JPS61192186A - Video and sound demodulation circuit - Google Patents

Abstract

PURPOSE:To prevent noise generation and sound deterioration by performing the envelope detection prior to the synchronization detection, obtaining a signal added with a signal differentiated the result and the above-mentioned envelope detection output signal, superposing this addition detection signal to the control voltage of a voltage control variable frequency oscillator VCO, and performing the operation to add the phase correction in the inverted direction to the phase distortion generated in a multiplication detection circuit to reduce generation of detection distortion. CONSTITUTION:A video intermediate frequency VIF signal added from an input terminal A is divided into two systems, one is added to an envelope detection circuit 28, the degree of modulation is detected, and further it is differentiated by a differentiation circuit 29. The signals obtained in the circuits 28 and 29 are added in an addition circuit 30, their output is superposed to the phase of the output of a VCO 5 is controlled to correct the phase shift generated at the amplification stage of a detection circuit 1. At the same time, that of other system of a VIF signal is delayed by a delay circuit 27, and after compensating the time lag in the envelope detection circuit 28, the multiplication detection is performed in the detection circuit 1 with an output signal of the VCO 5 made with phase shift, and an video signal and a sound intermediate frequency signal detected from a terminal B are obtained.

Description

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

As a conventional video/audio demodulation circuit for a television receiver, there is one shown as rPLL synchronous detection system VIF-IJ in the materials for the 1982 Annual National Conference of the Television Society of Japan. The main points are shown in Figure 4.

In FIG. 4, a video intermediate frequency signal (hereinafter referred to as VIP signal) is input from a terminal, divided into two,
One is supplied to the phase ratio *gR2.

The phase comparator 2 is a voltage controlled variable frequency oscillator (hereinafter referred to as vC
The oscillation output of 6 (referred to as O) is phase-shifted by i in the π/2 phase shift circuit 6 and the VIP signal is multiplied and detected, and when the phase difference between the two signals is other than π/2, it is detected with respect to the reference voltage. Generates a detection signal (APC voltage) that changes between positive and negative. This signal is DC-amplified by a DC amplifier 3, and the video component is removed by a low-pass filter 4.Then, it is applied to vCO6 as a frequency control voltage, and its oscillation frequency is changed to change the phase between the output signal of the one phase shift circuit and the VIP signal. Control is performed so that the phase difference is always π/2. The VIP signal is subjected to multiplication detection in the detection circuit 1 using the oscillation output signal of the VCO 6 controlled in this manner, and a demodulated signal of the video signal and the audio intermediate frequency signal from which high frequency components have been removed is obtained from the terminal B.

Problems to be Solved by the Invention However, in such conventional demodulation circuits, there is a problem in that detection distortion inevitably occurs in portions where the degree of modulation of the video intermediate frequency signal changes rapidly. The reason is as follows.

As the detection circuit 1, a double-balanced differential amplifier circuit as shown in FIG. 6 is generally used.

In the figure, 11 and 12 are inverters that amplify the VIF signal.
- The transistors 13 to 16 are switch circuits that switch the output signal of the transistor 11.12 using the output of the VCO 5, and the multiplication output, that is, the detection output, is obtained as a balanced signal between the common collectors of the transistors 13.15 and 14.16. It will be done. By the way, VI input from terminal B
Since the P signal is modulated by the video signal, the base potential of transistors 11 and 12 varies depending on the degree of modulation, whereas the carrier level of the oscillation output of VCO 5 input from the terminal is flat. The common emitter potential of transistors 13, 14 and 16, 16, that is, the collector potential of transistors 11, 12 is constant.

FIG. 6 shows an enlarged view of only the transistors 11 and 12. 'In the figure, the base of transistor 11 is connected to a person,
When the collector is at point B and its potential is vo, the modulated input from the mermaid is V, the DC potential at the point with high modulation is vl, and the DC potential at the low point is v2, then the base of transistor 11 -Collector potential V. , is (Vc−v+
) to (vc-v2). Then, NPN
Since the base-collector of the l-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 changes depending on the modulation degree, and since the carrier level at point A in Figure 5 is constant, significant detection distortion occurs in the part where the modulation degree changes rapidly. will occur.

As a result, phase distortion occurs in the output video signal or audio intermediate frequency signal. This phenomenon is particularly noticeable in the case of demodulated output of a television audio multiplex signal.

FIG. 7 shows the portion X of the equivalent pulse in the vertical blanking interval and the audio multiplex demodulation output Y. According to actual measurements, unnecessary noise due to phase distortion occurs in parts where the change in the equivalent pulse is large, and this is in the audible band of 6oHz.
This causes a significant deterioration of the S/N ratio of the audio. SUMMARY OF THE INVENTION An object of the present invention is to solve such problems and provide a video/audio demodulation circuit that generates less detection distortion.

Means for Solving the Problems In the video/audio demodulation circuit of the present invention, the video intermediate frequency signal is envelope-detected to detect its modulation degree, the envelope detection output signal is differentiated, and the differential output and By adding the envelope detection output and superimposing it on the VCO control voltage, the phase of the vCO output is changed, and the phase distortion generated in the process of amplifying the VIP signal is controlled to be corrected, and the VLF signal is Distortion after detection is corrected by setting a delay circuit to adjust the time delay that occurs during the process of detecting the modulation degree before passing through the detector.

