JPH0512798A - Video reproduction device - Google Patents

Abstract

PURPOSE:To increase the margin to an inversion phenomenon and to automatically control the whole characteristics of a double limiter to the best characteristics at all times eve's. frequency characteristics of a reproduced FM signal vary by using the double limiter for the correction of the frequency characteristics of the reproduced FM signal. CONSTITUTION:An FM signal which is modulated with a video signal where a signal of specific frequency is inserted in a period other than an effective scanning period is outputted from a preamplifier 8. A high-frequency emphasizing circuit 27 compensates the frequency characteristics of the reproduced FM signal. A level comparing means and an integrator 23 compares the levels of an Nth lower sideband wave and an Nth upper sideband wave of the FM signal which is modulated with the relevant specific frequency signal in the reproduced FM signal from the high-band emphasizing circuit and control the characteristics of the high-band emphasizing circuit according to the comparison result. The reproduced FM signal has its low band emphasized by a low-band emphasizing circuit 25 and is added by an adder 29 to the output of a limiter 28 which limits the output of the high-band emphasizing circuit 26 and further inputted to an FM demodulator 11 through a limiter 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video reproducing apparatus for reproducing an FM signal from a recording medium on which an FM signal frequency-modulated by the video signal is recorded, and more particularly to a frequency characteristic of the reproduced FM signal. The characteristics of the equalizer for compensating for are automatically controlled.

[0002]

2. Description of the Related Art Generally, a video tape recorder (hereinafter referred to as VT
In the video recording / reproducing apparatus including the R),
A method is used in which a carrier signal having a frequency higher than the base band of the video signal is frequency-modulated by the video signal to form a video FM signal, which is recorded on a recording medium. The reproduced FM signal in this video recording / reproducing apparatus has a so-called low-frequency emphasis characteristic, in which the higher the frequency, the greater the attenuation amount due to the effects of the gap loss related to the gap length of the magnetic head, the space loss related to the distance from the magnetic tape, and the like. Becomes When such a signal is demodulated by the FM demodulator as it is, a harmonic component of the reproduced FM signal drops to a low frequency in the limiter, which causes a so-called inversion phenomenon in which a rising portion of the reproduced image from black to white drops to black. .. Therefore, it is necessary to equalize the frequency characteristic of the reproduced FM signal and flatten the frequency characteristic of the video signal. Conventionally, the frequency characteristic of the reproduced FM signal is equalized by using an equalizer or a combination of an equalizer and a double limiter.

By the way, it is general that the equalization characteristics of the equalizer and the double limiter are adjusted before the product is shipped and fixed thereafter. However, in some commercial products, the operator adjusts it. An adjustment trimmer is provided so that However, when the tape recorded in the normal operation is reproduced, the frequency characteristic of the reproduction FM signal is not always constant, and the setting state of the recording current, the worn state of the magnetic head, the azimuth angle between the recording head and the reproducing head are And the change in the space between the head and the magnetic tape. Therefore, several methods have been devised to automatically adjust the frequency characteristics of the video signal during playback,
It has been put to practical use. FIG. 3 is a block diagram of a conventional video reproducing apparatus having an automatic equalization circuit.

The video signal introduced to the input terminal 1 is FM
It is input to the modulator 2 and frequency-modulated to become a video FM signal. The video FM signal is amplified by the recording amplifier 3 and is passed through the recording / reproduction changeover switch 4 to rotate the rotary drum 5.
It is supplied as a recording signal to the magnetic head 6 provided above.

On the other hand, at the time of reproduction, the video FM signal read by the magnetic head 6 from the magnetic tape 7 is supplied to the preamplifier 8 via the recording / reproducing switch 4, and a sufficient level necessary for the subsequent signal processing. Is amplified up to. The output of the preamplifier 8 is input to the variable equalizer 9 and the frequency characteristic is compensated. The output of the variable equalizer 9 is branched into two, one of which is connected to the FM demodulator 11 via the limiter 10.
And is demodulated to a video signal. Then, the demodulated video signal has its emphasis characteristic at the time of recording restored to its original value in the de-emphasis circuit 12 and is derived from the video output terminal 13.

The other output of the variable equalizer 9 is introduced into the level comparing means surrounded by a dotted line. First, it is input to the buffer amplifier 14, amplified, and further branched into two. One of the outputs of the buffer amplifier 14 is input to the trimmer 15 and gain-adjusted, and then input to the first lower sideband bandpass filter 16. The first lower sideband of the FM signal derived from the bandpass filter 16 is input to the envelope detector 17 and envelope-detected. The other output of the buffer amplifier 14 is input to the bandpass filter 18 for the first upper sideband, and the first upper sideband obtained from the bandpass filter 18 is input to the envelope detector 19 and envelope detected. ..

