JPS61258585A - Video/sound demodulation circuit - Google Patents

Abstract

PURPOSE:To correct distortions after detection by applying envelope detection to a video intermediate frequency signal prior to passing through a delay circuit then adding its output polarity to the control voltage of a voltage controlled variable oscillator so that a phase variation in a detection circuit generated by the variation of a modulation factor is cancelled. CONSTITUTION:VIF signals obtained from a terminal A are divided into two systems; as to the former system, the modulation factor is detected through peak detection circuits 8 in order to detect the above signals' envelope. In addition, detected signals are convoluted to controlled signals of the voltage controlled variable oscillator VCO 5 and VCO 5's outputs are controlled so that phase differences generated in amplification stages of detection circuits 1 are corrected. As regards the latter system, delay circuits 7 make its signal system delay and multiplication detection is applied by means of VCO 5 output signals as well as the detection circuits 1, phases of which are corrected after compensating delayed times in the peak detection circuits 8. And videos/sounds of the detected signals are obtained from a terminal B. Thus phase distortions of videos/sounds generated by rapid variation of the modulation factor of VIF signals can be mitigated.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a video/audio demodulation circuit for a television receiver.

Conventional technology There is a video/audio demodulation circuit for television receivers as shown in the ``PLL synchronous detection system VIP-ICJ'' described in the materials for the 1982 International Conference of the Television Society of Japan.The main points are shown in Figure 4. Shown below.

In FIG. 4, a video intermediate frequency signal (hereinafter referred to as VIP signal) is input from terminal A, divided into two parts, and one part is supplied to phase comparator 2. The phase comparator 2 multiplies and detects the oscillation output of the voltage-controlled variable oscillator (hereinafter referred to as VGO) 5, which has been phase-shifted by π/2 in a π/2 phase shifter 6, and the VIP signal, and generates two signals. When the phase difference of the signals is other than π/2, positive and negative No. 44 signals are generated with respect to the reference. This signal is DC amplified by the A and PC amplifiers 3, and the video component is removed by the low-pass filter 4, and then the VCO 5
is controlled so that the phase difference between the output signal of the π/2 transfer-sum circuit 6 and the V IF signal is always π/2 by changing the oscillation frequency of the π/2 transfer-sum circuit 6. Multiply detection is performed on the V'IF signal and the detection circuit using the output signal of the VCO 5 controlled in this way, and the video/audio (4, 5
Now you can get the old (z) signal.

Problems to be Solved by the Invention However, in such a demodulated signal, there is a problem in that detection distortion inevitably occurs in the detection portion in a portion where the degree of modulation changes rapidly.

The reason is as follows. As the detection circuit, a double-balanced differential amplifier circuit as shown in FIG. 5 is generally used. ], 1. ．． 1.2 is an inverting 1-transistor that amplifies the VIP signal, 13~]6 is a transistor that constitutes a switch circuit that switches the output signal of the inverting transistor 11.12 at the output of the VCO 5, and the multiplication output is the transistor 13゜. 15 and 1.4.16 as a balanced signal between the common collectors. By the way 1
Since the VTF signal input from the helangistor 13 is modulated by the video signal, the base potentials of the transistors II and 12 vary depending on the degree of modulation. The carrier level to the terminal is constant since there is no modulation, and the common emitter potential of transistor 131.4.1516, ie, the collector potential of transistor 1.1.12, is constant. FIG. 6 is an enlarged drawing of only the transistors 11, 1.2. In the figure, transistor 1
The base of 1 is a, the collector is b, and its potential is Vc.
Then, the modulated input waveform from a is 22, and the DC potential with a high degree of modulation is 1. If the low DC potential is v2, the base-collector potential Vcn of the transistor 11 is (,
V c V s ) to (Vc-V, ).

Since the base-collector of the NPNI transistor is reverse biased as a diode, the base-collector capacitance value changes depending on the reverse bias capacitance value.

As a result, the capacitance between the base and the collector varies according to the modulation degree, so as long as the carrier level at terminal A in Figure 5 is constant, significant detection distortion will occur in parts where the modulation degree changes rapidly. Occur. As a result, phase distortion occurs in the video signal or the 4.5 old Iz audio signal.

This phenomenon is particularly noticeable in the demodulated output of audio multiplexing.

FIG. 7 shows the equivalent pulse portion 31 and the audio multiplex demodulation output 32 in the vertical synchronization period. According to actual measurements, it was found that unnecessary nozzles were generated due to phase distortion in parts where the change in the equivalent pulse was large, and this was in the 60 Hz audible band, causing significant S/N deterioration. Conventional circuits have such drawbacks.

Means for Solving the Problems The video/audio demodulation circuit of the present invention divides a video intermediate frequency signal into two systems after passing it through a delay circuit, and applies one to a detection circuit using a double-balanced multiplier circuit. The other output of the delay circuit is applied to one terminal of the phase comparator, and a signal obtained by shifting the oscillation output of the voltage-controlled variable oscillator by π/2 is added to the other terminal of the phase comparator to calculate the phase difference between the two. The output of the phase difference is passed through a low 4 = one-pass filter and then applied as a control voltage to the voltage-controlled variable oscillator so that the output frequency of the voltage-controlled variable oscillator and the video intermediate frequency signal are in phase. The controlled signal is supplied to a switch circuit of the detection circuit to obtain a video/audio demodulated signal at the output of the detection circuit,
and controlling the voltage-controlled variable oscillator so as to perform envelope detection of the video intermediate frequency signal before passing through the delay circuit, and to cancel the amount of phase change in the detection circuit caused by a change in modulation degree by adjusting the polarity of the output. It is characterized by adding to the voltage.

