JPH0475579B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a tracking servo system used in a disc playback device, and particularly to improvements in its signal switching circuit. [Technical Background of the Invention] As is well known, in the fields of audio equipment, video equipment, etc., digital audio disk (DAD) playback devices, video disk playback devices, and the like are being actively developed. Among these disc playback devices, non-contact ones that use an optical pick-up in particular can move the pick-up itself freely in the radial direction (tracking direction) of the disc, so it is possible to read the desired data. This has the advantage that a search operation for high-speed selection can be easily performed. Here, the above-mentioned search operation is generally performed by a so-called lens kick, which moves the objective lens as a signal detection section in the pickup by a predetermined amount using a kick pulse. A major problem is how to quickly stop the movement of the beam spot and quickly converge the beam spot onto a desired track. Therefore, conventionally, after the search operation is completed, the tracking error signal is given a hysteresis characteristic such that a part of the acceleration region with respect to the objective lens becomes a deceleration region. Here, FIG. 5 shows tracking servo means and focus servo means in a conventional DAD reproducing apparatus, and a three-beam type optical pickup is used. That is, a main light beam and a pair of sub light beams reflected by a disk (not shown) are received by a main light detection section 11a and a pair of sub light detection sections 11b and 11c, respectively, in the optical pickup. Here, the main light detection section 11a is divided into four light receiving areas by substantially orthogonal boundary lines. Then, the output signals A and C and the output signals B and D from the light-receiving areas facing each other in point symmetry are added together and supplied to the current-voltage converters 12a and 12b, respectively. Here, the output signals from the current-voltage conversion sections 12a and 12b are calculated by the focus error detection section 13, and the level is adjusted by the level adjustment section 14, so that the focus becomes (A+C)-(B+D). An error signal (FE) is generated.
This focus error signal (FE) is sent to a focus actuator coil 18 for moving an objective lens (not shown) in the focus direction via a switch circuit 15, a phase compensation circuit 16, and an amplifier circuit 17, which will be described later. The focus servo is applied here. Further, the output signals from the current-voltage conversion sections 12a and 12b are added together in the calculation section 19, and an RF signal (RF) of A+B+C+D is generated. This RF signal (RF) is supplied to a data slice circuit (not shown) via the connection terminal 20, and is subsequently subjected to reproduction processing. In addition, the above RF signal (RF) is
The signal is supplied to a peak detection circuit 21, which will be described later, and is also supplied to a focus OK detection section 22 via a coupling capacitor (Ca). This focus OK detection unit 22 determines that the objective lens is located near the in-focus position when the level of the RF signal (RF) becomes higher than a predetermined value, and outputs a focus OK signal (FOK). This is what is output. Then, the switch circuit 15, while this focus OK signal (FOK) is being output,
The focus error signal (FE) is guided to the phase compensation circuit 16, and the output of the oscillator 23 is adjusted in phase when the focus OK signal (FOK) is not output, that is, when the objective lens is not located near the in-focus position. It is switched so as to lead to the compensation circuit 16. Therefore, at the start of playback, the oscillator 23
The objective lens is forcibly moved in the focus direction with the output of It will come to pass. Further, the focus OK signal (FOK) is supplied to the arithmetic processing circuit 25 via a note circuit 24a in a timing control logic circuit 24, which will be described later. When a search operation is requested from an operation unit (not shown) while a focus OK signal (FOK) is being generated, this arithmetic processing circuit 25 calculates the track jump amount, direction, etc., and generates a corresponding kick pulse. signal KP ₁ ,
It generates and outputs KP ₂ . On the other hand, the output signals E and F from the sub-light detection sections 11b and 11c are transmitted to the current-voltage conversion section 12, respectively.
c and 12d, the tracking error detection unit 26 calculates the tracking error signal (TE) as E-F. This tracking error signal (TE) is passed through a tracking error signal processing circuit 27 (details will be described later), then an adder circuit 28 and a phase compensation circuit 2.