The outputs of the envelope detectors 17 and 19 are
It is input to the subtractor 20 and level comparison is performed. The difference output signal of the subtractor 20 is input to the sample hold circuit 21, and the burst signal portion is sampled and held. The sampling and holding circuit 21 is supplied with the sampling pulse from the sample pulse generator 22. The sample pulse generator 22 creates a sampling pulse using the video signal of the output terminal 13. The output of the sampling and holding circuit 21 is input to the integrator 23 and integrated, and the integrated output is supplied to the control terminal of the variable equalizer 9 as a control voltage.

With such a video reproducing apparatus, adjustment of equalization characteristics is automated. However, considering reproduction of a tape recorded by overmodulation, equalization by a single equalizer has a low margin for the inversion phenomenon. Therefore, equalization by using an equalizer and a double limiter together is desirable.

The double limiter splits the reproduced FM signal into two after passing through a high-pass emphasis circuit such as an equalizer, one of which is input to a first limiter via a high-pass filter and the other is input to a low-pass filter. The outputs of the first limiter and the low-pass filter are added and input to the second limiter. By doing so, the margin against the inversion phenomenon becomes sufficiently high. However, due to the structure of the double limiter, it is connected after the equalizer, so even if the equalizer automatic adjustment means is applied to the equalizer, the equalization characteristic will change due to the double limiter, resulting in incomplete adjustment of the equalization characteristic. Will end up.

[0010]

As described above, according to the conventional circuit, even if an equalizer or a double limiter is used to compensate for the low-frequency emphasis characteristic of the reproduced FM signal, the equalization characteristic is reproduced due to various causes. The inversion phenomenon cannot be completely eliminated without following the frequency characteristics of the FM signal.

Therefore, an object of the present invention is to provide a video signal reproducing apparatus which increases the margin against the inversion phenomenon and automatically adjusts the equalization of the frequency characteristic of the reproduced FM signal without increasing the circuit scale. ..

[0012]

The present invention reproduces an FM signal from a recording medium on which an FM signal frequency-modulated by a video signal in which a signal of a specific frequency is inserted in a period other than the effective scanning period is recorded. Reproducing means, a high-frequency emphasis circuit for compensating the frequency characteristic of the reproduced FM signal obtained by the reproducing means, and a reproduction FM at the output of the high-frequency emphasis circuit.
FM frequency-modulated by the signal of the specific frequency in the signal
Level comparing means for comparing the levels of the Nth lower sideband and the Nth upper sideband of the signal, and a control circuit for controlling the characteristics of the high-frequency emphasis circuit based on the comparison result of the level comparing means,
A first limiter for making the output amplitude of the emphasizing circuit constant, a low frequency emphasizing circuit for emphasizing the low frequency band of the reproduced FM signal, and an output of the low frequency emphasizing circuit and an output of the first limiter are added. The adder, a second limiter that keeps the output amplitude of the adder constant, and an FM demodulation circuit that receives the output of the second limiter as an input.

[0013]

The following effects can be obtained by the above means. That is, in the frequency spectrum of the FM signal, sidebands are vertically symmetrical with respect to the carrier signal frequency. However, the reproduced FM signal of the VTR has a low-frequency emphasis characteristic. In order to correct this characteristic, the levels of the upper and lower sidebands are compared with the output of the high-frequency emphasizing circuit of the double limiter, and the characteristic of the high-frequency emphasizing circuit is controlled by the comparison result. As a result, the compensation of the entire frequency characteristic can be automated and the margin for the reversal phenomenon becomes high.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention and shows a block of a reproduction signal processing circuit of a video reproducing apparatus (VTR).

A magnetic head 6 mounted on the rotary drum 5.
Thus, the video FM signal read from the magnetic tape 7 is amplified by the preamplifier 8 to a sufficient level necessary for the subsequent signal processing. The reproduced FM signal output from the preamplifier 8 is input to the low-frequency emphasis circuit 25 and the high-frequency emphasis circuit 27. The output of the low-frequency emphasis circuit 25 is the amplitude adjustment circuit 26.
Via the limiter 28, and the output of the high frequency emphasizing circuit 27 is input to the adder 29 via the limiter 28. The output of the adder 29 is sent to the FM demodulator 1 via the limiter 10.
1 and FM demodulated, and the demodulated video signal is input to the de-emphasis circuit 12 to release the emphasis characteristic at the time of recording, returned to the original state, and output to the output terminal 13 as a reproduced video signal.

The output of the high-frequency emphasizing circuit 27 is also input to the buffer amplifier 14 of the level comparing means surrounded by the dotted line. The output of the buffer amplifier 14 is input to the bandpass filter 16 via the trimmer 15 and directly to the bandpass filter 18. The trimmer 15 is actually set at the adjustment stage, and is a semi-fixed gain adjuster. The bandpass filter 16 has a first
The lower passband for the lower sideband is extracted and the lower passband of the burst signal is extracted. The bandpass filter 18 is for the first upper sideband and extracts the upper sideband of the burst signal.