Function: With this configuration, the degree of modulation is detected by envelope detection of the VIP signal, and the detected signal is superimposed on the CO control voltage, thereby changing the phase of the CO output and amplifying the VIP signal. In addition to controlling to correct the phase distortion that occurs in the VIP signal, a delay circuit is set to adjust the time delay that occurs in the process of detecting the degree of modulation before the VIP signal passes through the detector, and the distortion after detection is corrected. It is possible.

Embodiment Hereinafter, one embodiment of the video/audio demodulation circuit of the present invention will be described with reference to the drawings.

FIG. 1 shows a configuration diagram of the present invention. Note that the detection circuit 12 phase comparator 2. APC amplifier 3. Low pass filter 4. V
The system configuration and operation of the CO5, π/2 phase shift circuit 6 are the same as those of the conventional example, and the principle of operation thereof has been explained in the conventional example, so a detailed explanation thereof will be omitted.

The key point of the present invention is to divide the VIP signal obtained from the terminal A into two systems, detect the modulation degree of one with the peak detection circuit 8 for detecting its envelope, and send the detected signal to the VCO5.
The output of ■C○5 is controlled so as to correct the phase shift generated in the amplification stage of the detection circuit 1 by being superimposed on the control signal of After compensating for the time delay in the peak detection circuit 8, multiplication detection is performed using the phase-corrected VCO 5 output signal and the detection circuit 1, and the video/audio (4
, 5Ml1z).

A specific example of the peak detection circuit is shown in FIG. V from terminal E
An IP signal is input, the amplifier AMP amplifies it to a level that allows transistor detection, and the output is subjected to envelope detection in a detection circuit consisting of transistors 1 to 41, resistors 42, 43, 44, and capacitor 45, The output is amplified by an amplifier circuit consisting of a transistor 46 and resistors 47 and 48, and is superimposed on the control voltage of the VCO 5 shown in FIG. 1 from a terminal via a resistor 49. FIG. 3 shows a specific example of the delay circuit 7, which is composed of a resistor 51 and a capacitor 52.

The degree of modulation in the VIP signal is obtained by detecting the VIP signal, but this detection must be done before the detection is performed. Therefore, in the present invention, envelope detection is performed in the peak detection circuit 8 before synchronous detection, and the detected signal is superimposed on the control voltage of the VCO 5, so that the phase distortion generated in the transistors 11 and 12 in FIG. The operation is performed so as to add phase correction. Furthermore, in order to compensate for the delay time generated in the peak detection circuit 8, the VIF before synchronous detection is
By delaying the signal and performing synchronous detection in the detection circuit 1 using this signal and the previous VC05 signal = 7 =, it is possible to reduce the phase distortion that occurred in the conventional example. This is something that comes out in a big way.

Effects of the Invention As described in Section 2, the video/audio demodulation circuit of the present invention is comprised of an envelope detection circuit and a delay circuit, and is capable of suppressing the phase distortion of video/audio caused by rapid changes in the degree of modulation of the IF signal. This can be reduced by inserting a phase compensation circuit, and in particular can reduce abnormal noise that occurs in the demodulated signal of audio multiplexing, and according to experiments, it can be improved from 30 dB to 37 dB. .

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Figs. 2 and 3 are block diagrams of main parts in Fig. 1, Fig. 4 is a block diagram of a conventional example, and Fig. 5 is a block diagram of a detection circuit. 6 is an explanatory diagram of the VIP signal amplification 1-transistor in FIG. 5, and FIG. 7 is a conventional circuit pairing of the vertical period equivalent pulse in the image detection output and the demodulation output of the audio multiplex signal. −8= It is an explanatory diagram of response. 1...Detection circuit, 2...Phase comparator, 3...A
PC amplifier, 4...Low pass filter, 5...Voltage controlled variable oscillator, 6...π/2 phase shift circuit, 7...Delay circuit, 8...Peak detection circuit

Claims (1)

[Claims] 1. After passing the video intermediate frequency signal through a delay circuit, divide it into two systems and feed one into a detection circuit using a double-balanced multiplication circuit, and add the output of the other delay circuit to one terminal of a phase comparator. A signal obtained by π/2 phase of the oscillation output of the voltage-controlled variable oscillator is applied to the other terminal of the phase comparator, the phase difference between the two is detected, and the phase difference output is passed through a low-pass filter before being applied to the voltage control circuit. A signal that is given as a control voltage to the voltage-controlled variable oscillator and controlled so that the output frequency of the voltage-controlled variable oscillator and the video intermediate frequency signal are in phase is supplied to the switch circuit of the detection circuit to output the video signal to the output of the detection circuit.・
It is configured to obtain a demodulated audio signal, and performs envelope detection on the video intermediate frequency signal before passing through the delay circuit, and detects the polarity of the output and the amount of phase change in the detection circuit caused by a change in the degree of modulation. A video/audio demodulation circuit added to the control voltage of the voltage controlled variable oscillator for cancellation.