JP2000331425A - Ac coupling circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the time required for a DC pull-in without generating a DC step by putting a time constant to a minimum at the instant when a switching is made from a low speed AC coupling to a high speed AC coupling and then, gradually increasing the time constant so that it becomes the time constant of the low speed AC coupling. SOLUTION: A control signal processing circuit 2 differentiates control signals to be supplied to a control terminal Tc with a high pass filter(HPF) 21. The differentiated signals are half wave rectified by a half wave rectifying circuit 22 to produce second control signals. Then, a transconductance value gm1 is changed in proportion to the second control signals so as to control the time constant of an HPF1. Thus, the time constant of the HPF1 becomes small at the instant of a DC fluctuation and then, the time constant is gradually increased. Therefore, there exists practically no change in the time constant while a switching is made from a high speed AC coupling to a low speed AC coupling even though the time constant of the HPF1 is made small during a high speed AC coupling for a high speed pull-in.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reproduction signal of a rewritable digital video disc, for example.
The present invention relates to an AC coupling circuit for eliminating a DC step of a reproduced signal having a DC (direct current) price difference.

[0002]

2. Description of the Related Art Rewritable DVD-RAM (digit)
al video disc-randam access memory) For disc playback, AC (AC) coupling is required. Before explaining the reason, the configuration of the DVD-RAM disc signal will be described.

Tracks of a DVD-RAM disk are divided into sectors as shown in FIG. At the head of each sector is a header area, followed by a recording area. An ID (identity) signal is recorded in the header area in advance, and the recording area is an area where a data signal is written. When playing a disc, the pickup moves along this track, irradiates the disc with laser light,
The reflected light of the laser light is detected by a photodiode to obtain a reproduced signal.

A photodiode cell of a pickup is generally divided into four areas as shown in FIG. The ID signal in the header area is arranged with a half track shift as shown in FIG. 10B, and when reading the ID signal, the push-pull signal obtained by calculating (A + D)-(B + C) is used. Used. Since the data signal of the recording area is on the track, a full addition signal obtained by performing the operation of A + B + C + D is used. That is, DVD-RA
When reproducing an M disk, it is necessary to switch and read a push-pull signal and a full addition signal in a header area and a recording area. However, if the signal is simply switched between the header area and the recording area, a reproduced signal having a DC step in the header portion as shown in FIG. Although the signal processing in the subsequent stage is possible as it is, it is necessary to increase the dynamic range of the circuit in the subsequent stage, and other performances such as S / N are sacrificed in order to secure the dynamic range. Therefore, the DC step is removed by the AC coupling circuit, a signal having no DC step as shown in FIG. 12 is generated and output, and the load on the circuit at the subsequent stage is reduced.

FIG. 13 shows a DVD-RAM reproducing system. The pickup 1 applies a laser beam to the disk 2 and detects and outputs the reflected light. The full adder 3 adds the pickup outputs A, B, C, and D and outputs the result to the signal switching circuit 4. Further, the PP signal generation circuit 5 calculates (A + C) −
A (B + D) push-pull signal is generated and output to the signal switching circuit 4. The signal switching circuit 4 selects, based on the control signal, an RF full addition signal when reproducing the data area or a push-pull signal when reproducing the header area, and outputs the signal to the AC coupling circuit 6. The header position detection circuit 7 detects the header position based on the RF full addition signal and the push-pull signal, and outputs a signal switching control signal to the signal switching circuit 4 and a time constant switching control pulse to the AC coupling circuit 6. The AC coupling circuit 6 removes a DC step generated in the header portion and outputs the result to the waveform equalization circuit 8. The waveform equalizing circuit 8 equalizes the waveform of the output of the AC coupling circuit 6 and performs RF equalization.
The output is output to the data slicer 9. Data slicer 9
Is obtained by slicing the RF signal at a certain reference potential and binarizing the signal.
Output to LL10.

The PLL 10 synchronizes the binary data,
Extract the bit clock. The synchronization detection circuit 11 outputs a timing signal for demodulation based on the bit clock.
The decoder 12 demodulates and outputs the binary data based on the timing signal.

