Optical head transfer circuit, optical head and reproducing apparatus introducing the same circuit

In the optical head transfer circuit of the related art, a sled error signal generated only via a low-pass filter has been used as the tracking error signal. However, the present invention provides a portable type reproducing apparatus which compensates a DC element corresponding to a shifting displacement of an objective lens generated depending on a change of attitude of the reproducing apparatus with a sled feed mechanism, and does not result in deterioration of reproducing characteristics depending on vertical or horizontal installation. The present invention and has realized much improvement in vibration characteristics, because a signal from which the DC element is eliminated corresponding to the shifting displacement of the objective lens is used as the tracking error signal and the sled error signal is generated depending on a signal from which the DC element is not yet eliminated.

FIELD OF THE INVENTION 
The present invention relates to a reproducing apparatus for reproducing a 
variety of disc type recording media and particularly to a reproducing 
apparatus of which cabinet attitude is changed in various ways like a 
portable and mobile reproducing apparatus. 
BACKGROUND OF THE INVENTION 
As an optical disc recording medium, CD (compact disc) and MD (mini-disc) 
(these are registered as trademarks) are widely spreading and used in 
various application fields such as audio, etc. 
In a disc reproducing apparatus for reproducing various types of recording 
media, in order to control the tracking of an optical spot, there are 
provided a double-axis mechanism for driving an objective lens of an 
optical head with a tracking error signal obtained from a reflected light 
beam indicating a track guide information such as pit train and group, 
etc. and a sled mechanism for displacing the relative position of the 
optical head as a whole and disc surface in the disc radius direction. 
As the sled mechanism, those for moving the optical head as a whole against 
a disc and for moving a turn-table on which a disc is mounted against the 
optical head which is fixed to the predetermined position are known. 
Moreover, as a control system of the sled mechanism, it has been proposed 
that a sled error signal generated by extraction of a low frequency 
element with a low-pass filter from the tracking error signal is amplified 
and is then applied to a sled motor as a sled drive signal. The sled error 
signal is changed to the signal indicating an offset of an objective lens 
driven for tracking by the double-axis mechanism of the optical head as a 
whole and the optical head. 
Here, FIG. 1 schematically shows an example of the structure of an optical 
block of the related art which is installed in an optical head of a 
reproducing apparatus for a disc such as CD, etc. 
In this figure, an optical block of the optical head is composed of a 
semiconductor laser 81, a collimator lens 82, a deflected beam splitter 
83, an objective lens 84 and a photo detector 85. For example, the laser 
beam radiated from the semiconductor laser 81 is paralleled by the 
collimator lens 82, and then reflected by the deflected beam splitter 83 
to radiate the recording surface of a disc 1 via the objective lens 84. 
When the laser beam is focused, the light beam reflected by the pit train 
provided on the disc 1 passes through the deflected beam splitter 83 and 
is then supplied to the photo detector 85 to provide the pit information 
of the disc 1 in the photo detector 85. 
FIG. 2 shows the concept of distribution of intensity of the reflected 
light beam in the relative positional relationship between the pits formed 
on the disc 1 and spot beam. In the so-called just tracking condition 
where the pit train of disc 1 is relatively matched with the position of 
the spot beam, the reflected beam indicated as (a) can be obtained on the 
photo detector 84. Namely, pit information having an equal intensity 
distribution in the right and left sides can be obtained on the photo 
detector 85. 
Moreover, if the pit of the disc 1 is relatively displaced in position from 
the spot beam, for example, when the spot beam position is relatively 
deviated in the left side from the pit train (de-track condition), the pit 
information having intensity distribution as shown in (b) can be obtained 
on the photo detector 85, while when the spot beam position is relatively 
deviated in the right side from the pit train (de-track condition), the 
pit information having intensity distribution as shown in (c) can be 
obtained on the photo detector 85. 
Namely, when positions of the pit provided on the disc 1 and spot beam are 
relatively deviated in the tracking direction, the pit information having 
different intensity distribution in the right and left sides can be 
obtained on the photo detector 85. 
