Apparatus for recording and reproducing information on an optical disk with a focus servo system for avoiding influence of traverse signal during search

An optical disk recording and reproducing appartus is provided with a signal processing circuit in a focus servo circuit for suppressing a traverse signal which has a frequency determined by the pitch of pre-grooves and the transversely shifting speed of a laser beam spot and which modulates a focus error signal during a search mode operation. This signal processing circuit successfully avoids an influence of the traverse signal focus servo system and thus assures accurate focus on the control.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an optical disk recording and 
reproducing apparatus for recording and reproducing information on an 
optical disk. More specifically, the invention relates to an optical disk 
recording and reproducing apparatus with a focus servo system. 
2. Description of the Background Art 
In order to facilitate a better understanding of the features of the 
present invention in relation to the background art, a brief discussion 
will be provided about the background art of the present invention with 
reference to FIGS. 1 and 2, which illustrate a circuit diagram of an 
optical disk recording and reproducing apparatus of the prior art. 
The shown optical disk recording and reproducing apparatus of FIG. 1 is 
designed for recording and reproducing information on tracks formed on an 
optical disk 1. As is well known, the optical disk is chucked on a disk 
drive mechanism including a motor driven spindle 2 which is driven by a 
spindle motor for rotatingly driving the disk. As shown in FIGS. 2 and 2A, 
the optical disk to be used in the shown apparatus is a so-called 
"pre-grooved type disc" which has a plurality of recording tracks defined 
by preliminarily formed grooves. In addition, as a recordable optical 
disk, the shown embodiment employs an optomagnetic disk. The disk 1 
chucked on the spindle 2 is thus driven at a given constant speed. 
An optical head 3 is provided in the vicinity of the optical disk for 
optically reading or writing information on the recording tracks. The 
optical head 3 is connected to an RF circuit 4. The RF circuit 4, 
operating in a reproducing mode, converts the information read from the 
recording track by means of the optical head 3 into an electric signal 
indicative of the read information to output. On the other hand, the RF 
circuit 4, operating in a recording mode, converts the information 
containing an electric signal into optical information data in a form 
recordable on the recording track. 
This RF circuit 4 is connected to a signal processing circuit 5. This 
signal processing circuit 5 performs a known signal processing operation. 
The signal processing circuit 5 is connected to an input/output circuit 6. 
The RF circuit 4 also outputs a focus error signal to a focus servo circuit 
7 to feed thereto a focus error signal. The RF circuit 4 is further 
connected to a tracking servo circuit 8 to feed a tracking error signal. 
The output of the focus servo circuit 7 is connected to the optical head 
3. The output of the focus servo circuit 7 is also connected to a thread 
servo circuit 9 which controls transverse shift of the optical head 3. 
Operation of the signal processing circuit 5, the focus servo circuit 7, a 
tracking servo circuit 8 and a thread servo circuit 9 are connected to a 
CPU 10 which serves as a system controller. The CPU 10 outputs a clock 
signal, a timing signal, an access signal and so forth. The CPU also 
outputs control signals for the aforementioned respective circuits for 
controlling operations thereof depending upon the operation modes thereof. 
The CPU also serves to control the driving speed of the spindle motor for 
controlling rotation speed of the optical disk 1. 
As shown in FIGS. 2 and 2(A), the grooves G are formed on the disk in 
concentric circular or helical fashion. The grooves will be hereafter 
referred to as "pre-grooves". Each pre-groove G has a width corresponding 
to where .lambda./8 (.lambda. is the wavelength of laser of the optical 
head). An adjacent pair of grooves G define a land which serves as a 
recording track T. As will be appreciated, the light intensity to be 
reflected from the pre-groove G and the land T is different from each 
other. Based on the difference of light intensity reflected from the 
groove and the land, a tracking error signal is generated so that a 
tracking servo system will control the optical head to place the light 
spot of the laser beam on a desired one of tracks for tracing therealong. 
