JPS6182378A - Drop-out detection circuit - Google Patents

Abstract

PURPOSE:To detect stably a drop-out without erroneous detection by selecting an RF signal subjected to the waveform processing in accordance with the detection of a low frequency information pulse signal overlapped with the RF signal and detecting the drop-out. CONSTITUTION:A low frequency component detection circuit to which a read-out RF signal through an RF amplifier 1 is supplied controls a changeover switch 14 of a waveform processing circuit 12, and the overlapping of a low frequency information pulse signal with the RF signal is not detected. At this time the output of the amplifier 1 is supplied to a drop-out detection circuit of a limiter amplifier, etc., as it is. When the circuit 13 detects the overlapped low frequency signal pulse from the RF signal, the RF signal where the low frequency component for passing through the waveform processing circuit 12 formed with a clip circuit 15, high frequency amplifier circuit 16 and HPF17 is removed is supplied to a limiter amplifier 4. A drop-out is stably detected without an erroneous detection in such a way that the envelope component of the RF signal becomes large and a pickup level will not comes more than and less than a center level.

Description

TECHNICAL FIELD The present invention relates to a dropout detection circuit, and more particularly to a circuit for detecting dropout of an RF (high frequency) signal read from a recording medium on which a plurality of signals are recorded in a superimposed manner.

BACKGROUND ART In an information reading device that reads information from a recording medium such as a video disk, a dropout detection circuit is essential to detect and compensate for dropouts caused by scratches, dirt, etc. on the recording medium. .

FIG. 1 is a block diagram showing an example of a conventional dropout detection circuit, in which the RF signal a read from the recording medium is R
After being amplified by the F amplifier 1, the signal is supplied to a video detection circuit 2 consisting of an FM demodulator and the like, and at the same time, it is supplied to a dropout detection circuit 3. In the dropout detection circuit 3, the RF signal a is amplitude limited by a limiter amplifier 4, and then amplified and converted into a pulse train signal. This pulse train signal is applied to one input terminal of the multiplier 5 and simultaneously delayed by the delay circuit 6 before being applied to the other input terminal of the multiplier 5.

The output C of the multiplier 5 is supplied to a time constant circuit 7 consisting of an integrating circuit and the like. The output voltage d of the time constant circuit 7 is compared with the reference voltage Vr by a comparator 8. When the output voltage d of the time constant circuit 7 becomes equal to or higher than the reference voltage Vr1, the comparator 8 outputs, for example, a high level signal, which becomes the dropout detection signal e. This dropout detection signal e is supplied to a correction command generation circuit 9. The correction command generation circuit 9 is configured to generate a correction command signal f which exists for a predetermined time Tl longer than the dropout detection signal e from the time when the dropout detection signal e is generated. The output of the correction command generation circuit 9 serves as a control input to the selector 10. selector 10
A video signal demodulated by the video detection circuit 2 is directly applied to one input terminal of the video detection circuit 2. Also, selector 1
A video signal that has passed through an IH (one horizontal synchronization period) delay circuit 11 is applied to the input terminal of 0. This selector 10 selectively outputs the video signal that has passed through the IH delay circuit 11 when the correction command signal f is generated, and selects the video signal directly supplied from the video detection circuit 2 when the correction command signal f disappears. It is configured to output the following information.

In the above configuration, the RF signal a has a center frequency of 8, 1
MHz video, FM signal, and center frequency of 2
．． Two audio F of 3 MHz and 2.8 MHz
It is assumed that the signal was obtained from a recording medium on which the 'M signal was recorded in a superimposed manner. The waveform of the RF signal a is as shown in FIG. 2 (8). When this RF signal a is supplied to the limiter amplifier 4 via the RF amplifier 1, the appearance timing of the rising edge and the falling edge is determined by the RF signal a.
A pulse train signal as shown in [F] in the same figure is obtained, which coincides with the zero crossing point of , that is, the point in time when the instantaneous level of the RF signal a becomes equal to the center level (zero level) Vo.

Here, if the signal delay time in the delay circuit 6 is made to be approximately equal to the time width V2 of the positive pulse forming the pulse train signal S, the output g of the delay circuit 6 becomes as shown in the figure (Q). When the pulse train signal S and the output g of the delay circuit 6 are multiplied in the multiplier 5, the appearance timing of both the rising and falling edges of the pulse train signal S is changed from the rising edge to the rising edge, as shown in FIG. A pulse train signal whose appearance timing coincides is formed and is supplied to the time constant circuit 7 as the output C of the multiplier 5.The appearance timing of the falling edge of this output C coincides with the rise of the output g of the delay circuit 6. This coincides with the appearance timing of both the and falling edges.

