JPS6158074B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dropout compensation device for angle-modulated video signals.

In information reproducing devices, such as magnetic recording and reproducing devices for video signals, processing for dropouts that occur during playback is an extremely important technology.
It is no exaggeration to say that the quality of the reproduced signal of the device is greatly influenced by this processing.

By the way, there are various reasons why such dropouts occur, but for example, in devices that use magnetic tape as a recording medium, there are dust attached to the tape surface, missing magnetic powder on the tape, and problems between the tape and the transducer. It is possible that there was a mistake in contact with Yusa.

In any case, when dropout occurs, the quality of reproduced information is seriously degraded, so various methods for compensating for dropout have been proposed and implemented.

By the way, in magnetic recording and reproducing devices for video signals, dropout compensation is generally performed by utilizing the correlation of information for each horizontal scan and embedding the signal of the previous horizontal scanning period during the period in which dropout occurs. It's summery.

Therefore, first, we will introduce a conventional example of a dropout compensation device in a magnetic recording/reproducing device for video signals.
This is shown in the block diagram shown in Figure.

In FIG. 1, 1 is an input terminal, 4 is a delay device for one horizontal scanning period (1H), 2 is a changeover switch, 3 is an output terminal, and 5 is a dropout detector. Usually, video signals are recorded on magnetic tape in the form of angle modulation (for example, frequency modulation), and when played back, FM demodulation is performed to restore the original video signal. Therefore, the FM extracted from the magnetic recording medium
A modulated signal is applied to the contact a of the switch 2 and the delay device 4 via the input terminal 1. The switch 2 selects one of the signal applied to the input terminal 1 and the signal passed through the delay device 4, and a dropout-compensated signal appears at the output terminal 3. Naturally, the switch 2 is controlled by the dropout detector 5, and is set so that the signal applied to the input terminal 1 is selected during normal times, and the signal passed through the delay device 4 is selected during the dropout period. There is.

However, this method has the disadvantage that a large level of noise is generated after demodulation because the phase becomes discontinuous immediately after switch 2 is switched.

The situation will now be explained with reference to the waveform diagram shown in FIG. In Figure 2, waveform 6 is the waveform of the signal to be reproduced, waveform 7 is the reproduced signal waveform with dropout (dotted line), waveform 8 is the signal delayed by delay device 4, and waveform 9 is the output terminal. 3, and 10 to 14 each indicate the time. Originally, the signal to be reproduced should be waveform 6, but due to dropout after time 12, the signal is deleted and becomes waveform 7. On the other hand, the signal passing through the delay device 4 has a waveform 8, and is led to the output terminal 3 via the switch 2 after time 12. Therefore, the signal appearing at output terminal 3 has waveform 9.

As is clear from waveform 9, phase discontinuity occurs immediately after the switch is switched at time 12. That is, the rising or falling cycle of the signal should be from time 10 to time 11, but the rising edge occurs at time 13 after time 11, which is extremely short. In a frequency modulated signal, it is thought that information is superimposed on the rising and falling edges of the signal or the zero-crossing time, so when such phase discontinuity occurs, a very large amount of noise is generated.

In view of these drawbacks, the present invention reduces the phase discontinuity that occurs when switching between the main signal and the delayed signal, and
The object of the present invention is to provide a dropout compensation device that can extremely reduce the noise accompanying the demodulated output signal at this time.

One embodiment of the present invention is shown in block diagram form in FIG.

In Figure 3, 15 is an input terminal, 16 and 21
is a frequency multiplier, 20 is a delay device, 17 is a switch, 18 is a frequency downshifter, 19 is an output terminal, 22
is a dropout detector. The reproduced video signal is led to a frequency multiplier 16 and a delay device 20 via an input terminal 15, and the signal multiplied by the frequency multiplier 16 is supplied to a contact a of a switch 17. On the other hand, the signal applied to the delay device 20 is also supplied to the contact b of the switch 17 through the frequency multiplier 21. The switch 17 is a dropout detector 22
Under normal conditions, frequency multiplier 1
6, and during the dropout period, the frequency multiplier 21 extracts the signal output from the frequency multiplier 21.
A signal whose frequency is lowered by a frequency lowerer 18 and whose dropout is compensated for appears at an output terminal 19.

This embodiment will be described in detail below, but first, the level of noise immediately after the switch is changed in the conventional system will be explained.

