JPH10276384A - Video detection circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce sound buzz noise produced in the video detection circuit. SOLUTION: The detection circuit is provided with a detection circuit 2 and a phase comparator circuit 3 that receive respectively a television signal whose frequency is converted into an intermediate frequency, a voltage controlled oscillator circuit 4 that gives an oscillation signal to the detection circuit 2 and the phase comparator circuit 3, and a low pass filter 5 that smooths an error voltage from the phase comparator circuit 3 and provides the smoothed voltage to the voltage controlled oscillator circuit 4 as its control voltage. Then part of a video signal whose level is fluctuated based on fluctuation in the level of the control voltage is superimposed on the control voltage so as to make a phase of the level fluctuation in the control voltage opposite to a phase of the level fluctuation in the video signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video detection circuit for a television receiver, and more particularly, to a synchronous detection circuit using a PLL detection method.

[0002]

2. Description of the Related Art A conventional video detection circuit is a PL detector shown in FIG.
A synchronous detection circuit using the L detection method will be described. In FIG. 2, an intermediate frequency (58.75M for a Japanese intermediate video frequency) is used in a mixing circuit of a television tuner (not shown).
Hz), and similarly, a television signal given a predetermined selectivity characteristic by a SAW filter (not shown)
After being amplified by the intermediate frequency amplification circuit 21, the signal is input to the detection circuit 22 and also input to the phase comparison circuit 23. As the detection circuit 22, a double-balanced detection circuit is used.

On the other hand, an oscillation signal from a voltage controlled oscillation circuit (hereinafter, referred to as VCO) 24 is input to a detection circuit 22 and a phase comparison circuit 23, and the phase comparison circuit 23 outputs a television signal from the intermediate frequency amplification circuit 21. The phases of the signal and the oscillation signal from the VCO 24 are compared, and an error voltage corresponding to the phase difference is output from the phase comparison circuit 23.

The error voltage output from the phase comparison circuit 23 is smoothed by a low-pass filter 25,
4 is input as a control voltage. Accordingly, the phase comparison circuit 23, the VCO 24, and the low-pass filter 25 constitute a closed loop of the PLL, and the VCO 24 is in phase with the carrier (video intermediate frequency) of the television signal whose frequency has been converted to the intermediate frequency (synchronous). Oscillation frequency (58.75 MHz)
And its output is input to the detection circuit 22.

Accordingly, the television signal whose frequency has been converted to the intermediate frequency and an oscillation signal having the same frequency as the carrier of the television signal are input to the detection circuit 22,
Here, the video signal is synchronously detected. The video signal detected by the detection circuit 22 is amplified by the video amplification circuit 26,
The processing is performed by a subsequent circuit (not shown).

[0006]

In the above-described conventional video detection circuit, the vestigial sideband system is employed as the television signal transmission system. Therefore, the characteristics of the audio system include the so-called Nyquist Buzz noise due to the slope occurs. That is, the television signal (R
F signal) is converted to an intermediate frequency signal via a tuner,
The signal passes through a Nyquist slope corresponding to a vestigial sideband which is a predetermined selectivity characteristic created by a SAW filter (not shown). As a result, the side band near the video carrier is cut off, so that the upper and lower upper side bands and the lower side band of the video carrier have different vector amounts, and phase distortion occurs in the combined vector amount.

When this phase distortion occurs, the phase comparison circuit 2
3 fluctuates. This variation is difficult to sufficiently reduce with the low-pass filter 25.
The control voltage input to the CO 24 fluctuates. Therefore, P
The LL VCO 24 is modulated in synchronization with the fluctuation of the control voltage. Therefore, the video signal detected by the detection circuit 22 is superimposed with the modulation component generated by the VCO 24, and includes a fluctuation component synchronized with the fluctuation of the control voltage input to the VCO 24. Since this modulation is FM modulation, it does not pose a problem in visual perception in television images, but it causes audio buzz noise. Therefore, an object of the present invention is to reduce audio buzz noise generated in a video detection circuit.

[0008]

In order to solve the above-mentioned problems, a video detection circuit according to the present invention comprises a detection circuit and a phase comparison circuit to which a television signal frequency-converted to an intermediate frequency is inputted, respectively, and the detection circuit A voltage-controlled oscillation circuit that inputs an oscillation signal to the phase comparison circuit; and a low-pass filter that smoothes an error voltage from the phase comparison circuit and supplies the voltage as a control voltage to the voltage-controlled oscillation circuit. A part of the video signal whose level fluctuates based on the level fluctuation of the voltage is superimposed on the control voltage, and the phase of the level fluctuation of the control voltage and the phase of the level fluctuation of the video signal are opposite to each other. did.

The video detection circuit according to the present invention further includes a video amplification circuit for amplifying the video signal detected by the detection circuit, wherein the video amplification circuit uses an amplifying transistor, and a collector of the transistor. The video signal superimposed on the control voltage was extracted and superimposed on the control voltage.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A video detection circuit according to the present invention will be described with reference to a synchronous detection circuit using a PLL detection system shown in FIG. In FIG. 1, a mixing circuit of a television tuner (not shown) uses an intermediate frequency (5
The television signal, which has been frequency-converted to 8.75 MHz) and given a predetermined selectivity characteristic by a SAW filter (not shown), is input to the intermediate frequency amplifier circuit 1. After the television signal is amplified by the intermediate frequency amplifier circuit 1, the television signal is input to a double balance type detection circuit 2 which is a detection circuit, and is also input to a phase comparison circuit 3.

On the other hand, the double balanced type detection circuit 2 and the phase comparison circuit 3 include a voltage controlled oscillation circuit (hereinafter referred to as VCO).
In the phase comparison circuit 3, the phases of the carrier of the television signal from the intermediate frequency amplification circuit 1 (here, the carrier of the video intermediate frequency signal) and the oscillation signal from the VCO 4 are adjusted. The phase comparison circuit 3 outputs an error voltage corresponding to the phase difference.

The error voltage output from the phase comparison circuit 3 is smoothed by a low-pass filter 25,
Is input as a control voltage. Therefore, the phase comparison circuit 3, VCO 4 and low-pass filter 5 constitute a closed loop of the PLL, and the oscillation frequency of the VCO 4 is controlled by the control voltage based on the error voltage from the phase comparison circuit 3. Oscillate at an oscillation frequency (58.75 MHz) that matches (synchronizes with) the carrier frequency of the television signal whose frequency has been converted to the intermediate frequency.

Therefore, the television signal whose frequency has been converted to the intermediate frequency and the oscillation signal having the same frequency as the carrier of this television signal are input to the double-balanced detection circuit 2, where they are synchronously detected. Gives a video signal.
The video signal detected by the double balanced detection circuit 2 is amplified by the video amplification circuit 6 and processed by a subsequent circuit (not shown).

Here, the video amplifying circuit 6 uses a PNP transistor 7 for amplification, and load resistors 8 and 9 are connected to the emitter and the collector. Then, the video signal from the emitter is guided to a subsequent circuit (not shown), and a video signal having the opposite phase to the video signal from the emitter is extracted from the collector. The video signal having the opposite phase is input to the VCO 4 via the coupling capacitor 10 as a control voltage.

By the way, the VCO 4 constituting the PLL is
Since FM modulation is applied due to phase distortion caused by the Nyquist slope, an FM component due to the phase distortion is superimposed on a video signal detected by video detection. Therefore, the level of the video signal fluctuates according to the frequency fluctuation of the VCO 4 that has been subjected to the FM modulation. Therefore, in the present invention, the level fluctuation of the video signal is superimposed on the control voltage based on the error voltage from the phase comparison circuit 3 so that the VCO
4 is input.

In this case, the phase of the level change of the video signal from the collector of the transistor 7 of the video amplifier 6 is
The phase of the control voltage from the phase comparison circuit 3 is opposite to the phase of the control voltage. As a result, the control voltage from the phase comparison circuit is canceled by the level fluctuation of the video signal from the video amplification circuit 6, and the VCO 4 does not receive the FM modulation. Accordingly, no FM component is superimposed on the video signal from the double balanced detection circuit 2, and no audio buzz noise occurs.

In the present invention, since the video signal is merely superimposed on the control voltage of the VCO 4 from the transistor 7 of the video amplifier circuit 6 via the coupling capacitor 10, the FM of the VCO 4 which causes audio buzz noise can be realized with a simple configuration. Modulation can be prevented. The signal e can be easily obtained from the video amplifier circuit 6.

[0018]

As described above, the video detection circuit according to the present invention comprises a detection circuit and a phase comparison circuit to which a television signal whose frequency has been converted to an intermediate frequency is input, and an oscillation signal to the detection circuit and the phase comparison circuit. A video signal having a voltage-controlled oscillating circuit to be input and a low-pass filter for smoothing an error voltage from the phase comparison circuit and supplying the same to the voltage-controlled oscillating circuit as a control voltage; Is superimposed on the control voltage, so that the phase of the level fluctuation of the control voltage and the phase of the level fluctuation of the video signal are opposite to each other, so that the control voltage input to the VCO is output from the video amplifier circuit. Is canceled out by the level fluctuation of the video signal of
No more M modulation. Therefore, no FM component is superimposed on the video signal from the detection circuit, and no audio buzz noise occurs.

Further, the video detection circuit of the present invention includes a video amplification circuit for amplifying the video signal detected by the detection circuit, uses an amplification transistor in the video amplification circuit, and converts the voltage from the collector of the transistor to the control voltage. Since the video signal to be superimposed is taken out and superimposed on the control voltage, the use of the video amplifier circuit makes it possible to easily obtain a video signal whose level fluctuation is opposite in phase to the control voltage fluctuation phase.

[Brief description of the drawings]

FIG. 1 is a diagram of a video detection circuit according to the present invention.

FIG. 2 is a diagram of a conventional video detection circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 intermediate frequency amplifier circuit 2 double balanced detector circuit 3 phase comparator circuit 4 voltage controlled oscillator circuit 5 low-pass filter 6 video amplifier circuit 7 transistor 8.9 load resistance 10 coupling capacitor

Claims (2)

[Claims] 1. A detection circuit and a phase comparison circuit to which a television signal frequency-converted to an intermediate frequency is input, respectively.
A voltage-controlled oscillation circuit that inputs an oscillation signal to the detection circuit and the phase comparison circuit; and a low-pass filter that smoothes an error voltage from the phase comparison circuit and supplies the voltage as a control voltage to the voltage-controlled oscillation circuit. Superimposing a part of the video signal whose level fluctuates based on the level fluctuation of the control voltage on the control voltage, and setting the phase of the level fluctuation of the control voltage and the phase of the level fluctuation of the video signal to be opposite to each other. A video detection circuit characterized in that: 2. A video amplifying circuit for amplifying the video signal detected by the detection circuit, wherein the video amplifying circuit uses an amplifying transistor, and the video is superimposed on the control voltage from a collector of the transistor or from the collector. 2. The video detection circuit according to claim 1, wherein a signal is extracted and superimposed on the control voltage.