JPH0746472B2 - Detection level adjustment method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reproduction audio signal in a VTR (video tape recorder) which reproduces a signal recorded by, for example, FM-modulating an audio signal and frequency-multiplexing it with a video signal. The present invention relates to a detection level adjusting method in a circuit for detecting dropout.

Background art and its problems In a VTR, the luminance signal in the video signal is FM-modulated on the high frequency side, and the chroma signal is low-frequency converted so as to be frequency interleaved between adjacent tracks. Recording is performed without tilting the guard band on the slanted track on the tape by using the rotating magnetic head.

In such a VTR, recording of an audio signal is conventionally performed on a continuous track in the tape transfer direction by using a fixed head by a method similar to what is called an audio tape recorder.

By the way, in the above-mentioned VTR, if the transfer speed of the tape is lowered in order to increase the recording density, the relative speed between the fixed head and the tape in recording the audio signal becomes slow, and it becomes difficult to record the audio signal well.

Therefore, conventionally, an audio signal is FM-modulated between an FM-modulated luminance signal and a low-frequency-converted chroma signal, frequency-multiplexed with a video signal, and recorded on an inclined track.

That is, in FIG. 1, the video signal recording circuit (1) is supplied with the video signal S _V from the terminal (2), and from this, the FM modulated luminance signal Y _FM and the low-frequency chroma signal C _D are obtained. (See FIG. 2) and supplied to the synthesizers (3A) and (3B), respectively.

Also, the audio signal S _A is supplied to the terminal (4) and this is FM.
It is supplied to the modulators (5A) and (5B). Then, in each of them, FM modulation is performed at carrier frequencies of frequencies f ₁ and f ₂ between the luminance signal Y _FM and the chroma signal C _D, and the modulated voice signals are respectively obtained.
S _AFM1 and S _AFM2 are obtained, and the band pass filter (6
It is supplied to the combiners (3A) and (3B) via A) and (6B).

The signals obtained from the synthesizers (3A) and (3B) are supplied to rotary magnetic heads H _A and H _B having different azimuth angles via recording amplifiers (7A) and (7B), respectively.

The heads H _A and H _B are arranged at an angular interval of about 180 °, and are rotated once in one frame, for example, and the heads H _A and H _B are
The magnetic tape is sequentially scanned for each field.

As described above, signals are sent to the head H _A [S _AFM1 , C _D , Y _FM ].
_And a signal [S _AFM2 , C _D , Y _FM ] is supplied to the head H _B. Therefore, as shown in FIG. 3, the recording tracks T _{A having} different azimuths are recorded on the magnetic tape (8). as well as
T _B are formed alternately.

In this way, the video signal S _V and the audio signal S _A are recorded.

Then, the signal thus recorded is reproduced as shown in FIG.

That is, during reproduction, the heads H _A and H _B scan the recording tracks T _A and T _B , respectively. Therefore, head H _A
And H _B from signals [S _AFM1 , C _D , Y _FM ] and [S _AFM2 , C _D , Y
_FM ] is played.

The reproduction signal of the head H _A is supplied to the terminal on the A side of the switch circuit (10) via the reproduction amplifier (9 A). Also the head
The reproduction signal of H _B is supplied to the B side terminal of the switch circuit (10) through the reproduction amplifier (9 B). Switch circuit (10)
Switching pulse P _SW obtained in accordance with the rotational position of the head H _A and H _B from terminal (11) is supplied to the head H _A
Is connected to the A-side terminal during one field period during which the track T _A is scanning the track T _A , while it is connected to the B-side terminal during one field period during which the head H _B is scanning the track T _B. The continuous signal obtained from the switch circuit (10) is supplied to the video signal reproducing circuit (12), from which the luminance signal Y and the chroma signal C are obtained.

Further, after the reproduced signal of the head H _A that is passing through the reproduction amplifier (9A), a band-pass filter (13 center frequency is f ₁
A). A modulated audio signal S _AFM1 is obtained from this band pass filter (13A), and this is a switching circuit (1
It is supplied to the A side terminal of 4). Also, the reproduction signal of the head H _B passes through the reproduction amplifier (9 B) and then has a center frequency of f ₂
Is supplied to the bandpass filter (13B). A modulated audio signal S _AFM2 is obtained from this band pass filter (13B), and this is supplied to the frequency converter (15). The converter (15) sends a signal (frequency f ₂
-F ₁ ) is supplied to the output side of which a modulated audio signal S _AFM1 whose carrier frequency has been converted to f ₁ is obtained, which is _supplied to the B side of the switch circuit (14) via the bandpass filter (17). Supplied to the terminal.

From the terminal (11) to the switch circuit (14), heads H _A and H _B
The switching pulse P _SW obtained according to the rotational position of the head H _A is connected to the terminal on the A side for one field period during which the head H _A scans the track T _A , and the head H _B scans the track T _B. During one field period, it is connected to the B side terminal. The continuous modulated audio signal S _AFM1 obtained from the switch circuit (14) is supplied to the FM demodulator (19) via the limiter (18), and the continuous audio signal S _A is obtained from the demodulator (19). .