When the output C of the multiplier 5 becomes a low level, the output voltage d of the time constant circuit 7 starts to rise with a time constant τ as shown in FIG. Further, the output voltage d of the time constant circuit 7 rapidly decreases when the output C of the multiplier 5 becomes high level. Here, the time from when the output voltage d of the time constant circuit 7 starts to rise until it becomes equal to the reference voltage Vr is RF
It is assumed that the time constant τ is set to be longer than the period corresponding to the frequency of the video FM signal component of the signal a. Then, when dropout occurs and no zero-crossing point occurs, as in the period T8 in FIG. It is longer than the period corresponding to the frequency of the video FM signal component. As a result, the output voltage d of the time constant circuit 7 becomes equal to or higher than the reference voltage Vr1 as shown in FIG.

When this dropout detection signal e is supplied to the correction command generation circuit 9, as shown in FIG. A correction command signal f is output.This correction command signal f causes the IH delay circuit 1 to
1 is selectively outputted from the selector 10 to perform dropout compensation.

Incidentally, it has recently been considered to record a PC'M signal consisting of low-frequency components of 1.75 MHz or less, together with audio and video signals, on a recording medium such as a video disk. The read RF multiplied signal is such that the signal level of the envelope component becomes extremely large and the upper peak level is below the center level, or the lower peak level is above the center level. If the doubled signal is supplied to the conventional dropout detection circuit as described above, a zero crossing point will not occur even if dropout does not occur, and a dropout detection signal will be erroneously generated.

In this case, since the conventional dropout compensation force formula only compensates for brightness, there arises a disadvantage that, for example, a highly saturated red color changes to gray, making the reproduced image unsightly.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to prevent false detection even when an RF' signal read from a recording medium on which a low frequency information pulse signal such as a PCM signal is superimposed and recorded together with audio and video signals is input. It is an object of the present invention to provide a dropout detection circuit that can perform stable dropout detection without any problems. RF multiplied signal waveform processing is performed according to this low frequency component detection signal, and a dropout detection signal is generated when the period in which the instantaneous level of the RF signal subjected to waveform processing is equal to a predetermined level is equal to or longer than a predetermined period. It is configured to do this.

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to FIGS. 3 to 8.

As shown in FIG. 3, the dropout detection circuit 3 according to the present invention includes a limiter amplifier 41, a multiplier 5, and a delay circuit 6.
1. The time constant circuit 7 and the comparator 8 are connected in the same way as the circuit shown in FIG. However, in the dropout detection circuit 3 according to the present invention, the RF signal is amplified by the RF amplifier 1 and is supplied to the limiter amplifier 4 via the external waveform processing circuit 12 . Also, at the same time,
This RF signal a is also supplied to the low frequency component detection circuit 13. In the waveform processing circuit 12, the skin F signal a is supplied to the changeover switch circuit 14 and simultaneously supplied to the high frequency amplifier circuit 16 via the clip circuit 15. The high-frequency amplifier circuit 16 has frequency characteristics that selectively amplify only high-frequency components of 1.75 MHz or higher. The output of this high frequency amplifier circuit 16 is the HPF (bypass filter) 17
It becomes the input of the changeover switch 14 via. HPF17
has frequency characteristics that remove low-frequency components such as PCM signal components of 1.75 MHz or less. These high frequency amplifier circuits 16 and HPF 17 form high frequency extraction means, and the clipped R
High frequency components above 1.75 MHz are extracted from the outside of the F signal. This high-frequency component, ie, the output of the I-I P F 17, serves as an input to the changeover switch circuit 14. The changeover switch circuit 14 is configured to selectively output two outputs, that is, the output of the RF signal and the output of the PF17, depending on the state of the control input.
The output of the waveform processing circuit 12 is supplied to the limiter amplifier 4 as the output of the waveform processing circuit 12.