Figure 5 is a waveform diagram that is a little more detailed than the waveform diagram shown in Figure 2, and waveforms 30 and 33 are the second waveform diagram.
Waveforms 31 and 34 correspond to waveform 8 in FIG. 2, waveforms 32 and 35 correspond to waveform 9 in FIG. 2, waveform 29 corresponds to waveform 6 in FIG.
1 indicates the time. Waveforms 30 to 32 are time 3
Waveforms 33 to 35 show the case where dropout occurs after time 40, and waveforms 33 to 35 show the case where dropout occurs after time 40, respectively. The phase discontinuity that occurs immediately after the switch 2 is switched when a dropout occurs varies greatly depending on the phase relationship of the delayed signal with the main signal and the rise time of the output signal of the dropout detector 5. It takes about twice as long as the original time to fall after rising, and it falls at time 39. On the other hand, in waveform 35, the time required from rising to falling at time 36 is extremely short and has already fallen at time 41.

Since the rising or falling cycle of the signal after dropout compensation is detected and the original video signal is restored, the magnitude of the generated noise is determined by the degree of discontinuity in the rising or falling cycle immediately after switching the switch 21. is determined. Therefore, in the conventional configuration as shown in FIG. 1, phase discontinuity occurs within a range from infinitesimal to twice the original period.

Therefore, the case of the present invention will be explained below with reference to FIG.

In FIG. 6, waveform 42 is the signal that should originally be reproduced, waveform 43 is the output signal of frequency multiplier 16,
The waveform 44 shows the output signal of the delay device 20, the waveform 45 shows the output signal of the frequency multiplier 21, the waveform 46 shows the output signal of the switch 17, the waveform 47 shows the output signal of the frequency multiplier 18, and 48 to 52 each represents the time. From time 48 to time 51 is the rising and falling cycle of the signal to be originally reproduced.
In the case of the signal waveform 43 multiplied by the frequency multiplier 16, its period is 1/4. By the way, dropout occurs and the dropout detector 22
It is assumed that the switch 17 outputs an output signal and the switch 17 switches at time 50. On the other hand, the signal waveform 44 that has passed through the delay device 20 is multiplied by the frequency multiplier 21 to form a waveform 44.
5, and after time 50, it is led to the frequency downgrader 18 via the switch 17. Therefore, the output signal of switch 17 becomes waveform 46,
The frequency is divided by the frequency downgrader 18 and a waveform 47 is obtained.
In this case, as in the case of the conventional example described above, time 5
Immediately after zero, the rising and falling cycles become discontinuous as shown in waveform 46, and the degree of this is in the range from infinitesimal to twice the original cycle from time 48 to time 49. However, since the waveform 46 is further frequency-divided by the frequency down-converter 18, this phase discontinuity is alleviated, resulting in a waveform 47. The degree of discontinuity in the rising and falling cycles of waveform 47 is different from the original cycle at time 4.
It is within plus or minus 1/4 of the time between 8 and 51. In other words, this is a great improvement over the conventional example, particularly in the direction where the period becomes narrower.

Although the frequency multiplier 21 is placed after the delay device 20 in FIG. 3, the same effect can be obtained even if this order is reversed.

Another embodiment of the present invention is shown in FIG.

In FIG. 4, 23 is an input terminal, 27 is a delay device, 24 is a switch, 25 is a frequency downshifter, and 26 is a
is an output terminal, and 28 is a dropout detector. Similarly to FIG. 3, the reproduced signal is led to a delay device 27 and a switch 24 via an input terminal 23. The signal output from the delay device 27 is also applied to the switch 24, which is controlled by a dropout detector 28 and is normally connected to the input terminal 23.
When the signal added to the signal is dropped out, delay device 2
7 is selected and supplied to the frequency downgrader 25. In the frequency down-converter 25, the signal selected by the switch 24 is frequency down-done and appears at the output terminal 26 as a drop-out compensated signal.
In this case, looking at the output signal of the switch 24, a phase discontinuity occurs immediately after the switch 24 is switched, and the waveform 32 in FIG. 5 is similar to the waveform 35. However, since the frequency is stepped down after the switch 24, the degree of phase discontinuity in the waveform at the input terminal 26 is reduced.
This signal will be similar to waveform 47 in FIG. However, the time axis is an expanded version of Figure 6. The operation of the embodiment shown in FIG. 4 is as described above, and is almost the same as the embodiment shown in FIG. Regarding the method shown in Fig. 4, for signals whose video information does not deteriorate even after frequency division, that is, for signals where the frequency ratio between the carrier signal frequency and the video signal band is larger than the frequency division ratio, , a frequency multiplier as shown in FIG. 3 is not required, and the configuration is extremely simplified.

