
both addition outputs A and B. Therefore, according to such a circuit 
which is constituted so as to always perform the phase adjustment, an 
unnecessary phase adjustment is also executed even if the pit whose depth 
d is equal to .lambda./4. 
SUMMARY OF THE INVENTION 
The present invention is made in consideration of such a problem and it is 
an object of the invention to provide an apparatus for optically 
reproducing the recorded information having a circuit in which the phase 
adjustment is performed only when necessary, and the RF signal having the 
excelent frequency characteristics such that the leading and trailing 
edges are always steep irrespective of the depth of pit and the amplitude 
is almost constant independently of the length of pit is obtained. 
According to the present invention, there is provided an apparatus for 
optically reproducing the recorded information in which at least a pair of 
photo sensing devices are arranged adjacently in the direction parallel 
with the tangential line of the recording track of the disc and receive 
the reflected light beam from the recording surface of the disc, the 
phases of both output signals of these photo sensing devices being made 
coincident, and the resultant output signals whose phases were equalized 
being added, thereby producing the RF signal. In this apparatus, the phase 
difference between both output signals from a pair of photo sensing 
devices is detected, and either one of phases of the output signals is 
shifted in accordance with the detection output of the phase difference.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described in detail 
hereinbelow with reference to the drawings. 
FIG. 1 is a block diagram showing an embodiment of the invention. In the 
diagram, the photo sensing surface of four-split type photo sensing device 
1 is divided into four surfaces by a split line 2 which is parallel with 
the tangential line of the recording track and by a split line 3 which is 
perpendicular to the split line 2. With respect to the split line 3 
perpendicular to the tangential direction of the track, both outputs of 
upper devices 1a and 1d in the diagram are added by an adder 4, and both 
outputs of lower devices 1b and 1c are added by an adder 5. The output of 
the adder 4 is directly supplied to one input terminal of a change-over 
switch 6. This output is also transmitted through a delay circuit 7 for 
phase adjustment and supplied to the other input terminal of the switch 6. 
The output of the switch 6 and the output of the adder 5 are added to each 
other by an adder 8, so that an RF signal is picked up. 
The outputs of the adders 4 and 5 are also supplied to a phase difference 
detection circuit 9, so that the phase difference between both outputs is 
detected by the detection circuit 9. A detection circuit having a 
well-known contsitution consisting of a combination of a multiplier and a 
level comparator or the like may be used as the phase difference detection 
circuit 9. 
In such a constitution, when the depth d of pit is smaller than .lambda./4, 
a diffracted image (intensity distribution) 10 of the reflected light on 
the photo sensing surface of the photo sensing device 1 changes as shown 
in FIG. 2 with respect to the positional relation between a pit 11 and a 
light beam spot 12 for reproduction. FIG. 2 (A) is a diagram at times 
t.sub.1 to t.sub.3 when the position of the light spot 12 to the pit 11 is 
slightly deviated to the right in the direction perpendicular to the 
recording track. FIG. 2 (B) is a diagram at times t.sub.1 to t.sub.3 when 
the positions of the pit 11 and light spot 12 are correct. FIG. 2 (C) is a 
diagram at times t.sub.1 to t.sub.3 when the position of the light spot 12 
to the pit 11 is slightly deviated to the left in the direction 
perpendicular to the recording track. 
Thus, the waveforms of both addition outputs of the adders 4 and 5 are made 
coincident and there is a phase difference t.sub.0 between them as shown 
in FIG. 3. FIG. 3 (A) shows an output waveform of the adder 4. FIG. 3 (B) 
shows an output waveform of the adder 5. FIG. 3 (C) shows an addition 
output waveform when the outputs of the adders 4 and 5 are directly added. 
When the outputs of waveforms of FIGS. 3 (A) and (B) having the phase 
difference t.sub.0 therebetween are directly added to form the RF signal, 
the leading and trailing edges of the resultant RF signal are dull as will 
be seen from the waveform of FIG. 3 (C), and therefore, the amplitude of 
the output waveform corresponding to a pit of a smaller pit length is 
smaller by A.sub.0 than the amplitude of the output waveform corresponding 
to a pit of a larger pit length. 
When there is the phase difference t.sub.0 between both outputs of the 
adders 4 and 5, the detection circuit 9 detects the phase difference and 
then turns the switch 6 to the side b. Thus, the output of the adder with 
a waveform of FIG. 4 (A) is delayed by a delay time t.sub.0 through the 
delay circuit 7, so that the phases of the delayed output with a waveform 
FIG. 4 (A') and of the output with a waveform FIG. 4 (B) of the adder 5 
coincide with each other. Therefore, the RF signal with a waveform of FIG. 
4 (C) which is derived by adding both outputs of the adder 8 has a 
waveform free from any distortions at the leading and trailing edges 
thereof and any decrease in amplitude or the like. 