The outputs of the bandpass filters 16 and 18 are
Input to envelope detectors 17 and 19, respectively, and F
The envelope of the M signal is detected. Detector 17, 19
The output of is input to the subtractor 20 and the levels are compared to derive the difference signal. The output signal of the subtractor 20 is input to the sample hold circuit 21. Here, the burst signal portion is sampled and held by the sampling pulse from the sampling pulse generator 22. This hold signal is input to the integrator 23 and integrated into a direct current, and the output thereof is supplied to the control terminal of the high frequency emphasis circuit 27 as a control voltage. As a result, the high frequency emphasis circuit 2
In No. 7, the frequency characteristics are compensated optimally for the reproduced FM signal. That is, this compensation loop operates so that the upper or lower band component around the burst signal frequency is adjusted and the difference output in the subtractor 20 becomes zero.

The trimmer 15 sets the ratio of the level of the first lower sideband and the level of the first upper sideband of the burst signal portion in the reproduced FM signal, and the trimmer 15 adjusts the high frequency range. The frequency characteristic of the reproduced FM signal output from the emphasis circuit 27 is determined.

The high frequency adjusting circuit 27 can be realized by a combination of a cosine equalizer and an AGC amplifier as shown in FIG. The input signal is input to the delay line 32 via the buffer amplifier 30 for impedance conversion and the resistor 31 for input impedance matching. The output of the delay line 32 is input to the first automatic gain control (AGC) amplifier 33, and the output of the resistor 31 is also input to the second automatic gain control (AGC) amplifier 35. The first AGC amplifier 33 is composed of a fixed gain non-inverting amplifier, and the second AGC amplifier 35 is an inverting amplifier whose gain is changed by the control voltage supplied from the high frequency emphasis circuit 27 being given as the AGC voltage. The outputs of these AGC amplifiers 33 and 35 are added by the adder 34 and become the output of the high frequency emphasizing circuit 27. In this case, by using the AGC amplifiers 33 and 35 having the same delay characteristics, the delay characteristics of the AGC amplifier can be canceled and the characteristics of the equalizer can be improved.

In the video reproducing apparatus configured as shown in FIG. 1, the output voltage of the sample and hold circuit 21 becomes zero, that is, the first lower band wave and the first upper band wave of the burst signal portion in the reproduced FM signal. The high-frequency adjusting circuit 2 so that the wave level difference becomes a constant value determined by the adjustment of the trimmer 15.
7 characteristics are controlled. As a result, even if the frequency characteristics of the video FM signal change during the recording / reproducing process, the high frequency emphasizing circuit 2
7, the input signal of the limiter 28 is automatically optimally compensated, and the inversion phenomenon can be avoided.
Further, with such a configuration, the circuit scale can be reduced.

[0021]

As described above, according to the present invention,
In the correction of the frequency characteristic of the reproduced FM signal, it is possible to increase the margin for the inversion phenomenon by using the double limiter. Further, in the present invention, the first FM signal
Since the levels of the lower sideband and the first upper sideband are detected and compared, and the characteristics of the high-frequency emphasis circuit of the double limiter are controlled based on the level comparison result, the entire characteristics of the double limiter are reproduced FM. Even if the frequency characteristic of the signal changes, it can always be automatically controlled to the optimum characteristic.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing a specific example of a high-frequency emphasis circuit shown in FIG.

FIG. 3 is a block configuration diagram of a conventional video recording / reproducing apparatus.

[Explanation of symbols]

5 ... rotary drum, 6 ... magnetic head, 7 ... magnetic tape, 8
... Preamplifier, 25 ... Low-frequency emphasis circuit, 26 ... Amplitude adjustment circuit, 27 ... High-frequency emphasis circuit, 10, 28 ... Limiter, 29
... Adder, 11 ... FM demodulator, 12 ... De-emphasis circuit, 14 ... Buffer amplifier, 15 ... Trimmer, 16, 1
8 ... Band pass filter, 17, 19 ... Envelope detector, 20 ... Subtractor, 21 ... Sample and hold circuit, 2
2 ... Sampling pulse generator.

Claims (1)

Claims: 1. A specific frequency is set in a period other than the effective scanning period.
The FM signal frequency-modulated by the video signal with the signal inserted
Playback of the FM signal from the recorded recording medium
And a frequency characteristic of the reproduced FM signal obtained by this reproducing means.
And a high-frequency emphasis circuit for compensating for the
The Nth lower part of the FM signal frequency-modulated with the signal of the specific frequency
Level comparer to compare the levels of sideband and Nth upper sideband
And the high-frequency emphasis based on the comparison result of this level comparison means.
A control circuit for controlling the characteristics of the circuit, and a first limiter for keeping the output amplitude of the emphasizing circuit constant.
A low frequency emphasizing circuit for emphasizing the low frequency band of the reproduced FM signal, and an output of the low frequency emphasizing circuit and an output of the first limiter.
An adder for adding, a second limiter for making the output amplitude of the adder constant, and an FM demodulation circuit for receiving the output of the second limiter as an input
An image reproducing apparatus comprising: