JP2005252936A - Video image voice receiving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a video image voice receiving circuit which functions AFT (Automatic Frequency Tuning) as much as possible even in an area where an electric wave is weak, and can clearly receive the video image and voice. <P>SOLUTION: The circuit is provided with a limiter 42 which amplifies a voice intermediate frequency signal, a signal strength detector 46 which is connected to the limiter 42, and detects strength of the voice intermediate frequency signal, and a comparison circuit 31 in which output voltage of the signal strength detector 46 is connected to one input terminal, a reference voltage is applied to another input terminal, and its output terminal is connected to the AFT circuit 30. The comparison circuit 31 comprises a switch circuit which changes over the output voltage when strength of the voice intermediate signal is weak, and makes operation of the AFT circuit 30 stop. Since the reference voltage of the comparison circuit 31 is possible to be optimally set, the AFT function can continuously operate as long as possible even when the input signal is weak. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an AFT circuit that belongs to a video / audio reception circuit and automatically adjusts an intermediate frequency.

  In recent years, television receivers have been reduced in weight and size, and are being installed in in-vehicle televisions and car navigation systems. Under such circumstances, there is an increasing market demand for receiving video signals and audio signals as clearly as possible even in areas where radio waves are weak. An AFT (automatic frequency adjustment) circuit is known as a technique for solving these problems, and its functions are roughly divided into two.

  The first function is that the video intermediate frequency input to the video / audio signal processing circuit (VIF / SIF) for receiving the video signal and audio signal clearly is always constant 58.75 MHz in Japan. The AFT circuit outputs an S-characteristic as shown in FIG. 6 and feeds it back to the local oscillator of the tuner circuit to automatically adjust the frequency. Since the frequency of the local oscillator is affected by changes in ambient temperature and time, for example, if the frequency of the local oscillator is shifted to a lower side, the video intermediate frequency is higher than the normal 58.75 MHz in Japan. The output voltage of the AFT circuit becomes lower than the normal center voltage, and this voltage is sent to the local oscillator via the microcomputer, and the frequency of the local oscillator is corrected to the higher side to return to the normal frequency. As a result, the video intermediate frequency becomes a regular 58.75 MHz, and the best received video and audio can be obtained.

  The second function is a function called auto-tuning or auto-preset that uses the S-characteristic of the AFT circuit to automatically select channels that can be received from all channels. For example, if the channel selection frequency is f0, the signal presence / absence determination discriminates the level of the AFT output signal at f0 ± Δf to determine whether or not reception is possible, and if possible, this receivable frequency / receivable channel Store the number in memory.

  As described above, the S-characteristic of the output voltage of the AFT circuit is used to perform automatic adjustment and auto-tuning of the video intermediate frequency. However, when the radio wave becomes very weak, the S-characteristic is disturbed and unstable. Therefore, the auto tuning and automatic frequency adjustment functions may malfunction. In order to eliminate such an adverse effect, there is an AFT defeat function for forcibly stopping the AFT operation by forcing the AFT output signal to the center voltage at the time of weak input signal as shown in FIG.

  The market demand is to receive clear video and audio signals using the automatic frequency adjustment function even in areas where the radio wave is weak, and to identify as many receivable channels as possible using the auto tuning function. Therefore, it is important that the AFT circuit continues to operate as much as possible even if it becomes a weak input signal. However, if the AFT circuit continues to operate until the input signal becomes very weak, auto-tuning and automatic frequency adjustment may malfunction. Therefore, it is very important to set a switching level for turning on / off the defeat function for stopping the operation of the AFT circuit.

  Hereinafter, defeat control for stopping the operation of the AFT circuit in the conventional weak input signal will be described with reference to FIG.

  Conventional systems for controlling the AFT circuit for weak input signals are broadly divided into an antenna 10 and a tuner circuit 101 that selects a desired channel frequency from a television high-frequency signal received by the antenna 10 and converts it into a video intermediate frequency and an audio intermediate frequency. The video / audio signal processing circuit 100 detects video and audio from the intermediate frequency signal of the video and audio.

  The operation of the circuit for controlling the output characteristics of the AFT circuit in the conventional weak input signal configured as described above will be described.

  First, the UHF band or VHF band television high-frequency signal received by the antenna 10 is amplified by selecting a desired channel frequency by the high-frequency amplifier circuit 11, and this high-frequency signal and a local oscillator (Local Oscillator) signal are 1st mixer. For example, in Japan, video is converted to an intermediate frequency signal of 58.75 MHz, and audio is converted to an intermediate frequency signal of 54.25 MHz.