A difference voltage obtained from intensity distribution difference in the 
right and left sides of the pit information obtained on the photo detector 
85 is supplied to the double-axis mechanism to drive the objective lens 84 
as a tracking error signal, a low frequency element of this tracking error 
signal is extracted to generate a sled error signal and it is then 
supplied to the sled mechanism for moving the optical head as a whole in 
order to control the tracking of the optical head to the ON-track 
condition. A method of detecting such tracking error signal is generally 
called a push-pull method and a tracking error signal obtained by this 
push-pull method is called a push-pull error signal in this specification 
of the present invention. 
When, when the tracking of the reproducing apparatus is controlled by the 
push-pull method shown in FIG. 1, if the objective lens 84 of the optical 
head is shifted in the lateral direction (tracking direction) indicated by 
a broken line in FIG. 1 due to the tracking servo operation, etc., the 
reflected beam obtained on the photo detector 85 is also shifted in the 
tracking direction as indicated by the broken line. 
Therefore, the push-pull error signal generated from the pit information 
obtained on the photo detector 85 includes, even under the just tracking 
condition, a DC offset voltage depending on the shift of the objective 
lens 84, resulting in the problem that this DC offset voltage disables 
accurate tracking control. 
In view of solving such a problem, a detecting method called the top hold 
push-pull method has been proposed. In this method, the tracking error 
signal is detected after removing the DC offset voltage for detecting the 
tracking error signal. Such Top Hold Push-Pull (TPP) method has been 
disclosed in the Japanese Laid-Open Patent Number HEI 4-23234 presented 
(filed May 18, 1990) by the same applicant of the present invention. In 
this case, a DC offset removing circuit for removing a DC offset voltage 
included in the push-pull error signal is provided to obtain the tracking 
error signal after having removed the DC offset voltage. Thereby, accurate 
tracking control can be realized by eliminating influence of the shift 
operation in the tracking direction of the objective lens 84 of the 
optical head. In the related art, the sled error signal has been generated 
by extracting a low frequency element of the tracking error signal 
obtained by the top hole push-pull method explained above. 
In general, since the reproducing apparatus for reproducing a disc such as 
CD or MD is restricted in installation space, it is expected that the 
apparatus may be installed vertically or horizontally and is reduced in 
size. The reproducing apparatus is required, even if it is installed 
vertically or horizontally, that the optical block in the optical head (at 
least the objective lens) may be held reliably and an arm forming the 
double-axis mechanism is never shifted in the tracking direction 
(horizontal direction) due to its gravity. Namely, if the reproducing 
apparatus is installed, for example, vertically, it is just preferable as 
shown in FIG. 3A that the disc 1 is arranged in vertical, the sled axis 91 
of the optical head 90 is arranged in vertical and the arm 85 holding the 
objective lens 84 of the optical head 90 is arranged to cross diagonally 
the sled axis 91 so that the position in the tracking direction of the 
objective lens 84 is never influenced by gravity. 
However, in this case, since the sled mechanism for moving the optical head 
90 in the radius direction of the disc 1 becomes larger than the external 
shape of the disc 1 as indicated as the shaded area of FIG. 3A, it has 
impeded reduction in size of the reproducing apparatus. 
Therefore, when the sled mechanism is inclined for arrangement, for 
example, as shown in FIG. 3B, the sled mechanism can be accommodated in 
the internal side as much as .DELTA.1 in comparison with FIG. 3A to 
realize reduction in size of the reproducing apparatus. However, since the 
sled axis 91 holding the objective lens 84 is also inclined, the arm 85 is 
deviated with its self weight and the objective lens 84 is shifted in the 
tracking direction (horizontal direction) under the natural condition 
where the tracking servo is not effectuated. 
In the case where the tracking error signal is obtained by the top 
push-pull method in the reproducing apparatus in which the sled mechanism 
is inclined as shown in FIG. 3B, the DC offset voltage indicating the 
amount of shift due to the gravity of the objective lens 84 can be 
eliminated. Therefore, as schematically shown in FIG. 4B, the arm 85 
holding the objective lens 84 does not return up to the actual mechanical 
center (hereinafter referred to as mechacenter) and thereby the arm 85 is 
shifted by its gravity to the external or internal circumference side of 
the disc 1. 