On the other hand, when the depth d of pit is equal to .lambda./4, as shown 
in FIG. 5, the diffracted image 10 of the reflected light on the photo 
sensing surface of the photo sensing device 1 is symmetrical with respect 
to the center 0 of the photo sensing surface irrespective of the 
positional relation between the pit 11 and the light beam spot 12 for 
reproduction. Therefore, the phases of both addition outputs of the adders 
4 and 5 are coincident. In this case, no detection output is generated 
from the detection circuit 9 and the switch 6 is turned to the side a. 
Therefore, the output of the adder 4 is not unnessarily delayed but is 
directly added to the output of the adder 5 by the adder 8. Thus, 
similarly to the above case, the led out RF signal has a waveform free 
from any deteriorations at the leading and trailing edges and any decrease 
in amplitude or the like. 
In the foregoing embodiment, the output of the adder 4 is selectively 
transmitted through the delay circuit 7 in dependence on he presence or 
absence of the phase difference between both addition outputs. However, 
the line l which bypasses the switch 6 and delay circuit 7 can be omitted, 
the delay time of the delay circuit 7 can be set to two values of 0 and 
t.sub.0, and the delay time (phase adjustment amount) can be also set in 
accordance with the detection output of the phase difference detection 
circuit 9. 
FIG. 6 is a block diagram showing another embodiment of the invention. In 
FIG. 6, the same parts and components as those shown in FIG. 1 are 
designated by the same reference numerals. In FIG. 6, the output of the 
adder 4 is supplied to one input terminal of an adder 8a through the delay 
circuit 7 and is also directly supplied to one input terminal of an adder 
8b. The output of the adder 5 is supplied to the other input terminals of 
the adders 8a and 8b. Either one of the outputs of the adders 8a and 8b is 
selectively output by the switch 6 and is led out as an RF signal. 
The outputs of the adders 8a and 8b are also supplied to a level comparator 
20 and the level difference between them is detected by the comparator 20. 
The level comparator 20 functions as means for detecting the phase 
difference between both outputs of the adders 8a and 8b. Namely, by 
reference to the waveforms corresponding to the short pit length in FIGS. 
3 and 4, it will be understood that the amplitude of the output signal 
when there is a phase difference is smaller than the amplitude when there 
is no phase difference. Therefore, by detecting the level difference, the 
phase difference between both outputs can be detected. FIGS. 3 and 4 show 
the waveforms in the case of the digital audio disc. The phase difference 
can be detected only when the pit length is short. However, in the case of 
the video disc, the phase difference can be always detected because the 
video data is recorded by pits having short lengths. 
The comparator having such a constitution as shown in, e.g., FIG. 7 can be 
used as the level comparator 20. Namely, the output of the adder 8a which 
is input through a capacitor C.sub.1 is half-wave rectified by a diode 
D.sub.1. The rectified output is transmitted through a resistor R.sub.1 
and integrated by an integrator consisting of a resistor R and a capacitor 
C.sub.2. Thereafter, the integrated output signal is supplied through a 
resistor R.sub.3 to a non-inverting input teminal of a comaprator 21. On 
the other hand, the output of the adder 8b which is input through a 
capacitor C.sub.3 is half-wave rectified by a diode D.sub.2. The rectified 
output is transmitted through a resistor R.sub.4 and integrated by an 
integrator consisting of a resistor R.sub.5 and a capacitor C.sub.4. 
Thereafter, the integrated output signal is supplied through a resistor 
R.sub.6 to an inverting input terminal of the comparator 21. The output of 
the cmparator 21 becomes the phase difference detection output. A power 
source voltage V is applied to each output terminal of the capacitors 
C.sub.1 and C.sub.3 through resistors R.sub.7 and R.sub.8, respectively. 
The foregoing embodiments have been described with respect to the case 
where the four-split type photo sensing device is used. However, it is 
also possible to use a two-split type photo sensing device such that the 
photo sensing surface is divided into two parts by only the split line 3 
which is perpendicular to the recording track. Further, the split line 3 
is not limited to only the straight line. On the other hand, although only 
one of the addition outputs has been delayed in the embodiments, it is 
also possible to delay both addition outputs with proper delay times so as 
to make the phases of both outputs coincide with each other. 
As described above, according to the invention, at least a pair of photo 
sensing devices are arranged adjacently in the directions which are 
parallel with the tangential line of the recording track of the disc and 
receive the reflected light from the recording surface of the disc. The 
phase difference between both output signals from the photo sensing 
devices is detected. The adjustment amount of the phase adjusting means 
for making the phases of both output signals coincident is set in 
accordance with the detection output of the phase difference. Therefore, 
the RF signal such that the leading and trailing edges are always steep 
irrespective of the depth of pit and the amplitude is almost constant 
independently of the length of pit can be produced. Thus, the error ratio 
in reproduction of the signal can be reduced.