In such optical disk recording and reproducing apparatus, the reflected 
light intensity frequently varies every time the laser beam spot moves 
across the pre-groove during a search operation, in which the optical head 
is shifted transversely to the tracks. As a result, a high frequency 
signal St modulated by the pre-grooves, which is shown in FIG. 3(a) and 
will be hereafter referred to as "traverse signal", tends to be 
superimposed on the focus error signal. 
The focus servo circuit 7 in FIG. 1 employs a phase compensation circuit 71 
(FIG. 4) for enhancing a high frequency component of the focus error 
signal St for improving response characteristics. The output of the phase 
compensation circuit 71 is fed to a driver circuit 72. The driver circuit 
72 generates a drive signal S.sub.FD for driving a focus actuator 73. 
In this circuit arrangement, when the traverse signal superimposes on the 
focus error signal St, the traverse signal may be enhanced in the focus 
servo loop set forth above. As a result, the peak of the enhanced focus 
error signal tends to saturate to cause distortion of the waveform in the 
drive signal S.sub.FD, as shown in FIG. 3(b). This distortion of the 
waveform of the drive signal S.sub.FD causes variation of the direct 
current level. Variation of the direct current level tends to degrade the 
accuracy of a focusing operation of the focus servo system. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide an optical 
disk recording and reproducing apparatus which can effectively and stably 
eliminate influence of a traverse signal for accurate focus control. 
In order to accomplish the aforementioned and other objects, an optical 
disk recording and reproducing apparatus, according to the present 
invention, is provided with a signal processing circuit in a focus servo 
circuit for suppressing a traverse signal which has a frequency determined 
by the pitch of pre-grooves and the transversely shifting speed of a laser 
beam spot and modulates a focus error signal otherwise, during a search 
mode operation. 
This signal processing circuit successfully avoid influence of the traverse 
signal for focus servo system and thus assures accurate focus control. 
According to one aspect of the invention, an optical disk recording and 
reproducing apparatus comprises an optical head scanning a light beam on 
an optical disk formed with a plurality of essentially circumferentially 
extending grooves for reproducing an information signal including a focus 
error signal, a focus servo system including a focus actuator operable for 
driving an object lens of the optical head for focusing a light beam on 
the optical disk, means associated with the focus actuator for deriving a 
focus control signal on the basis of the focus error signal in order to 
control the focus actuator, and means, active in an access mode operation 
of the optical disk recording and reproducing apparatus in which light 
beam shifts transversely across at least one of the grooves, for removing 
a signal component modulated by the groove, superimposing on the focus 
error signal. 
According to another aspect of the invention, a focus control system for an 
optical disk recording and reproducing apparatus including an optical head 
scanning a light beam on an optical disk formed with a plurality of 
essentially circumferentially extending grooves for reproducing an 
information signal including a focus error signal and a focus servo system 
including a focus actuator operable for driving an object lens of the 
optical head for focusing a light beam on the optical disk, comprises a 
focus control signal generator means, associated with the focus actuator, 
for deriving a focus control signal on the basis of the focus error signal 
in order to control the focus actuator, and a traverse signal component 
absorbing means, provided upstream of the focus control signal generator, 
for absorbing fluctuation of the focus error signal within a predetermined 
fluctuation range in order for removing a traverse signal superimposing on 
the focus error signal. 
The signal component removing means comprises a deadband circuit and a 
sample/hold circuit, the deadband circuit defining a deadband for the 
focus error signal for absorbing fluctuation of the focus error signal 
within the deadband so as to hold a held value in the sample/hold circuit 
unchanged. 
The focus control system further comprises means for adjusting-the 
deadband. The deadband adjusting means detects of the level of the signal 
component for adjusting the width of the deadband depending thereon. The 
deadband adjusting means detects the signal component superimposing on an 
output of the sample/hold circuit for adjusting the deadband. 
In addition, the focus control system further comprises means for defining 
a signal path by-passing the deadband circuit for directly feeding an 
input focus error signal to the sample/hold circuit, the signal path 
including a switch operable between a conductive state for establishing 
the path and a non-conductive state for establishing the path and a 
non-conductive state for breaking the path and switching at the conductive 
state in response to a signal indicative of one of a tracking On state and 
focus search state. 