When the optical block 84 as a whole of the optical head 90 is shifted to 
the external or internal circumference side of the disc 1 as explained 
above, in regard to the visual field indicated by the peak to peak level 
of the reproduced RF signal as shown in FIG. 11A and jitter element, the 
reproduced RF signal will be deteriorated because the peak to peak level 
of the reproduced RF signal is reduced and jitter element is increased 
depending on the movement of lens. As a result, when a disc is reproduced 
using a disc reproducing apparatus, there has been the problems that the 
vibration proof characteristic which is indicated by an applied vibration 
frequency showing the limit for not generating track skip may be 
deteriorated or the S/N ratio of the RF signal is deteriorated to generate 
noise. 
SUMMARY OF THE INVENTION 
The present invention has been proposed considering the problems explained 
above and it is therefore an object of the present invention to provide a 
reproducing apparatus which can realize reduction in size without 
deteriorating the vibration proof characteristic and more specifically to 
provide an optical head transfer circuit, comprising: 
a first envelope detector for holding a received light beam output signal 
from a first light receiving element in a light receiving means which is 
formed of a plurality of light receiving elements for radiating an optical 
disc with an optical beam emitted from an objective lens and receiving the 
light beam reflected from the optical beam, 
a first differential amplifier for obtaining the difference between a 
holding output from the first envelope detector and a receiving output 
signal from the first light receiving element, 
a second envelope detector for holding a receiving output signal from the 
other light receiving element of the light receiving means, 
a second differential amplifier for obtaining the difference between the 
holding output of the second envelope detector and a light receiving 
output signal from the second light receiving element, 
a tracking error signal generator for generating a tracking error signal 
indicating a relative shift in the radius direction of the objective lens 
and the optical disc by obtaining the difference between the differential 
output of the first differential amplifier and the differential output of 
the second differential amplifier, and 
a sled error signal generator for generating a sled error signal for 
relatively shifting the optical head as a whole in the radius direction of 
the optical disc by obtaining the difference between the light receiving 
output signal from the first light receiving element of the light 
receiving means and the light receiving output signal from the other light 
receiving element of the light receiving means. 
Moreover, the present invention also provides an optical head and a 
reproducing apparatus comprising the optical head transfer circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 5 is a block diagram of a reproducing apparatus for a disc such as a 
CD provided with a changer function as a preferred embodiment of the 
present invention. 
A disc 1 is driven for rotation by a spindle motor (M) 2. Under this 
condition, information can be read by means of an optical head 3. The 
optical head 3 radiates the laser to the disc 1 and the information which 
is recorded, for example, in the pit condition on the disc 1 can be read 
from the reflected light beam. 
The optical head 3 includes an optical system composed of a laser diode 31 
as a laser output means, described below, for reading data from a disc 1, 
a deflected beam splitter and a 1/4 wavelength plate, an objective lens 34 
as a laser output end and a photodetector 37 for detecting the reflected 
light beam, while the objective lens 34 is held to be displaced in both 
the disc radius direction (tracking direction) and in the direction to 
become near and far from the disc (focusing direction) by means of a 
double-axis mechanism 4. Moreover, the optical head 3 as a whole may be 
moved in the radius direction of the disc 1 by means of a sled mechanism 
5. 
The information detected by the reproducing operation from the disc 1 with 
the optical head 3 is supplied to an RF amplifier/arithmetic unit 6. The 
RF amp/arithmetic unit 6 executes the arithmetic process of the reproduced 
RF signal, tracking error signals TE1, TE2, and focus error signal FE, 
etc. from the information supplied. 
The tracking error signals TE1, TE2 and focus error signal FE obtained by 
the RF amp/arithmetic unit 6 are then supplied to a servo processor 7. The 
servo processor 7 generates, for example, the servo signal as the PWM 
modulated signal depending on the focus error signal TE and then supplies 
this servo signal to the servo driver 8. The servo driver 8 generates the 
focus drive signal FD based on the PWM modulated signal supplied and 
impresses this signal to a focus coil of the double-axis mechanism 4. 
Namely, the drive of the objective lens in the focus direction is 
controlled by the focus error signal FE. 
The servo processor 7 generates a servo signal as a PWM modulated signal 
depending on the tracking error signal TE1 and then supplies this signal 
to the servo driver 8. The servo driver 8 generates the tracking drive 
signal TD based on the PWM modulated signal supplied and then impresses 
this signal to the tracking coil of the double-axis mechanism 4. Namely, 
the drive of the objective lens in the tracking direction is controlled on 
the basis of the tracking error signal TE1 in the double-axis mechanism 4. 