Here, a conventional AC coupling circuit will be described with reference to FIG. The transistor Q1 is a saturation switch, and when the control voltage at the control terminal is sufficiently low and the transistor Q1 is OFF, the resistor R2 is in an open state, and the capacitor C1 and the resistor R1 constitute a high-pass filter.

Also, if the control voltage of the control terminal is sufficiently high,
When the transistor Q1 is ON, the resistor R2 is connected in parallel with the resistor R1, forms a high-pass filter with C1 and R1 // R2, reduces the time constant of the high-pass filter, and controls the Lo / Hi of the control signal. Switches the time constant of the high-pass filter.

FIG. 15 shows a case where a DVD-RAM reproduction signal including a header signal is input to the AC coupling circuit.
The signal waveform of each part is shown. The control signal output from the header position detection unit 7 is a signal that becomes Hi for a certain time from the moment of a large DC fluctuation as shown in FIG. In the AC coupling circuit 6,
As described above, the time constant of the AC coupling is switched by Lo / Hi of the control signal, and when Lo, the time constant is increased (low speed A).
C), the time constant is reduced in the case of Hi (high-speed AC
Binding). When a large DC fluctuation occurs, the DC is drawn at high speed, and after sufficiently drawn, the time constant is reduced so as not to follow the AC component or noise component superimposed on the input signal.

By switching the time constant of AC coupling in this way, large DC fluctuations can be absorbed. However, in a DVD-RAM reproducing system, 15 μs
It is necessary to operate including the PLL within c.
Has become more difficult due to the expansion of the operation range, and it has become necessary to shorten the DC pull-in time of AC coupling in order to secure the PLL pull-in time.

However, in the conventional AC coupling system, the DC coupling time of the AC coupling cannot be shortened. Because, in order to make DC pull of high-speed AC coupling fast,
The time constant at the time of high-speed AC coupling may be reduced. But,
If the time constant of the high-speed AC coupling is reduced, the charge flowing into and out of the capacitor increases due to the AC component superimposed on the input signal, and a DC offset occurs due to the charge accumulated in the capacitance when switching to the low-speed AC coupling. , A DC step appears.

If a DC step occurs, the distortion of the signal in the portion where the DC step occurs increases, and data may not be read under the worst conditions. Also, with data slicer
When the RF signal is binarized, the jitter increases. The DC step is caused by an excessively large ratio of the time constant between the high-speed AC coupling and the low-speed AC coupling. The only way to prevent this step while keeping the time constant at the time of high-speed AC coupling small is to reduce the time constant at the time of low-speed AC coupling and reduce the ratio of the time constant at the time of high-speed AC coupling to that at low-speed AC coupling. Absent.

However, if the time constant at the time of the low-speed AC coupling is too small, the AC coupling responds to the noise component of the signal at the time of the low-speed AC coupling, and the jitter is affected by the noise when binarizing the data with the data slicer. Therefore, it is difficult to reduce the time constant at the time of low-speed AC coupling.

[0014]

As described above, in the conventional AC coupling circuit, if the time constant at the time of high-speed AC coupling is reduced in order to speed up the direct current pull-in, when switching from high-speed AC coupling to low-speed AC coupling is performed. Causes a DC step in the output. The only way to prevent this is to reduce the time constant at the time of low-speed AC coupling and reduce the ratio of the time constant at the time of high-speed AC coupling to that at low-speed AC coupling. However, if the time constant at the time of low-speed AC coupling is too small, the AC coupling responds to the noise component of the signal at the time of low-speed AC coupling, and when binarizing with a data slicer, jitter increases due to the influence of noise. Therefore, it is difficult to reduce the time constant at the time of low-speed AC coupling.

According to the present invention, there is provided a DVD capable of shortening a DC pull-in time with almost no DC step.
Providing an AC coupling circuit for the RAM;

[0016]

In order to solve the above-mentioned problem, the AC coupling circuit according to the present invention minimizes the time constant at the moment when switching from low-speed AC coupling to high-speed AC coupling is performed, The time constant is gradually increased until the time constant of.