9 and an amplifier circuit 30, the signal is supplied to a tracking actuator coil 31 for moving the objective lens in the tracking direction, where tracking servo is applied. Here, the tracking error signal processing circuit 27 causes the tracking error signal (TE) to have hysteresis characteristics and to make a part of the acceleration region a deceleration region at the end of the search operation. (TE). That is, in a normal reproduction state, the tracking error signal (TE) is transmitted through the switch circuit 27a, the hold circuit 27b, and the amplifier circuit 27.
c, level adjustment circuit 27d and switch circuit 27
The signal is supplied to the adder circuit 28 via e. Further, during a search, the kick pulse signals KP ₁ and KP ₂ outputted from the arithmetic processing circuit 25 are supplied to the kick pulse generation circuit 32 to generate a kick pulse (KPO). This kick pulse (KPO) is transmitted to the adder circuit 28 and the phase compensation circuit 2.
9 and an amplifier circuit 30 to a tracking actuator coil 31, where track jumping, that is, a search operation is performed. At this time, the above-mentioned hard pulse signals KP ₁ , KP ₂
is connected to the set-reset flip-flop circuit (hereinafter referred to as S-RFF circuit) 2 via the OR circuit 24b and NOT circuit 24c in the logic circuit 24.
Since it is supplied to the set input terminal S of 4d, the S-
The RFF circuit 24d is in a set state, and its output terminal Q is at H level. And this S
- When the output terminal Q of the RFF circuit 24d is at H level, the switch circuit 27e is switched to the ground side, and during a search, the tracking servo is turned off and the objective lens is driven only by the kick pulse (KPO). It is a matter of fact. Further, the peak detection circuit 21 extracts an envelope component of the RF signal (RF),
This RF envelope signal is supplied to the on-track detection section 33. This on-track detection section 33
holds the peak value and bottom value of the RF envelope signal, generates a signal at the midpoint level between both hold levels, and compares this midpoint level signal and the RF envelope signal to the level comparator 33.
By comparing the levels at a, a pulse-like on-track signal (ONTR) is generated. On the other hand, the tracking error signal (TE) output from the tracking error detection section 26 is
Tracking error zero cross edge detection section 34
is supplied to The tracking error zero cross edge detection unit 34 generates a differential waveform every time the tracking error signal (TE) crosses the zero level, and shapes the waveform into a pulse to generate a zero cross edge pulse (TEG). It is. The on-track signal (ONTR), zero-crossing edge pulse (TEG), etc. are supplied to the logic circuit 24, whereby a control signal (SHSW) is generated and the switch circuit 27
a is now controlled to turn on and off. In this case, the switch circuit 27a outputs the control signal (SHSW)
It is assumed that it is in the on state when it is at the H level, and is in the off state when it is at the L level. Here, the operation will be explained with reference to the timing chart shown in FIG. 6. First, during the search operation when the kick pulse signals KP ₁ and KP ₂ are at H level, the output terminal Q of the S-RFF circuit 24d is is H
level, the tracking servo is turned off, and a control signal (SHSW) synchronized with the on-track signal (ONTR) is generated. Then, the hard pulse signals KP ₁ and KP ₂
When both become L level and the search operation is completed, the next zero cross edge pulse (TEG)
In synchronization with this, the output terminal Q of the S-RFF circuit 24d becomes H level, and the tracking servo is turned on. Then, the next fall of the control signal (SHSW), which turned on the tracking servo,
In other words, when the tracking error signal (TE) is at its peak value, the switch circuit 27a is turned off, and the peak value is held in the hold circuit 27b, resulting in the tracking error signal (TEO).
It is output to the adder circuit 28 as an addition circuit. This hold state continues until the next rise of the control signal (SHSW), that is, when the tracking error signal (TE) is at the other peak value, and at this point, the tracking error signal (TE) is in the acceleration region. The objective lens is provided with a hysteresis characteristic so that a part thereof becomes a deceleration region, so that the objective lens can be stopped rapidly. Note that after the tracking servo is turned on, the period during which the control signal (SHSW) can alternately take H level and L level, that is, the period during which hysteresis characteristics can be given to the tracking error signal (TE), is determined by the resistors 24e, Time constant of the time constant circuit consisting of capacitor 24f and knot circuit 24g, 24h
is defined by the threshold voltage _VH . This operation can be easily understood from the waveform shown in FIG. 6, where the anode voltage of the diode 24i is (C-HYS) and the output of the knot circuit 24g is (HYS). Therefore, in a normal reproduction state, the output (HYS) of the note circuit 24g is set to H level, and the switch circuit 27a is kept in the on state. Further, FIG. 7 shows a timing chart when the lens is tilted in the opposite direction to that shown in FIG. 6, and since it is substantially the same as FIG. 6, the explanation thereof will be omitted. [Problems with the Background Art] However, in the conventional tracking servo system as described above, the switch circuit 27a and the tracking servo are turned on and off to give the tracking error signal (TE) a hysteresis characteristic.
