JPH02208869A - Drop out detecting circuit for recording and reproducing device - Google Patents

Abstract

PURPOSE:To accurately generate a drop out detecting pulse by inputting a signal passing the full-wave rectifier circuit of a regenerative signal to a re- triggerable monostable multivibrator after applying on a waveform arranging circuit. CONSTITUTION:After an FM signal regenerated from a magnetic tape 1 is full-wave rectified at a full-wae rectifier circuit 9, waveform arrangement 10 is applied with the center level of AC amplitude, and inputted to the re- triggerable monostable multivibrator 11. By applying full-wave rectification in such a manner, the DC level of a white noise existing in a drop out period can be lowered to the lower side of the AC amplitude. After that, connection is performed by AC coupling with a capacitor, and the waveform arrangement is performed with the center level of the amplitude of the FM signal. Thereby, the drop out detecting pulse without being disturbed with the white noise in the drop out period can be accurately obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a dropout detection circuit in a recording/reproducing apparatus.

[Conventional technology]

VTR (Video Tape Recorder)
), etc., the signal is recorded in the form of a frequency-modulated carrier wave (hereinafter abbreviated as FM signal). The original video signal is obtained through a predetermined signal processing circuit such as an FM demodulation circuit.By the way, when reproducing this video signal, there are no problems such as dirt or dust attached to the magnetic tape or scratches on the tape itself. This may cause signal loss called dropout.The period of this dropout may be as short as several microseconds, or as long as several microseconds.
() (is - horizontal scanning period), and during this period,
The black level of the playback screen dropped, significantly deteriorating the image quality. In such a case, in most cases, this drop-out error is detected and replaced with another signal, such as a signal from 1H earlier, to effectively improve the image quality.

Here, a conventional example of the dropout detection circuit as described above is shown in FIG. 2 and will be described below. Note that FIG. 5 shows the main waveforms of FIG. 2.

In FIG. 2, an FM signal reproduced from a magnetic tape 1 by a magnetic head 2 is transmitted to a preamplifier 3° equalizer circuit 4. FM demodulation circuit5. De-emphasis circuit 6. The original video signal 8 is reproduced through an amplifier circuit 7. On the other hand, F.M.
In order to detect dropouts in the signal, the FM signal is connected to the output of the equalizer circuit 4 through AC coupling using a capacitor, and then the FM signal is sent to the limiter circuit 14. The waveform is shaped to the operating level of the retriggerable mono-multivibrator 11 via. Incidentally, a capacitor is often used at an intermediate stage of the preamplifier 3, equalizer circuit 4, etc., but for convenience, it is provided at the stage before the limiter circuit 13.This retriggerable mono multivibrator 1
1 determines whether the input frequency is larger or smaller than a predetermined limit frequency fn, as described in Japanese Patent Application Laid-open No. 55-35515, and this limit frequency fn is usually determined by an externally attached 01 time constant. You can change it with In other words, if a signal is input at a cycle shorter than the output pulse width τ determined by this 01 time constant, it will be retriggered and the same output level will be maintained, and the signal will be input at a cycle longer than τ like dropout. If it is, the output level changes to the other side with a delay of τ. Therefore, a dropout shorter than τ cannot be detected, and the dropout detection pulse 12 is output with a delay of τ.

Also, among the main waveforms at this time, the reproduced FM signal AC-coupled by the capacitor is as shown in Fig. 3 2A.
During the dropout period, it appears as white noise. Furthermore, the output signal that passes through this signal to the limiter circuit 14 is saturated with white noise during the dropout period, as shown in FIG. 2B. When this 2B signal is input to a retriggerable mono multivibrator, it becomes a dropout detection pulse 12 as shown in Figure 2C, which is disturbed by white noise during the dropout period, or a pulse that appears only in some places (2σ in the figure). appears.

[Problem to be solved by the invention]

The above conventional technology has the following problems.

In other words, 1. Since the path leading to the dropout detection circuit is connected by AC coupling using a capacitor, if the input is input to the retriggerable mono multivibrator via the limiter amplifier circuit or waveform shaping circuit, the dropout period will be so-called white. This appeared as noise and the dropout detection circuit did not generate accurate dropout detection pulses.

2. The minimum detection width of dropout determined by the 01 time constant of the retriggerable mono multivibrator and the output delay time of the mono multivibrator are large;
It was not possible to detect short dropouts, and the playback quality deteriorated significantly.

Therefore, the objects of the present invention are: 1. To generate an accurate dropout detection pulse that is not disturbed by white noise during the dropout period even if the path leading to the dropout detection circuit is connected by AC coupling using a capacitor. 2. Output delay of the retriggerable monomultivibrator By reducing the time and further shortening the detectable dropout width, it is possible to obtain good playback image quality without dropouts.

[Means to solve the problem]

In order to achieve the above purpose, a full-wave rectifier circuit is provided in the path leading to the dropout detection circuit. In other words, the FM signal reproduced from the magnetic tape is full-wave rectified by the full-wave rectifier circuit. By doing so, the DC level of white noise during the dropout period can be lowered (Figure 4 (B)). Then, by performing waveform shaping at the center level of the AC amplitude, the retriggerable mono-multivibrator output During the dropout period, an accurate dropout detection pulse can be obtained that is not disturbed by white noise.

In addition, by performing full-wave rectification, the FM signal is multiplied by 2, and the CR time constant of the retriggerable mono multi-bi break can be halved, allowing for even shorter dropout detection. becomes.

Furthermore, in the case where a dropout detection pulse is obtained by envelope detection of the output signal of the full-wave rectifier circuit instead of the retriggerable mono-multipipe rake, in order to achieve the above object, the envelope detection circuit CR time constant can be shortened.

[Effect]

When the F'M signal reproduced from the magnetic tape is full-wave rectified in a full-wave rectifier circuit, the DC level of white noise that existed during the dropout period can be lowered to below the AC loss width (Figure 4). IB).

After that, connect them using AC coupling using a capacitor.
By performing waveform shaping at the center level of the amplitude of the FM signal, an FM signal waveformed to a constant amplitude level undisturbed by white noise appears at the input of the retriggerable mono multivibrator during the dropout period. Thereby, an accurate dropout detection pulse can be obtained at the output of the retriggerable monomultivibrator, which is not disturbed by white noise during the dropout period.

In addition, by performing full-wave rectification, the FM signal reproduced from the magnetic tape can be multiplied by 2.
The time constant can be halved. Therefore, it is possible to detect a dropout in a shorter period of time, and the output delay time of the dropout detection pulse is also halved. therefore,
Good playback quality without dropouts can be obtained.

Furthermore, when obtaining a dropout detection pulse by envelope-detecting the output of a full-wave rectifier circuit instead of the retriggerable mono-multi-bi-break, the input FM signal is multiplied by 2, so the envelope detection The CR time constant can be shortened. Therefore, it becomes possible to detect dropouts that are shorter f)).

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. In addition, the parts shown in FIG. 2 having the same functions as those of the prior art shown in FIG. Further, FIG. 4 is a waveform diagram of the mounting portion of FIG. 1.

First, the FM signal reproduced from the magnetic tape 1 by the magnetic head 2 is transmitted to the preamplifier 3. Equalizer circuit 4. FM
@Tune circuit 5. De-emphasis circuit 6. After passing through the amplifier circuit 7, the original video signal 88 is reproduced. On the other hand, in order to detect dropout during FM (no. The reason why the rectifier circuit 9 is provided before the rectifier circuit 9 is for the reason mentioned above in the prior art.By full-wave rectifying the input FM signal in this full-wave rectifier circuit 9,
The signal shown in A becomes as shown in FIG. 1B. As a result, the white noise in the dropout period that appeared near the center of the signal amplitude in FIG. 4 1A falls below the signal amplitude as shown in FIG. 4 1B. Furthermore, by operating the waveform shaping circuit 10 at the level of the average value a, it is possible to obtain as its output a signal in which the white noise does not saturate during the dropout period, as shown in FIG. 10.

This signal is input to the retriggerable mono multivibrator 11, and the dropout detection pulse 1 shown in FIG.
You can get 2. Note that the waveform shaping circuit 10 may be replaced with a limiter circuit.

In the embodiment shown in FIG. 5, the output (#!
6 (FIG. 6, 5B) is input to the envelope detection circuit 15, and its output (FIG. 6, 5C) is supplied to the waveform shaping circuit 10 to generate a dropout detection pulse 128 as shown in FIG. 6, 5D.
obtain.

In the embodiment shown in FIG. 7, the output of the half-wave rectifier circuit 15 (FIG. 8, 7B) is input to the waveform shaping circuit 10, and the output (FIG. 8, 7C) is applied to the retriggerable mono-multivibrator 11. As a result, a dropout detection pulse 12 as shown in FIG. 8, 7D is obtained.

Note that the half-wave rectifier circuit 15 can be realized by a circuit as shown in FIG. 49, as an example. Vref in the figure may be set to zero V.

In the embodiment shown in FIG. 10, the output of the half-wave rectifier circuit 15 is input to the envelope detection circuit 15, and then
A dropout detection pulse 12 is obtained by shaping the waveform.

In the embodiment shown in FIG. 11, the output of the envelope detection circuit 13 (FIG. 12 11A) is input to the adder circuit 166c, and the signal at the input stage of the envelope detector circuit 15 is also input to the adder circuit 16. are doing. The output signal of the envelope detection circuit 15 is in the form of a pulse during the dropout period as shown in FIG. 11B, and this signal is converted into the original signal (FIG.
1A), the signal as shown in Fig. 12 11C becomes
It can be obtained from the adder circuit 16 output.

Furthermore, the waveform shaping circuit 10. Dropout detection pulse 12 via retriggerable monochrome vibrator 118
(Fig. 12 11D) 8 obtained.

In the embodiment shown in FIG. 13, the signal output from the adder circuit 16 is input to the envelope detection circuit 15. A dropout detection pulse 12 is obtained through a waveform shaping circuit 10.

〔Effect of the invention〕

Since the present invention is configured as described above, it produces the effects described below.

1. Input FM! Since the signal is multiplied by 2 using a full-wave rectifier circuit, the white noise that appears during the dropout period will fall below the signal amplitude, and even if the dropout is detected by waveform shaping, the white noise will not increase. An undisturbed and accurate dropout detection pulse 8 can be obtained.

2. When using either a retriggerable mono multivibrator or an envelope detection circuit to detect dropout detection pulses, reduce the CR time constant for detection by half (
Therefore, it becomes possible to detect even shorter drops. Therefore, a playback screen without dropouts can be obtained, and the image quality is significantly improved.

[Brief explanation of the drawing]

41 is a diagram showing a dropout detection circuit according to an embodiment of the present invention, FIG. 2 is a diagram showing a conventional dropout detection circuit, FIG. 3 is a waveform diagram of the main part of FIG. 2, and FIG. 1 is a waveform diagram of the main parts, FIG. 5 is a diagram showing the second embodiment of the present invention, and FIG. 6 is a diagram showing the second embodiment of the present invention.
The figure shows the main part waveform diagram of Fig. 45, Fig. 7 shows the third embodiment of the present invention, Fig. 8 shows the main part waveform diagram of Fig. 7, and Fig. 9 shows an example of the half-wave rectifier circuit. Figure 10 is a diagram showing 44th embodiment of the present invention, Figure 411 is a diagram showing Example 5 of the present invention, Figure 1g12 is a waveform diagram of the main part of Figure 11, and Figure 15 is a diagram showing the embodiment of the present invention. It is a figure which shows the Example of durability -6. 1...Magnetic tape 2...Magnetic head 9...
Full wave rectifier circuit 10... Waveform shaping circuit 11... Retriggerable mono multivibrator 12... Dropout detection pulse 13... Envelope detection circuit 15... Half wave rectifier circuit 1
6...addition circuit gruel 1 figure ff12 figure child 4Z 155 figure gti5 figure 5! "Fig. 16!! Fig. f'18 Closed Fig. 12 Fig. 5 F'5 Fig. 15

Claims (1)

[Claims] 1. In a dropout detection circuit of a recording and reproducing apparatus that records an information signal on a recording medium or reproduces an information signal from the recording medium, a full-wave rectifier circuit ( 9), the signal passed through the full-wave rectifier circuit (9) is applied to a waveform shaping circuit (10), and then input to a retriggerable mono-multivibrator (11). dropout detection circuit. 2. In the dropout detection circuit for a recording/reproducing apparatus according to claim 1, in place of the waveform shaping circuit (10) and the retriggerable mono-multivibrator (11), an envelope detection circuit (15) and the envelope detection circuit (15) are provided. 1. A dropout detection circuit for a recording/reproducing apparatus, comprising a waveform shaping circuit (10) that inputs an output signal of a detection circuit (13). 5. A dropout detection circuit for a recording and reproducing apparatus according to claim 1, characterized in that the dropout detection circuit for a recording and reproducing apparatus comprises a half-wave rectifier circuit (15) in place of the full-wave rectifier circuit (9). detection circuit. 4. A dropout detection circuit for a recording and reproducing apparatus according to claim 2, characterized in that the dropout detection circuit for a recording and reproducing apparatus comprises a half-wave rectifier circuit (15) in place of the full-wave rectifier circuit (9). detection circuit. 5. In the dropout detection circuit for a recording/reproducing apparatus according to claim 1, in place of the full-wave rectifier circuit (9), an envelope detection circuit (13) and an input stage of the envelope detection circuit (13) are provided. 1. A dropout detection circuit for a recording/reproducing apparatus, comprising an addition circuit (16) for adding a signal and a signal at an output stage of the envelope detection circuit (13). 6. In the dropout detection circuit for a recording/reproducing apparatus according to claim 2, in place of the full-wave rectification circuit (9), an envelope detection circuit (13) and an input stage of the envelope detection circuit (13) are provided. 1. A dropout detection circuit for a recording/reproducing apparatus, comprising an addition circuit (16) for adding a signal and a signal at an output stage of the envelope detection circuit (15).