JPH0371779A - Detection circuit - Google Patents

Abstract

PURPOSE:To reduce buzz noise by using a high frequency component of a video signal after detection so as to reduce the parasitic PM component due to the Nyquist lobe of a video intermediate frequency IF signal generated from a phase comparator circuit. CONSTITUTION:A level change in a video signal after the detection is detected by an HPF 9 and its detection output is used to eliminate the parasitic PM component generated from a phase comparator circuit 10. That is, a mixer circuit 11 is formed to subtract an output signal of the HPF 9 from a ripple component resulting from the PM component included in the output signal of the phase comparator circuit 10. Thus, a signal representing a substantial phase deviation is generated from the mixer circuit 11, smoothed by a smoothing circuit 12 and applied to a VCO 4. Thus, the VCO 4 makes stable oscillation without being affected with the parasitic PM component.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a detection circuit used for audio detection in TV (television) receivers, and particularly relates to a detection circuit for detecting low frequency buzz sounds generated in audio signals. Concerning a detection circuit that measures

(b) Conventional technology The intercarrier system has been used as an audio reception system for TV broadcasting. The intercarrier system is superior to the separate carrier system in terms of simplicity of circuit configuration, stability, and cost. However, the intercarrier method has a problem in that it is affected by the Nyquist slope caused by the filter placed before the video IF (intermediate frequency) amplification circuit, and a buzz sound is generated in the audio signal after detection. Therefore, a television receiver that reduces the generation of such buzz noise was developed in Japanese Patent Application Laid-Open No. 59-20058.
It is described in Publication No. 7. According to the above publication, in order to prevent the influence of the Nyquist slope of the filter, a correction filter having filter characteristics axially symmetrical to the filter characteristics of the filter is arranged to remove the PM component of the video IF signal. There is. Then, the image I from which the PM component has been removed is
4 and 5 are the beat components of the F signal and the audio IF signal.
A MHz audio FM signal is detected and the signal is subjected to FM detection to obtain an audio signal. By doing so, it is possible to obtain audio that does not include buzz sounds.

(c) Problem to be solved by the invention However, when using the method described in the above publication,
A filter having a coil or the like is required as a correction filter, which is unsuitable for C conversion.

Further, the filter characteristics must be set to exactly the opposite characteristics to the existing filter characteristics, and there is a drawback that adjustment is required.

(d) Means for Solving the Problems The present invention has been made in view of the above points, and includes a video IF amplification circuit that amplifies a video IF signal, a VCO that oscillates at the video IF signal frequency, and a a video detection circuit that detects the output video IF signal of the video IF amplification circuit according to an oscillation output signal; a filter that extracts a high frequency component in the detection output of the video detection circuit; and an output video of the video IF amplification circuit. a phase comparison circuit that performs a phase comparison between the IF signal and the oscillation output signal of the VCO; a mixing circuit that mixes the output signal of the phase comparison circuit with the output signal of the filter; and a smoothing circuit that smooths the output signal of the mixing circuit. and a smoothing circuit that applies a smoothed output to the VCO as a control voltage;
It is characterized in that the audio signal is detected in accordance with the oscillation output signal of O.

(E) Function According to the present invention, the parasitic PM component due to the Nyquist slope of the video IF signal generated from the phase comparator circuit is reduced using the high frequency component of the video signal after detection. For that reason,
The oscillation of the VCO is stabilized by suppressing the parasitic PM component, and the buzz noise is reduced.

(f) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention. 5 with a slope
AW (Surface Acoustic Wave) filter, (3) is the image from the SAW filter <2>! A video IF amplifier circuit that amplifies the signal by F times, (4> is a VCOl that oscillates at the video IF signal frequency
(5> is a video detection circuit that detects the video IF signal from the video IF amplifier circuit (3) according to the oscillation output signal of the VCO (4) and outputs the video signal and audio IF signal to the terminal (6). , (8> is a video amplification circuit that amplifies the video signal from the terminal (6), (9) is an HPF that extracts the high frequency component of the output video signal of the video amplification circuit (8), and (10) is a phase comparison circuit. circuit, (11> is a mixing circuit that mixes the output signal of the phase comparison circuit (10) and the output signal of the HPF (9);
(12> is a smoothing circuit, (13> is an output terminal (7>) is a BP that extracts a 4.5 MHz audio IF signal from the output signal.
F, (14) is a limiter amplifier, and (15) is 4,
This is an FM detection circuit that performs FM detection of a 5 MHz audio IF signal.

In Fig. 1, the output signal of the tuner (1) is applied to a SAW filter (2) having frequency characteristics as shown in Fig. 2, and then amplified by a video IF amplification circuit (3>.The amplified video The IF signal is applied to the phase comparison circuit (10) and the video detection circuit (5>. The oscillation output signal of the VCO (4) that oscillates at the video IF signal frequency is applied to the phase comparison circuit (10)
10> and a video detection circuit (5>), and the phase comparison circuit (10) compares the phases of both human input signals.