The audio signal S _A is obtained at the output terminal (23) through the low pass filter (20), the sample hold circuit (21) and the low pass filter (22).

By the way, if the tape has scratches or defects, dropouts occur in the reproduction signals of the heads H _A and H _{B at} the places of scratches or defects. However, when the audio signal S _A is recorded on the audio track of the tape by the fixed head without being modulated, the width of the audio track is usually relatively large, and scratches and defects are spread over the entire width direction of the audio track. Since there is no such thing, the dropout of the reproduction signal is hardly a problem. However, when the FM-modulated audio signals S _AFM1 and S _AFM2 are multiplexed on the video signal as described above and recorded on the track inclined with respect to the tape running direction by the rotary magnetic heads H _A and H _B , Since the width of the track is small, if there are scratches or defects, the reproduced signals of the FM-modulated audio signals S _AFM1 and S _AFM2 will drop out, and the output of the FM demodulator (19), that is, the reproduced audio signal S Pulsed noise is generated on _A.

The sample-and-hold circuit (21) described above pre-holds the audio signal S _A in such a dropout portion to avoid noise appearing in the audio signal S _A due to the dropout.

The detection of this dropout is conventionally performed as follows, for example. That is, band pass filter (13A)
And (13B) modulated audio signals S _AFM1 and S _AFM2
Are respectively supplied to the A side and the B side of the switch circuit (24). The switching pulse P _SW is supplied to the switch circuit (24) from the terminal (11), and is connected to the terminal on the A side for one field period during which the head H _A scans the track T _A , and the head H _B is connected to the track T _A. During one field period in which _B is scanned, it is connected to the B side terminal. The modulated audio signals S _AFM1 and S _AFM2 obtained from the switch circuit (24) are supplied to a variable gain amplifier (25) which constitutes an AGC circuit. In this case, the output of the amplifier (25) is supplied to the level detection circuit (26), which is compared with the reference voltage V _REF, and the gain of the amplifier (25) is controlled and AGC is applied by this comparison error signal. The output of the amplifier (25) is supplied to the comparator (29) via the full-wave rectification circuit (27) and the smoothing circuit (28), and the threshold level Vth for detecting dropout is detected in the comparator (29). When the output V ₀ of the smoothing circuit (28) is lower than this threshold level Vth, the dropout detection signal S _D is output. The dropout detection signal _SD is supplied to the sample hold circuit (21) described above as a control signal.

According to the example of FIG. 4, the input of the amplifier (25) is as shown in FIG.
When you have a dropout as shown in
On the output side, an output as shown in FIG. 9B is obtained in which AGC is also applied to this dropout portion. Therefore, no matter how the threshold level Vth is adjusted in the comparator (29), the dropout detection signal S _D does not correspond to the original dropout portion, and as shown in FIG. However, the sample-and-hold circuit (21) cannot sufficiently prevent pulse noise due to dropout.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is possible to easily adjust the dropout detection level and obtain a good dropout detection signal corresponding to the dropout.

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention supplies an FM-modulated audio signal to a comparator via a variable gain amplifier and a rectifying / smoothing circuit, and an output signal of this comparator is used as a dropout detection signal. A circuit is provided, and the threshold level of the comparator is set to a level that is a predetermined amount lower than the reference playback level of the FM-modulated audio signal, and the reference tape is played back so that the output signal of the variable gain amplifier is the reference The gain of the variable gain amplifier is adjusted so that the reproduction level is achieved.

Since the dropout detection level is adjusted in this manner, the adjustment is easy and a good dropout detection signal corresponding to the dropout can be obtained.

Example 6 FIG. 6 shows an FM modulation signal S reproduced by a magnetic head.
_It shows the relationship between the _AFM and the S / N of the demodulated signal S _A. That is, in FIG. 7, (30) is a magnetic head and (3)
1) is reproduction amplifier and bandpass filter, (32) is FM
A magnetic tape that is a demodulator and changes the recording current in sequence (33)
Relationship between the level of the FM modulation signal S _AFM on the input side of the FM demodulator (32) and the S / N of the signal S _A on the output side when the FM modulation signal S _AFM is reproduced by the magnetic head (30) Is shown. According to the experiment, it was found that the relationship between the FM modulation signal S _AFM and the S / N of the demodulation signal S _A is the same regardless of the type of the magnetic tape (33). This means that if there is no variation in the gain of the heads and amplifiers of each set, the relationship between the FM modulation signal S _AFM and the S / N of the demodulation signal S _A at that time is as shown in FIG. Means to become. In FIG. 6, when the level of the FM modulation signal S _AFM becomes equal to or lower than E _D , the S / N is extremely deteriorated, and it can be regarded equivalently as a dropout state, and this level E _D is the dropout detection level. do it. That is, assuming that there is no variation in the gain of the head and amplifier of each set, the dropout detection level is uniquely this level B _D.