A switch circuit 27e and the like for turning off the switch are required, and an amplifier circuit 27c and an adder circuit 2 are also required.
Since the operational amplifiers OP ₁ , OP ₂ , and OP ₃ are used for the 8 and the hard pulse generation circuit 32, the configuration is complicated, and especially when integrated circuit (IC) is implemented, the chip size becomes large. There is. Furthermore, since the number of stages of operational amplifiers OP ₁ , OP ₂ , and OP ₃ is large, there is a problem that a large number of offset voltage components are added to the tracking error signal (TE) or kick pulse (KPO) supplied to the phase compensation circuit 29. There is also. [Object of the Invention] The present invention has been made in consideration of the above circumstances, and provides an extremely good signal switching circuit for a tracking servo system that has a simple configuration, reduces offset voltage, and is especially suitable for IC implementation. The purpose is to [Summary of the Invention] That is, the signal switching circuit of the tracking servo system according to the present invention includes a signal detection section facing the signal recording surface of the disk, and a tracking error signal according to the output signal from the signal detection section. a tracking error signal generation section that generates a tracking error signal; an amplification section that amplifies the tracking error signal output from the tracking error signal generation section; and a signal detection section that generates a tracking error signal according to the tracking error signal output from the amplification section. In the tracking servo system, the tracking servo system includes a drive section that performs tracking servo on the disk, and a tracking error signal outputted from the tracking error signal generation section in the playback state of the disk is transmitted to the disk at a stage before the amplification section. The level is adjusted so that the tracking servo gain corresponding to the reproduction state of the signal is obtained in the drive section, and the level is adjusted to be guided to the amplification section, and the tracking error signal generation section generates a tracking error signal in response to a signal requesting a change in the tracking servo gain. The level of the output tracking error signal is adjusted so that the required tracking servo gain can be obtained in the drive section, and the level is adjusted and guided to the amplifier section, and the tracking error signal is adjusted in accordance with a signal requesting a track jump. Instead, by providing a switching circuit that switches the track skipping signal to the amplification section, the offset voltage can be reduced with a simple configuration, and especially the IC
It is designed to be suitable for various applications. [Embodiment of the Invention] Before describing an embodiment of the present invention, the basic configuration of a tracking control circuit for converging an objective lens after a search operation to which this invention is applied will be briefly explained. I'll leave it there. That is, in FIG. 8, parts and signals that are the same as those in FIG. This is the input terminal that is supplied with. This input terminal 35 is connected to an operational amplifier OP _a and resistors R ₁ to R _{3 .}
and an amplifier circuit 37 consisting of a switch circuit 36,
It is connected to an input terminal 39 of the adder circuit 28 via a switch circuit 38 in series. Here, the switch circuit 36 is controlled to be in an on state when a signal (TSGUP) to be described later is at an H level, and to be in an off state when it is at an L level, thereby switching the amplification factor (gain) of the amplifier circuit 37. It is something. That is, when the switch circuit 36 is in the OFF state, the amplification factor is R ₁ +R ₃ /R ₃ , and when the switch circuit 36 is in the ON state, the amplification factor is R ₁ +R ₂ R ₃ /R ₂ R ₃ , and the switch circuit 36 is in the OFF state. 36 is in the off state. The tracking servo gain is changed by changing the amplification factor of the amplifier circuit 37 that amplifies the tracking error signal (TE). Further, the switch circuit 38 switches to the ground side when a signal (TSGOF), which will be described later, is at H level.
When the level is L, the output of the amplifier circuit 37 is input to the input terminal 3.
9. In other words, the tracking servo loop is opened and closed to control the tracking servo into the off state and on state. Further, the input terminal 35 is connected to the zero cross edge detection circuit 4 of the tracking error signal (TE).
