JPH0552588B2 - - Google Patents

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 indispensable in order to detect and compensate for dropout caused by scratches, dirt, etc. on the recording medium.

FIG. 1 is a block diagram showing an example of a conventional dropout detection circuit.
After the RF signal a is amplified by the RF 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 b. This pulse train signal b 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 this time constant circuit 7
is compared with reference voltage Vr1 by comparator 8. The output voltage d of this time constant circuit 7 is the reference voltage
When the voltage exceeds 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 generates a predetermined period of time from the time when the dropout detection signal e is generated.
It is configured to generate a correction command signal f that exists for a time longer than _T1 . The output of this correction command generation circuit 9 serves as a control input of a selector 10. A video signal demodulated by the video detection circuit 2 is directly applied to one input terminal of the selector 10. In addition, the other input terminal of the selector 10 is connected to a 1H (one horizontal synchronization period) delay circuit 1.
1 is applied. This selector 10 selects 1H when the correction command signal f is generated.
It is configured to selectively output the video signal that has passed through the delay circuit 11, and to selectively output the video signal directly supplied from the video detection circuit 2 when the correction command signal f disappears.

In the above configuration, the RF signal a has a center frequency
It is assumed that the signal is obtained from a recording medium in which a video FM signal of 8.1 MHz and two audio FM signals with center frequencies of 2.3 MHz and 2.8 MHz are superimposed and recorded. The waveform of such RF signal a is as shown in FIG. 2A. This RF signal a
When supplied to the limiter amplifier 4 via the RF amplifier 1, the appearance timing of the rising edge and the falling edge coincides with the zero crossing point of the RF signal a, that is, the RF
The instantaneous level of signal a is the center level (zero level)
A pulse train signal b as shown in FIG. 3B is obtained, which coincides with the time point when the pulse train becomes equal to V ₀ .

Here, the signal delay time in the delay circuit 6 is 1/ of the time width of the positive pulse forming the pulse train signal b.
2, the output g of the delay circuit 6 becomes as shown in FIG. When these pulse train signal b and the output g of the delay circuit 6 are multiplied in the multiplier 5, the pulse train signal b is generated as shown in FIG.
A pulse train signal in which the appearance timing of the rising edge coincides with the appearance timing of both the rising and falling edges of is formed and is supplied to the time constant circuit 7 as the output c of the multiplier 5. Incidentally, the appearance timing of the falling edge of this output c is based on the output g of the delay circuit 6.
This coincides with the appearance timing of both the rising and falling edges of .

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 reaches a high level. Here, the time constant τ is set so that 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 longer than the period corresponding to the frequency of the video FM signal component of the RF signal a. It is assumed that this has been set. Then, when a dropout occurs and the zero crossing point no longer occurs, as in the case of period _T3 in FIG.
It is longer than the period corresponding to the frequency of the video FM signal component of signal a. 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. 5E, and the dropout detection signal e shown in FIG.

When this dropout detection signal e is supplied to the correction command generation circuit ₉ , as shown in FIG. f is output. In response to this correction command signal f, the video signal that has passed through the 1H delay circuit 11 is selectively outputted from the selector 10 to perform dropout compensation.

By the way, in addition to audio and video signals, PCM also consists of low-frequency components of, for example, 1.75MHz or less.
Recently, consideration has been given to recording signals on recording media such as video disks. In such a case, the RF signal read from the recording medium is a signal in which the signal level of the envelope component becomes extremely large such that the upper peak level is below the center level or the lower peak level is above the center level. becomes. If such an RF 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, the conventional dropout compensation method only compensates for the brightness, so
For example, a highly saturated red color changes to gray, resulting in an inconvenience in which the reproduced image becomes unsightly.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to prevent erroneous 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 with audio and video signals is input. An object of the present invention is to provide a dropout detection circuit capable of performing stable dropout detection.

The dropout detection circuit according to the present invention
A low frequency component detection signal is generated by detecting the low frequency information pulse signal in the signal, and different clip waveform processing is performed on the RF signal depending on the presence or absence of this low frequency component detection signal. The device is configured to generate a dropout detection signal when the period at which the instantaneous level of the processed RF signal becomes equal to a predetermined level is equal to or longer than a predetermined period.