  Then, only the video intermediate frequency signal is discriminated and passed by the video SAW filter (surface acoustic wave filter) 13 having the characteristics of the band pass filter of 58.75 MHz of the video intermediate frequency signal, and the video intermediate frequency signal is passed through the video intermediate frequency amplifier 21. And is applied to the video detector 22. The output signal of the video intermediate frequency amplifier 21 includes a phase detector (PD: Phase Detection) 26, a low pass filter (LPF) 27, a video voltage controlled oscillator (VCO) 25, and a phase shifter (PS: Phase Shift). ) Is also applied to a PLL circuit 29 comprising 28.

  In this PLL circuit 29, first, the output of the video voltage controlled oscillator 25 is input to the phase detector 26 with the phase shifted by 90 degrees by the phase shifter 28 so that phase detection is possible. A phase comparison with the input signal is performed, and the output is smoothed by the low-pass filter 27 to become a frequency control voltage and fed back to the video voltage controlled oscillator 25. Next, a signal whose phase is shifted by 0 degrees so that synchronous detection can be performed by the phase shifter 28 is input to the video detector 22. As a result, the PLL circuit 29 locks the phase of the output signal from the video intermediate frequency amplifier 21 and the signal of the video voltage controlled oscillator 25 via the phase shifter 28, the video detector 22 performs synchronous detection, and the video Output a signal.

  The IFAGC circuit 24 determines the strength of the intermediate frequency signal from the synchronously detected video signal. If the intermediate frequency signal is weak, the gain of the video intermediate frequency amplifier 21 is increased. The gain of the intermediate frequency amplifier 21 is controlled to be low, and the intensity of the video signal output from the video detector 22 is always constant. Then, it is amplified by the video amplifier 23 and a video signal is output to the video signal output terminal 36.

  The input of the AFT circuit 30 that performs automatic frequency adjustment is connected to the phase detector 26 through the low-pass filter 27, and the difference frequency between the normal 58.75 MHz and the signal frequency that has passed through the video intermediate frequency amplifier 21. A DC signal corresponding to is applied, and this signal is amplified by the AFT circuit 30. This output characteristic has an S-characteristic as shown in FIG. Is fed back to the local oscillator 15. As a result, even when the reception frequency changes or when the frequency of the local oscillator 15 changes with the ambient temperature or with time, the video intermediate frequency is automatically adjusted so that this frequency is always the regular 58.75 MHz. Operate. Further, at the time of auto-tuning, the microcomputer 16 determines the presence / absence of the channel selection by the presence / absence of the AFT output signal having the S-characteristic, and selects the frequency selected in the memory 17 connected to the microcomputer 16. Store.

  On the other hand, the audio intermediate frequency of 54.25 MHz mixed by the 1st mixer circuit 12 is amplified by the audio intermediate frequency amplifier 39 through the audio SAW filter 14 having the band pass filter characteristic of 54.25 MHz, and the signal is supplied to the 2nd mixer circuit. 40 is mixed with the signal 58.75 MHz of the video voltage controlled oscillator 25 input via the phase shifter 28 and down-converted to a 4.5 MHz audio signal, and 4.5 MHz by the band pass filter (BPF) 41. , And is amplified by the limiter 42, detected by the FM detector 43 connected to the second audio voltage controlled oscillator 45 having an oscillation frequency of 4.5 MHz, and the audio signal passes through the audio amplifier 44 at the final stage. An audio signal is output to the output terminal 55.

  As described above, the characteristics of the output signal AFTOUT33 of the AFT circuit 30 of FIG. 8 are stable S-shaped characteristics as shown in FIG. 6 when the input radio wave is relatively strong, and the video intermediate signal frequency of the output of the 1st mixer circuit 12 is Control to be constant is performed. However, when the input radio wave is very weak, the ratio of the noise intensity to the signal intensity increases, and the PLL circuit 29 does not normally phase lock, and the output signal of the LPF 27 becomes very unstable. For this reason, the AFT circuit 30 cannot supply a stable voltage to the local oscillation circuit 15, the frequency thereof becomes very unstable, the video intermediate frequency of the 1st mixer circuit 12 is not constant, and the video SAW filter 13 Has a characteristic of a band-pass filter that passes only the video intermediate frequency of 58.75 MHz. Therefore, the video intermediate frequency amplifier 21 receives an attenuated video intermediate frequency signal that is different from the normal intermediate frequency. As a result, there is a possibility that the image is not normally detected and malfunctions. Therefore, a defeat function is provided for forcibly stopping the operation of the AFT circuit 30 when the input radio wave is weak.