It is connected to the input terminal of 0. This zero cross edge detection circuit 40 includes level comparators 40a, 40
b, constant voltage sources 40c, 40d and OR circuit 40e
It is composed of The output voltage level of the constant voltage sources 40c and 40d is the zero cross level of the tracking error signal (TE),
In other words, the on-track level is set to about 1/8 to 1/4 of the positive and negative peak levels. Here, as shown in FIG. 9, the level comparators 40a and 40
If the voltage levels at the inverting input terminal (-) and non-inverting input terminal (+) of the input terminal b are V _TH+ and V _TH- , respectively, the outputs CMP a and CMP _a of each level comparator 40a and 40b are respectively
CMP _b is as shown in FIG. Therefore, the OR circuit 40e generates a zero-cross edge pulse (TEG) that generates an L-level pulse every time the tracking error signal (TE) crosses the zero level.
is what is output. Then, as shown in FIG. 8 again, the kick pulse signal outputted from the arithmetic processing circuit 25
KP ₁ , KP ₂ , the on-track signal (ONTR) and the zero-cross edge pulse (TEG) output from the on-track detection section 33 are sent to the buffer circuit 41.
a to 41c, OR circuits 41d to 41h, not circuits 41i, 41j, and S-RFF circuit 41k
The signals (TSGUP) and (TSGOF) are generated under the control of a logic circuit 41 consisting of the following. The operation of the basic configuration as described above will be explained below with reference to the timing chart shown in FIG. Here, signal KP ₁ for Kitsukuku pulse
The track jumping operation will be described in the case where the signal KP2 first becomes an H level, and in synchronization with this inversion to an L level, the kick pulse signal _KP2 becomes an H level. That is, in a state where the kick pulse signal KP ₂ is still at H level before time T ₁ in FIG. 10, the set input terminal S and reset input terminal R of the S-RFF circuit 41k are at L level and H level, respectively. It's summery. For this reason, S-
Since the output terminal Q of the RFF circuit 41k is at H level and the output of the buffer circuit 41c, that is, the signal (TSGOF) is at H level, the switch circuit 38 is switched to the ground side, and a tracking error signal (TEO) is generated. This is something that has not been done yet. That is, during track jumping, the tracking servo is stopped. In this state, when the kick pulse signal _KP2 is inverted to L level at time _T1 , the set input terminal S of the S-RFF circuit 41k becomes H level in synchronization with this.
However, its output terminal Q is held at H level. After that, S-RFF is activated during the L level period of the on-track signal (ONTR).
A zero cross edge pulse (TEG) is supplied to the set input terminal S of the circuit 41k, and the S-RFF circuit 41k is activated during the H level period of the on-track signal (ONTR).
A zero-crossing edge pulse (TEG) is now supplied to the reset input terminal R of. For this reason, S
-RFF circuit 41k is reset to L level at times T ₂ , T ₅ , and T ₈ and set to H level at times T ₄ and T ₇ . Here, the output of the S-RFF circuit 41k, that is, the signal (TSGOF), has an L level period of
In this case, corresponding to the deceleration region of the tracking error signal (TE) with respect to the moving direction of the objective lens,
H level period is tracking error signal (TE)
It corresponds to the acceleration region of During the H level period of the signal (TSGOF), since the switch circuit 38 is switched to the ground side, the tracking error signal (TH) is not outputted to the input terminal 39 (0 [V] level). Also, during the L level period of the signal (TSGOF),
A tracking error signal (TE) output from the amplifier circuit 37 is guided to the input terminal 39. During the period when this signal (TSGOF) is at the L level and the on-track signal (ONTR) is at the L level (times _T3 to _T4 , _T6 to _T7 ), the output of the note circuit 41j, that is, the The signal (TSGUP) becomes H level and switch circuit 3
6 is turned on, and the amplification factor of the amplifier circuit 37 is increased. Therefore, after time _T1 , the input terminal 39 receives a tracking signal that increases the tracking servo gain in a part of the deceleration region and turns off the tracking servo in the acceleration region, as shown in FIG. An error signal (TEO) can now be obtained. Then, when the objective lens is converged and the zero cross edge pulse (TEG) is held at the L level, both the signals (TSGOF) and (TSGUP) are stabilized at the L level, and from then on, tracking is performed using the normal tracking servo gain. Servo is applied. Therefore, after the search is completed, the tracking servo gain is increased in part of the deceleration region, and the tracking servo is turned off in the acceleration region.