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

As shown in FIG. 3, in the dropout detection circuit 3 according to the present invention, a limiter amplifier 4, a multiplier 5, a delay circuit 6, a time constant circuit 7, and a comparator 8 are connected in the same way as in the circuit shown in FIG. However, in the dropout detection circuit 3 according to the present invention, the RF signal amplified by the RF amplifier 1 is
Signal a is supplied to limiter amplifier 4 via waveform processing circuit 12. 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 RF signal a becomes one input of the changeover switch circuit 14, and simultaneously passes through the clip circuit 15 to the high frequency amplifier circuit 16.
supplied to 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 becomes another input to the changeover switch 14 via an HPE (high pass filter) 17. HPF17 is
It has frequency characteristics that remove low frequency components such as PCM signal components below 1.75MHz. These high frequency amplifier circuit 16 and HPF 17 form a high frequency extraction means, and high frequency components of 1.75 MHz or more are extracted from the RF signal a clipped by the clipping circuit 15. This high frequency component, HPF1
The output of 7 serves as another input to the changeover switch circuit 14. The changeover switch circuit 14 has two inputs, ie, the RF signal a and the HPF signal, depending on the state of the control input.
It is configured to selectively output 17 outputs. The output of the changeover switch circuit 14 is supplied to the limiter amplifier 4 as the output of the waveform processing circuit 12.

On the other hand, in the low frequency component detection circuit 13, the RF
The signal a is supplied to a de-emphasis circuit 19 via an LPF (low pass filter) 18. LPF
18 has frequency characteristics that allow only low frequency information pulse signals such as PCM signals of 1.75 MHz or less to pass through. This LPF18 allows the RF signal a
The lower frequency information pulse signal is separated and extracted and supplied to the de-emphasis circuit 19. In the de-emphasis circuit 19, the components emphasized during recording are restored. This de-emphasis circuit 1
The low frequency information pulse signal outputted from 9 is supplied to an envelope detector 20 consisting of an AM detector or the like, and the envelope component of the low frequency information pulse signal is detected. The envelope component detected by the envelope detector 20 is transmitted to the comparator 2.
1. In the comparator 21, the envelope component of the low frequency information pulse signal is set to the reference voltage.
Compared to Vr2. When the instantaneous level of the envelope component of this low frequency information pulse signal 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 there is no low frequency component detection signal, the changeover switch circuit 14 outputs the RF signal a.
is selectively output and a low frequency component detection signal is present, the selector switch circuit 14 outputs the HPF 17.
It is assumed that the output of is selectively output. Then, if a low frequency information pulse signal such as a PCM signal is not superimposed on the RF signal a, no low frequency component detection signal is generated, and the RF signal a is directly supplied to the limiter amplifier 4, making it possible to use the conventional circuit. Dropout detection is performed in the same way. Further, when a PCM signal is superimposed on the RF signal a as a low frequency information pulse signal, the RF signal a is supplied to the limiter amplifier 4 via the clip circuit 15, the high frequency amplifier circuit 16, and the HPF 17.

When the PCM signal is superimposed on the RF signal a, the waveform of the RF signal a becomes as shown in FIG. 4A, and the upper peak level may become lower than the center level V ₀ (time t ₁ ). When this RF signal a is supplied to the clip circuit 15, the portion of the RF signal a that exceeds the clip level Vc, which is a predetermined distance away from the center level _V0 , is clipped and is caused by dropout.
A clip signal h as shown in FIG. 1B is obtained by removing minute level fluctuations such as those during period _T3 during which the RF signal a occurred. This clip signal h passes through a high frequency amplifier circuit 16 and an HPF 17, and low frequency components such as PCM signals are removed, thereby obtaining a signal i as shown in FIG. In this signal i, the portion corresponding to the section _T3 which is equal to the clip level Vc of the clip signal h has the DC component removed and is at the center level.
The point where V is equal to ₀ , that is, the zero crossing point is 1
It becomes a place. This signal i is the changeover switch circuit 14
When the generation period of each pulse forming the output c of the multiplier 5 becomes longer than the period corresponding to the frequency of the video FM signal component of the RF signal a in the interval _T3 , a dropout is detected. Signal e is generated and dropout detection is made error-free.

When the low frequency information pulse signal is not superimposed on the RF signal a, the RF signal a is sent to the high frequency amplifier circuit 16.
Since the signal is directly supplied to the limiter amplifier 4 without passing through the HPF 17, low-frequency components are not removed, and dropout detection ability is prevented from deteriorating.

Incidentally, when a PCM signal is superimposed on the RF signal a, the RF signal a is passed through only the high frequency amplifier circuit 16 and HPF 17 without passing through the clip circuit 15, so that only the PCM signal component is removed, and then the limiter amplifier It is conceivable to supply it to 4. 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 level of the limiter amplifier 4 approaches the center level V ₀ . It will appear in In this case, the generation cycle of the zero-cross point at the input of the limiter amplifier 4 becomes equal to the cycle corresponding to the frequency of the video FM signal component of the RF signal a, and the dropout detection signal e is not generated, resulting in a decrease in detection ability.

FIG. 5 is a block diagram showing another embodiment of the present invention.
The other parts are constructed in the same manner as the circuit shown in FIG. 3, except for the part connected so that the output of the clip circuit 15 is supplied instead of the signal a. 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, the limiter amplifier 4, multiplier 5, delay circuit 6, time constant circuit 7, comparator 8, waveform processing circuit 12, and low frequency component detection means 13 are
Connected in the same way as the circuit shown. However, in the waveform processing circuit 12 in this example, the RF
The RF signal a amplified by the amplifier 1 is passed through the variable gain amplifier 30 to the clip circuit 15 and
It is supplied to an AGC (automatic gain control) voltage generator 31. The AGC voltage generator 31 is configured to generate an AGC voltage according to the amplitude of the output of the variable gain amplifier 30. This AGC voltage generator 31
The output voltage is supplied to the variable gain amplifier 30 via the DC amplifier 32 as a gain control voltage.
The AGC 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 controlled by an offset control circuit 33. The offset control circuit 33 controls, for example, a DC voltage according to the output of the low frequency component detection circuit 13.
The configuration is such that one of two different voltages is selectively applied to the input of the amplifier 32.

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 amplified by the RF amplifier 1. If the offset of the DC amplifier 32 is controlled by the offset control circuit 33 so that it becomes larger than the amplitude of the signal a, the clip level Vc is constant, so it is possible to clip it to the center level of the RF signal a. . 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 between recording disks and
The amplitude of the RF signal a becomes small depending on the radial position of the information detection point on a CAV (Constant Angular Velocity) disk, and even if dropout occurs, the instantaneous level of the RF signal a may not reach the clip level Vc. arise. At this time, if this RF signal a is supplied as is to the clip circuit 15, unnecessary minute level fluctuation components caused by dropout will not be removed, and the dropout detection ability will deteriorate.

When a low frequency component detection signal occurs, i.e.
When the PCM signal is superimposed on the RF signal a, the offset control circuit 33 controls the DC output so that the amplitude of the output of the variable gain amplifier 30 is smaller than the amplitude of the RF signal a amplified by the RF amplifier 1. If the offset of the amplifier 32 is controlled by switching, it is possible to prevent the level fluctuation portion caused by the superimposition of the PCM signal from being clipped. As a result, the level fluctuation portion due to the superimposition of the PCM signal will not be erroneously detected as a dropout. For example, there may be a case where the amplitude of the RF signal a becomes large due to variations among recording disks, and the level fluctuation portion caused by superimposition of the PCM signal exceeds the clip level Vc. At this time, if RF
If the signal a is supplied as is to the clip circuit 15, the level fluctuation part due to the superimposition of the PCM signal is flattened, and then passes through the high frequency amplifier circuit 16 and HPF 17, where it is treated in the same way as a dropout part, thereby preventing erroneous dropouts. It will be detected.

FIG. 7 is a block diagram showing still another embodiment of the present invention, in which an amplifier 21 is connected instead of the comparator 21, and the offset control circuit 33 is
The other parts are constructed in the same manner as the circuit shown in FIG. 6, except that the offset of the DC amplifier is continuously controlled in accordance with the output of the amplifier 21. In this configuration, the amplitude of the input to the clip circuit 15 is controlled to be equal to the amplitude that changes continuously in response to changes in the amplitude of the PCM signal in the RF signal a, and the same effect as in the circuit of FIG. 6 is obtained. can get.