  FIG. 7 shows the relationship between the video intermediate frequency and the output voltage of the AFT circuit 30. The defeat function of the AFT operation is the input frequency as indicated by an arrow 70 in the figure for an input signal having a certain input intensity Vin1 or higher. A weak input signal of Vin1 or less is forcibly set to the center voltage regardless of the input frequency and the AFT operation is stopped. The AFT defeat switching input intensity Vin1 utilizes the characteristics of the output voltage 32 corresponding to the input intensity of the IFAGC circuit 24 shown in FIG. 4, and the output voltage 32 of the IFAGC circuit 24 is input to one end and the other end is input. Is connected to the AFT circuit 30 so that the threshold voltage Vref1 is preset so that the output inverts when the input is weakly input Vin1, so that the operation of the AFT circuit 30 is stopped when the input signal is weak. .

  With such a configuration, if the switching level for turning on / off the AFT defeat function for stopping the operation of the AFT circuit 30 is set high, the AFT circuit does not operate with weak input, and it is desired to receive video and audio even with weak input. Can't satisfy market demand. On the other hand, if the level is set low, the intermediate frequency signal becomes very unstable at a weak input, and the video signal and the audio signal cannot be received clearly, and a malfunction may occur. For this reason, it is particularly important how much the input level is set as the switching level.

For example, Patent Document 1 is known as prior art document information relating to the invention of this application.
Japanese Unexamined Patent Publication No. 1-311778

  In Japan, the video bandwidth is about 6 MHz per channel, for example, while the audio bandwidth is about 100 kHz at the maximum per channel, and the video bandwidth is wider than the audio bandwidth. Worse than. For this reason, the video gain needs to be set lower than the audio gain. For this reason, the output voltage 32 of the video IFAGC circuit 24 is such that the gain of the video intermediate frequency amplifier 21 is at a constant level so that noise components are not amplified when the input signal is weak, as shown in FIG. It has the characteristic that the gain can be varied above a certain intensity.

  However, in the conventional configuration, the threshold voltage 49 of the defeat function for stopping the operation of the AFT circuit 30 is set using the output voltage 32 of the IFAGC circuit 24 as described above, and the threshold voltage 49 is close to the maximum voltage. The input intensity Vin1 corresponding to Vref1 is the limit of weak input, and the input intensity becomes a relatively high level. For this reason, especially when a receiver is used for a mobile body such as a vehicle, the auto preset function and the automatic frequency adjustment function of the AFT circuit cannot be fully utilized in an area where radio waves are weak.

  The present invention solves the above-mentioned conventional problems, and particularly in an in-vehicle application, even in an area where the radio wave is weak, the auto preset function and automatic frequency adjustment function of the AFT circuit are operated as much as possible to cope with an area where the radio wave is weak. It is an object of the present invention to provide a video / audio receiving circuit that can be used.

  In order to solve the above problems, the means of the invention of claim 1 includes a voltage controlled oscillation circuit and a phase comparison circuit for comparing the phase of the output signal of the voltage controlled oscillation circuit and the video intermediate frequency signal. A video receiving circuit that performs video detection using a phase synchronization circuit, an AFT circuit that amplifies the output voltage of the phase comparison circuit, a limiter that amplifies an audio intermediate frequency signal, and an audio intermediate frequency signal connected to the limiter A signal intensity detector for detecting the input intensity of the signal, and connecting the output voltage of the signal intensity detector to the first input terminal, the second input terminal is applied with a reference voltage capable of adjusting the threshold voltage, The output terminal is connected to the AFT circuit, and is composed of a comparison circuit provided with a switch means for stopping the operation. The threshold voltage of the comparison circuit is adjusted to operate / stop the AFT circuit. A video audio receiving circuit, characterized in that to adjust the switching level.

  According to the first aspect of the present invention, especially in an in-vehicle application, even in an area where the radio wave is weak, the threshold voltage is adjusted to operate the auto preset function and the automatic frequency adjustment function of the AFT circuit as much as possible. Can also be provided.

  According to a second aspect of the present invention, the signal intensity detector includes an amplifier circuit that amplifies the input intensity of the audio intermediate frequency signal and a peak hold circuit that converts the signal into a DC voltage, and the gain of the amplifier circuit is increased. 2. The video / audio receiving circuit according to claim 1, wherein the switching intensity of the operation / stop of the AFT circuit can be adjusted.