Detection of the degree of modulation in the active VIF signal is obtained by detecting the vIF signal, but this detection must be done before coherent detection is performed.

Therefore, in the present invention, envelope detection is performed before synchronous detection, a signal obtained by adding this differentiated signal and the aforementioned envelope detection output signal is obtained, this addition detection signal is superimposed on the control voltage of the VCO, and multiplication is performed. In order to compensate for the delay time that occurs in the detection circuit, the VIP signal before synchronous detection is delayed, and synchronous detection is performed between this signal and the output signal of the previous VCO. Distortion can be reduced. This effect is particularly noticeable in the demodulated output of audio multiplexing.

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

Figure 1 shows its configuration. In the figure, the detection circuit 12
Phase comparator 2. kPG amplifier 3. low pass filter 4
, VCOs, and the π/2 phase shift circuit 6, their configurations and operations are the same as those of the conventional example, and their operating principles are explained in the conventional example, so a description thereof will be omitted.

In this circuit, the video intermediate frequency (4IF) signal applied from the input terminal is divided into two systems, and in addition to the envelope detection circuit 28 for detecting one, the degree of modulation is detected and further differentiated. Differentiation is performed in a circuit 29. circuits 28 and 29
The signals obtained in are added in an adder circuit 3o, and the output is expressed as v
It is superimposed on the cos control signal to control the phase of the vcos output so as to correct the phase shift occurring in the amplification stage of the detection circuit 1. At the same time, other systems of VIP signals are delayed by the delay circuit 27, and the envelope detection circuit 28
After compensating for the time delay in the phase-corrected vcos
Multiply detection is performed on the output signal of the detector circuit 1, and the terminal B
The detected video signal and audio intermediate frequency signal are obtained.

A specific example of the envelope detection circuit 28 and the differentiation circuit 29 is shown in FIG. Input the VIP signal from the terminal shown in the figure, amplify it to a level that allows transistor detection in the amplifier circuit (MP), and send the output to the transistor 31 and resistor 32, 33.
．． Envelope detection is performed by a detection circuit 28 consisting of a capacitor 34 and a capacitor 35.

Furthermore, the output is differentiated by a differentiating circuit consisting of a capacitor 40 and a resistor 41.42, and the output signal is transferred to a transistor 36.
Further, the envelope detection output obtained by the transistor 31 is amplified by a resistor 43, and added by a resistor 44 and a resistor 39. Superimposed on control voltage.

FIG. 3 shows a specific example of the delay circuit 27, in which a resistor 51 and a capacitor 52 constitute the delay circuit.

Effects of the Invention As described above, according to the video and audio demodulation circuit of the present invention, the phase distortion of the video signal and the audio intermediate frequency signal of the demodulated output, which occurs due to a sudden change in the degree of modulation of the video intermediate frequency signal, can be compensated for. By inserting a circuit, it can reduce the abnormal noise that occurs in the demodulated signal of audio multiplexing, and according to experiments, the S/N has improved from 30 dB to 37 dB. It was made. In particular, the differential circuit set in the modulation degree detection section can correct only the portion where the modulation degree changes sharply, which is very effective.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a video/audio demodulation circuit according to an embodiment of the present invention, FIGS. 2 and 3 are specific circuit diagrams of the main parts thereof, and FIG.
The figure is a block diagram of the conventional example, Figure 5 is a specific circuit diagram of the detection circuit, Figure 6 is the circuit diagram and waveform diagram of the VIP signal amplification transistor, and Figure 7 is the equivalent vertical period during the detection output. FIG. 3 is a waveform diagram of pulses and a demodulated output of an audio multiplexed signal. 1...Detection circuit, 2...Phase comparator,
3... Direct R amplifier, 4... Low pass filter, 5... Voltage variable frequency oscillator, 6...
....pi/2 phase shift circuit, 27..delay circuit, 28.....envelope detection circuit, 29..
Differential circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 zo cc Figure 3 Figure 4 Figure 5 Figure 6 Figure 7

Claims (1)

[Claims] The video intermediate frequency signal is applied to a delay circuit, and the output signal of the delay circuit is applied to the base of a transistor of a detection circuit using a double-balanced multiplier circuit and one input terminal of a phase comparator, and the above-mentioned phase comparison is performed. A signal obtained by shifting the oscillation output of the voltage-controlled frequency variable oscillator by π/2 is added to the other input terminal of the device, the phase difference between them is detected, and the phase comparison output is passed through a low-pass filter to generate the voltage-controlled frequency variable oscillator. The output of the voltage-controlled frequency variable oscillator is applied to the oscillator as an oscillation frequency control voltage, and the output of the voltage-controlled frequency variable oscillator is controlled to be in phase with the video intermediate frequency signal, and this oscillator is supplied to the detection circuit to detect the video signal and the audio intermediate frequency signal. In addition to obtaining a demodulated signal, the video intermediate frequency signal before being passed through the delay circuit is envelope-detected and differentiated, and a sum signal of the differential output and the envelope detection output is detected by changing the modulation degree. A video/audio demodulation circuit characterized in that a control voltage of the voltage controlled frequency variable oscillator is applied to cancel phase changes occurring in the circuit.