According to a further aspect of the invention, a focus control system for 
an optical disk recording and reproducing apparatus including an optical 
head scanning a light beam on an optical disk formed with a plurality of 
essentially circumferentially extending grooves for reproducing 
information signal including a focus error signal and a focus servo system 
including a focus actuator operable for driving an object lens of the 
optical head for focusing a light beam on the optical disk, comprises a 
focus control signal generator means, associated with the focus actuator, 
for deriving a focus control signal on the basis of the focus error signal 
in order to control the focus actuator, and a focus error smoothing means, 
disposed upstream of the focus control signal generator, for smoothing the 
focus error signal for removing a traverse signal superimposing on the 
focus error signal. 
The focus error signal smoothing means comprises a peak hold circuit for 
holding a peak value of the focus error signal, a bottom hold circuit for 
holding a bottom of the focus error signal and an adder adding outputs of 
the peak and bottom hold circuits. The peak and bottom hold circuits 
respectively include diodes and the apparatus further comprises means for 
compensating for non-linear characteristics of the diodes. 
In this case, the focus control system also comprises means for defining a 
signal path by-passing the focus error signal smoothing means for directly 
feeding an input focus error signal to the sample/hold circuit, the signal 
path including a switch operable between a conductive state for 
establishing the path and a non-conductive state for establishing the path 
and a non-conductive state for breaking the path and switching at the 
conductive state in response to a signal indicative of one of a tracking 
On state and a focus search state.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the preferred embodiments of the present invention, and 
particularly to FIG. 5, a traverse signal eliminating circuit 11 is 
interposed between an RF circuit 4 (shown in FIG. 1) and a phase 
compensation circuit 71 (shown in FIG. 4). The traverse signal eliminating 
circuit 11 includes an operational amplifier 111 which forms a voltage 
follower circuit. The operational amplifier 111 has a non-inverting input 
terminal connected to the RF circuit 4 to receive therefrom a focus error 
signal St. The operational amplifier 111 also has an inverting input 
terminal connected to the output of the operational amplifier to 
constitute a voltage follower circuit. 
The output of the operational amplifier 111 is also connected to a deadband 
circuit 112. The deadband circuit 112 is connected to the phase 
compensation circuit 71 via a sample/hold circuit 116 which comprises a 
resistor 113, a capacitor 114 and an operational amplifier 115 which form 
a voltage follower circuit. 
The deadband circuit 112 comprises a pair of diodes 117 and 118. The pair 
of diodes 117 and 118 are arranged in parallel relationship to each other 
and in opposite polarity. This deadband circuit 112 is designed to cancel 
the traverse signal superimposing on the focus error signal St with a 
potential difference of the rising voltage of the diodes. 
Namely, a held voltage V.sub.H of the sample/hold circuit 116 is normally 
applied to the non-inverting input terminal of the operational amplifier 
115. When the input voltage V.sub.FE to the deadband circuit 112 which 
contains the traverse superimposing signal, fluctuates in a fluctuation 
range (V.sub.H -V.sub.FE) smaller than the rising voltage V.sub.D1 or 
V.sub.D2 of the diodes 117 and 118, both of the diodes are held OFF. 
Therefore, the held voltage V.sub.H of the sample/hold circuit 116 is 
transferred to the phase compensation circuit 71. 
On the other hand, when the voltage fluctuation range (V.sub.H -V.sub.FE) 
is greater than the rising voltage of the diodes 117 and 118, both diodes 
turn ON. By this the held voltage V.sub.H is varied in a magnitude 
corresponding to a difference of voltage determined by subtracting the 
riding voltage V.sub.D1 or V.sub.D2 of the diodes 117 or 118 from the 
fluctuated voltage (V.sub.H -V.sub.FE). 
As will be appreciated, with the arrangement set forth above, when the 
input voltage V.sub.FE fluctuates in relation to the held voltage V.sub.H, 
the held voltage will not be varied as long as the voltage fluctuation is 
in a range define by the rising voltages V.sub.D1 and V.sub.D2 of the 
diodes 117 and 118. This voltage fluctuation range will be hereafter 
referred to as "deadband". The traverse signal superimposing on the focus 
error signal St is successfully removed utilizing this deadband. 
