JP2005268860A - Video/audio demodulation circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To fully remove buzz appearing in an audio signal due to fluctuation in the frequency of an oscillation circuit for AM detection. <P>SOLUTION: The frequency of an audio signal outputted by an AM detection circuit 16 is (f<SB>S</SB>+Δf<SB>VCO</SB>) by the fluctuation of the oscillation frequency of VCO 19. The output signal of the VCO 19 of an oscillation frequency (f<SB>VCO</SB>+Δf<SB>VCO</SB>) is inputted into a multiplier 41. When a signal of (f<SB>VCO</SB>+Δf<SB>VCO</SB>) and a signal of (f<SB>P</SB>-f<SB>S</SB>-f<SB>C</SB>) from a VCO 55 are inputted, the multiplier 41 outputs the component of a frequency difference (f<SB>S</SB>+f<SB>C</SB>+Δf<SB>VCO</SB>) via an LPF 61. A PLL circuit 60 outputs the signal of the oscillation frequency (f<SB>S</SB>+f<SB>C</SB>+Δf<SB>VCO</SB>) using the component of this frequency difference as reference. If the audio signal of the frequency (f<SB>S</SB>+Δf<SB>VCO</SB>) and the signal of the frequency (f<SB>S</SB>+f<SB>C</SB>+Δf<SB>VCO</SB>) inputted via an HPF 31 is provided, a frequency converter 32 outputs an audio signal converted into a frequency to be f<SB>C</SB>. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a video / audio demodulation circuit, and more particularly to a video / audio demodulation circuit mounted on a television receiver.

  FIG. 4 is a block diagram showing an example of a configuration of a conventional video / audio demodulation circuit mounted on a television receiver (see, for example, Patent Document 1).

  Referring to FIG. 4, the video / audio demodulation circuit is roughly composed of a video receiver 100 and an audio receiver 300.

  The video receiver 100 includes antennas 101 and 102, a tuner circuit 103, a variable gain amplifier 104, an automatic gain control (AGC) unit GC, and an automatic phase control (APC) unit PC. And a sound removal filter (STF: Sound Trap Filter) 111.

  The tuner circuit 103 selectively receives one channel of VHF or UHF to be received, and amplifies both video and audio carrier waves simultaneously and uniformly. Once amplified, the video and audio signals are converted to intermediate frequencies for further amplification.

  Here, as a circuit system for receiving an audio signal, there are a split carrier system that amplifies and extracts the audio signal separately from the video signal, and an intercarrier system that amplifies the audio signal by the same amplifier circuit without separating the audio signal. is there. In recent years, the split carrier method has been widely adopted because it has a large number of amplifying elements and is unstable with respect to fluctuations in the local oscillation frequency of the tuner circuit. Also in this embodiment, it is assumed that an audio signal is received using this intercarrier method.

  In the intercarrier method, a video signal and an audio signal are amplified by the same amplifier circuit, and a beat signal of a difference between the video carrier and the audio carrier is used as an intermediate frequency of the audio signal. For this reason, the sound intermediate frequency is stable against fluctuations in the local oscillation frequency of the tuner circuit.

  In general, in the TV channel, the video carrier has a frequency 4.5 MHz lower than the audio carrier, but when converted to an intermediate frequency, the video carrier frequency is reversed, and the video intermediate frequency is 4.5 MHz higher than the audio intermediate frequency. .

In the following, it is assumed that the notation video intermediate frequency at f _P. Furthermore, in the intercarrier system, the audio intermediate frequency corresponds to the difference between the video carrier and the audio carrier, and is 4.5 MHz in the above-described TV channel. When this audio intermediate frequency is set to f _S , the carrier frequency of the audio carrier is given by (f _P −f _S ).

Therefore, the variable gain amplifier 104 receives a mixed signal of the video intermediate frequency signal (carrier frequency f _P ) and the audio intermediate frequency signal (carrier frequency f _P −f _S ).

  The variable gain amplifier 104 amplifies the mixed signal of the video intermediate frequency signal and the audio intermediate frequency signal. The amplification degree at this time is controlled by the AGC detection circuit 105 which is the automatic gain control unit GC. The AGC detection circuit 105 constitutes a negative feedback circuit, and controls the gain of the variable gain amplifier 104 so that the output peak value of the video intermediate frequency signal applied to the AM detection circuit 106 is always constant.

  The automatic phase control unit PC includes an APC detection circuit 107, a voltage controlled oscillation circuit (VCO: Voltage Controlled Oscillator) 109, a -45 ° phase shifter 108, and a + 45 ° phase shifter 110.