  According to the invention of claim 2, for example, when the attenuation characteristics of the SAW filter for video and audio differ depending on the country such as Japan and the United States, or the intensity ratio (PS ratio) between the video intermediate frequency signal and the audio intermediate frequency signal is Even in different cases, it is possible to optimize the input level for stopping the AFT circuit.

  As described above, in the video / audio reception circuit according to the first aspect of the present invention, the comparator in which the threshold voltage is optimally set in advance so that the AFT circuit is stopped with respect to the weak input is provided between the signal intensity detector and the AFT circuit. By providing it in between, the output voltage of the AFT circuit outputs the S-characteristic of AFT as stably as possible even with weak input, and the local oscillator operates so that the output of the mixer has a normal video intermediate frequency. In particular, an AFT defeat circuit having an excellent characteristic that a clear video signal and audio signal can be received even in a weak input signal region is possible, especially in mobile applications such as in-vehicle use.

  In the video / audio reception circuit according to claim 2 of the present invention, the gain of the signal intensity detector can be adjusted, and as a result, the input intensity level for stopping the AFT circuit can be adjusted. Optimizes the input level for stopping the AFT circuit even when the attenuation characteristics of the SAW filter for video and audio differ due to differences in the difference, or when the intensity ratio (PS ratio) between the video intermediate frequency signal and audio intermediate frequency signal is different Is possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a block diagram showing a configuration of an AFT defeat circuit according to Embodiment 1 of the present invention, and the same reference numerals are used for the same parts as in the conventional example.

  In this configuration, the limiter 42 is connected to the signal intensity detector 46, the output voltage 47 of the signal intensity detector 46 is input to one end, and the threshold voltage is applied to the other end so that the output is inverted when the input radio wave is weak. A comparator 31 in which Vref2 is optimally set in advance is provided. The output of the comparator 31 is connected to the AFT circuit 30 and has a defeat function for stopping the operation of the AFT circuit 30 when the input radio wave is weak. Compared to the conventional AFT defeat function using the output voltage 32 of the IFAGC circuit 24, the threshold voltage 49 of the comparator 31 can be set so that the AFT defeat function operates when a weaker input signal is applied. . Antenna 10, tuner circuit 101, video SAW filter 13, audio SAW filter 14, video intermediate frequency amplifier 21, video detector 22, video amplifier 23, IFAGC circuit 24, PLL circuit 29, AFT circuit 30, microcomputer according to the present embodiment 16, the memory 17, the audio intermediate frequency amplifier 39, the 2nd mixer 40, the band pass filter 41, the limiter 42, the FM detector 43, the audio amplifier 44, and the audio voltage controlled oscillator 45 have the same configuration as the conventional one.

  The operation of the AFT defeat circuit of the present invention configured as described above will be described. Note that the configuration and functions other than the AFT defeat are the same as in the prior art.

  First, as in the prior art, the output signal AFTOUT 33 of the AFT circuit 30 of FIG. 1 has a stable S-characteristic as shown in FIG. 6 when the input radio wave is relatively strong. Control is performed so that the frequency is constant. However, when the input radio wave is very weak, the ratio of the noise intensity to the signal intensity increases, and the PLL circuit 29 does not normally phase lock, and the output signal of the LPF 27 becomes very unstable. For this reason, the AFT circuit 30 cannot supply a stable voltage to the local oscillation circuit 15, the frequency thereof becomes very unstable, the video intermediate frequency of the 1st mixer circuit 12 is not constant, and the video SAW filter 13 Has a characteristic of a band-pass filter that passes only the video intermediate frequency of 58.75 MHz. Therefore, the video intermediate frequency amplifier 21 receives an attenuated video intermediate frequency signal that is different from the normal intermediate frequency. As a result, there is a possibility that the image is not normally detected and malfunctions. Therefore, a defeat function is provided for forcibly stopping the operation of the AFT circuit 30 when the input radio wave is weak.

  FIG. 7 shows the relationship between the video intermediate frequency and the output voltage of the AFT circuit 30. The defeat function of the AFT operation is an input frequency as indicated by an arrow 70 in the figure for an input signal having a certain input intensity Vin2 or higher. A weak input signal of Vin2 or less is forcibly set to the center voltage regardless of the input frequency and the AFT operation is stopped. This AFT defeat switching input intensity Vin2 utilizes the characteristics of the output voltage 47 corresponding to the input intensity of the video intermediate frequency of the signal intensity detector 46 shown in FIG. By connecting the comparator 31 with the threshold voltage Vref2 optimally set in advance so that the output is inverted when the weak input Vin2 is input to the other end, the AFT circuit 30 is used for the weak input signal. The operation of is to stop.