First, in the low frequency component detection circuit 13, the RF signal is de-emphasized through the LPF (low pass filter) 18! ! 19. LPF'1
8 has a frequency characteristic that allows only low frequency information pulse signals such as PCM signals of 1.75 MHz or less to pass through. The RF signal af and nine low frequency information pulse signals are separated and extracted by the LPF 18 and supplied to the de-emphasis circuit 19. In the de-emphasis circuit 19, the components emphasized during recording are restored. The low frequency information signal output from the de-emphasis circuit 19 is processed by an envelope detector 20 consisting of an AM detector or the like.
The envelope component of the low frequency information pulse signal is detected. The envelope component detected by the envelope detector 20 is supplied to a controller 21, and the envelope component of the low frequency information pulse signal is compared with a reference voltage Vr2 in a zero comparator 21.

When the instantaneous level of the envelope component of the pulse signal (low frequency information) exceeds the reference voltage Vr2, the comparator 21 outputs, for example, a high level low frequency component detection signal, which becomes the control input of the changeover switch circuit 14. In the above configuration, when the low frequency component detection signal does not exist, the changeover switch circuit 14 selectively outputs the 9RF signal a, and when the low frequency component detection signal exists, the changeover switch circuit 14 selectively outputs the output of the pHPFL7. It shall be output. Then, if a low frequency information pulse signal such as an RF signal a K P CM signal is not superimposed, a low frequency component detection signal will not be generated, and the RF signal a will be directly supplied to the limiter amplifier 4. Dropout detection is performed in the same way as the circuit in . Still RF signal a
When a PCM signal is superimposed as a low frequency information pulse signal on the BP' signal a, the BP' signal a is sent to the clip circuit 15. When the PCM signal is superimposed on the ORF signal a that is supplied to the limiter amplifier 4 via the high-frequency amplifier circuit 16 and HPF 17, the waveform of the RF signal a becomes as shown in FIG. The peak level is the center level V. The following may occur (time 11). When this RF signal a is supplied to the clip circuit 15, the center level of the RF signal a becomes ■.

The portion exceeding the clip level Vc, which is a predetermined value from Vc, is clipped, and the minute level fluctuations that occurred in the RF' signal a during the period T8 due to dropout are removed, resulting in a clipped signal as shown in FIG. can get. This clip signal passes through the high frequency amplifier circuit 16 and the HPF 17, where low frequency components such as PCM signals are removed, and a signal i as shown in the same figure (Q) is obtained.In this signal i, the clip level Vc of the clip signal Equal interval T
The part corresponding to 8 has the DC component removed and is at the center level■
.

There is one point where the value is equal to , that is, one zero-crossing point. This signal i is supplied to the limiter amplifier 4 via the changeover switch circuit 14, so that the generation period of each pulse forming the output C of the multiplier 5 is changed to the video and FM signal components of the RF signal a in the interval T8. The dropout detection signal e is generated with a period longer than the period corresponding to the frequency, and dropout detection is prevented from occurring.O When the low frequency information pulse signal is not superimposed on the RF signal a, the RF signal a is in the high frequency range. Amplification circuit 16 and HPF 17
Since the signal is directly supplied to the limiter amplifier 4 without going through the filter, low-frequency components are not removed, and dropout detection ability is prevented from deteriorating.

Similarly, when the PCM signal is superimposed on the RF signal a, the RF
It is conceivable to send the signal a to the limiter amplifier 4 after only removing the PCM signal component by passing the signal a through only the high frequency amplifier circuit 16 and HPF 17 without passing through the clip circuit 15.

However, in this case, the minute level fluctuation component caused by dropout in the RF signal a loses its DC component due to the action of the high frequency amplifier circuit 16 and the HPF 17, and the input center level V of the limiter amplifier 4 is reduced. 0 will appear in the vicinity.Then, the frequency of occurrence of the zero-crossing point of the input of the limiter amplifier 4 will be the same as that of the video F of the RF signal a.
If the period is not equal to the frequency corresponding to the frequency of the M signal component, the dropout detection signal e will not be generated, and the detection ability will be reduced.

FIG. 5 is a block diagram showing another embodiment of the present invention, in which a changeover switch 1401 is connected so that the output of the clip circuit 15 is supplied to the RF' signal a which is supplied manually. Except for this part, the other parts are constructed in the same way as the circuit shown in FIG. It is clear that the circuit of FIG. 5 can also provide the same effect as the circuit of FIG. 3.