In addition, the video detection circuit (5) detects the video IF signal and generates a video signal at the terminal <6>.
z audio IF signal is generated. The signal from the terminal (6) is amplified by the video amplification circuit (8) and then sent to the output terminal (
7> obtained.

As mentioned above, the video IF signal from the video IF amplifier circuit (3) is affected by the SAW filter (2) and has a parasitic PM component. includes the ripple voltage due to the parasitic PM component in addition to the original phase shift component.

The ripple voltage due to this parasitic PM component causes buzz. The parasitic PM component changes depending on the degree of modulation, that is, the level, of the video signal, and the two are in a proportional relationship. When the level of the video signal changes, the parasitic PM component changes accordingly, which causes the oscillation frequency of the VCO to fluctuate and generate buzz. Therefore, in the present invention, the level change of the video signal after detection is detected by the HP F (9), and the detected output is used to remove the parasitic PM generated from the phase comparator circuit (10). That is, the mixing circuit (11) is a phase comparator circuit (
The output signal of HP F (9) is configured to be subtracted from the ripple caused by the parasitic PM included in the output signal of 10). Therefore, a signal indicating the original phase shift is generated from the mixing circuit (11), and after being smoothed by the smoothing circuit (12), it is applied to the VCO (4).

Therefore, VCO (4) is not affected by the above-mentioned parasitic PM and oscillates stably. Therefore, the output terminal (7) has a 4 or 5 MHz audio IF that is not affected by Tsuji parasitic PM.
After a signal is generated and unnecessary components are removed by B P F (13), it is amplified via a limiter amplifier (14) and converted to FM.
FM detection is performed by a detection circuit (15). FM detection circuit (1
Since the audio FM signal applied to 5) is not affected by parasitic PM, no buzz is generated in the audio signal after FM detection.

Since the phase comparison circuit <10> performs control such that the phase difference between the two input signals is exactly 90 degrees, the comparison output does not include level information of the input signals. Therefore, V
The oscillation output signal of CO(4) is not affected by the level of the video signal, and a video carrier component of a constant level can be obtained.

Figure 3 shows a specific circuit example of the mixing circuit (11>) in Figure 1, and the same circuit elements as in Figure 1 are given the same reference numerals. A control signal having an opposite phase to the first and second transistors (17) and (
18>, and a corresponding current flows. On the other hand, the high-frequency component of the detected video signal from the video amplification circuit (8) is extracted by the HPF (9), and the extracted signal is transmitted to the third transistor (20> constituting the current mirror circuit (19)). is applied to the collector, where HP F
(9) The signal generated from the first transistor <17
The level is set so that the level is twice the ripple caused by parasitic PM generated in the collector of . Therefore, the ripple generated at the collector of the first transistor (17) is canceled out, and the ripple generated at the collector of the second transistor (18)
A current equal to the current corresponding to the ripple generated in the collector of the fourth transistor (21) flows through the collector of the fourth transistor (21).

Therefore, the ripple occurring at the collector of the second transistor (18>) is also canceled out.As a result, only the original phase comparison output is smoothed by the smoothing circuit (12>) and applied to the VCO (4).

Although the circuit shown in FIG. 1 has been described with reference to the case where it is used in an intercalary carrier type detection circuit, the present invention is not limited to this. It may also be used in circuits.

(G) Effects of the Invention As described above, according to the present invention, it is possible to oscillate the VCO without being affected by parasitic PM caused by the Nyquist slope, so it is possible to obtain an audio signal that does not include buzz noise. . Furthermore, the present invention has the advantage that it is possible to prevent an increase in the number of elements because it is only necessary to add a simple filter.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram for explaining FIG. 1, and FIG. 3 is a circuit diagram showing a specific example of the circuit shown in FIG. . (3)...Video IF amplification circuit, (4>...VCo
1 (5>...Video detection circuit, (9>...HPF,
(10)...Phase comparison circuit, (11)...Mixing circuit, (12)...Smoothing circuit.

Claims (1)

[Claims] (1) A video IF amplification circuit that amplifies a video IF signal, a VCO that oscillates at the video IF signal frequency, and a video detection circuit that detects the output video IF signal of the video IF amplification circuit according to the oscillation output signal of the VCO. and a filter for extracting high-frequency components in the detection output of the video detection circuit, and an output video IF signal of the video IF amplification circuit and the VCO.
a phase comparison circuit that performs a phase comparison with the oscillation output signal of the filter; a mixing circuit that mixes the output signal of the phase comparison circuit with the output signal of the filter; and a mixing circuit that smoothes the output signal of the mixing circuit to produce a smoothed output. Said VCO
and a smoothing circuit that applies the control voltage to the V
A detection circuit characterized in that an audio signal is detected according to an oscillation output signal of a CO.