The automatic phase control is to lock the VCO oscillation frequency and phase with the incoming video intermediate frequency signal as a reference oscillation signal for tracking. Specifically, the APC detection circuit 107 detects a phase difference between the video intermediate frequency signal and the output signal of the _VCO 109, and outputs a control signal for increasing or decreasing the oscillation frequency f _VCO of the _VCO 109. At this time, the APC detection circuit 107 controls the VCO 109 so that both have the same frequency and the phase difference is 90 °.

Here, as shown in FIG. 4, the output signal of the VCO 109 is input to the APC detection circuit 107 with a phase of 45 ° delayed by the −45 ° phase shifter 108. _{The VCO} is controlled to be a signal equal to the carrier frequency f _P of the video intermediate frequency signal and having a phase shifted by 135 °. The output signal of the oscillation frequency f _VCO (= f _P ) output from the _VCO 109 is given to the AM detection circuit 106 via the + 45 ° phase shifter 110 with a further 45 ° phase difference.

  When an output signal synchronized with the hybrid signal from the + 45 ° phase shifter 110 is input together with the hybrid signal of the video intermediate frequency signal and the audio intermediate frequency signal, the AM detection circuit 106 integrates these two signals. The so-called synchronous detection is performed. At this time, since the video intermediate frequency signal and the output signal of the + 45 ° phase shifter 110 have the same frequency and a phase difference of 180 °, the video signal that is AM-negatively modulated can be demodulated. .

On the other hand, the voice intermediate frequency signal (carrier frequency f _P −f _S ) of the hybrid signal is integrated with the output signal of the frequency f _VCO (= f _P ) of the 45 ° phase shifter, so that the carrier frequency is ( f _P −f _S ) −f _P = f _S (1)
Is converted into an audio signal.

When the demodulated video signal and the audio signal having the audio intermediate frequency f _S are supplied to the audio removal filter (STF) 111, only the audio signal component is removed. As a result, the video signal is output as a video output from a video reproduction unit (not shown). The video signal and the audio signal are further transmitted to the audio receiving unit 300.

  The voice receiving unit 300 includes a band-pass filter (BPF) 301 that extracts only the voice signal from the output signal of the AM detection circuit 106, and an FM detection circuit 302 that demodulates the extracted voice signal.

The audio signal having the audio intermediate frequency f _S is demodulated by the FM detection circuit 302 and then output as an audio output from a speaker (not shown).
Japanese Patent Laid-Open No. 3-158082

  In the conventional audio / video demodulation circuit shown in FIG. 4, if the output of the VCO 109 of the automatic phase control unit PC is stable, the AM detection circuit 106 can always obtain a stable output.

  However, in practice, the output of the variable gain amplifier 104 input to the APC detection circuit 107 is subjected to AM modulation and the amplitude is not constant. Normally, the detection efficiency of the APC detection circuit 107 decreases when the amplitude of the input signal becomes extremely small. When the detection efficiency decreases, the phase error of the APC detection circuit 107 increases, and the automatic phase control loop changes the oscillation frequency of the VCO 109 in an attempt to adjust the error.

  Therefore, the output of the AM detection circuit 106 is not stabilized with the fluctuation of the oscillation frequency of the VCO 109, and a signal with a frequency fluctuation appears. Since the audio signal is frequency-modulated, when it is demodulated by the FM detection circuit 302, the frequency fluctuation is superimposed on the audio signal and appears in the audio output.

This phenomenon will be described in more detail. Now, let the oscillation frequency of the _VCO 109 be f _VCO, and its variation be Δf _VCO . The frequency component of the input signal of the APC detection circuit 107 has a video intermediate frequency signal component f _P and an audio intermediate frequency signal component (f _P −f _S ). The automatic phase control unit operates so as to lock its own oscillation frequency to the video intermediate frequency signal that is the reference oscillation signal, that is, f _VCO = f _P. The AM detection circuit 106 outputs a frequency difference component between two input signals. That is, the frequency component output from the AM detection circuit 106 is (f _VCO + Δf _VCO ) −f _P = Δf _VCO (2) as a video signal component after detection.
(F _VCO + Δf _VCO ) − (f _P −f _S ) = f _S + Δf _VCO (3)
It consists of. In this audio signal component, the variation Δf _VCO behaves as if frequency modulation is applied, so that it appears as it is in the output of the FM detection circuit at the subsequent stage that demodulates the audio signal. This is a so-called buzz and is a typical phenomenon in which the influence of the video signal appears in the audio output.

  SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a video / audio demodulation circuit having high reception performance by completely removing buzz appearing in an audio signal due to frequency fluctuation of an AM detection oscillation circuit.

  According to an aspect of the present invention, an image includes an image receiving unit that amplitude-demodulates an image intermediate frequency signal and outputs an image output, and an audio receiving unit that demodulates an audio intermediate frequency signal and outputs an audio output. An audio demodulation circuit, wherein the video receiving unit uses the first oscillation signal and a phase control unit that generates a first oscillation signal having the same frequency as the video intermediate frequency signal and having a predetermined phase relationship And an AM detection circuit for amplitude-demodulating the video intermediate frequency signal. The audio receiving unit extracts, from the output of the AM detection circuit, an audio signal extraction circuit that extracts a first audio signal that is a frequency difference between the video intermediate frequency signal and the audio intermediate frequency signal, and a first audio signal A frequency converter that mixes with a reference signal having a frequency of 2 and converts the frequency into a second audio signal that is a difference in frequency between the first audio signal and the reference signal, and an FM detection circuit that frequency-demodulates the second audio signal Including. The video graduate student demodulation circuit further includes a phase locked loop unit that generates a reference signal based on the first oscillation signal and supplies the reference signal to the frequency converter. The phase-locked loop unit outputs a second oscillation signal having the same frequency as the difference between the frequency of the audio intermediate frequency signal and the second audio signal based on the internal local oscillation signal; A multiplier that multiplies the first oscillation signal and the second oscillation signal and outputs the multiplication result; and a difference in frequency between the first oscillation signal and the second oscillation signal of the multiplication result is a reference signal. As a reference signal extraction circuit.

  According to one aspect of the present invention, by adopting a configuration in which the audio signal is frequency-converted using the reference signal generated by the phase-locked loop unit, the frequency fluctuation component of the VCO superimposed on the audio signal is canceled, and the buzz Occurrence can be completely suppressed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of a video / audio demodulation circuit according to the first embodiment of the present invention.

  Referring to FIG. 1, the video / audio demodulation circuit includes a video reception unit 10, an audio reception unit 30, and a phase-locked loop unit (PLL: Phase Locked Loop) 40.

  The video receiver 10 includes antennas 11 and 12, a tuner circuit 13, a variable gain amplifier 14, an AGC detection circuit 15 that is an automatic gain control unit GC, an AM detection circuit 16, and an automatic phase control (APC) unit PC. And a sound removal filter (STF) 21.

  The automatic phase control unit PC includes an APC detection circuit 17, a −45 ° phase shifter 18, a VCO 19, and a + 45 ° phase shifter 20.

  The voice receiving unit 30 includes a high pass filter (HPF) 31, a frequency converter 32, a band pass filter (BPF) 33, and an FM detection circuit 34.

  In the video / audio demodulation circuit shown in FIG. 1, the configurations of the video reception unit 10 and the audio reception unit 30 are the same as the configurations of the video reception unit 100 and the audio reception unit 300 included in the conventional video / audio demodulation circuit shown in FIG. Each is basically the same.

To summarize the operation of each unit, the video receiver 10 receives the video intermediate frequency signal (carrier frequency f _P ) and audio intermediate frequency signal (carrier frequency f _P −f _S ) amplified by the variable gain amplifier 14 as AM. The detection circuit 16 detects in accordance with the output signal of the _VCO 19 having the oscillation frequency f _VCO (= f _P ). From AM detection circuit 16, and the audio signal of the video signal and the intermediate frequency f _S is output.

  The voice receiving unit 30 extracts a voice signal component from the output of the AM detection circuit 16, demodulates it by the FM detection circuit 34, and outputs it as a voice output.

  However, the video / audio demodulation circuit of FIG. 1 is provided with a phase-locked loop unit 40 in addition to these parts, and frequency conversion based on the output signal of the phase-locked loop unit 40 to the audio receiving unit 30 The only difference is that the frequency converter 32 is provided. Therefore, detailed description of other common circuit parts is omitted.

  The phase-locked loop unit 40 includes two PLL circuits (a PLL circuit 50 and a PLL circuit 60) and a multiplier 41.

  The PLL circuit 50 includes a crystal oscillator 51 that oscillates at a predetermined frequency, a 1 / n frequency divider 52 that divides the frequency of the output signal of the crystal oscillator 51 by n (n is a natural number), and an output signal that has been divided by n. And a phase comparator 53 that compares the phase of the signal fed back via the 1 / m (m is a natural number) frequency divider 54 and a VCO 55 that outputs a signal having a frequency corresponding to the comparison result of the phase comparator 53 And a 1 / m frequency divider 54 that divides the frequency of the output signal of the VCO 55 by m.