It should be noted that the input/output characteristics of the deadband 
circuit 112 are shown in FIG. 6. As will be seen from FIG. 6, the opposite 
connection of the diodes 117 and 118 will provide a substantially great 
absorption magnitude for small level signals, i.e. for the signals in a 
range of V.sub.D1 and V.sub.D2. 
The foregoing deadband circuit 112 is associated with a control circuit 120 
which controls the deadband circuit between an active state and an 
inactive state. In order to control the deadband circuit operational state 
between an active state and an inactive state, a switch 119 is provided. 
The control circuit 120 has input terminals 121 and 122 which are 
connected to a system controller (not shown). When a HIGH level input is 
applied to one of the input terminals 121 and 122, the switch 119 is 
operated to an open state to make the deadband circuit 112 inactive. 
The input level at the input terminal 121 represents an operational state 
of the tracking servo system and is held HIGH while the recording and 
reproducing apparatus operates in a reproduction mode and thus the 
tracking servo system is an ON state. This turns the switch 119 ON to 
establish a by-pass circuit for by-passing the focus error signal S.sub.t 
through the switch 119. Similarly, the input level of the input terminal 
122 represents a focus search operational state. When a focus search is 
ON, a HIGH level input is applied to the input terminal 122. By this, both 
of transistors 123 and 124 are turned ON to turn the switch 119 ON. This 
establishes the by-pass circuit for by-passing the focus error signal 
S.sub.t through the switch 119. 
On the other hand, when the optical head is driven transversely to the 
tracks on the optical disk in a search mode operation, the tracking servo 
system turns into an OFF position to apply a LOW level to the input 
terminal 121. As a result, the transistor 123 is turned OFF to turn the 
switch 119 OFF. Therefore, the by-pass circuit through the switch 119 is 
broken to apply the focus error signal St to the deadband circuit 112. At 
this time, since the input/output characteristic of the deadband circuit 
112 is as illustrated in FIG. 6, the focus error signal St (shown in FIG. 
7(a)) as the input of the deadband circuit is absorbed as to the 
superimposing traverse signal component to output the signal having a 
waveform shown in FIG. 7(b). Since the output of the deadband circuit 112 
shown in FIG. 7(b) has successfully removed the traverse signal component, 
the driver circuit 72 will never become saturated by the output of the 
phase compensation circuit 71. 
On the other hand, at a ON setting of the recording and reproducing 
apparatus, or when the focus servo becomes an output of control, a focus 
search signal is applied to a driver circuit 72 for driving an object lens 
vertically for focus control. During a focus search, a HIGH level signal 
is applied to the input terminal 122 from the system controller. In 
response to the HIGH level input at the input terminal 122, the transistor 
124 turns ON. This raises the output level of a differential amplifier 125 
to turn the switch 119 ON. This establishes the aforementioned by-pass 
circuit through the switch 119 to pass the focus error signal St to the 
phase compensation circuit therethrough. Therefore, a focus search can be 
done accurately and precisely. 
In the aforementioned first embodiment of the optical disk recording and 
reproducing apparatus, the deadband circuit is designed to be active only 
when the optical head is driven transversely to the tracks of the optical 
disk to shift the laser beam spot transversely across the pre-grooves, for 
absorbing or removing the traverse signal which can superimpose on the 
focus error signal. In other words, since the deadband circuit is held 
inactive while the tracking servo is ON. This avoids a possibility to 
activate the focus servo system in an off-focused condition due to an 
influence of the deadband circuit. 
FIG. 8 shows another embodiment of the traverse signal eliminating circuit 
according to the invention. The shown embodiment of the traverse signal 
eliminating circuit is generally represented by the reference numeral 220. 
A deadband circuit 223 comprises a pair of diodes 217 and 218 and 
resistors 221 and 222 which are connected in series to associated ones of 
the diodes. Similarly to the former embodiment, the diodes 217 and 218 are 
arranged in parallel and in opposite polarity. The resistors 221 and 222 
are provided with a resistance of R.sub.1. A current source 225 is 
respectively connected to a junction between the series of the diode 217 
and the resistor 221. Similarly, a current source 226 is connected to a 
junction between the series of diode 218 and the resistor 222. The current 
sources 225 and 226 are adapted to supply currents of I.sub.1 and I.sub.2 
respectively. The currents I.sub.1 and I.sub.2 of the current sources 225 
and 226 serve for causing a voltage drop V.sub.R1 and V.sub.R2 at the 
resistors 221 and 222 so as to apply an offset voltage (V.sub.FE -V.sub. 
R1, V.sub.FE +V.sub.R2) relative to the input voltage V.sub.FE to the 
diodes 217 and 218. 
Therefore, the diodes 217 and 218 turn ON when the following formulae are 
established: 
EQU V.sub.D1 &lt;(V.sub.FE -V.sub.R1)-V.sub.H (1) 
EQU V.sub.D2 &lt;V.sub.H -(V.sub.FE +V.sub.R2) (2) 
The foregoing formulae (1) and (2) are modified as: 
EQU V.sub.D1 +V.sub.R1 &lt;V.sub.FE -V.sub.H (3) 
EQU -(V.sub.D2 +V.sub.R2)&gt;V.sub.FE -V.sub.H (4) 
As will be seen from the foregoing formulae, the diodes 217 and 218 turn ON 
when the fluctuation magnitude of the input voltage V.sub.FE becomes out 
of the range defined by (V.sub.D1 +V.sub.R1) and -(V.sub.D2 +V.sub.R2). 
When the diodes 217 and 218 turn ON, the held voltage V.sub.H of a 
sample/hold circuit 216 varies. In other words, as long as the fluctuation 
magnitude of the input voltage V.sub.FE is maintained within the range 
defined by (V.sub.D1 +V.sub.R1) and -(V.sub.D2 +V.sub.R2), the held 
voltage V.sub.H can be held constant. 
Here, as will be seen, since the deadband in a range defined by (V.sub.D1 
+V.sub.R1) and -(V.sub.D2 +V.sub.R2) is variable depending upon the 
voltage drop V.sub.R1 and V.sub.R2, it can be adjusted by adjusting the 
currents I.sub.1 and I.sub.2 to be applied from the current sources 225 
and 226. 
In the shown embodiment, the current sources 225 and 226 are designed to 
vary the output currents I.sub.1 and I.sub.2 depending upon the level of 
the traverse signal. This adjusts the deadband range depending upon the 
traverse signal level for assured by remaining of the traverse signal, 
superimposing on the focus error signal and preventing the focus error 
signal level from being excessively lowered. 
For this purpose, a high-pass filter circuit 232 which comprises a 
capacitor 230 and a resistor 231 is connected to the output of the 
sample/hold circuit 216 in order to receive focus error signal St.sub.1 
output therefrom. The high-pass filter 232 extracts the traverse signal 
S.sub.M1 from the focus error signal St.sub.1. The traverse signal 
S.sub.M1 extracted by the high-pass filter 232 is fed to a full-wave 
rectification amplifier 233. As a result, when the traverse signal 
S.sub.M2 superimposed on the focus error signal, as shown in FIG. 9(a), is 
input to the traverse signal eliminating circuit 220, and when the 
traverse signal S.sub.M2 causes the input signal fluctuation beyond the 
deadband defined in the deadband circuit 223, the excess magnitude of the 
traverse signal S.sub.M1 is extracted by the high-pass filter circuit 232 
and input to the full-wave rectification amplifier 233. 
The full-wave rectification amplifier 233 comprises an input resistor 234, 
a feedback resistor 235 and an operational amplifier 237 including a 
rectification diode 236. The resistance of the input resistor 234 and the 
feedback resistor 235 are set at an equal value. With this circuit 
arrangement, the full-wave rectification amplifier 233 outputs a full-wave 
rectified output signal S.sub.MA, as shown in FIG. 9(c). The output signal 
S.sub.MA of the full-wave rectification amplifier 233 is fed to an envelop 
detector circuit 243 which comprises a resistor 241, a capacitor 242 via 
an operational amplifier 240. As will be seen from FIG. 8, the envelop 
detector circuit 243 is in a form of a low-pass filter. Through this 
envelop detector circuit 243, an envelop signal S.sub.E (shown in FIG. 
9(d)) can be obtained from the traverse signal S.sub.M1 extracted by the 
high-pass filter 232. 
The foregoing high-pass filter 232, the full-wave rectification amplifier 
233, the operational amplifier 240, and the envelop detector circuit 243 
form a traverse signal detector circuit. 
The output of the envelop detector circuit 243 is connected to an inverting 
amplifier 248. The inverting amplifier 248 comprises an operational 
amplifier 247 having an input resistor 245 and a feedback resistor 246. 
The input resistor 245 and the feedback resistor 246 are provided with the 
same resistance value. With this arrangement, the inverting amplifier 248 
receives the envelop signal S.sub.E as the output of the envelop detector 
circuit 243 and inverts the received envelop signal to provide an output 
to the current source circuits 225 and 226. Here, assuming the voltage 
level of the traverse signal S.sub.M1 input to the inverting amplifier 248 
is Vc, the voltage level of the envelop signal becomes -Vc. 
The current source circuit 225 includes an operational amplifier 253 having 
an inverting input terminal, a non-inverting input terminal and an output 
terminal. A feedback resistor 250 is disposed between the non-inverting 
input terminal and the output terminal. On the other hand, a feedback 
resistor 252 is disposed between the inverting input terminal and the 
output terminal. The non-inverting input terminal of the operational 
amplifier 253 is also connected to a reference voltage source 257 
comprising a resistor 255 and a temperature compensation diode 256, to 
receive therefrom a reference voltage V.sub.D3. On the other hand, the 
inverting input terminal of the operational amplifier 253 is connected to 
the inverting amplifier 248 to receive the inverted envelop signal via a 
resistor 258. The output terminal of the operational amplifier 253 is 
connected to the resistor 221 of the deadband circuit 223 via an output 
resistor 251 to supply the current I.sub.1. The resistance of the output 
resistor 251 is set at a value equal to the resistance R.sub.1 of the 
resistor 221. On the other hand, the resistances of the feedback resistors 
250 and 252 and the input resistors 254 and 258 are selected to be the 
equal in values to R.sub.2 and to each other. With this circuit 
arrangement, the following equation can be established at the output 
resistor 251 with respect to input voltages Vc and V.sub.D3 and the output 
current I.sub.1 : 
EQU I.sub.1 R.sub.1 =Vc-V.sub.D3 (5) 
On the other hand, the voltage drop V.sub.R1 at the resistor 221 in 
relation to the current I.sub.1 can be illustrated by: 
EQU V.sub.R1 =R.sub.1 I.sub.1 (6) 
Therefore, the voltage drop V.sub.R1 can be illustrated by: 
EQU V.sub.R1 =Vc-V.sub.D3 (7) 
From the foregoing result, the voltage V.sub.F1 defining the deadband and 
determined by the resistor 221 and the diode 217 can be illustrated by: 
V.sub.F1 =V.sub.D1 +V.sub.R1 =V.sub.D1 +Vc-V.sub.D3 (8) 
Assuming the diodes 217, 218 and 256 are provided with the same rising 
voltage, the following equation can be derived from the foregoing equation 
(8): 
EQU V.sub.F1 =Vc (9) 
Therefore, the voltage V.sub.F1 defining the deadband, which voltage is 
determined by the diode 217 and the resistor 221, can be controlled in 
proportion to the output voltage Vc. 
On the other hand, the current source circuit 226 has an operational 
amplifier 264 having a non-inverting input terminal, an inverting input 
terminal and an output terminal. A feedback resistor 260 is connected to 
the output terminal via a resistor 261 at one end and to the non-inverting 
input terminal at the other end. On the other hand, a feedback resistor 
262 is disposed between the output terminal and the inverting input 
terminal. The non-inverting input terminal of the operational amplifier 
264 is also connected to the reference voltage source 257 via an input 
resistor 265. The inverting input terminal is, on the other hand, 
connected to the inverting amplifier 248 to receive therefrom the inverted 
envelop signal -Vc. In the shown circuit construction, the resistor 261 
serves as an output resistor. This output resistor 261 has a resistance 
R.sub.1 which is the same as that of the resistor 222. On the other hand, 
the resistances of the feedback resistors 260 and 262 and the input 
resistors 265 and 266 are set at the same value R.sub.2. 
