JPH0477514B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Industrial Application Field The present invention relates to a detection circuit used in intercarrier television receivers, etc., and is particularly suitable for receiving audio multiplex broadcasts. Regarding. (b) Conventional technology In intercarrier type television receivers, the SIF signal located 4.5MHz away from the carrier wave of the video signal is extracted as a 4.5MHz beat signal, and the audio signal is detected by FM detection. I'm getting a signal. FIG. 2 is a circuit diagram showing a video synchronous detection circuit using a PLL circuit. The RF signal received by the antenna 1 is applied to the tuner 2 and converted to an IF signal, and then passed through the SAW filter 3 to the IF signal.
The signal is applied to the amplifier circuit 4 and amplified. The amplified video IF signal is then applied to a PLL circuit 8 consisting of a phase comparison circuit 5, a low-pass filter 6, and a VCO 7, and the carrier wave is reproduced in the PLL circuit 8 . Then, the IF signal is detected in the video synchronous detection circuit 9 based on the carrier wave.
In Japan, the video IF frequency is set to 58.75MHz and the audio IF frequency is set to 54.25MHz, and when the video IF signal is AM detected, the video signal is output to terminal 10.
Further, an SIF signal of 4.5 MHz, which is the beat frequency of the audio IF frequency and the video IF frequency, is taken out from the terminal 11. Therefore, an audio signal can be obtained by performing FM detection on the SIF signal. Intercarrier type television receivers are known, for example, from Japanese Patent Application Laid-Open No. 57-58476.
It is stated in the No. (c) Problems to be solved by the invention However, in the circuit shown in Figure 2, since the television signal transmission method is a vestigial sideband method, the low frequency is emphasized at the output end of the video synchronous detection circuit. In order to prevent the generation of such signals, the SAW filter characteristics are determined so that the video carrier frequency is located in the attenuation range of the SAW filter 3 as shown in Figure 3.
It is necessary to provide a filter 3. However, in this case, an imbalance occurs between the upper and lower sidebands of the video carrier wave, and the video IF signal after passing through the SAW filter 3 is subjected to abnormal phase and amplitude modulation, resulting in distortion. Then, since the distorted video IF signal is phase-compared with the oscillation output signal of the VCO 7 in the phase comparison circuit 5, unnecessary video signal components ( _H , _2H ...) are generated in the error output. . Then, the video signal component passes through the low-pass filter 6 and the VCO 7
is applied to At that time, f _H in the video signal component
and 2f _H components have a low frequency and a large amplitude, so they have a particularly large effect on the VCO 7. For that reason,
There is a problem in that a carrier wave that has been abnormally modulated from the VCO 7 is applied to the video synchronous detection circuit 9, resulting in an abnormal output that causes a buzz in the SIF signal. The buzz is particularly noticeable in audio signals of audio multiplex broadcasts. For example, in the audio multiplex broadcasting currently being carried out in the United States, the stereo pilot signal is set equal to the horizontal synchronization signal frequency _H , and the subcarrier frequency is selected to be _2H , so that the horizontal synchronization signal is There is a problem in that the second harmonic _2H of the horizontal synchronization signal interferes with the subcarrier in the stereo pilot signal, resulting in a buzz in the frequency of the vertical synchronization signal in the audio signal. (d) Means for Solving the Problems The present invention has been made in view of the above points, and includes a VCO that oscillates at a video IF frequency, an oscillation output signal of the VCO, and an IF signal from an IF amplifier circuit. a low-pass filter that smoothes the output signal of the phase comparison circuit and controls the oscillation frequency of the VCO, and video synchronous detection that synchronously detects the IF signal using the oscillation output signal of the VCO. The detector circuit is characterized in that a trap circuit is inserted between the phase comparator circuit and the VCO to attenuate signals at frequencies of _H and _2H to a predetermined level. (E) Effect According to the present invention, since _{the H} and _2H frequency signals that cause buzz are simultaneously attenuated from the output signal of the phase comparison circuit, regardless of the content of the video signal. The VCO's oscillation output signal is no longer affected by phase modulation, and the correct 4.5MHz SIF signal can be detected. (F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention.
12 is a VCO that oscillates at the video carrier frequency, 13
is the oscillation output signal of the VCO 12 and the IF amplifier circuit 4
a phase comparison circuit that performs a phase comparison with a video IF signal from and generates an output signal according to the phase difference ;
is a low-pass filter that smoothes the output signal; 1
5 is a trap circuit that attenuates unnecessary components in the output signal of the low-pass filter 14 and applies it to the VCO 12; 16 is an FM demodulation circuit; and 17 is an audio multiplex demodulation circuit. In addition, in the circuit of Fig. 1, the second
Circuit elements that are the same as those in the figures are given the same reference numerals, and their explanations will be omitted. Next, the operation will be explained. The video IF signal of frequency f _P (58.75MHz) from IF amplifier circuit 4 is sent to VCO 12.
Oscillation output signal of frequency f _P from and phase comparison circuit 1
3, and an output signal corresponding to the phase difference is generated at the output terminal. The output signal is smoothed by a low-pass filter 14 and then applied to the VCO 12 via a trap circuit 15 .
Here, due to the factors mentioned above, the video IF signal is subjected to phase modulation at the horizontal synchronizing signal frequency _H and its harmonic frequencies ( _2H, etc.), and the actual frequency is _H and its harmonics in addition to _P. It has a harmonic frequency. And the signal of such frequency is VCO1
2, resulting in a buzz in the audio signal. Therefore, the present invention focuses on the existence of the frequency _H and its harmonic frequencies, which are the main cause of buzz, and attempts to eliminate the frequencies.
That is, at the output end of the phase comparison circuit 13, in addition to an error output according to the phase difference, output signals of frequency _H and its harmonic frequencies are generated. Therefore, a trap circuit 15 that attenuates signals of frequency _H and second harmonic _2H , which have a particularly large influence, is connected to the phase comparator circuit 15 .
3 and the VCO 12 to prevent the oscillation output signal of the VCO 12 from being affected by abnormal phase modulation. By the way, a lag lead filter is generally used as the low-pass filter 14 due to the capture range of the PLL circuit, and its frequency versus output level characteristic is as shown by the solid line A in FIG. Therefore, in the past, the frequency _H and its harmonic frequencies were not sufficiently smoothed and had an influence. Therefore, it is necessary to use the trap circuit 15 to attenuate the frequency _H and its harmonic frequencies. In this case, the Q of the trap circuit 15 needs to be low. The use of a high Q trap circuit cannot attenuate the frequency _H and _2H components simultaneously. For example, a high Q trap circuit with a center frequency _H cannot attenuate the _2H component, and conversely, a trap circuit with a center frequency _2H cannot attenuate _{the H} component. The above problem can be solved by inserting two high Q trap circuits with center frequencies of _H and _2H . However, this requires two trap circuits, increasing the number of elements. In addition, a new problem arises in that when detecting a video signal with a high white level, it is not possible to remove components whose frequencies are slightly shifted from _H and _2H generated by the phase comparison circuit. For example, when a video IF signal with a high white level is applied from the IF amplifier circuit 4 in FIG. descend. Then, the level of the control voltage applied to the VCO 12 decreases, and the input video IF of the APC circuit consisting of the VCO 12, phase comparison circuit 13, low-pass filter 14 , and trap circuit 15 decreases .
Response sensitivity to signals decreases. Therefore, a signal with a slightly permissible frequency deviation from _H and _2H is generated at the output end of the phase comparison circuit 13, and this signal passes through a high Q trap circuit and adversely affects the VCO 12. For example, the capacitance of the capacitor 18 of the trap circuit 15 is 0.47 μF, and the inductance of the coil 19 is
0.47μH, resistance 20 of low pass filter 14
If the capacitance of the capacitor 21 is set to 1K [Ω], the capacitance of the capacitor 21 is set to 0.47 μF, and the resistance 2 is applied to 47 [Ω], the filter characteristic becomes as shown by the solid line B in FIG. In this case, _H and
It can be seen that the frequency component of _2H is sufficiently attenuated, and even if there is a frequency fluctuation, it can be sufficiently attenuated. The center frequency of solid line B is set to _2H , and the frequencies at points 3dB higher than the center are ( _2H - 1/ _2H ) and ( _2H + 1/ _2H ), respectively, so its Q is , Q= _2H /( _2H +1/ _2H )−( _2H −1/ _2H )=
2, and it is understood that if Q is 2 or less, _{the H} and 2 _H components can be sufficiently attenuated. In addition, commercially available chiyoke coils are 22μH, 33μH,
It is discrete like 39μH, 47μH, 51μH,
According to the applicant's experiments, a 0.47μF capacitor 18 and a 47μH coil 19 were used.
When Q = 2, _{the H} and _2H components could be sufficiently suppressed, but the 0.47μF capacitor 18 and the 51μH
When using the coil 19, _H and 2 _H still remain.
The influence of the ingredients remained. Now, the oscillation output signal of VCO12 has a frequency of _P
The signal is synchronized with the video IF signal. Therefore, when the video IF signal is applied to the video synchronous detection circuit 9, the video signal can be obtained at the terminal 10, and the correct 4.5 MHz SIF signal can be obtained at the terminal 11.
The SIF signal does not include frequency _H and harmonic frequencies. Therefore, even if the SIF signal is an audio multiplex signal demodulated by the FM demodulation circuit 16 and then applied to the audio multiplex demodulation circuit 17, no buzz will occur in the demodulated audio signal. (G) Effects of the Invention As described above, according to the present invention, _H and 2 _H , which cause the buzz generated at the output terminal of the phase comparator circuit
Since the frequency components of the trap are simultaneously attenuated by a single trap circuit, it is possible to prevent the VCO from receiving abnormal modulation. Therefore, the demodulated audio signal, especially
It is possible to prevent the generation of buzz sounds in audio multiplex broadcasts including _H and _2H .

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is a circuit diagram showing a conventional intercarrier type video synchronous detection circuit, and Fig. 3 is a characteristic diagram showing filter characteristics of a SAW filter. And, Figure 4 is
3 is a characteristic diagram showing filter characteristics of a low-pass filter 11 and a trap circuit 12. FIG. 9...Video synchronous detection circuit, 12...VCO, 1
3... Phase comparator circuit, 14 ... Low pass filter, 15 ... Trap circuit.

Claims (1)

[Claims] 1. A VCO that oscillates at the video IF frequency, a phase comparison circuit that compares the phases of the oscillation output signal of the VCO and the IF signal from the IF amplifier circuit, and a phase comparison circuit that smoothes the output signal of the phase comparison circuit to determine the oscillation frequency of the VCO. In a detection circuit comprising a low-pass filter to control and a video synchronous detection circuit that synchronously detects the IF signal using the oscillation output signal of the VCO, there is a signal between the phase comparison circuit and the low-pass filter, or between the low-pass filter and the video synchronous detection circuit. _H between the VCO and
and a detection circuit characterized by inserting a trap circuit that attenuates a signal with a frequency of _2H to a predetermined level.