JPH0516100B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a playback device suitable for application to a video tape recorder (VTR) that plays back a signal in which an audio signal is FM-modulated and recorded together with a video signal.

Background technology and its problems The luminance signal in the video signal is FM modulated on the high frequency side, and the chroma signal is converted to a low frequency so as to interleave the frequency between adjacent tracks, and two rotating heads with different azimuth angles are used. Recording is now being done without placing a guard band on the inclined track on the tape.

In such a VTR, audio signals are recorded by
This was done using a conventional fixed head on a continuous track in the tape transport direction, in a manner similar to what is known as an audio tape recorder.

However, as the recording density of the above-mentioned VTRs has been increased, the tape transport speed has been made extremely slow, and for so-called 8mm video, the tape transport speed has been reduced to approximately 1.4 mm.
cm/sec. For this reason, when recording audio signals, the relative speed between the fixed head and the tape becomes slow, making it impossible to record audio signals in good quality.

Therefore, it has been proposed to FM-modulate the audio signal in a band between the FM-modulated luminance signal and the low-frequency converted chroma signal, superimpose it on the video signal, and record it on a slope track.

That is, FIG. 1 shows an example of a so-called 8 mm video recording system. In the figure, a video signal from an input terminal 1 is supplied to an FM modulator 5 through an AGC amplifier 2, a pre-emphasis circuit 3, and a clamp circuit 4, and is subjected to FM modulation such that the sync chip is 4.2 MHz and the white peak is 5.4 MHz.
A luminance signal Y _FM is formed. This signal _YFM is supplied to a mixer 7 through a high pass filter 6.

Further, the video signal from the input terminal 1 is supplied to the low frequency conversion circuit 10 through the bandpass filter 8 and the pre-emphasis circuit 9, and the subcarrier frequency is changed.
A low frequency converted chroma signal C _D of 743.44KHz is formed. This signal C _D is supplied to the mixer 7 through the low pass filter 11.

Furthermore, the audio signal from the input terminal 12 is supplied to the low-pass filter 14 through the AGC amplifier 13, and the band is limited to 15 kHz or less. This signal is supplied to an encoder 15 for noise reduction. This encoded signal is supplied to the FM modulator 17 via the limiter 16 to form an FM audio signal _AFM having a center frequency of 1.5 MHz, for example.
This signal _AFM is supplied to the mixer 7 through a bandpass filter 18 of 1.5MHz±100kHz.

Further, a pulse signal synchronized with the rotation from a rotating head drum (not shown) is supplied to a multiplier 20 through a terminal 19 to form a signal that makes one revolution in four tracks. This signal is supplied to a four-frequency pilot signal forming circuit 21, and each track has approximately 102.54kHz, 118.95kHz, 165.21kHz,
A pilot signal varying to 148.69kHz is formed. This signal is fed to mixer 7.

Therefore, a frequency-multiplexed recording signal is taken out from the mixer 7, as shown in FIG. 2, for example. Note that only one pilot signal is provided for each track.

This recording signal is transmitted through the recording amplifier 22 to the rotary heads 23a and 2 having different azimuth angles.
3b.

In this way, video signals and audio signals are recorded.

Furthermore, in order to reproduce the signal recorded in this way, for example, the following procedure is performed.

That is, in FIG. 3, the rotating heads 23a,
The reproduction signal of 23b is transmitted to the amplifiers 31a and 31, respectively.
b. This amplifier 31a, 3
The signal from 1b is supplied to the switch circuit 32,
This switch circuit 32 is controlled by a head switching signal from a terminal 33. This switch circuit 3
The signal from YFM is supplied to a high-pass filter 34 to extract an FM luminance signal _YFM , and this signal is supplied to a limiter 35, an FM demodulator 36, and a de-emphasis circuit 37 to demodulate the luminance signal. Further, the signal from the switch circuit 32 is supplied to a low-pass filter 38 to extract a low-frequency converted chroma signal C _D , and this signal is supplied to a high-frequency conversion circuit 39, a de-emphasis circuit 40, and a band-pass filter 41 to convert the chroma signal into a low-pass filter. taken out. The luminance signal and chroma signal are mixed in a mixing circuit 42 to form a video signal, which is output to a video output terminal 43.

