JPS6248953B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a synchronization signal detection circuit that detects a synchronization signal from a reproduced FM signal obtained by reproducing a video signal that has been modulated and recorded.
It is ideal for use in a synchronization signal separation circuit for the playback system of video signal recording and playback devices such as VTRs.

The video signal is now FM modulated and recorded.
VTR is known. Figure 1A looks like this
Block diagram of VTR playback system, Figure 1B is 1A
It is a waveform chart of each part of a figure. In FIG. 1A, a reproduced RF signal (reproduced FM signal) output from a video head 2 obtained by reproducing a recorded signal on a tape 1 is amplified by a reproduced RF amplifier 3 and then supplied to a limiter 4. FIG. 1B a shows the output signal a of the regenerative RF amplifier 3, and b shows the output b of the limiter 4. As shown in FIGS. 1B a and 1b, if there is a dropout in the reproduced RF signal, this dropout portion is compensated by the dropout compensation circuit DC.

That is, the output b of the limiter 4 is supplied to the dropout detection circuit 7 via the fixed contact f ₁ and the movable contact m of the switch circuit 5, and the dropout detection signal detected by this circuit is supplied to the switch circuit 5. Therefore, in the dropout part, the movable contact m of the switch circuit 5 is connected to the fixed contact _f2 ,
The reproduced RF signal 1H before, which is stored in the 1H delay line 6, is selected, and the dropout is compensated for as shown in FIG. 1B (c). The output c of the dropout compensation circuit DOC is supplied to the FM demodulator 8, and a video signal d shown in FIG. 1B d is obtained.

In this way, even if there is a dropout in the reproduced RF signal, this dropout part is connected to the compensation circuit DOC.
However, at the boundary between the original RF signal part in Figure 1B and the compensated 1H previous RF signal, a small RF signal dropout or phase discontinuity of the RF signal may occur. Sometimes. in this case,
Whisker-like noise N as shown in FIG. 1B d occurs in the FM demodulated video signal d. If this video signal d is synchronously separated, a problem arises in that noise N is detected as a synchronous signal in addition to the original synchronous signal. If a synchronization signal containing such a pseudo synchronization signal is supplied to a drum servo circuit, AFC circuit, etc. of a VTR, the VTR will malfunction.

Furthermore, even if the dropout section is designed to replace the RF signal from 1H before, if a dropout occurs at the same location over several hours due to scratches on the tape, the output of the 1H delay line at the dropout section will be significantly attenuated. The S/N ratio may drop and a pseudo synchronization signal may be generated.

Yet another problem is that a disturbance signal with a large period may be superimposed on the reproduced RF signal, causing significant disturbance in the output of the synchronization separation circuit. For example, VTRs have been known that record audio signals with a video head. Such a VTR has the advantage of being able to record and reproduce audio signals with relatively high quality even at low tape speeds. FIG. 2A is a partial block diagram of a VTR in which the video head also records an audio signal, and FIG. 2B is a waveform diagram of each part of FIG. 2A.

In the VTR shown in Fig. 2A, Fig. 2B S
As shown in , an audio signal SA subjected to pulse modulation (for example, pulse amplitude modulation) is now recorded during recording or playback (so-called dubbing) in a predetermined section t of the back porch of the synchronization signal of the video signal. There is. The reproduced RF signal obtained from the video head 2 is supplied to the reproduced RF amplifier 3 via the movable contact m and the fixed contact _f1 of the switch circuit 14, and then to the limiter 4. On the other hand, the audio signal AUDIO is supplied to the pulse amplitude modulator 12, where it is modulated into a PAM signal, and then sent to the recording amplifier 11, the fixed contact f ₂ of the switch circuit 14,
It is supplied to the video head via a movable contact m.
Note that the movable contact m of the switch circuit 14 receives a synchronization signal.
It is controlled by a timing signal S _t of the output of the timing signal forming circuit 13 formed on the basis of SYNC, and this timing signal S _t corresponds to the interval t of FIG. 2B.

In FIG. 2A, the switch circuit 14 is usually a transistor circuit, and its separation is 80 dB or less. Therefore, when the movable contact m of the switch circuit 14 is switched to the fixed contact _f2 side in order to record the audio signal S _A during playback, a part of the recording current is transferred to the playback amplifier 3 as shown by the dashed line L. It may leak. The leakage current is considerably larger than the reproduced signal, so the reproduced RF amplifier 3 becomes saturated (clipped), and its output reproduced RF signal a
In the case of a period longer than 1H (several
KHz) waviness components (disturbance signals) may be superimposed. When such an RF signal is applied to the limiter 4, the limiter output causes a dropout that cannot be compensated for by the dropout compensation circuit DOC, as shown in FIG. 2B, and the output of the synchronizing signal separation circuit is also disturbed.

The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a synchronization signal detection circuit that is extremely less affected by dropouts of reproduced FM signals (reproduced RF signals), relatively long-period disturbance signals, etc. purpose.

The synchronization signal detection circuit of the present invention includes a reproduction amplifier that amplifies a reproduction signal of a video signal recorded by FM modulation, and a tuning point having a tuning point at an FM frequency corresponding to the synchronization signal tip level of the output signal of the reproduction amplifier. circuit, and a detection circuit for detecting the envelope of the output signal from the tuning circuit. An extremely stable synchronization signal is obtained based on the output signal of this detection circuit.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 3 is a block diagram of a reproduction system of a VTR showing an embodiment of the present invention, and FIG. 4 is a waveform diagram of each part of FIG. 3. In FIG. 3, the same parts as in FIG. 1A are given the same reference numerals.

In FIG. 3, the reproduced RF signal obtained from the video head 2 is transmitted to the FM demodulator 8 via the reproducer amplifier 3, the limiter 4, and the dropout compensation circuit DOC.
supplied to FM demodulated video signal VIDEO
is led out to the outside, and the synchronization separation circuit 16
The synchronizing signal SYNC1 is obtained separately.

On the other hand, the reproduced RF signal a output from the reproduced RF amplifier 3
is supplied to the tuned amplifier 17. This tuning amplifier 17 determines the level (sync chip level) of the leading edge of the synchronization signal in the video signal that is FM modulated and recorded.
The frequency of the FM wave corresponding to (FM carrier frequency
It is an amplifier with a tuning point at _p ). Therefore,
The output of the tuning amplifier 17 becomes an RF signal having a peak in the synchronizing signal portion, as shown in FIG. 4e. Note that the tuning amplifier 17 may be a bandpass filter. The output e of the tuned amplifier 17 is supplied to a limiter 18 (or an AGC circuit) and adjusted to a constant amplitude as shown in FIG. 4f.

The output f of the limiter 18 is supplied to a tuned amplifier 19 having the same tuning frequency as the tuned amplifier 17,
It is further tuned and amplified. The output of the tuned amplifier 19 (FIG. 4g) is supplied to an envelope detection circuit 20, where the envelope of the RF signal is detected. Therefore, a synchronizing signal SYNC2 shown in FIG. 2h is obtained from the envelope detection circuit 20. Since the synchronization signal SYNC2 is obtained from the reproduced RF signal using the tuned amplifiers 17 and 19 in this way, the noise N etc. shown in Figure 1B (d) generated due to the dropout of the reproduced RF signal can be used as the synchronization signal. Furthermore, the detection of the synchronizing signal is not affected by the undulation component of the RF signal as shown in FIG. Therefore, an extremely stable synchronization signal can be obtained.

Note that the synchronization signal SYNC2 obtained by tuned amplification
can be used as a synchronization signal as is, but in order to operate the VTR more stably, this synchronization signal SYNC2 is supplied as a control signal to the previously described synchronization signal separation circuit 16, as shown by the dotted line in Figure 3. You may.

Figure 5 shows the synchronization signal SYNC2 in the synchronization separation circuit 16.
FIG. 6 is a block diagram showing an embodiment in which the signal is used as a clamp signal and/or a muting signal. FIG. 6 is a waveform diagram of each part of FIG.

In FIG. 5, a video signal d (FIG. 6 d) output from the FM demodulator 8 is supplied to a clamp circuit 23 via a low-pass filter 22 for removing noise. In conventional VTRs, for example, a diode clamp circuit is used as the clamp circuit 23, and the input video signal is clamped to its sync chip level (minimum level).

This video signal d may contain noise N caused by dropouts in the reproduced RF signal, as shown in FIG. 1B d and FIG. 6 d. As shown in FIG. 6d, the level of this noise N is a level K lower than the sync chip level.
Then, the video signal d is clamped to this level K, and the subsequent synchronization signal detection level of the sync separator 25 changes, and the separated synchronization signal
The falling position of SYNC1 may become unstable. Therefore, the synchronization signal SYNC2 shown in FIG.
is supplied from the envelope detection circuit 20 described above to the clamp circuit 23 (the circuit diagram is shown in FIG. 7),
Clamp circuit 2 only in this SYNC2 part
3 is now working. Therefore, since the level K of the noise N contained in the portion other than the synchronization signal is not clamped, the subsequent level detection operation of the sync separator 25 is extremely stable.

The output of the clamp circuit 23 is supplied to a muting circuit 24. This muting circuit 24
may be a switch circuit that clamps the signal line to the pedestal level of the video signal. On the other hand, the output SYNC2 of the envelope detection circuit 20 is supplied to the mono multi 26.
From the output of SYNC2, as shown in Figure 6i,
A signal i is obtained which remains at a high level for a period of, for example, 6 μsec from the rise of the signal i. This signal i is further supplied to a monomulti 27, and from the output of this monomulti 27, as shown in FIG. 6j, a signal j is obtained which remains at a high level for a period of, for example, 58 μsec from the fall of the signal i. Since this signal j is supplied to the muting circuit 24 of FIG. 5, the section video signal of this signal j is clamped (or muted) to the pedestal level.

Therefore, the output of the muting circuit 24 is one in which the video signal other than the synchronization signal and the noise component N are removed, as shown in FIG. 6h. Since this signal k is supplied to the level detection type sync separator 25, this separator 25 prevents the noise component N contained in the video signal d from being mistakenly separated and detected as a pseudo synchronization signal, and is extremely stable. A synchronization signal SYNC1 can be obtained.
Note that either one of the clamping and muting performed based on SYNC2 may be performed, or they may be used together.

FIG. 7 is a circuit diagram showing an example of the clamp circuit 23 of FIG. 5. As shown in FIG. 7, the video signal VIDEO (output of LPF22) is transmitted through transistor _T1
and an emitter follower circuit consisting of a resistor _R1 , and the output of this emitter follower circuit is supplied via a capacitor _C1 to an emitter follower circuit consisting of a transistor _T3 and a resistor _R4 . capacitor
A transistor _T2 is connected to one end of _C1 . Therefore, the synchronization signal SYNC2 supplied to the base of this transistor _T2 causes the transistor T2 to
T ₂ is turned on and one end of capacitor C ₁ is connected to resistor R ₂
and R ₃ to be clamped to a predetermined DC level at the connection point. That is, the synchronization signal SYNC
Since the clamp circuit 23 operates only in the section 2, there is no problem such as clamping at the noise level K shown in FIG. 6h.

FIG. 8 shows another embodiment of the synchronous separation circuit of FIG. In this example, the synchronization signal SYNC2
The output of the monomulti 27 formed based on the synchronization signal is supplied as a muting signal to the sync separator 25 to prevent the sync separator 25 from malfunctioning due to noise components N other than the synchronization signal. At the same time, the synchronizing signal SYNC2 may be supplied to the clamp circuit 23 as in FIG. 5.