The objective lens can be rapidly and stably focused on the on-track position. FIG. 11 shows an embodiment in which the circuit shown in FIG. 8 is applied to an actual tracking servo means. In this case, a level adjustment circuit 4 is provided between the amplifier circuit 37 and the switch circuit 38.
2 is interposed. Further, the arithmetic processing circuit 2
Kick pulse signals KP ₁ and KP ₂ output from 5
are the OR circuits 41l, 41 in the logic circuit 41.
m and knot circuits 41n and 41o, the signal
KP ₁ ′, KP ₂ ′, and the hard pulse generation circuit 3
2. Furthermore, the focus OK signal (FOK) output from the focus OK detection section 22 is supplied to the OR circuit 41d of the logic circuit 41. The output of this OR circuit 41d is the time constant circuit 4.
It is used to turn on and off the switch circuit 43a of No. 3. In other words, this switch circuit 43a
is off when the output of the OR circuit 41d is at L level, and is on when the output of OR circuit 41d is at H level. The time constant circuit 43 allows the tracking error signal (TEO) to be generated for a predetermined period of time after the kick pulse signals KP ₁ and KP ₂ both become L level, and after the predetermined period of time has elapsed. , the switch circuit 36 is restricted from being turned on. The predetermined time of the time constant circuit 43 is defined by the output current of the constant current source 43b, the capacitance of the capacitor 43c, and the output V _T of the constant voltage source 43e applied to the level comparator 43d. Here, FIGS. 12 and 13 show specific timing charts of each part of FIG. 11 depending on the direction of the lens angle. Note that the voltage level of the non-inverting input (+) of the level comparator 43d is
Let C _GUP be the signal at the output end. With the basic configuration as described above, one embodiment of the present invention will be described in detail below with reference to the drawings. That is, in FIG. 1, 44 is an input terminal to which a tracking error signal (TE) outputted from the tracking error detection section 26 is supplied. This input terminal 44 is connected to a resistor.
It is connected to the first fixed terminal 46a of the switching circuit 46 via a level adjustment section 45 made up of R ₁₁ and R ₁₂ , and is also directly connected to the second fixed terminal 46b of the switching circuit 46. Further, the third fixed terminal 46c of this switching circuit 46 is grounded via a resistor _R13 , and is also connected to a connection point between constant current sources 47 and 48. These resistor _R13 and constant current sources 47 and 48 constitute the kick pulse generating circuit 32 together with an amplifier circuit 49 which will be described later. Further, the common terminal 46d of the switching circuit 46 is
It is connected to the input terminal 51 of the phase compensation circuit 29 via the amplification circuit 49 comprising an operational amplifier 50 and resistors R ₁₄ and R ₁₅ . Here, the constant current sources 47 and 48 are driven to output a constant current when the signals KP ₁ ′ and KP ₂ ′ are at H level, respectively, and the current obtained from the connection point flows through the resistor. It is converted into a voltage level by R ₁₃ and then guided to the amplifier circuit 49 . The switching circuit 46 has its common terminal 4
A movable contact piece 46e that is always in contact with 6d is
Each of the aforementioned signals (TSGUP), (TSGOF), KP ₁ ′,
Predetermined fixed terminals 46a to 46c according to KP ₂ '
The switch is controlled so that it is connected to the That is, each signal (TSGUP), (TSGOF),
When both KP ₁ ′ and KP ₂ ′ are at the L level, that is, when normal reproducing operation is being performed, the movable contact piece 46e of the switching circuit 46 is connected to the first fixed terminal 46a. Therefore, the tracking error signal (TE) supplied to the input terminal 44 is transmitted to the level adjustment section 45.
After level adjustment, the signal is amplified by the amplifier circuit 49 and supplied to the phase compensation circuit 29 via the input terminal 51, whereupon stable tracking servo is performed. Further, when only the signal (TSGUP) is at H level, that is, when the tracking servo gain is increased as described above in a part of the deceleration region of the tracking error signal (TE), the movable contact piece 46e of the switching circuit 46 is It is connected to the fixed terminal 46b of.