However, in reality, the gains of heads and amplifiers in each set are not constant and have variations, and the detection level of dropout is usually not uniquely determined.

By the way, in this case, the reference tape on which the FM modulation signal is recorded at the standard level is reproduced, the reproduction output is supplied to the variable gain amplifier, and the FM modulation signal S obtained by this variable gain amplifier is reproduced.
_{When the} gain of this variable gain amplifier is adjusted so that the _AFM level becomes a constant level B _REF (see Fig. 6), the FM modulation signal S _AFM and the demodulation signal S _A obtained from this variable gain amplifier are adjusted.
All relationships with S / N are as shown in FIG. Therefore, as long as the dropout is detected based on the FM modulation signal S _AFM obtained from the variable gain amplifier adjusted in this way,
Even if the gains of the heads and amplifiers of each set vary, the dropout detection level can be uniquely set to _BD .

The present invention takes this point into consideration.

FIG. 8 shows a reproducing system to which the method of the present invention is applied. The parts corresponding to those in FIG. 4 are designated by the same reference numerals, and detailed description thereof will be omitted.

In the figure, the modulated audio signals S _AFM1 and S _AFM2 obtained from the switch circuit (24) are supplied to the variable gain amplifier (34). The output of this amplifier (34) is supplied to the comparator (29) via the full-wave rectification circuit (27) and the smoothing circuit (28), and the threshold level for dropout detection is detected in this comparator (29). Output V of the smoothing circuit (28) compared with Vth
_{When 0} is lower than this threshold level Vth, the dropout detection signal S _D is output. And this detection signal
S _D is supplied to the sample hold circuit (21) as a control signal.

Others are the same as those in the example of FIG.

In such a circuit, the dropout detection level is adjusted as follows.

First, the threshold level Vth of the comparator (29)
Modulated audio signal obtained at the output side of the variable gain amplifier (34)
Dropout detection level E for S _AFM1 and S _AFM2 levels
Output of smoothing circuit (28) when _D (see FIG. 6)
Make it equal to V ₀ ′.

Next, the heads H _A and H _B reproduce the reference tape (35) on which the signals are recorded at the standard level. At this time, the modulated audio signals S _AFM1 and S _AFM1 obtained at the output side of the amplifier (34)
The gain of the amplifier (34) is adjusted while watching the level meter (36) so that the level of _AFM2 becomes the reference reproduction level E _REF (see FIG. 6), and the adjustment of the dropout detection level is completed. When the levels of the modulated audio signals S _AFM1 and S _AFM2 are different from each other, adjustment is performed with the lower level as a reference. For example, the dropout detection level E _D is set to about −20 dB of the reference reproduction level E _REF .

According to such an adjusting method, the relationship between the modulated audio signals S _AFM1 and S _AFM2 obtained from the amplifier (34) and the S / N of the demodulated signal S _A is
Because of the relationship shown in FIG. 6, the dropout detection level is uniquely determined by E _D. And comparator (29)
The threshold level of Vth is V corresponding to this level E _D
Since it is set to ₀ ', a good dropout detection signal S _D corresponding to the dropout can be obtained from this comparator (29).

Further, according to such an adjusting method, only the steps described above are performed, and the adjustment can be performed extremely easily.

EFFECTS OF THE INVENTION As is clear from the embodiments described above, the method of the present invention makes it possible to easily adjust the dropout detection level. Moreover, it is possible to reliably obtain a good dropout detection signal corresponding to the dropout.

[Brief description of drawings]

FIG. 1 is a block diagram of a VTR recording system, FIGS. 2 and 3 are diagrams for explaining each, FIG. 4 is a block diagram of a VTR reproducing system, and FIG. 5 is a diagram for explaining the same. 6 and 7 are diagrams for explaining the present invention, and FIG. 8 is a block diagram of a VTR reproduction system to which the method of the present invention is applied. (19) is an FM demodulator, (21) is a sample and hold circuit,
(27) is a full-wave rectifier circuit, (28) is a smoothing circuit, (29) is a comparator, (34) is a variable gain amplifier, (35) is a reference tape,
(36) is a level meter.

Claims (1)

[Claims] 1. An FM-modulated audio signal reproduced by a magnetic head is output from a magnetic track formed by contacting a magnetic tape and a magnetic head at a predetermined relative speed through a demodulation circuit and a sample hold circuit. Together with the FM-modulated audio signal is supplied to the comparator via the variable gain amplifier, rectification, smoothing circuit without passing through the automatic gain control amplifier, and the output signal of this comparator is the dropout detection signal. When a dropout is detected by the detection signal, the sample-hold circuit is controlled according to the duration of the detection signal to perform pre-hold during the dropout period. Set the hold level to a level that is lower than the reference playback level of the FM-modulated audio signal by a predetermined amount, A detection level adjusting method comprising reproducing and adjusting a gain of the variable gain amplifier so that an output level of the variable gain amplifier becomes a reference reproduction level which is larger than the threshold level by a predetermined amount.