Moreover, the servo processor 7 generates a sled servo signal as the PWM 
modulated signal from a low frequency element of the tracking error signal 
TE2 and then supplies this signal to the servo driver 8. The servo driver 
8 generates the sled drive signal SLD based on the PWM modulated signal 
supplied to drive the sled mechanism 5. 
Further, the servo processor 7 generates a spindle servo signal on the 
basis of the spindle error signal SPE from a decoder 9 and a spindle kick 
and spindle brake command, etc. from a system controller 14 and then 
supplies this spindle servo signal to the servo driver 8. The servo driver 
8 impresses the spindle drive signal SPD based on the spindle servo signal 
to a spindle motor 2. Thereby, rotation and stop and the constant linear 
velocity (CLV) control during rotation of the spindle motor 2 can be 
executed. 
In the reproducing apparatus of this preferred embodiment, the RF 
amp/arithmetic unit 6 and servo processor 7 are assumed to be integrally 
constituted with an IC, etc. 
Meanwhile, the reproduced RF signal extracted by the RF amp/arithmetic unit 
6 is supplied to the decoder 9 and the reproduced RF signal supplied is 
subjected, in the decoder 9, to the processes such as EFM demodulation 
(Eight Fourteen Demodulation) and CIRC decoding (Cross Interleaved Reed 
Solomon Decoding) and thereafter the reproduced RF signal is once stored 
in a buffer memory 11 under the control of the main controller 10. The 
digital audio data read from the buffer memory 11 in the predetermined 
timing is converted to an analog signal by a D/A converter 12, supplied to 
the predetermined amplifying circuit from a terminal 13 and is then 
reproduced as an output, for example, as the right and left channel audio 
signals. The buffer memory 11 is formed, for example, of a D-RAM 
(Dynamic-Random Access Memory) having the capacity for storing the digital 
audio data of about four seconds as the reproduced audio signal. 
The decoder 9 extracts the subcode information, namely TOC (Table of 
Contents) and address data, recorded on the disc 1 together with an audio 
data and then supplies these data to the system controller 14. Moreover, 
the reproduced clock synchronous to the reproduced data is generated by 
supplying the EFM signal to a PLL (Phase Locked Loop) circuit and is used 
for various processes such as decoding. But, since this reproduced clock 
is synchronous to a disc rotation speed, the reproduced clock is compared 
with the reference clock generated from the master clock to obtain the 
spindle error signal SPE as the difference signal. This spindle error 
signal SPE is then supplied to the servo processor 7. 
The system controller 14 is formed of a microcomputer to control various 
circuits and supply the master clock. In addition to the control explained 
above, setting or changing of each servo gain can also be done for the 
servo processor 7. 
The disc 1 can be accommodated in a plurality of pieces within a disc 
magazine 21, a certain disc is selected by a transfer mechanism 20 and it 
is then loaded to the position where it is reproduced by means of the 
optical head 3. 
Here, the structure of the optical head 3 provided in the reproducing 
apparatus of the preferred embodiment of present invention will be 
explained with reference to FIG. 6 to FIG. 9. 
FIG. 6 is a plan view of the optical head 3 which is composed of a laser 
source 31, a deflected beam splitter 32, a 1/2 wavelength plate 33, an 
objective lens 34, a condensing lens 35, a cylindrical lens 36 and a 
photodetector 37. The optical head 3 constituted as explained above 
allows, in comparison with the optical head of the related art shown in 
FIG. 1, elimination of the collimator lens and therefore the optical head 
3 as a whole can be reduced in size. 
Next, a practical example of the structure of the optical head 3 will be 
explained with reference to FIG. 7. In FIG. 7, a laser coupler 40 is 
composed of the laser source 31, deflected beam splitter 32 and 
photodetector 37 which are shown in FIG. 6 and mounted on the same silicon 
substrate, a couple of silicon mirrors 41, 42 for deflecting the light 
axis O-O' of the light emitted from this laser coupler 40, objective lens 
34 which condenses the light reflected from the silicon mirror 42 and then 
radiates the light beam to the recording surface 1a of the disc 1 and 1/2 
wavelength plate 33 which is disposed between the laser coupler 40 and 
disc 1 and has the regions 33a, 33b which are divided at least into two 
regions at the plane orthogonal to the light axis O-O' resulting in 
different light axes in these regions. 