Therefore, the tracking error signal (TE) supplied to the input terminal 44 is directly guided to the amplifier circuit 49 without level adjustment, and the tracking servo gain is increased. Furthermore, when the signal (TSGOF) is at H level, that is, in the state where the tracking servo is off (including the search state), the movable contact piece 46e of the switching circuit 46 is
is connected to the fixed terminal 46c. Therefore, the tracking error signal (TE) is not supplied to the amplifier circuit 49, and the tracking servo is turned off. Furthermore, in the search operation state where either the signals KP ₁ ′ or KP ₂ ′ is at H level, the amplifier circuit 49 converts the current corresponding to the signals KP ₁ ′ or KP ₂ ′ into a voltage level by the resistor R ₁₃ . A kick pulse (KPO) is generated and lens kick is performed. Here, the following table shows each signal (TSGUP) mentioned above,
(TSGOF), KP ₁ ', KP ₂ ' and the selected fixed terminals 46a to 46c of the switching circuit 46 are summarized.

〔Effect of the invention〕

Therefore, as detailed above, according to the present invention, the offset voltage can be reduced with a simple configuration, and
In particular, it is possible to provide an extremely good signal switching circuit for a tracking servo system that is suitable for IC implementation.

[Brief explanation of the drawing]

FIG. 1 is a block circuit configuration diagram showing an embodiment of a signal switching circuit of a tracking servo system according to the present invention, and FIGS. 2 and 3 are timing charts for explaining the operation of the same embodiment, respectively.
FIG. 4 is a circuit configuration diagram showing the same embodiment constructed with specific circuit elements, FIG. 5 is a block circuit diagram showing a conventional tracking servo means, and FIG.
7 and 7 are timing charts for explaining the operation of the conventional tracking servo means, respectively, FIG. 8 is a block circuit configuration diagram showing the basic configuration of a tracking control circuit to which the present invention is applied, and FIG. 9 10 is a timing chart for explaining the operation of the zero cross edge detection circuit of the tracking control circuit, and FIG. 10 is a timing chart for explaining the operation of the tracking control circuit.
FIG. 11 is a block circuit configuration diagram showing an embodiment in which the tracking control circuit is applied to an actual tracking servo means, and FIGS. 12 and 13 are timing diagrams for explaining the operation of the embodiment, respectively. It's a chat. 11... Photodetection section, 12... Current-voltage conversion section, 13
...Focus error detection unit, 14...Level adjustment unit, 15...Switch circuit, 16...Phase compensation circuit,
17... Amplification circuit, 18... Focus actuator coil, 19... Arithmetic unit, 20... Connection terminal, 21
...Peak detection circuit, 22...Focus OK detection section, 23...Oscillator, 24...Logic circuit, 25...
Arithmetic processing circuit, 26... Tracking error detection section, 27... Tracking error signal processing circuit, 2
8...Addition circuit, 29...Phase compensation circuit, 30...Amplification circuit, 31...Tracking actuator coil, 32...Kick pulse generation circuit, 33...On-track detection section, 34...Tracking error zero cross edge detection section, 35...Input Terminal, 36...Switch circuit, 37...Amplification circuit, 38...Switch circuit, 39...Input terminal, 40...Zero cross edge detection circuit, 41...Logic circuit, 42...Level adjustment circuit, 43...Time constant circuit, 44...Input terminal , 45
...Level adjustment section, 46...Switching circuit, 47, 48...
constant current source, 49... amplifier circuit, 50... operational amplifier,
51...Input terminal.

Claims (1)

[Claims] 1. A signal detection section facing the signal recording surface of the disk, a tracking error signal generation section that generates a tracking error signal according to an output signal from the signal detection section, and a tracking error signal generation section that generates a tracking error signal according to an output signal from the signal detection section. Tracking comprising: an amplification section that amplifies a tracking error signal output from a signal generation section; and a drive section that performs tracking servo on the signal detection section according to the tracking error signal output from the amplification section. In the servo system, the tracking servo gain corresponding to the playback state of the disk drives the tracking error signal output from the tracking error signal generation unit in the playback state of the disk, upstream of the amplification unit. The level of the tracking error signal output from the tracking error signal generation section is adjusted in response to a signal requesting a change in the tracking servo gain, and the tracking error signal is outputted from the tracking error signal generation section to the tracking servo gain. The level is adjusted so that a king servo gain is obtained in the drive section, and the signal is guided to the amplifier, and in response to a signal requesting track skipping, a track skipping signal is guided to the amplifier instead of the tracking error signal. A signal switching circuit for a tracking servo system, comprising a switching circuit for switching.