Further, the signal from the switch circuit 32 is supplied to a bandpass filter 44 having a center frequency of 1.5MHz, and an FM audio signal _AFM is obtained. This signal A _FM
is supplied to the FM demodulator 46 through the limiter circuit 45, and the audio signal is demodulated. This audio signal is supplied to a low pass filter 47. The signal from this low-pass filter 47 is sent to a pre-hold circuit 48 for dropout compensation, and then to a low-pass filter 4.
9, the signal is outputted to the audio output terminal 52 through the de-emphasis circuit 50 and the output amplifier 51.

The sample signal supplied to the hold circuit 48 is, for example, a signal from the switch circuit 32 supplied to an envelope detection circuit 53, and supplied to this detection output schmit circuit 54 to detect a period below a predetermined level. The signal is supplied to a monostable multivibrator 55, and the output signal of this multivibrator 55 and the signal from the Schmitt circuit 54 are combined in an OR circuit 56 to form a signal. In other words, when a dropout (DO) occurs in the reproduced signal as shown in Fig. 4A, Fig. 4B
An envelope output as shown in Figure 4C and a Schmitt output as shown in Figure 4C are formed, and a hold signal as shown in Figure 4E is generated from the multivibrator output as shown in Figure 4D and the Schmitt output described above. It is formed.

Further, the signal from the switch circuit 32 is supplied to a pilot signal detection circuit 57, and a four-frequency pilot signal is detected. This detection signal is supplied to a tracking control signal forming circuit 58, and a tracking control signal is formed according to the level of each pilot signal. This control signal is supplied through terminal 59 to a servo circuit (not shown).

In this way, the video signal and the audio signal are reproduced.

In this case, since the audio signal is FM modulated and recorded and reproduced together with the video signal by the rotary head, the audio signal will not deteriorate even if the tape transport speed is reduced.

By the way, in this device, if the demodulated output of the FM demodulator 46 is as shown in FIG. 4F due to dropout (DO), the signal obtained when this signal passes through the low-pass filter 47 is as follows. As shown in FIG. 4G, the signal is delayed to some extent and continues to oscillate after the pulse due to dropout. For this reason, in order to compensate for dropout, it is necessary to hold up to this vibrating part, and the multivibrator 55 is provided as described above, and hold is performed as shown in FIG. 4H.

However, when held for a long time in this way, the signal waveform becomes distorted and the signal deteriorates.
Especially when many dropouts occurred, the sound became extremely harsh.

On the other hand, it was considered to hold the signal before passing through the low-pass filter 47, but
In this case, there is a slight time delay in detecting the dropout, and if a hold is performed using a signal with a time delay as shown in Figure 4I, the dropout will be held in the middle of the pulse as shown in Figure 4J. There was a risk that this would cause a large level of noise.

Therefore, the applicant of the present application previously proposed a device that solved this problem. That is, in Figure 5,
The demodulated output of the FM demodulator 46 is sent to the low pass filter 4.
7 to a hold circuit 48, and this hold output and the demodulated output from the FM demodulator 46 are supplied to a switch 60. Further, the output signal of the Schmitt circuit 54 is directly supplied to the hold circuit 48 for holding, and this signal also switches the switch 60, and normally the demodulated signal is taken out, and when a dropout is detected, the hold output is taken out. . A signal from this switch 60 is supplied to a low pass filter 49.

Note that although only the path of the audio signal is shown above, the other components are constructed in the same manner as in FIG. 3.

In this device, regarding a reproduced signal having a dropout (DO) as shown in FIG. 6A,
If this demodulated output is as shown in Figure 6B,
The signal obtained by passing this signal through a low-pass filter is
If a dropout detection signal as shown in FIG. 6C is held, a signal as shown in FIG. 6D is obtained. By switching this signal to the FM demodulation output, a signal as shown in FIG. 6E is obtained. That is, the held signal is close to the original signal level, and the dropout is compensated for by using the signal for which the holding period is short.

Note that a spike-like signal appears during the delay time of dropout detection, but this is extremely short-lived.
The noise is not much different from the switching noise of the switch 60, and is removed by the low-pass filter 49 as shown in FIG. 6F.

Dropout compensation is performed in this way, and according to this device, there is less risk of a long hold period or generation of large-level noise, and it is possible to perform good compensation.

However, this device requires two low-pass filters, 47 and 49. In this case, if you try to obtain steep characteristics in the audio band, a large-scale 6th to 7th-order filter circuit is required, and a capacitor with a large capacity is required in the audio band, which is external to the IC. I end up.