The laser coupler 40 is composed, for example, as shown in FIG. 8, of the 
laser source 31 provided on the silicon substrate 51, a microprism 52 
having a deflection surface disposed along the light axis O-O' between the 
laser source 31 and objective lens 34 shown in FIG. 7 to isolate the 
predetermined deflected element of the reflected light beam reflected by 
the recording surface 1a of the disc 1 and generate the focus error signal 
FE, and photodetectors 53, 54 for respectively detecting the amount of 
light at positions an equal distance from the focusing point formed on the 
silicon substrate 51. 
The light receiving regions of the photodetectors 53, 54 are divided 
respectively into four regions 53a, 53b, 53c, 53d and four regions 54a, 
54b, 54c, 54d, for example, as shown in FIG. 9. Therefore, at the plane 
orthogonally crossing the light axis O-O' of the 1/2 wavelength plate 33, 
the light axes at the regions at least divided into two sections, for 
example, at the regions 33a, 33b, are set at different angles and the 
reflectivities for the P wave and S wave of the surface 52a of the 
microprism 52 are respectively set to 0% and 100% so that the reflected 
light beam having passed the first region 33a of the 1/2 wavelength plate 
33 of the output beam from the laser source 31 and reflected by the 
recording surface 1a of the disc 1 passes the second region 33b of the 1/2 
wavelength plate 33 and the reflected light beam having passed the second 
region 33b of the 1/2 wavelength plate 33 of the output light beam from 
the laser source 31 and reflected by the recording surface 1a of the disc 
1 passes the first region 33a of the 1/2 wavelength plate 33. 
The reflected beams detected respectively in the regions 53a, 53b, 53c, 53d 
of the photodetector 53 and the regions 54a, 54b, 54c, 54d of the 
photodetector 54 shown in FIG. 9 are supplied to the RF amp/arithmetic 
unit 6 and thereby the reproduced RF signal, tracking error signals TE1, 
TE2 and focus error signal FE, etc. can be extracted. 
FIG. 10 shows a practical example of the structure of the tracking control 
circuit block for controlling the tracking among the circuit structure of 
the RF amp/arithmetic unit 6 and servo processor 7 formed by an IC. The 
current signals detected in the regions 53a, 53b, 53c, 53d of the 
photodetector 53 and in the regions 54a, 54b, 54c, 54d of the 
photodetector 54 are respectively supplied to the current-voltage 
converters 55 (hereinafter referred to as "I-V converters") provided in 
the respective regions. For example, the current signal detected in the 
region 53a of the photodetector 53 is converted to the voltage signal A1, 
while the current signal detected in the region 53c is converted to the 
voltage signal A3, the current signal detected in the region 53d is 
converted to the voltage signal A4 and the current signal detected in the 
region 53b is converted to the voltage signal A2, respectively. Moreover, 
the current signal detected in the region 54a of the photodetector 54 is 
converted to the voltage signal B1, while the current signal detected in 
the region 54c is converted to the voltage signal B3, the current signal 
detected in the region 54d is converted to the voltage signal B4 and the 
current signal detected in the region 54b is converted to the voltage 
signal B2, respectively and these are then output. 
An addition amplifier 56 is composed of adders 56a, 56b, 56c, 56d. In the 
adder 56a, the voltage signals Al, A2, B3, B4 converted by the I-V 
converter 55 are added and then output as the PD1 signal, while in the 
adder 56b, the voltage signals A3, A4, B1, B2 from the I-V converter 55 
are added and then output as the PD2 signal. The reproduced RF signal and 
focus error signal FE, etc. are generated and then output from the PD1 
signal and the PD2 signal. For example, the reproduced RF signal is 
obtained by adding the PD1 signal and the PD2 signal, while the focus 
error signal is generated depending on the difference between the PD1 
signal and the PD2 signal. 