In the circuits shown in FIGS. 6 and 7, the amplitude of the input to the clip circuit 15 was controlled by the low frequency information pulse signal component, but the clip level Vc was controlled by the PCM signal component. An example circuit is shown in FIG. The dropout detection circuit 3 shown in FIG. 8 may output a voltage that continuously changes depending on the amplitude of the limiter amplifier 4 and the multiplier component detection signal. By doing so, the same effect as the circuit shown in FIG. 7 can be obtained.

In the above embodiment, the output c of the multiplier 5 is supplied to the time constant circuit 7, and the obtained output is used as the reference voltage.
Dropout is detected by comparing it with Vr1, but it is possible to detect dropout by supplying the output c of the multiplier 5 to a retriggerable mono multivibrator as disclosed in Japanese Patent Laid-Open No. 139506/1983. The present invention can also be applied to the case where dropout detection is performed.

Furthermore, in the above embodiment, the PCM signal is superimposed on the RF signal as a low frequency information pulse signal, but a digital signal or the like corresponding to data input to a computer or the like may be superimposed as this low frequency information pulse signal. The present invention can also be applied to such cases.

Effects of the Invention As detailed above, the dropout detection circuit according to the present invention includes a low frequency information pulse signal 5 in an RF signal, a delay circuit 6, a time constant circuit 7, a comparator 8, a waveform processing circuit 12, and a 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 means 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. ing.

In such a configuration, when the low frequency component detection signal does not exist, the voltage between the clip level Vc and the center level _V0 of the RF signal a becomes large, and when the low frequency component detection signal exists, the voltage between the clip level Vc and the center level _V0 becomes large. If the voltage between them is made small, an effect similar to that of the circuit shown in FIG. 6 can be obtained.

Note that the low frequency component detection circuit 13 in FIG. 8 has the same configuration as the circuit in FIG. 7, and generates a low frequency component detection signal when a low frequency is detected by the clip level control circuit 34. Dropout detection is performed using an RF signal that has been subjected to different clip waveform processing depending on the presence or absence of this low frequency component detection signal, so dropout detection is performed when no low frequency information pulse signal is superimposed. At the same time, when a low frequency information pulse signal is superimposed, it is possible to prevent erroneously detecting a level fluctuation part of the RF signal due to the low frequency information pulse signal as a dropout. Therefore, the best dropout compensation can be achieved regardless of whether or not the low frequency information pulse signal is superimposed.
Furthermore, by detecting the amplitude level of the low frequency information pulse signal and changing the clip level of the clip circuit for waveform processing in response to the change in the amplitude level, or changing the amplitude of the input of the clip circuit, Even if the amplitude of the low frequency information pulse signal changes, stable dropout compensation can be achieved.

[Brief explanation of the drawing]

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 shown in FIG. 1, and FIG. 3 is a block diagram showing an embodiment of the present invention. 4 is a waveform diagram showing the operation of each part of the device shown in FIG. 3, FIG. 5 is a block diagram showing another embodiment of the present invention, and FIG. 6 is a waveform diagram showing another embodiment of the present invention. FIG. 7 is a block diagram showing still another embodiment of the present invention, and 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 (1)

[Scope of Claims] 1. A dropout detection circuit that emits a dropout detection signal based on a change in the instantaneous level of an RF signal read from a recording medium, the dropout detection circuit generating a dropout detection signal when detecting a low frequency information pulse signal of the RF signal. a low frequency component detection means 13 that generates a component detection signal; a clip waveform processing circuit 12 that applies different clip waveform processing to the RF signal depending on the presence or absence of the low frequency component detection signal; A dropout detection circuit comprising a dropout detection circuit 3 which generates the dropout detection signal when it detects that the period in which the instantaneous level of the output signal of the waveform processing circuit exceeds a predetermined level exceeds a predetermined period. 2. The low frequency component detection signal has a level corresponding to the signal level of the low frequency information pulse signal, and the clip waveform processing means
amplification means for amplifying the signal to a signal having an amplitude corresponding to the level of the low frequency component detection signal; clipping means for clipping the output of the amplification means at a second predetermined level; 2. The dropout detection circuit according to claim 1, further comprising high frequency extraction means for extracting high frequency components of a predetermined frequency or higher. 3. The low frequency component detection signal has a level corresponding to the signal level of the low frequency information pulse signal, and the clip waveform processing means
A patent characterized in that the clipping means comprises clipping means for clipping a signal at a level corresponding to the level of the low frequency component detection signal, and high frequency extraction means 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.