With this circuit arrangement, the current source circuit 226 generates a 
current having the same amplitude as, and an opposite polarity to current 
i.sub.2. As set forth, this current I.sub.2 is applied to the resistor 
222. 
As will be appreciated, since the same or similar equations as discussed 
with respect to the current source circuit 225, apply the following 
relationship can be established: 
##EQU1## 
As will be appreciated herefrom, as controlled by the opposite polarity 
and the same amplitude of current I.sub.2, the voltage V.sub.F2 having the 
identical voltage value and having an opposite polarity, to define the 
deadband can be obtained, and can be controlled. 
The current source circuits 225 and 226, the inverting amplifier 248 and 
the reference voltage source 257 constitute a deadband control circuit for 
controlling the width of the deadband according to the level of the 
envelop signal S.sub.E. Furthermore, in the shown embodiment, the diodes 
217 and 218 in the deadband circuit are provided with the same 
characteristics to that of the temperature compensation diode in the 
reference voltage source 257. Since a closed loop is formed as a whole of 
the traverse signal eliminating circuit for controlling the width of the 
deadband, temperature characteristics of the diodes 217 and 218 can be 
stably and effectively compensated for practical use. 
As will be appreciated herefrom, the shown embodiment extracts the traverse 
signal maintained in the focus error signal St output from the sample/hold 
circuit, and controls the width of the deadband by adjusting the voltages 
V.sub.F1 and V.sub.F2. Therefore, a traverse signal can be effectively 
removed or absorbed from the focus error signal to obtain the waveform 
shown in FIG. 9(e). 
Namely, as shown in FIG. 10, the deadband circuit 223 is provided with 
input/output characteristics variable of the width by variation of the 
voltages V.sub.F1 and V.sub.F2 with taking the input/output voltage 
difference (V.sub.H -V.sub.FE) of OV as a center according to a traverse 
signal and since a closed loop is formed as a whole of the traverse signal 
eliminating circuit for controlling the width of the deadband, temperature 
characteristics of the diodes 217 and 218 can be compensated. As a result, 
the input/output characteristics of the deadband circuit, in which the 
voltages V.sub.F1 and V.sub.F2 defining the deadband varies with the held 
voltage V.sub.H of the sample/hold circuit 216, can be obtained. 
Since the voltages V.sub.F1 and V.sub.F2 have opposite polarities and the 
same magnitude of voltage difference relative to the held voltage V.sub.H, 
and the width of the deadband is variable depending upon the traverse 
signal level superimposing on the focus error signal so that the entire 
range of the traverse signal can be removed from the focus error signal, 
and influence of the traverse signal for the focus servo system can be 
successfully avoided. 
In the practical construction, the gain of the focus control circuit is 
adjusted to be as great as possible in a range where oscillation of the 
focus servo circuit is avoided. The deadband circuit serves for preventing 
the focus servo from oscillating by adjusting the width of the deadband. 
This makes adjustment of gain of the focus control circuit easier and more 
simple. 
It should be noted that though the shown embodiment employs resistors 221 
and 222 having the same resistance as the resistors 251 and 261 of the 
current source means, it may possible to set the resistances of those 
resistors at mutually different values or to supply a different amplitude 
of current to the resistors 221 and 222 which may cause offset of the 
deadband with respect to the held voltage V.sub.H. 