A further embodiment of the invention is shown in block diagram form in FIG. In FIG. 7, 53 is an input terminal, 54 is a frequency multiplier, 55 is a switch, and 57
58 is a frequency down-converter, 56 is a dropout detector, and 59 is an input terminal. In the embodiments of FIGS. 3 and 4, only dropouts shorter than one horizontal scanning period can be compensated, whereas the embodiment of FIG. 7 can compensate for dropouts longer than one horizontal scanning period. It is also possible to compensate for this and significantly reduce the noise immediately after the switch. The input signal applied to the input terminal 53 and the output signal of the delay device 57 are guided to the switch 55 and controlled by the dropout detector 56. The output of the multiplier 54 is selected and supplied to the frequency lowerer 58 and the delayer 57. The signal whose frequency is lowered by the frequency lowerer 58 has been subjected to dropout compensation and is led to an output terminal 59. It should be noted that the operation in this case is based on the same principle as shown in FIG. 3, and it is clear that the noise immediately after the switch is changed is significantly improved, so the explanation thereof will be omitted here. On the other hand, since the output signal of the switch 55 is input to the delay device 57, it is possible to compensate for a dropout that is longer than one horizontal scanning period.

By the way, in the configuration of FIG. 7, the signal input to the delay device 57 is a frequency-multiplied signal of the original signal, so the configuration of the delay device 57 is a little complicated. Therefore, another embodiment of the present invention is shown in a block diagram in FIG. 8, in which the frequency band of the signal applied to the delay device 57 is configured to be within the original frequency band. In Fig. 8, 60 is an input terminal;
1 is a frequency multiplier, 62 is a switch, 63 is a frequency lowerer, 64 is an output terminal, 67 is a delay device, 66
is a frequency multiplier. In the configuration of FIG. 8, the input signal to the delay device 57 in FIG. It was returned. Therefore, the operation is similar to that shown in FIG. 7, and the configuration of the delay device 67 is also simplified. Naturally, with this configuration, it is possible to compensate for dropouts that are longer than one horizontal scanning period, and the noise immediately after the switch is also significantly improved.

Also, similar to FIG. 4, a configuration as shown in FIG. 9 is also possible without using a frequency multiplier.

In FIG. 9, 68 is an input terminal, 69 is a switch, 70 is a frequency downshifter, 71 is an output terminal, and 72
is a dropout detector, and 73 is a delay device.
In this configuration, the frequency multiplier 54 in FIG. 7 is omitted, and since it basically works in the same way as the embodiment in FIG. 7, it is possible to compensate for dropouts that are longer than one horizontal scanning period. On the other hand, it is clear that the noise immediately after the switch is also extremely reduced by the same principle as shown in FIG.

In the above description of the embodiments of the present invention, the frequency multiplier includes a balanced modulator, a multiplier, a harmonic distortion detector, etc.; The same result can be obtained even if the frequency is converted by .

Further, although the present invention has been described only in the case immediately after the dropout occurs, according to the present invention, spikes can be extremely reduced even immediately after the dropout ends.

Naturally, the effects of the present invention are greatly influenced by the frequency down ratio.

By the way, in the waveform diagrams of FIGS. 5 and 6, non-modulated signals are shown to simplify the explanation, but the effect is the same even when frequency modulation is applied.

The present invention can also be applied to the removal of spikes that occur when switching heads in magnetic recording and reproducing devices.

Furthermore, if necessary, a shaper may be inserted after the delay device in FIGS. 4, 8, and 9.

As is clear from the above description, the present invention enables a dropout correction device that extremely reduces spike noises that occur immediately after a dropout occurs and immediately after a dropout ends.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional example of a dropout compensator, FIG. 2 is a waveform diagram for explaining FIG. 1, and FIGS. 3, 4, and 7.
8 and 9 are block diagrams showing various embodiments of the present invention, FIG. 5 is an operation waveform diagram of a conventional example, and FIG. 6 is an operation for explaining various embodiments of the present invention. FIG. 16, 21, 54, 61, 66... Frequency multiplier, 18, 25, 58, 63, 70... Frequency down converter, 20, 27, 57, 67, 73... Delay device, 17, 24, 55 ,62,69...Switch.

Claims (1)

[Scope of Claims] 1. Dropout for supplying a main signal angle-modulated with a video signal and a compensation signal created by delaying the main signal with a delay device to a switching means, and detecting a dropout period of the main signal. A dropout compensator configured to control the switch means by the output of a detector, characterized in that the output of the switch means is guided to an angle modulation demodulator via a frequency step-down means. 2. The dropout compensator according to claim 1, wherein the main signal and the compensation signal input to the switch means are each frequency-multiplied by a frequency multiplier. 3. Claim 1, wherein the compensation signal input to the switch means is the output of the switch means delayed by a delay device.
Dropout compensator as described in Section. 4. The frequency of the main signal input to the switch means is multiplied by the frequency multiplier, and the output of the switch means is applied as a compensation signal to the input signal via the frequency step-down means, the delay device, and the frequency multiplier. A dropout compensation device according to claim 3, characterized in that: