JPH0822085B2 - Secondary beat-cancell circuit - Google Patents

Description

The present invention relates to a secondary beat cancel circuit in a VTR reproducing circuit.

[Outline of Invention]

The present invention, in the secondary beat cancel circuit provided in the reproduction circuit of the VTR, forms a signal having a frequency twice as high as the low-frequency carrier frequency from the reproduced converted color signal having the low-frequency carrier frequency. By adding a frequency signal to the FM demodulated luminance signal in anti-phase, the secondary beat mixed in the FM demodulated luminance signal is removed.

[Prior Art] In a VTR that records a low-frequency-converted carrier color signal on a magnetic tape together with an FM-modulated luminance signal, a frequency double the carrier frequency of the low-frequency converted color signal due to the non-linear characteristics of the tape head system. The so-called secondary beat is mixed in the reproduced FM modulated luminance signal. That is, the carrier wave (co
s ω _Y t) and the low-frequency carrier (cos ω _C t) are recorded in multiplex, due to the limiter characteristics during electromagnetic conversion, (cos ω _Y t + c
The term of os ω _C t) ³ is included in the reproduced signal, and when this term is expanded, the term of (3cos ω _Y t cos ² ω _C t) appears. This means that a component of cos (ω _Y ± 2ω _C ) t is generated, and due to the characteristics of the RF system of the reproducing system, (cos (ω _Y + 2ω _C ))
Since t <cos (ω _Y −2ω _C ) t), a secondary beat (2ω _C ) with the carrier (cos ω _Y t) of the FM modulated luminance signal is generated.

Such secondary beats cause diagonal stripes in the reproduced image, and the image quality is significantly deteriorated. Therefore, in the conventional VTR,
A secondary beat cancel circuit for canceling the secondary beat is provided. As the secondary beat cancel circuit, for example, Japanese Patent Application No. 58-159461 or Japanese Patent Application No. 59-22.
Some are described in the specification of No. 353.

The conventional secondary beat cancel circuit has a configuration in which the secondary beat is extracted from the FM-demodulated reproduction luminance signal and the extracted secondary beat is added in antiphase to the reproduction luminance signal. When the secondary beat is extracted from the luminance signal, the mere frequency separation causes deterioration of the luminance signal in the vicinity of the frequency. Therefore, a comb filter using vertical correlation has been used.

[Problems to be solved by the invention]

However, there is a problem that the luminance signal having no vertical correlation and the secondary beat cannot be distinguished, the edge waveform of the luminance signal is deteriorated, and the contour of the reproduced image is blurred. Therefore, a method has been proposed in which the output signal of the comb filter is modulated by the envelope of the reproduced carrier color signal to form the cancel signal. However, even in the case of this method, there is a problem that the edges of the colored diagonal stripes are blurred.

Therefore, an object of the present invention is to remove only the secondary beat without degrading the luminance signal itself in the case of a video signal containing a luminance signal having no vertical correlation such as colored diagonal stripes. It is to provide a beat cancel circuit.

[Means for solving problems]

The present invention is a secondary beat canceller provided in a VTR reproduction circuit in which a carrier color signal is converted to a low carrier frequency, a luminance signal is FM-modulated, and a converted color signal and an FM-modulated luminance signal are mixed and recorded. In the circuit, a means for separating the converted color signal from the VTR reproduction signal, a double balanced modulator that is supplied with the converted color signal and generates a signal with a frequency twice the low frequency carrier frequency, and a double balanced modulator A gain controller that supplies a signal and generates a cancel signal at its output, a phase adjuster that adjusts the phase of the cancel signal, and an adder that adds the cancel signal in reverse phase to the FM demodulated luminance signal. , A cancellation signal and a luminance signal from the adding means, and a synchronous detector whose output signal is supplied to the gain controller as a gain control signal. It is a cell circuit.