  As described above, in the configuration of the present invention, the threshold voltage 49 of the defeat function for stopping the operation of the AFT circuit 30 is set by using the output voltage 47 of the signal intensity detector 46, and the voice bandwidth is set as described above. The intermediate frequency audio SN is better than the video SN because it is narrower than the video bandwidth. Unlike the characteristics of the video IFAGC circuit 24, the signal strength detector 46 is used even in the case of weak input signals where the intermediate frequency audio SN deteriorates. The output voltage 47 is output corresponding to the input intensity as shown in FIG. 5, and the threshold voltage 49 can be optimally set so as to correspond to a very weak input signal. For this reason, particularly in an in-vehicle application, the auto preset function and the automatic frequency adjustment function of the AFT circuit 30 can be fully utilized even in an area where radio waves are weak, and the threshold voltage 49 can be optimized.

  As described above, the present invention provides a video / audio receiving circuit that can operate in an area where radio waves are weak by operating the auto preset function and automatic frequency adjustment function of the AFT circuit as much as possible even in areas and places where radio waves are weak. Can do.

  Next, FIG. 2 is a circuit diagram showing in detail a portion of the signal intensity detector 46 shown in FIG. FIG. 3 is a circuit diagram showing details of the comparison circuit 31 connected to the signal intensity detector 46 of FIG.

  The signal intensity detector 46 includes a differential amplifier circuit 66 and a peak hold circuit 67, and a DC voltage proportional to the input intensity is output between the output terminals 63 and 64. The differential amplifier circuit 66 and the peak hold circuit 67 are general circuits, and include transistors Q10, Q11, Q12, Q13, resistors R10, R11, R12, R13, R14, R15, R16, R17, a capacitor C10, and a constant current source. It is comprised by I10, I11, I12, and the constant voltage source V10.

  The output voltage 50 of the limiter circuit 42 in FIG. 1 is connected to the input 50 of the differential amplifier circuit 66 in FIG. 2. First, the input signal is amplified by the differential amplifier circuit 66. One output voltage connected to the load resistor R17 is output to the output terminal 64 via the transistor Q13. However, since the current value of the constant current source I12 is small and the value of the capacitor C10 is large, the AC amplitude is changed to the DC voltage. It plays the role of a peak hold circuit that converts to. On the other hand, the center voltage of the output voltage of the differential amplifier circuit 66 is taken out from the connection terminal of R14 and R15 having the same resistance value with one end connected to the load resistors R16 and R17 and the other end connected to each other. To the output terminal 63. In this way, a DC voltage proportional to the input intensity is output between the output terminals 63 and 64.

  The gain of the differential amplifier circuit 66 is adjusted by adjusting the constant current value I10, the emitter resistance values R12 and R13, and the load resistance values R16 and R17. As a result, the AFT circuit operation / stop switching level is set. Can be adjusted. If the threshold voltage Vref2 shown in FIG. 5 is set to be constant, for example, if the switching level is set high, the gain of the differential amplifier is set low. Conversely, if the switching level is set low, the differential amplifier What is necessary is just to set a gain high.

  3 controls the operation / stop of the AFT circuit 30, the output signal 51 is connected to the AFT circuit 30, one input terminal thereof is connected to the signal intensity detector 47, and the other The input terminal is connected to a threshold voltage 52 that can adjust the threshold voltage, and the switching strength of the operation / stop of the AFT circuit 30 can be adjusted. This circuit includes transistors Q20 and Q21, a resistor R20, and constant current sources I20 and I21.

  The threshold voltage 49 is determined by Vref2 = R20 * I20. For example, if the switching strength is to be increased, the value of the resistor R20 or the constant current source I20 is increased to increase the threshold voltage Vref2 shown in FIG. In order to lower the threshold voltage Vref2, the value of the resistor R20 or the value of the constant current source I20 may be reduced.

  In the present invention, as described above, for example, when the attenuation characteristics of video and audio SAW filters differ depending on the country such as Japan and the United States, or the intensity ratio (PS ratio) between the video intermediate frequency signal and the audio intermediate frequency signal is different. Even in this case, it is possible to optimize the input level for stopping the AFT circuit.