Therefore, requiring two such low-pass filters makes the circuit configuration extremely large, which is disadvantageous in terms of design, and also requires a large number of external components and connection pins, which is a big problem in practice. It was hot.

OBJECTS OF THE INVENTION In view of the above-mentioned points, the present invention is intended to provide good dropout compensation with a simple configuration.

Summary of the Invention The present invention detects a dropout in the reproduced signal when reproducing a recorded audio signal by FM modulation, and passes the FM demodulated signal from the reproduced signal to a low-pass filter through a switch circuit. and a series circuit of the pre-hold circuit, or a series circuit in which these are arranged in reverse, outputting a signal from this series circuit, and placing the pre-hold circuit in a hold state with the dropout detection signal, The reproducing device is such that the output signal is fed back to the series circuit via the switch circuit, and according to this, good dropout compensation can be performed with a simple configuration.

Embodiment In FIG. 7, the demodulated output of the FM demodulator 46 is supplied to a low-pass filter 47 through a switch 60, the output of a hold circuit 48 is supplied to a de-emphasis circuit 50, and the output of this hold circuit 48 is supplied to a switch 60. will be returned. The hold circuit 48 is driven by a signal from the Schmitt circuit 54, and the switch 60 is switched to the feedback loop side.

Note that although only the audio signal path is shown above, the other components are configured in the same manner as in FIG. 3.

In this device, when a playback signal with dropout (DO) as shown in FIG. 8A occurs, distortion as shown in FIG. 8B occurs in the output of the FM demodulator 46 in FIG. 7, and detection delay occurs. Therefore, a dropout detection signal is generated from the Schmitt circuit 54 of FIG. 7 as shown in FIG. 8C. When a dropout detection signal is generated, the hold circuit 48 in FIG. 7 does not output a distorted waveform due to the dropout due to the group delay time of the low-pass filter 47 in FIG. Hold the signal.

On the other hand, the input of the switch 60 in FIG. 7 is connected to the hold circuit 48 in FIG. 7, but due to the delay in the dropout detection signal, the output of the FM demodulator 46 in FIG. A part of the distortion is output.

Furthermore, this held signal is fed back via the switch 60, so that although a spike-like signal remains at the input of the low-pass filter 47 (D in FIG. 8) at the beginning of the dropout, a constant signal thereafter remains. supplied, low pass filter 4
Although some vibration occurs in the output of No. 7 (Fig. 8E), it stops immediately and a stable signal can be obtained when the hold is released.

Dropout compensation is performed in this way, but with this device, there is little risk of a long hold period or large-level noise, and it is extremely simple, requiring only one low-pass filter circuit. Compensation can be made in the configuration.

In other words, according to this device, only one low-pass filter circuit is required, so the circuit can be miniaturized.
It is easy to convert into an IC, and costs can be reduced.

Note that the above-described device can also be used to remove noise when switching rotary heads.

Furthermore, even if the circuit configuration is such that the hold circuit 48 and the low-pass filter 47 are connected in reverse as shown in FIG. 9, the effect is the same. In this case, to ensure that the hold is performed after switching the switch 60,
An extremely short delay circuit 61 may be provided between the Schmitt circuit 54 and the control terminal of the hold circuit 48.

This device can also be applied to VTRs that perform FM modulation on the audio signal and record it deep into the magnetic layer of the tape using a head with a wide gap.

Effects of the Invention According to the present invention, it has become possible to perform good dropout compensation with a simple configuration.

[Brief explanation of the drawing]

1 to 4 are diagrams for explaining a conventional device, FIGS. 5 and 6 are diagrams for explaining a device previously proposed by the applicant, and FIG. 7 is an example of the present invention. Fig. 8 is a waveform diagram for explanation, Fig. 9 is a system diagram of
The figure is a system diagram of another example. 46 is an FM demodulator, 47 is a low-pass filter,
48 is a pre-hold circuit, 54 is a Schmitt circuit for detecting dropout, and 60 is a switch.

Claims (1)

[Claims] 1. When reproducing a recorded audio signal with FM modulation, a dropout in the reproduced signal is detected, and the FM demodulated signal from the reproduced signal is passed through a switch circuit to a low-pass filter and a pre-hold circuit. The signal is supplied to a series circuit or a series circuit in which these are arranged in reverse, and the signal from this series circuit is output, and the pre-hold circuit is put into a hold state by the dropout detection signal, and the output signal is sent to the switch. A reproducing device configured to feed back to the series circuit via a circuit.