Moreover, the adder 56c outputs a push-pull signal E obtained by adding the 
voltage signals A2, A4, B1, B3 from the I-V converter 55, while the adder 
56d outputs a push-pull signal F by adding the voltage signals A1, A3, B2, 
B4 from the I-V converter 55. These push-pull signal E and the push-pull 
signal F are assumed to include the DC offset voltage which is generated 
when the objective lens 34 of the optical head 3 is shifted in the 
tracking direction. These push-pull signals E, F are respectively supplied 
to the holding circuits 57a, 57b. Thereby, the peak voltages are held and 
the voltage signals T1, T2 obtained by multiplying the peak voltages by 
predetermined constant (0.8, for example) are then supplied to one end 
terminals of the amplifiers 58a, 58b. 
The voltage signals T1, T2 output from the holding circuits 57a, 57b and 
the push-pull signals E, F output from the adder amplifiers 56c, 56d are 
respectively input to the amplifiers 58a, 58b. The amplifier 58a outputs a 
difference voltage between the voltage signal T1 from the holding circuit 
57a and the push-pull error signal E from the adder amplifier 56c and the 
amplifier 58b outputs a difference voltage between the voltage signal T2 
from the holding circuit 57b and the push-pull error signal F from the 
adder amplifier 56d. As a result, the amplifiers 58a, 58b respectively 
output the top hold push-pull signals TPPE, TPPF from which the DC offset 
voltage included in the push-pull error signals E, F is canceled. The TPP 
circuit of the present invention can be formed of the holding circuits 
57a, 57b and amplifiers 58a, 58b, 58c. 
The amplifier 58c outputs a difference voltage between the top hold 
push-pull signals TPPE and TPPF output from the amplifiers 58a, 58b as the 
tracking error signal TE1. 
Therefore, this tracking error signal TE1 is supplied to the double-axis 
mechanism 4 to control the objective lens 34. Accordingly, if the 
objective lens 34 is shifted in the horizontal direction (tracking 
direction), the DC offset voltage may be eliminated depending on the 
amount of shift and thereby the double-axis mechanism 4 can accurately 
control the tracking. 
Moreover, in this preferred embodiment, the push-pull signal E output from 
the adder amplifier 56c and the push-pull signal F output from the adder 
amplifier 56d are supplied to the amplifier 58d and the amplifier 58d 
outputs a difference voltage as the tracking error signal TE2. A low 
frequency element (2 Hz to 3 Hz or lower) is extracted, to generate the 
sled error signal, by means of a low-pass filter not illustrated from the 
tracking error signal TE2 containing the DC offset voltage. 
Therefore, when the sled error signal is supplied to the sled mechanism 5 
to control the tracking of the optical head 3 as a whole, the sled 
mechanism 5 is controlled by the sled error signal to which the DC offset 
voltage is added depending on the amount of shift even if the arm forming 
the double-axis mechanism 4 to hold the objective lens 34 is shifted in 
the tracking direction due to its gravity. Accordingly, the optical head 3 
can be returned to the actual mechacenter. 
As a result, a reproducing operation is realized while the visual field is 
maintained in well improved condition even when the optical head of the 
reproducing apparatus is disposed with a certain inclination angle as 
shown in FIG. 3B. The vibration proof characteristic of the related art as 
indicated by a broken line in FIG. 11 can be improved as indicated by a 
solid line in FIG. 11. That is, even when the optical head is arranged 
with a certain inclination angle, the vibration proof characteristic which 
is almost equal to that obtained when the optical head 3 is arranged in 
vertical can be obtained. Accordingly, it is now possible to arrange the 
optical head of the reproducing apparatus with a certain inclination 
angle, and the reproducing apparatus as a whole can be reduced in size. 
As explained previously, the reproducing apparatus of the present invention 
is provided with a tracking control signal generating means which can 
generate a first tracking control signal to control displacement of the 
objective lens and a second tracking control signal to extract the sled 
control signal to control displacement of the optical head. Here, the 
first tracking control signal is generated as the signal not including the 
DC offset voltage corresponding to displacement of the objective lens, 
while the second tracking control signal as the signal including the DC 
offset voltage corresponding to displacement of the objective lens. 
Therefore, deterioration of the vibration proof characteristic can be 
prevented even if the optical head is disposed with a certain inclination 
angle. As a result, it is now possible to arrange the optical head with a 
certain inclination angle and the reproducing apparatus can be reduced in 
size.