  In the present invention, the video intermediate frequency and the audio intermediate frequency are described as 58.75 MHz and 54.25 MHz in Japan, respectively. However, in the United States, for example, they are 45.75 MHz and 41.25 MHz, respectively. Needless to say, the oscillation frequency is 45.75 MHz.

  Further, in the present invention, the gain of the differential amplifier 66 and the threshold voltage Vref2 are set by setting constants inside the IC, but it goes without saying that they can be controlled from outside the IC.

  As described above, in the video / audio reception circuit according to the first aspect of the present invention, the comparator in which the threshold voltage is optimally set in advance so that the AFT circuit is stopped with respect to the weak input is provided between the signal intensity detector and the AFT circuit. By providing it in between, the output voltage of the AFT circuit outputs the S-characteristic of AFT as stably as possible even with weak input, and the local oscillator operates so that the output of the mixer has a normal video intermediate frequency. In particular, an AFT defeat circuit having an excellent characteristic that a clear video signal and audio signal can be received even in a weak input signal region is possible, especially in mobile applications such as in-vehicle use.

  In the video / audio reception circuit according to claim 2 of the present invention, the gain of the signal intensity detector can be adjusted, and as a result, the input intensity level for stopping the AFT circuit can be adjusted. Optimizes the input level for stopping the AFT circuit even when the attenuation characteristics of the SAW filter for video and audio differ due to differences in the difference, or when the intensity ratio (PS ratio) between the video intermediate frequency signal and audio intermediate frequency signal is different Is possible.

The figure of the AFT defeat circuit in Embodiment 1 of this invention The figure of the circuit of the signal strength detector in Embodiment 2 of the present invention The figure of the circuit of the comparator connected to the signal strength detector in Embodiment 2 of this invention The figure which shows the intensity | strength dependence of the video intermediate frequency signal of an IFAGC output signal The figure which shows the intensity dependence of the video intermediate frequency signal of the signal intensity detector output signal The figure which shows the video intermediate frequency dependence of an AFT output signal The figure which shows the intensity dependence of the video intermediate frequency signal of an AFT output signal Figure of conventional AFT defeat circuit

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Antenna 11 High frequency amplifier circuit 12 1st mixer 13 Video SAW filter 14 Audio SAW filter 15 Local oscillation circuit 16 Microcomputer 17 Memory 21 Video intermediate frequency amplifier 22 Video detector 23 Video amplifier 24 IFAGC circuit 25 Video voltage control oscillator 26 Phase comparator 27 LPF (low pass filter)
28 Phase shifter 29 PLL circuit 30 AFT circuit 31 Comparator 32 IFAGC output voltage 33 AFT output voltage 36 Video signal output terminal 39 Audio intermediate frequency amplifier 40 2nd mixer 41 BPF (band pass filter)
42 limiter 43 FM detector 44 audio amplifier 45 audio voltage controlled oscillator 46 signal intensity detector 47 signal intensity detector output voltage 49 threshold voltage power supply 50 signal intensity detector input signal 51 comparator output signal 52 threshold voltage 55 audio signal output terminal 60 VCC terminal 61 GND terminal 66 Differential amplifier 67 Peak hold circuit 101 Tuner circuit 100 Video / audio signal processing circuit Q10 to Q13 transistor R10 to R17 resistor C10 capacitance I10 to I12 constant current source V10 constant voltage source Q20 to Q21 transistor R20 resistor I20 ~ I21 Constant current source

Claims (2)

Video receiving circuit for performing video detection using a phase synchronization circuit including a voltage controlled oscillation circuit and a phase comparison circuit for comparing the phase of the output signal of the voltage controlled oscillation circuit and the video intermediate frequency signal, and the phase comparison circuit An AFT circuit that amplifies the output voltage of the audio signal, a limiter that amplifies the audio intermediate frequency signal, a signal intensity detector connected to the limiter to detect the input intensity of the audio intermediate frequency signal, and an output voltage of the signal intensity detector Is connected to the first input terminal, the second input terminal is supplied with a reference voltage capable of adjusting the threshold voltage, and the output terminal is connected to the AFT circuit, and has a switch means for stopping its operation. And a switching level of the operation / stop of the AFT circuit by adjusting a threshold voltage of the comparison circuit. . The signal intensity detector includes an amplifier circuit that amplifies the input intensity of the audio intermediate frequency signal, and a peak hold circuit that converts this signal into a DC voltage, adjusts the gain of the amplifier circuit, 2. The video / audio receiving circuit according to claim 1, wherein the switching strength of operation / stop is adjustable.

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