FIG. 6 is a block diagram showing still another embodiment of the present invention. In the dropout detection circuit 3 in this example, a limiter amplifier 41 and a multiplier 5. Delay circuit 69 Time constant circuit 7. Comparator 8. The waveform processing circuit 12 and the low frequency component detection circuit 13 are connected in the same way as the circuit shown in FIG.

However, in the waveform processing circuit 12 in this example, the RF signal a amplified by the RF amplifier 1 is supplied to the clip circuit 15 and the AQC (automatic gain control) voltage generator 31 via the variable gain amplifier 30. The device 31 is configured to generate an AGC voltage according to the amplitude of the output of the variable gain amplifier 30. This AG
The output voltage of the C voltage generator 31 is supplied to the variable gain amplifier 30 via the DC amplifier 32 as a gain control voltage. The AQC voltage generator 31 and DC amplifier 32 control the amplitude of the output of the variable gain amplifier 30 to be constant. The offset voltage of the DC amplifier 32 is switched and controlled by an offset control circuit 33. The offset control circuit 33 is configured to selectively apply one of two different voltages to the input of the DC amplifier 32, for example, depending on the output of the low frequency component detection circuit 13.

In the above configuration, when the low frequency component detection signal is not present, that is, when the low frequency information pulse signal is not superimposed on the RF signal a, the amplitude of the output of the variable gain amplifier 30 is the RF signal a amplified by the RF amplifier 1. If the offset of the DC amplifier 32 is switched and controlled by the offset control circuit 33 so that the amplitude of
Since the clip level W is constant, the RF' signal can be clipped to the center level. As a result, the dropout detection ability is improved, which is effective when reproducing information from a recording disk with many dropouts. for example,
Variations among recording discs and CAV (Consta
The amplitude of the RF signal a may be small depending on the radial position of the information detection point on a disk using the nt Angular Velocity method, and even if dropout occurs, the instantaneous level of the RF signal a may not reach the clip level c. arise. At this time, if this RF signal a is clipped as it is, j! 315, unnecessary minute level fluctuation components caused by drop alerts will not be removed and the dropout detection ability will deteriorate.

When the low frequency component detection signal is generated, that is, the RF signal a
When the PCM signal is superimposed on the variable gain amplifier 3
RF whose output amplitude is amplified by RF amplifier 1
Offset control circuit 3 so that the amplitude of signal a is smaller than that of signal a.
If the offset of the DC amplifier 32 is switched and controlled by 3, it is possible to prevent the level fluctuation part caused by the superimposition of the PCM signal from being clipped.0 As a result, the level fluctuation part caused by the superimposition of the PCM signal can be prevented from being clipped. For example, R
The amplitude of the F signal a becomes large due to variations between recording disks, etc., and the level fluctuation portion caused by the superimposition of the PCM signal may exceed the clip level VC.In this case, if the RF signal a is 15, the level fluctuation part due to the superimposition of the PCM signal is flattened, and then passed through the high frequency amplifier circuit 16 and HPF 17, so that it is treated in the same way as a dropout part, and a dropout can be erroneously detected. becomes.

FIG. 7 is a block diagram showing still another embodiment of the present invention, in which an amplifier 21 is connected in place of the comparator 21, and an offset control circuit 33 controls the offset of the DC amplifier according to the output of the amplifier 21. The other parts are constructed in the same manner as the circuit shown in FIG. 6, except that the circuit is continuously controlled. In this configuration, the amplitude of the input to the clip circuit 150 is controlled to be equal to the amplitude that continuously changes in accordance with the change in the amplitude of the PCM signal in the RF signal a, and the same effect as the circuit shown in FIG. 6 can be obtained. It will be done.