[Action]

The secondary beat occurs when the converted color signal converted to the low carrier frequency is mixed with the FM modulated luminance signal and recorded on the tape. Therefore, the secondary beat is nothing but a converted color signal component. Therefore, by supplying the converted color signal separated from the reproduction signal to the double balanced modulator, a frequency (2f _C ) that is twice the low-frequency carrier frequency f _C is formed, and the level and phase of this signal are adjusted. By doing so, a cancel signal can be formed. In the conventional secondary beat cancel circuit, since the secondary beat is extracted from the FM demodulated luminance signal, the luminance signal in the vicinity of twice the low carrier frequency is deteriorated. However, in the present invention, such deterioration of the luminance signal does not occur.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. First reference example to which this invention can be applied
Shown in the figure. In FIG. 1, 1 is a rotary head type VT
This is an input terminal to which the R reproduction signal is supplied. The reproduction signal is supplied to the high pass filter 2 and the low pass filter 3. The FM-modulated luminance signal is separated into the output of the high-pass filter 2, and this modulated luminance signal is supplied to the FM demodulator 4. The output signal of the FM demodulator 4 is supplied to the low-pass filter 5 that removes unnecessary signals, and the reproduction luminance signal is obtained from the low-pass filter 5. This reproduced luminance signal is supplied to the adder 6. In the adder 6, a cancel signal formed as described below is added to the luminance signal in anti-phase.
The output signal of the adder 6 is supplied to the adder circuit 7, the reproduction luminance signal and the reproduction carrier color signal are mixed, and a reproduction color video signal is obtained at the output terminal 8.

The low-pass filter 3 allows the low carrier frequency f _C (for example,
The converted color signal converted to (688 kHz) is separated. This converted color signal is supplied to the frequency converter 9. The frequency converter 9 uses the carrier signal for conversion formed by the APC circuit to convert the converted color signal into the original carrier frequency (3.58 MHz).
Return to. The output signal of the frequency converter 9 is supplied to the comb filter 11 via the bandpass filter 10. The comb filter 11 is provided to remove crosstalk from adjacent tracks. The reproduction carrier color signal obtained from the comb filter 11 is supplied to the adding circuit 7 and mixed with the reproduction luminance signal.

The converted color signal from the low-pass filter 3 is a phase adjustment circuit.
12 and the output signal of the phase adjustment circuit 12 is supplied to the double balanced modulator 13. The phase adjustment circuit 12 is configured to add the output signals of the collector and the emitter of the transistor via the variable resistor and the capacitor, for example. The output signal of the phase adjustment circuit 12 is supplied to the two input terminals of the double balanced modulator 13, and the multiplication calculation force signal of the output signal of the phase adjustment circuit 12 is obtained at the output terminal of the double balanced modulator 13.

The output signal of the double balanced modulator 13 is supplied to the tuning circuit 14. The tuning circuit 14 is provided to extract the component of the frequency 2f _C in the output signal of the double balanced modulator 13.
The output signal of the tuning circuit 14 is level-adjusted in the gain control amplifier 15, and the cancel signal is obtained from the gain control amplifier 15. This cancel signal is supplied to the adder circuit 6 in anti-phase, so that the secondary beat in the reproduced luminance signal is removed.

The phase adjustment circuit 12 and the level adjustment circuit 15 need not be adjusted once they are adjusted at the time of shipment, as long as they are recorded and reproduced on the same VTR. Further, even if the adjustment of each VTR is not performed, even if it is adjusted so as to cancel the average value of the generated secondary beats, there is a considerable effect.

FIG. 2 shows the basic configuration of the present invention. That is, the reproduction conversion color signal from the input terminal 16 is supplied to the frequency doubler 17, a cancellation signal is obtained from the frequency doubler 17, and the cancellation signal is inverted in the adder circuit 6 with respect to the reproduction luminance signal from the input terminal 19. A reproduction luminance signal in which the secondary beats are canceled is obtained at the output terminal 18 by adding in phases.