FIG. 13 shows a further embodiment of the traverse signal eliminating 
circuit in the optical disk recording and reproducing apparatus. The shown 
embodiment of the traverse signal eliminating circuit is generally 
represented by the reference numeral 300. The traverse signal eliminating 
circuit 300 has an amplifier 311 which is of a voltage follower type 
construction and receives the focus error signal St. The traverse signal 
eliminating circuit 300 also include a pair of peak and bottom hold 
circuits 318 and 319. The peak hold circuit 318 comprises a diode 312, a 
resistor 314 and a capacitor 316. On the other hand, the bottom hold 
circuit 319 comprises a diode 313, a resistor 315 and a capacitor 317. The 
outputs of the peak and bottom hold circuits 318 and 319 are connected to 
an adder circuit 322 including resistors 320 and 321. The diodes 312 and 
313 are connected to a respective constant voltage source -Vcc and +Vcc 
via the resistors 314 and 315. With the voltages -Vcc and +Vcc supplied 
via the resistors 314 and 315, suitable forward current flow occurs 
through the diodes 312 and 313. The resistance of the resistors 320 and 
321 are set at the same values. 
The focus error signal St is applied to the diodes 312 and 313 of the peak 
and bottom hold circuit 318 and 319 via the voltage follower type 
amplifier 311. The peak value of the focus error signal St is rectified by 
the diode 312 and changes the capacitor 316. On the other hand, the bottom 
value of the focus error signal St is rectified by the diode 313 and 
charges the capacitor 317. 
Assuming the time constant of the resistor 314 and capacitor 316, and the 
resistor 315 and capacitor 317 is T, and when this time constant T is 
sufficiently greater than the period of the traverse signal S.sub.M2, the 
terminal voltages of the capacitors 316 and 317 varies as shown as the 
peak hold voltage e.sub.1 and bottom hold voltage e.sub.2, as shown in 
FIG. 14(a). Since the peak hold voltage e.sub.1 and the bottom hold 
voltage e.sub.2 are applied through the resistors 320 and 321 of the same 
resistance, the voltage at the intersection point becomes (e.sub.1 
/2+e.sub.2 /2) which becomes substantially equal to the pure focus error 
signal e.sub.f, as shown in FIG. 14(b). The output of the adder 322 is 
output through an operational amplifier 323. 
As will be appreciated herefrom, it is necessary to set the resistance of 
the resistors 314 and 315 and the capacitors 316 and 317 to provide a 
sufficiently great time constant in relation to the period of the traverse 
signal. 
FIG. 15 shows a modification of the foregoing embodiment of the traverse 
signal eliminating circuit of FIG. 13. In this modification, operational 
amplifiers 330 and 331 are added in the peak and bottom hold circuits 318 
and 319. These operational amplifiers 330 and 331 are provided for 
improving non-linear characteristics of the diodes 312 and 313. For this 
purpose, the operational amplifiers 330 and 331 are disposed between the 
amplifier 311 and the diodes 312 and 313 of the peak and bottom hold 
circuits 318 and 319. 
Furthermore, the shown modification employs a by-pass circuit by-passing 
the traverse signal eliminating circuit to directly feed the focus error 
signal to the phase compensation circuit. A switch 333 is disposed in the 
by-pass circuit for establishing and blocking the by-pass circuit. The 
position of the switch 333 is controlled by a gate signal of an OR gate 
336. The OR gate is connected to one input terminal 334, to which a 
tracking ON state indicative signal is applied. The OR gate 336 is 
connected to the other input terminal 335, to which a focus search state 
indicative signal is applied. The OR gate 336 is responsive one of the 
tracking ON state indicative signal and the focus search state indicative 
signal to operate the switch 333 to the closed position for establishing 
the by-pass circuit. 
As will be appreciated, a by-pass circuit with the switch 333 and the OR 
gate 336 to control the switch position between open and closed positions, 
will serve as a control circuit equivalent to that illustrated in FIG. 5, 
as the control circuit 125. 
Therefore, in the embodiments of FIGS. 13 and 15, influence of the traverse 
signal can be successfully avoided by holding the peak and bottom values 
and obtaining average values thereof. 
While the present invention has been disclosed in terms of the preferred 
embodiment in order to facilitate better understanding of the invention, 
it should be appreciated that the invention can be embodied in various 
ways without departing from the principle of the invention. Therefore, the 
invention should be understood to include all possible embodiments and 
modifications to the shown embodiments which can be embodied without 
departing from the principle of the invention set out in the appended 
claims.