FIG. 9 is a circuit diagram showing an example of the sync separator 25. In Figure 9, the transistor
A differential switch circuit is configured by T ₄ and T ₅ , and a video signal (Fig. 6 d) is supplied to the base of T ₄ .
Further, a DC comparison voltage is supplied to the base of _T5 . Therefore, when the input of T ₄ becomes below a predetermined level in the synchronization signal part of the video signal, T ₄ turns off and T ₅ turns off.
turns on. As a result, a synchronization signal of negative polarity is obtained separately from the collector of transistor _T5 . This synchronization signal is led out to the outside as a positive polarity synchronization signal SYNC1 via an emitter follower (transistor T ₆ ) and an inverter (transistor T ₇ ).

At the base of the transistor T ₅ there is a diode
The output of the monomulti 27 in FIG. 8 (an inverted signal of the signal j in FIG. 6) is supplied via D ₁ and resistor R ₅ . Therefore, since the sync separator 25 is inactive during the period when the signal is at a low level, there is no possibility that the sync separator 25 will erroneously detect the noise N of the video signal shown in FIG. 6d. In addition, as shown by the dotted line in Fig. 9, transistor _T8 is connected between the signal line of synchronization signal SYNC1 and the ground, and this transistor
The signal j in FIG. 6j (the Q output j of the monomulti 27 in FIG. 5) may be supplied to the base of _T8 , and the output of the sync separator 25 may be muted in areas other than the sync signal portion.

Note that when recording a video signal with FM modulation, it is generally recorded with pre-emphasis applied.
In this case, the "whiskers" caused by pre-emphasis may extend to near the sink chip level (below the pedestal level). For this reason, the reproduced RF signal may contain a signal close to the frequency _p corresponding to the sync chip level, and in Fig. 3,
Noise components corresponding to the whiskers may be mixed into the synchronization signal SYNC2 obtained by performing tuned amplification and envelope detection of the reproduced RF signal.

FIG. 10 is a block diagram showing a time width detection circuit which detects the pulse width of a synchronization signal obtained by tuned amplification and envelope detection to obtain SYNC2 in order to solve the above-mentioned problem. Also the 11th
The figure is a waveform diagram of each part in FIG. 10.

As shown in FIG. 11h, the output h of the envelope detection circuit 20 in FIG. 3 may contain relatively narrow pulse-like noise N' due to pre-emphasis during recording, in addition to the synchronization signal. 10th
In the figure, the signal h is supplied to differentiating circuits 32 and 33, respectively, and the differentiating circuit 32 obtains a differentiated pulse l (FIG. 11 l) at the rising position of the signal h, and the differentiating circuit 33 provides the signal h. A differential pulse n (n in FIG. 11) at the falling position of is obtained. The differential pulse l is supplied to the trigger input of the monomulti 34, and the differential pulse n is supplied to the trigger input of the monomulti 34.
4 reset input. Therefore, as shown in FIG. 11m, the output of the monomulti 34 has a low level for a certain period of time (for example, 1 μsec) in the synchronization signal part of the signal h, and a low level in the noise part N'. A signal m whose period is extremely short is obtained.

This signal m is applied to the trigger input T of the D flip-flop 35, and the differential pulse n is applied to the reset input R of this flip-flop 35. Therefore, in the synchronization signal part of signal h, signal m
The flip-flop 35 is set at the rising edge of
It is also reset by the differential pulse n. Therefore, the output SYNC2 of the flip-flop 35 is as shown in FIG.
become that way. Also, in the noise portion N' of the signal h, the D of the flip-flop 35 is turned off at the rising edge of the signal m.
Since the input is low level, flip-flop 3
5 does not enter the set state. As a result, from the Q output of the flip-flop 35, as shown in FIG.
is obtained. That is, if the pulse width of the signal h is not greater than the time width (1 μsec) of the monomulti 34,
This pulse is removed as noise, and a pulse lasting 1 μsec or more is separated and obtained as a regular synchronization signal.

Note that the output of the envelope detection circuit 20 in Fig. 3
SYNC2 or flip-flop 35 in Figure 10
The output SYNC2 can be used in place of the synchronization signal supplied to the VTR drum servo circuit, chroma reproduction system AFC, APC circuit, etc. Figure 12 shows a normal sink separator 25 as shown in Figure 9.
FIG. 2 is a block diagram of a synchronization signal switching circuit which switches between the output SYNC1 of the SYNC1 and the SYNC2 described above.