According to the above-mentioned means, the time constant of the AC coupling hardly changes when switching from the high-speed AC coupling to the low-speed AC coupling, so that a DC step is not generated and the time required for direct current pull-in can be reduced.

[0018]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a block diagram for explaining a first embodiment of the present invention, and FIG. 2 is a signal waveform of each part of the block diagram.

In FIG. 1, reference numeral 1 denotes a high-pass filter for AC coupling, which removes a DC component of an input signal before AC coupling and outputs the signal. The high-pass filter 1 has an input terminal V
i, one end of the resistor R1 is connected, and the other end is connected to the output terminal Vo.
And a non-inverting input of a transconductance amplifier GM1 having an inverting input connected to the reference potential Vref and an output of a transconductance amplifier GM2 having an inverting input grounded are connected to the output terminal Vo. The output of the transconductance amplifier GM1 is grounded via the capacitor C1, and is connected to the non-inverting input of the transconductance amplifier GM2. The high-pass filter 1 performs feedback so that the output DC voltage becomes the reference potential Vref.

The control signal processing circuit 2 has a control terminal Tc
Is connected to the input of the high-pass filter 21, and its output is connected to the input of the half-wave rectifier circuit 22. The output of the half-wave rectifier circuit 22 controls the transconductance amplifier GM1 of the high-pass filter 1. The control signal processing circuit 2 converts the control signal supplied to the control terminal Tc into a high-pass filter 21.
, And the half-wave rectification circuit 22 half-wave rectifies the differentiated signal to generate a second control signal. The time constant of the high-pass filter 1 is controlled by the second control signal.

As shown in FIG. 2, the control signal is a large DC
This signal is Hi for a certain period of time from the moment of the fluctuation. The control signal processing circuit 2 generates a second control signal that rapidly increases from the control signal at the moment of the DC fluctuation and then gradually decreases, and outputs the second control signal to the high-pass filter 1. The time constant of the high-pass filter 1 is set to DC based on the second control signal.
It suddenly decreases at the moment of the change, and then gradually increases.

As described above, when the time constant of the high-pass filter 1 is controlled, even if the time constant of the high-pass filter 1 at the time of high-speed AC coupling is reduced for high-speed pull-in, the high-speed AC
When switching from the coupling to the low-speed AC coupling, there is almost no change in the time constant of the high-pass filter 1, so that the DC pull-in time can be reduced without generating a DC step.

Here, the time constant of the AC-coupling high-pass filter 1 is gm1 · gm2 · R1 / C1 (1). The transconductance value gm1 of the transconductance amplifier GM2 is proportional to the second control signal.
Is changed to control the time constant of the high-pass filter 1.

As a result, the time constant of the high-pass filter 1 rapidly decreases at the moment of the DC fluctuation, and thereafter gradually increases. Therefore, even if the time constant of the high-pass filter 1 at the time of high-speed AC coupling is reduced for high-speed pull-in, there is almost no change in the time constant of the high-pass filter 1 when switching from high-speed AC coupling to low-speed AC coupling. The DC pull-in time can be shortened without any occurrence.

FIG. 3 is a circuit diagram for explaining a second embodiment of the present invention. That is, one end of the capacitor C1 is connected to the input terminal Vi, the other end is connected to the output terminal Vo, and one end of the resistor R1 is grounded.
Is connected to the collector of the transistor Q1 which is grounded.

The control terminal Tc is connected to one end of each of the resistors R2 and R3 via the capacitor C2. The other end of the resistor R3 is grounded, and the other end of the resistor R2 is connected to the base and collector of the transistor Q2 whose emitter is grounded. The base of transistor Q2 is commonly connected to the base of transistor Q1.

Here, the output resistance Ro of the transistor Q1
Has the relationship Ro = Vk / I (2) where I is the collector current of the transistor Q1. Here, Vk is a voltage which is a constant peculiar to the transistor.

