JPH0595520A - Synchronism detecting circuit - Google Patents

Abstract

PURPOSE:To provide a synchronism detecting circuit capable of good synchronism detection by matching the delay time between the output signal of a video filter and the output signal of a video carrier wave extracting filter especially. CONSTITUTION:For generating the video carrier P, a video carrier P component is extracted from an intermediate frequency signal before passing through a video filter 3 by a video carrier extracting filter 9 with the symmetrical amplitude characteristic. Moreover, by waveform-shaping and amplitude-limiting the video carrier P component with the delay of the video filter 3 delayed by a delay circuit 10 by a synchronism detection signal generation circuit 5, a synchronism detecting signal (multiplication signal) is obtained. Thus, the influence of the Nyquist rope of the video filter 3 can be removed, the delay time between the synchronism detecting signal and a PIF signal can be matched, thereby enabling the good synchronism detection.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a TV (television).
In receivers and VTRs (video tape recorders),
The present invention relates to an intercarrier type synchronous detection circuit. And, the present invention, in particular, the output signal of the video filter,
An object of the present invention is to provide a coherent detection circuit capable of achieving better coherent detection by matching the delay time with the output signal of the video carrier extraction filter.

[0002]

2. Description of the Related Art In a tuner section of a TV receiver, synchronous detection of a video signal and an audio intermediate frequency (SIF) signal is often performed by an intercarrier method.
A block diagram of a conventional synchronous detection circuit is shown in FIG. The intermediate frequency signal obtained by the tuner 1 of the receiver is taken into the video filter 3 via the buffer amplifier 2. The characteristics of this video filter 3 are that the video carrier (video carrier) is used to attenuate the low-frequency modulation component of the vestigial sideband modulation.
It is set to an asymmetric filter characteristic that has a Nyquist slope in the range. The signal that has passed through the video filter 3 is amplified by a video intermediate frequency (hereinafter referred to as PIF) signal amplifier 4, and the video carrier P included in the signal is extracted by the synchronous detection signal generation circuit 5. This video carrier P is a video synchronous detection circuit.
6 and SIF Video signal and SIF in the synchronous detection circuit 7
It becomes a multiplication signal for signal synchronous detection. Then, the video signal is taken out by the video synchronous detection circuit 6, and further,
The output of the SIF synchronous detection circuit 7 is passed through the audio filter 8 (4.5 MHz filter in the well-known M system)
The F 2 signal (ie the audio FM signal) is taken out.

The conventional synchronous detection circuit shown in FIG. 3 uses the video carrier P through the video filter 3.
Since this video carrier P is located on the Nyquist slope of the video filter 3, it is AM depending on the video modulation content.
Will be subject to frequency modulation. Therefore,
The SIF component (4.5 MHz component in the case of M method), which is the difference between the video carrier P and the audio carrier S, is also affected by the video modulation component, and the effect appears as a noise component in the SIF signal. there were.

In order to solve this problem, JP-A-61-161
In FIG. 1 of 198882, a video carrier extraction filter 9 which has a symmetrical selection characteristic and extracts a video carrier from the intermediate frequency signal before the video filter 3 is provided, and a synchronous detection signal is generated from the output of this filter 9. The circuit 5 obtains the synchronous detection signal (multiplication signal). As a result, the influence of the phase modulation due to the asymmetrical characteristic of the Nyquist slope of the video filter 3 on the extraction of the video carrier is eliminated, and the noise component in the SIF signal is reduced. Figure 4
FIG. 4 shows the state of phase modulation caused by the modulation content of the video carrier P in the symmetric filter (corresponding to the video carrier extraction filter 9) and the asymmetric filter (corresponding to the video filter 3). In the case of the symmetrical filter shown in FIG. 4A, the upper sideband and the lower sideband have the same vector level, and as shown in FIG. 4B, no phase modulation occurs in the combined carrier vector. On the other hand, in the case of the asymmetric filter shown in (c), since the upper sideband and the lower sideband have different levels, as shown in (d), the phase of the angle θ is added to the combined carrier vector. Receive modulation.

Using the video carrier extraction filter 9,
With the above advantages, even if the signal for synchronous detection (multiplication signal) is directly obtained from the video carrier extracted by the video carrier extraction filter 9, the output of the video filter 3 is because the video filter 3 generally has a delay characteristic. And the output of the video carrier extraction filter 9 has a difference in delay time. Therefore, the synchronous detection circuit disclosed in Japanese Patent Laid-Open No. 61-198882 is
As intended, good coherent detection cannot be performed.

[0006]

The problem to be solved by the present invention lies in what kind of means should be taken to perform good synchronous detection while making the most of the advantages of the video carrier extraction filter. is there.