An embodiment of the present invention is shown in FIG. The output signal of the double-balanced modulator 13 is supplied to the tuning circuit 14, and the signal of the frequency of 2f _C is extracted from the tuning circuit 14 as in the above-described embodiment. This signal is supplied to the gain control amplifier 21 and the 90 ° phase shift circuit 22. The output signal of the 90 ° phase shift circuit 22 is supplied to the gain control amplifier 23. Gain control amplifiers 21 and 23
The respective output signals of the above are supplied to the adding circuit 6 through the capacitors 24 and 25, respectively. That is, the gain control amplifiers 21 and 23
A cancellation signal is formed by adding the output signals of the above.

The output signal obtained from the adder circuit 6 is the double balanced modulator 26.
And 29. These double balanced modulators 26,29
Constitute a synchronous detection circuit. The output signal of the double balanced modulator 26 is supplied as a control voltage to the gain control amplifier 21 via the low pass filter 27 and the DC amplifier 28. The output signal of the double balanced modulator 29 is supplied as a control voltage to the gain control amplifier 23 via the low-pass filter 30 and the DC amplifier 31. The gain control amplifiers 21 and 23 are capable of arbitrarily changing the gain from negative to positive corresponding to the control voltage.

The operation of the above-described embodiment will be described with reference to FIG.
A signal corresponding to the secondary beat a included in the luminance signal A supplied to the adder circuit 6 is supplied to the gain control amplifier 21, and a signal c of a component orthogonal to this is supplied to the gain control amplifier 23. Therefore, the gain of each of the gain control amplifiers 21 and 23 is changed.
Assuming K1 and K2, the signal obtained by adding the signals passed through the capacitors 24 and 25 is (K1b + K2c) as shown by the broken line in FIG. Therefore, the output signal of the adder circuit 6 is as shown below.

A + a- (K1b + K2c) The gains K1 and K2 of the gain control amplifiers 21 and 23 are feedback-controlled so that the quadrature component of the secondary beat becomes zero. In other words, it is controlled so that [a = K1b + K2c], and the secondary beat a in the output signal of the adder circuit 6 is removed by the cancel signal.

〔The invention's effect〕

According to the present invention, since it is not necessary to extract the secondary beat from the reproduction luminance signal, the reproduction luminance signal
There is no deterioration of frequency components near 2f _C. Further, as in the conventional case, in order to prevent the deterioration of the waveform of the reproduction luminance signal due to the non-correlation component of the reproduction luminance signal, the secondary beat extracted from the reproduction luminance signal is modulated by the envelope of the converted color signal, etc. Is unnecessary, and the circuit configuration can be simplified. Further, according to the present invention, since the gain of the cancel signal is feedback-controlled by the synchronous detection output of the luminance signal from which the secondary beat has been removed and the cancel signal, a reliable cancel operation can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram of a reference example to which the present invention can be applied, FIG. 2 is a block diagram used for explaining the principle of the present invention, and FIG. 3 is a block diagram of an embodiment of the present invention.
FIG. 1 is a schematic diagram for explaining one embodiment of the present invention. Description of main symbols in the drawings 1: Input terminal, 3: Low-pass filter, 4: FM demodulator, 6,7: Adder circuit, 8: Output terminal, 12: Phase adjustment circuit, 13: Double balanced modulator, 15 : Level adjustment circuit, 21, 23: Gain control amplifier, 22:
90 ° phase shift circuit, 26, 29: Double balanced modulator for synchronous detection.

Claims (1)

[Claims] 1. A carrier color signal is converted to a low carrier frequency,
A means for separating the converted color signal from the VTR reproduction signal in the secondary beat cancel circuit provided in the VTR reproduction circuit in which the luminance signal is FM-modulated and the converted color signal and the FM-modulated luminance signal are mixed and recorded. And a double-balanced modulator that is supplied with the converted color signal to generate a signal having a frequency twice that of the low-frequency carrier frequency, and a signal having a frequency that is twice the double frequency is supplied to generate a cancel signal at its output. A gain controller, a phase adjusting means for adjusting the phase of the cancel signal, an adding means for adding the cancel signal in reverse phase to the FM demodulated luminance signal, and a cancel signal from the cancel signal and the adding means. A secondary beat cancel circuit, comprising a synchronous detector to which a luminance signal is supplied and whose output signal is supplied to the gain controller as a gain control signal.