When the low-speed AC coupling is performed, that is, when the control signal is Lo, the transistor Q2 is turned off. Therefore, the transistor Q1 forming a current mirror with the transistor Q2 is also turned off, and the collector current I of the transistor Q1 becomes 0.
And the output resistance Ro becomes infinite, and the high-pass filter is composed of the capacitor C1 and the resistor R1.

Conversely, when the transistor Q1 is turned on, the time constant of the high-pass filter is determined by the resistance Ro determined by the equation (2), the parallel resistance of the resistance R1 and the capacitor C1. When a pulse as shown in FIG. 4 is input as a control signal, a signal shown in FIG. 4 is obtained at V1 by a high-pass filter composed of a capacitor C2 and a resistor R3. This V1
Flows through the resistor R1 and the diode of the transistor Q2, so that the collector current of the transistor Q2 is I2,
Since I2 · R2 + Vbe (Q2) = V1, I2 = (V1−Vbe (Q2)) / R2 (3), and the base-emitter voltage V of the transistor Q2 is
Since the change due to the collector current I2 is small and can be considered as a fixed voltage, be (Q2) increases as the control signal pulse becomes Hi as shown in FIG. 4 and gradually decreases with time. It becomes a current. Since transistors Q1 and Q2 constitute a current mirror, I
1 = I2. At the moment when the control signal becomes Hi, the transistor Q1 is turned on, the value of the collector current I1 of the transistor Q1 becomes the largest, the resistance Ro becomes the minimum according to the relationship of the equation (2), and the high-pass filter 1 for the AC coupling is used.
Is the minimum time constant.

Thereafter, as the collector current I1 decreases, the resistance Ro gradually increases, and the time constant of the high-pass filter 1 increases. Eventually, I
When 1 = 0, Ro = infinity, the resistance R1 and the capacitor C
The time constant of the high-pass filter 1 is determined by 1 alone. Therefore, even in this case, the time constant of the AC coupling changes to a minimum at the moment when the control signal becomes Hi, and to gradually increase thereafter.

Also in this example, even when the time constant of the high-pass filter 1 at the time of high-speed AC coupling is reduced for the high-speed pull-in as described above, the time of the high-pass filter 1 at the time of switching from high-speed AC coupling to low-speed AC coupling is reduced. Since there is almost no constant change, the DC pull-in time can be shortened without generating a DC step.

FIG. 5 is a circuit diagram for explaining a third embodiment of the present invention. In FIG. 5, one end of a capacitor C1 is connected to the input terminal Vi, the other end is connected to the output terminal Vo, and one end is connected to the other end of the grounded resistor R1. Further, a resistor R2 and a switch S1, and a resistor R3 and a switch S are connected between the output terminal Vo and the ground.
2. Connect the resistor R4 and the switch S3 in parallel.

Tc is a control terminal. The switch S1 is connected from the control terminal Tc via the capacitor C4 to the capacitor C4.
3 and a control signal for controlling the switch S1 via the capacitor C2. A resistor R5 is provided between the connection point of the capacitor C2 and the switch S3 and the ground, a resistor R6 is provided between the connection point of the capacitor C3 and the switch S2 and the ground, and the capacitor C4 is connected to the switch S3.
A resistor R7 is connected between the connection point 1 and the ground.

When the low-speed AC coupling is performed, that is, when the control signal is Lo, the switches S1 to S3 are all open, and the high-pass filter includes a capacitor C1 and a resistor R1. Next, at the moment when the control signal becomes Hi, the switches S1 to S3 are all closed, and the high-pass filter is constituted by the parallel resistance of the capacitor C1 and the resistors R2 to R4, and the time constant of the high-pass filter becomes the smallest. .

Thereafter, as time passes, the switches S1 to S1
S3 opens in order, and the time constant of the high-pass filter increases stepwise as shown in FIG.
It converges to the time constant at the time of C coupling. In order to open the switches S1 to S3 over time, the capacitors C2 to C4
And the resistors R5 to R7 constitute a control signal processing circuit.
Capacitor C2 and resistor R5, capacitor C3 and resistor R
6, the capacitor C4 and the resistor R7 constitute high-pass filters having different time constants, respectively, and have different responses to control signals as shown in FIG. V1 is for switch S1,
V2 is a control signal for the switch S2, and V3 is a control signal for the switch S3.

Here, the threshold values for opening and closing the switches S1 to S3 are all the same, and when the control voltage becomes lower than the threshold value, the switches are opened. After the control signal becomes Hi,
The control signal V1 becomes the earliest voltage lower than the threshold value,
Thereafter, the voltages become lower than the threshold value in the order of the control signals V2 and V3. Therefore, the switches are all closed at the moment when the control signal becomes Hi, and then opened in the order of the switches S1 to S3. For this reason, the time constant of the AC coupling changes stepwise as shown in FIG.

When the time constant of the AC coupling is controlled in this way, even if the time constant of the high-pass filter at the time of the high-speed AC coupling is reduced for high-speed pull-in, the time constant increases and then increases stepwise. Therefore, when switching from the high-speed AC coupling to the low-speed AC coupling, the change in the time constant of the high-pass filter can be reduced, and the DC pull-in time can be reduced with almost no DC step.

Here, the step-like time constant change is described as having four stages. However, if the number of switches, the corresponding resistors, and the number of high-pass filters are changed, the number of time constant changes can be set to any number of three or more stages. .

FIG. 8 is a circuit diagram for explaining a fourth embodiment of the present invention. A of this embodiment
The time constant of the C-coupling changes stepwise as in the case of the configuration example of FIG. However, in this example, the high-pass filter for the control signal includes only the capacitor C2 and the resistor R5, and sets the threshold voltages of the switches S1 to S3 to:
Vth1 to Vth3 are changed as shown in FIG.

As a result, similarly to the configuration example of FIG. 5, the switches S1 to S3 switch after the control signal becomes Hi.
By opening the switches S1 to S3 in this order, a stepwise time constant change similar to the configuration example of FIG. 5 can be realized.

[0041]

As described above, the DVD of the present invention
-In the AC coupling circuit of the RAM, the generation of the DC step is suppressed,
DC pull-in time can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram for explaining a first embodiment of the present invention.

FIG. 2 is a waveform chart for explaining the operation of FIG.

FIG. 3 is a circuit diagram for explaining a second embodiment of the present invention.

FIG. 4 is a waveform chart for explaining the operation of FIG. 3;

FIG. 5 is a circuit diagram for explaining a third embodiment of the present invention.

FIG. 6 is a characteristic diagram for explaining the operation of FIG. 5;

FIG. 7 is a waveform chart for explaining the operation of FIG. 5;

FIG. 8 is a circuit diagram for explaining a fourth embodiment of the present invention.

FIG. 9 is a waveform chart for explaining a threshold value of the switch shown in FIG. 8;

FIG. 10 is an explanatory diagram for describing a signal configuration of a DVD-RAM disk.

FIG. 11 is an explanatory diagram for explaining a configuration of a DVD-RAM pickup.

FIG. 12 is a waveform chart for explaining DC step difference removal by conventional AC coupling.

FIG. 13 is a circuit configuration diagram for explaining a conventional AC coupling circuit.

FIG. 14 is a circuit diagram for explaining a conventional AC coupling circuit.

FIG. 15 is a waveform chart for explaining the operation of FIG. 14;

[Explanation of symbols]

1: high-pass filter, 2: control signal processing circuit, Vi:
Input terminal, Vo: output terminal, GM1, GM2: transconductance amplifier, Vref: reference potential, Tc: control terminal, 21: high-pass filter, 22: half-wave rectifier circuit, C1 to C4: capacitor, R1 to R7: resistor, S1
S3: switch, Q1, Q2: transistor.