[0007]

In order to solve the above problems, the present invention is a synchronous detection circuit for extracting a video carrier from a television signal and synchronously detecting a video signal and an audio intermediate frequency signal. A video carrier extraction filter for extracting a video carrier having a symmetrical selection characteristic from the intermediate frequency signal in the preceding stage of the video filter, and outputting the output of the video carrier extraction filter through a synchronous detection signal generation circuit In a synchronous detection circuit that uses a multiplied signal of synchronous detection to obtain a frequency signal, a delay circuit that delays an input signal or an output signal of the video carrier extraction filter to match a delay time of a signal that has passed through the video filter. The present invention provides a synchronous detection circuit characterized by being provided.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, a delay circuit 10 for compensating for the signal delay of the video filter 3 is provided at the input or output of the video carrier extraction filter 9 to realize good synchronous detection operation.

FIG. 1 is a block diagram showing an embodiment of the present invention. 3 is different from FIG. 3 in that the input of the synchronous detection signal generation circuit 5 is taken in from the output of the buffer amplifier 2 via the video carrier extraction filter 9 and the delay circuit 10. That is, in this embodiment, in order to generate the video carrier P, the video carrier P component is extracted from the intermediate frequency signal before passing through the video filter 3 by the filter 9 having a symmetrical amplitude characteristic, and the delay of the video filter 3 is delayed. The video carrier P component delayed by the delay circuit 10 is subjected to waveform shaping and amplitude limitation by the synchronous detection signal generating circuit 5 to obtain a synchronous detection signal (multiplication signal). Since the symmetrical image carrier extraction filter 9 is provided, as described above, the image signal and the S signal by the image carrier P from which the influence of the Nyquist slope of the image filter 3 is removed.
Synchronous detection with the IF signal can be performed. In the present embodiment, differential gain (DG) and differential phase (D) of audio noise and video system are used.
It is possible to improve characteristics such as P).

Here, most of the SAW filters are currently used as the video filter 3, and
3 is generally composed of a circuit element having a delay time. Therefore, if the synchronous detection signal (multiplication signal) is generated as it is from the extracted video carrier P, the generated synchronous detection signal is generated between the generated synchronous detection signal and the PIF signal that has passed through the video filter 3 and the PIF amplifier 4. However, due to the difference in the group delay characteristics of the respective paths, a phase difference occurs, and good synchronous detection characteristics cannot be obtained. Therefore, in order to eliminate the phase difference due to the difference in delay time, in this embodiment, the delay circuit 10 is provided in the generation path of the signal for synchronous detection. The delay time of the delay circuit 10 is (delay time of the video carrier extraction filter 9) +
Set so that (delay time of delay circuit 10) = (delay time of video filter 3).

An example of the delay circuit 10 is shown in FIG. The figure shows a variable delay circuit using a variable capacitance diode. As is well known, the delay time can be changed by applying a control bias with a potentiometer or the like. In FIG. 2, CR1 to CR5 are variable capacitance diodes, C1 to C4 are capacitors, L1 to L6 are coils, and R1 and R2 are resistors. Further, the delay circuit 10 may be a fixed delay circuit as long as variations in delay time can be suppressed within a practical range. Further, a delay line or a delay element may be used as the delay circuit 10. Delay circuit 10 so that the above relational expression holds
By setting the delay time of
The phase difference from the PIF signal can be eliminated, and good synchronous detection characteristics can be obtained. The delay circuit 10 may be provided before the video carrier extraction filter 9.

[0012]

As described above, according to the present invention, the image carrier extracted by the image carrier extraction filter having the symmetrical characteristic from the intermediate frequency signal in the preceding stage of the image filter for the generation of the synchronous detection signal (multiplication signal). Since the video carrier is used, synchronous detection can be performed without the video carrier being phase-modulated by the video modulation content, and the interference of the video modulation content with audio is improved. Further, distortion in image detection is improved, generation of harmonic components is reduced, and color fringing or the like caused by interference with a color signal is improved. Further, this synchronous detection circuit can eliminate the phase difference due to the difference in delay time between the synchronous detection signal and the PIF signal, and a better synchronous detection characteristic can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment.

FIG. 2 is a diagram showing an example of a variable delay circuit.

FIG. 3 is a block diagram of a conventional example.

FIG. 4 is a diagram for explaining phase modulation by a symmetric filter and an asymmetric filter in video carrier extraction.

[Explanation of symbols]

 1 tuner 2 buffer amplifier 3 video filter 4 PIF amplifier 5 synchronous detection signal generating circuit 6 video synchronous detection circuit 7 SIF synchronous detection circuit 8 audio filter 9 video carrier extraction filter 10 delay circuit 11 potentiometer

Claims (1)

[Claims] 1. A synchronous detection circuit for synchronously detecting a video signal and an audio intermediate frequency signal by extracting a video carrier from a television signal, and having a symmetrical selection characteristic from the intermediate frequency signal in the preceding stage of the video filter. In a synchronous detection circuit that has a video carrier extraction filter for extracting a video carrier, and outputs the output of the video carrier extraction filter through a synchronous detection signal generation circuit as a multiplication signal of synchronous detection for obtaining a video signal and an audio intermediate frequency signal. A synchronous detection circuit comprising a delay circuit for delaying an input signal or an output signal of the video carrier extraction filter to match a delay time of a signal passed through the video filter.