As mentioned above, play as shown in Figure 2B a.
When long frequency undulations are superimposed on the RF signal,
As shown in Fig. 2B h, the output of limiter 4 is
Dropout may occur over a period of 1H or more (several hours). Since such a dropout cannot be fully compensated for by the dropout compensation circuit DOC, the synchronization signal SYNC1 output from the sync separator 25 is lost.

Therefore, in FIG. 12, the dropout detection circuit 7 similar to that in FIG. 1A or FIG.
The dropout of the RF signal is detected and this detection signal is supplied to the time width detection circuit 29, for example, 1H.
It is detected that the dropout continued for the above period. If the dropout continues for 1H or more, the output of the time width detection circuit 29 also changes the switch circuit 30 from the SYNC1 side to the SYNC2 side.
The configuration is such that the synchronization signal is not substantially lost.

In addition, in FIG. 3, the envelope detection circuit 2
Based on the output SYNC2 of 0, the time span t in Figure 2B S
It is also possible to form a timing signal SYNC2' (signal S _t in FIG. 2A) corresponding to the timing signal SYNC2', supply this signal SYNC2' to the limiter 4, and make the limiter 4 inactive during this signal interval. good.
In this case, as shown in Figure 2A, when the VTR is put into playback mode and only the audio signal is rewritten (dub-recording), even if the recording current leaks into the playback system, this will prevent the playback from limiter 4 in Figure 3. It is possible to prevent the system circuit from becoming saturated as much as possible.

As described above, the present invention obtains an output tuned to a frequency corresponding to the synchronization signal tip level of the video signal from a reproduced RF signal obtained by reproducing a video signal recorded with FM modulation, and detects the envelope of this output. now detects the synchronization signal. Therefore, regeneration
Detection of the synchronization signal is rarely affected by noise caused by FM signal dropouts, disturbance signals with a relatively long period (horizontal period or more) superimposed on the reproduced FM signal, and so an extremely stable synchronization signal can be obtained. Obtainable. In particular, if the synchronization signal detected in this way is used as a control signal (clamp signal, muting signal for the video signal section, gate signal, etc.) for the reproduction circuit (including the synchronization separation circuit) of the video signal reproduction device, The video signal reproducing device can operate more stably.

[Brief explanation of the drawing]

Figure 1A is a block diagram of a conventional VTR playback system, Figure 1B is a waveform diagram of each part in Figure 1A, and Figure 2A is a block diagram of a conventional VTR playback system.
The figure shows a partial block diagram of a conventional VTR that uses a video head to record audio signals.
FIG. 2B is a waveform diagram of each part of FIG. 2A, FIG. 3 is a block diagram of a VTR showing an embodiment of the present invention, and FIG.
The figure is a waveform diagram of each part in Figure 3, Figure 5 is a block diagram of the synchronous separation circuit in Figure 3, Figure 6 is a waveform diagram of each part in Figure 5, and Figure 7 is a diagram of the clamp circuit in Figure 5. Circuit diagram, FIG. 8 is a block diagram of a sync separator circuit showing a modification of FIG. 5, FIG. 9 is a circuit diagram of a sync separator in FIG. 8, and FIG.
11 is a block diagram of the time width detection circuit that detects the time width of 2. FIG. 11 is a waveform diagram of each part of FIG.
The figure is a block diagram of a synchronization signal switching circuit that switches between the synchronization signals SYNC1 and SYNC2 shown in FIG. 3. In addition, in the symbols used in the drawings, 1
7, 19... Tuning amplifier, 20... Envelope detection circuit.

Claims (1)

[Claims] 1. A playback amplifier that amplifies a playback signal of a video signal recorded by FM modulation; and a tuning circuit having a tuning point at an FM frequency corresponding to a synchronization signal tip level of an output signal of the playback amplifier. , a detection circuit that detects an envelope of an output signal from the tuning circuit, and a synchronization signal detection circuit that obtains a synchronization signal based on the output signal of the detection circuit.