In the circuits shown in FIGS. 6 and 7, the amplitude of the input to the clip circuit 15 is controlled by the low-frequency information pulse signal component, but an example of a circuit in which the clip level vc is controlled by the PCM signal component is shown below. It is shown in Figure 8. In the dropout detection circuit 3 of FIG. 8, a limiter amplifier 49, a multiplier 5, a delay circuit 61, a time constant circuit 7. Comparator 8° waveform processing circuit 12 and low frequency component detection circuit 1
3 are connected in the same way as the circuit shown in FIG. However, in the waveform processing circuit 12 in this example, the output of the low frequency component detection circuit 13 is supplied to the clip level control circuit 34. The clip level control circuit 34 is configured to selectively supply, for example, one of two different voltages to the clip circuit 15 as a voltage for determining the clip level VC according to the output of the low frequency component detection circuit 13. In this configuration, when there is no low frequency component detection signal, the clip level ■ and the center level ■ of the RF multiplied signal. When the voltage between is large and a low frequency component detection signal is present, the clip level VC and the center level ■. The same effect as the circuit shown in FIG. 6 can be obtained by reducing the voltage between the low frequency component detection circuit 13 in FIG. Alternatively, the clip level control circuit 34 may output a voltage that continuously changes depending on the amplitude of the low frequency component detection signal. By doing so, an effect similar to that of the circuit shown in FIG. 7 can be obtained.

In the above embodiment, dropout detection is performed by supplying the output C of the multiplier 5 to the time constant circuit 7 and comparing the obtained output with the reference voltage Vr1. The present invention can also be applied to the case where dropout detection is performed by supplying the output C of the multiplier 5 to a retrigger pull mono multi-bicycle as disclosed in Japanese Patent No. 139506.

In addition, in the above embodiment, a PCM signal is superimposed on the RF signal as a low frequency information pulse signal, but in the case where a digital signal, etc. corresponding to data input to a conbeater etc. is superimposed as this low frequency information pulse signal. The present invention can also be applied to.

Effects of the Invention As detailed above, the dropout detection circuit according to the present invention generates a low frequency component detection signal when detecting a low frequency information pulse signal in an RF signal, and performs waveform processing in accordance with this low frequency component detection signal. Since the configuration is such that dropout detection is performed using the RF multiplied signal that has been subjected to R by low frequency information pulse signal if
It is possible to prevent the F-fold signal level variation section from being erroneously detected as a dropout. Therefore, the best dropout compensation can be achieved regardless of whether or not a low frequency information pulse signal is superimposed. In addition, the amplitude level of the low frequency information pulse signal is detected, and the clip level of the clip circuit for waveform processing is changed in response to the change in the amplitude level, or the amplitude of the input of the clip circuit is changed. Furthermore, stable dropout compensation can be achieved even if the amplitude of the low frequency information pulse signal changes.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a conventional dropout detection circuit, FIG. 2 is a waveform diagram showing the operation of each part of the device in FIG. 1, and FIG. 3 is a block diagram showing an embodiment of the present invention. , FIG. 4 is a waveform diagram showing the operation of each part of the device in FIG. 3, and FIG.
FIG. 6 is a block diagram showing another embodiment of the invention, FIG. 6 is a block diagram showing still another embodiment of the invention, and FIG. 7 is a block diagram showing still another embodiment of the invention. , FIG. 8 is a block diagram showing still another embodiment of the present invention. Explanation of symbols of main parts 4... Limiter amplifier 5... Multiplier 6...
Delay circuit 7... Time constant circuit 8°°° Comparator 12... Waveform processing circuit 13... Low frequency component detection circuit

Claims (3)

[Claims] (1) A low frequency signal that generates a low frequency component detection signal by detecting a low frequency information pulse signal in an RF signal read from a recording medium on which a high frequency carrier signal and a low frequency information pulse signal are recorded in a superimposed manner. component detection means; and waveform processing means for processing the waveform of the RF signal in accordance with the low frequency component detection signal, and the period at which the instantaneous level of the output of the waveform processing means is equal to a first predetermined level is a predetermined period. A dropout detection circuit characterized in that it generates a dropout detection signal when a dropout detection signal is exceeded. (2) The low frequency component detection signal has a level corresponding to the signal level of the low frequency information pulse signal, and the waveform processing means processes the RF signal according to the level of the low frequency component detection signal. amplification means for amplifying the signal to have a certain amplitude; clipping means for clipping the output of the amplification means at a second predetermined level; and high frequency extraction for extracting high frequency components of a predetermined frequency or higher from the output of the clipping means. A dropout detection circuit according to claim 1, characterized in that it comprises means. (3) The low frequency component detection signal has a level corresponding to the signal level of the low frequency information pulse signal, and the waveform processing means processes the RF signal according to the level of the low frequency component detection signal. 2. The dropout detection circuit according to claim 1, comprising: clipping means for clipping at a level; and high-frequency extraction means for extracting high-frequency components of a predetermined frequency or higher from the output of the clipping means.