Continued on the front page F term (reference) 5D044 AB05 AB07 BC06 CC04 FG05 5D090 AA01 BB04 CC04 EE13 5K029 AA01 AA11 BB01 CC07 DD02 HH01 JJ01 LL01

Claims (10)

[Claims] 1. A time constant of a reproduced signal in which an AC component is superimposed on a DC potential that changes with a step-like step is changed by a control signal synchronized with the step of the DC potential to remove the step of the DC potential. An AC coupling circuit according to claim 1, wherein a time constant immediately after the occurrence of the DC potential difference is minimized, and thereafter the time constant is gradually increased to converge to a certain time constant. 2. The method according to claim 1, wherein, after minimizing the time constant, the rate of change of the time constant with respect to time is increased so as to decrease with time and converges to a certain time constant. AC coupling circuit. 3. The AC coupling circuit according to claim 1, wherein, after minimizing the time constant, the time constant is increased stepwise to converge to a certain time constant. 4. A high-pass filter that supplies a reproduced signal in which an AC component is superimposed on a DC potential that changes with a step-like step, removes the DC component of the reproduced signal, and outputs the resulting signal. Process the synchronized control signal to
And a control signal processing circuit that generates a control signal of the above, and an AC coupling circuit that controls a time constant of the high-pass filter based on the second control signal. 5. A high-pass filter that supplies a reproduced signal in which an AC component is superimposed on a DC potential that changes with a step-like step, removes the DC component of the reproduced signal, and outputs the resulting signal. Process the synchronized control signal to
And a control signal processing circuit for generating a control signal of the above, and an AC coupling circuit for controlling a time constant of the high-pass filter and the control signal processing circuit based on the second control signal. 6. The control signal processing circuit comprises: a high-pass filter for differentiating the control signal; and a half-wave rectifier circuit for half-wave rectifying the output of the high-pass filter. 5. The AC coupling circuit according to claim 4, wherein the control signal is two control signals. 7. A first GM that amplifies a difference voltage between two signals, converts the current into a current, and outputs the amplified voltage, using a resistor connected between an input terminal and an output terminal, and the output terminal and a first reference potential point as inputs. An amplifier; a capacitor connected between the output of the first GM amplifier and a second reference potential point; and an input of the output of the first GM amplifier and the second reference potential point to amplify a difference voltage between two signals. The output terminal is connected to a second GM amplifier connected to the output terminal, and the input terminal is connected to a control terminal. The AC component is superimposed on a DC potential that changes with a step-like step. A reproduced signal, a high-pass filter for differentiating a control signal synchronized with the level difference of the DC potential, and a half-wave rectifier circuit for half-wave rectifying the output of the high-pass filter. GM amplifier conductor An AC coupling circuit, wherein the AC coupling circuit controls a sense value. 8. A first capacitor having one electrode connected to an input terminal and the other electrode connected to an output terminal; a first resistor connected between an output terminal and a reference potential point; A first transistor having a collector terminal connected to the output terminal; a second capacitor having one end connected to the control terminal; and a second capacitor connected between the other end of the second capacitor and the reference potential point. A third resistor connected to the other end of the second capacitor and the base terminal of the first transistor; a base terminal and a collector terminal of the first transistor connected to an emitter terminal as a reference potential point; An AC coupling circuit, comprising: a second transistor connected to a base. 9. A capacitor having one end connected to the input terminal and the other end connected to the output terminal, a first resistor connected between the output terminal and a reference potential point, and a connection between the output terminal and the reference potential point. n (n ≧ 2)
And a series circuit of a resistor and a switch, and a control signal synchronized with the step of the DC potential of a reproduced signal in which an AC component is superimposed on a DC potential that changes with a step-like step, and inputs the control signal to the switch. AC switch comprising n (n ≧ 2) high-pass filters for outputting differential signals of the signals, wherein the switches have the same threshold value, and the high-pass filters have different time constants.
Coupling circuit. 10. A capacitor having one end connected to the input terminal and the other end connected to the output terminal, a first resistor connected between the output terminal and a reference potential point, and a connection between the output terminal and the reference potential point. N (n ≧
2) a series circuit of a resistor and a switch, and a control signal synchronized with the step of the DC potential of a reproduction signal in which an AC component is superimposed on a DC potential that changes with a step-like step is input to the switch. An AC coupling circuit comprising a high-pass filter that outputs a differential signal of the control signal, wherein the switches have different threshold values.