Crystal oscillator 51, a predetermined local oscillation frequency (hereinafter referred to as f _X) for generating a reference oscillation signal. The reference oscillation signal having the frequency f _X is frequency-divided to a frequency of f _X / n by the frequency divider, and then supplied to the phase comparator 53.

The VCO 55 generates a signal having a frequency corresponding to the phase comparison result (hereinafter referred to as an oscillation frequency f _VCOA ). The output signal of the frequency _{f VCOA,} after being divided in 1 / m frequency divider 54 to a frequency of _{f VCOA} / m, applied to a phase comparator 53.

The phase comparator 53 compares the phase of the reference oscillation signal having the frequency f _X / n and the signal having the frequency f _VCOA / m, and adjusts the oscillation frequency f _VCOA so that the phase difference between the two becomes zero. Is output.

The VCO ₅₅ increases or decreases its own oscillation frequency f _VCOA according to the control signal. Such a series of operations is repeated until the frequency and phase of both signals coincide.

When the phases of both signals finally match, the frequency f _X and the oscillation frequency f _VCOA are
f _X / n = f _VCOA / m (4)
It becomes the relationship. In this embodiment, setting the local oscillation frequency f _X and the frequency division ratio n of the crystal oscillator 51, the m as established the following relationship.

_{f X / n = (f P} -f S -f C) / m (5)
Here, (f _P −f _S ) is the carrier frequency of the audio intermediate frequency signal. F _C corresponds to the frequency of the audio signal converted by the frequency converter 32 of the audio receiving unit 30 described later.

From the equations (4) and (5), in the PLL circuit 50, the oscillation frequency f _VCOA of the VCO ₅₅ is
f _VCOA = f _P −f _S −f _C (6)
Will be controlled. A signal of frequency f _VCOA output from the VCO ₅₅ is input to the multiplier 41.

The multiplier 41 multiplies the output signal (frequency f _VCOA ) of the _VCO 55 of the PLL circuit 50 by the output signal (frequency f _VCO ) of the _VCO 19 of the automatic phase control unit, and outputs the multiplication result to the PLL circuit 60.

  The PLL circuit 60 includes a low pass filter (LPF) 61, a phase comparator 62, and a VCO 63.

The multiplier 41 outputs a frequency difference component (f _VCO −f _VCOA ) between two input signals and a frequency sum (f _VCO + f _VCOA ) component. Since the frequency sum component of the output of the multiplier 41 is removed by the LPF 61, only the frequency difference component is supplied to the phase comparator 62.

Here, regarding the frequency difference component, the two signals have the relationship of the oscillation frequency f _VCO = f _{P of} the _VCO 19 and the oscillation frequency f _VCOA = f _P −f _S −f _C of the _VCO 55, respectively.
_{f VCO -f VCOA = f P -} (f P -f S -f C) = f S + f C (7)
It is represented by The PLL circuit 60 performs phase synchronization control using this frequency difference component as a reference oscillation signal.

The phase comparator 62 compares the phase of the signal of the frequency (f _S + f _C ) and the feedback signal of the oscillation frequency (hereinafter referred to as f _VCOB ) from the VCO _63, and outputs a control signal corresponding to the comparison result to the VCO 63. Output to.

The VCO 63 makes its own oscillation frequency f _VCOB and phase coincide with the signal of the frequency (f _S + f _C ) according to the control signal. As a result, the oscillation frequency f _VCOB is adjusted to the frequency (f _S + f _C ) of the reference oscillation signal. Further, the output signal of the VCO 63 having the oscillation frequency f _VCOB (= (f _S + f _C )) is given to the frequency converter 32 of the voice receiving unit 30.

In the voice receiving unit 30, the frequency converter 32 receives the signal of the frequency f _VCOB = (f _S + f _C ) and the voice signal of the intermediate frequency f _S given from the AM detection circuit 16 through the HPF 31. Is done.

The frequency converter 32 mixes two input signals, and a circuit is added on the output side to extract the frequency component of the difference as a frequency conversion signal. Therefore, the frequency converter 32 is the difference between the two frequencies.
(F _S + f _C ) −f _S = f _C (8)
The audio signal frequency-converted into is output.

Further, the band pass filter 33 extracts an audio signal having a frequency f _C and transmits it to the FM detection circuit 34. The FM detection circuit 34 demodulates the audio signal having the frequency f _C and outputs it as an audio output from a speaker (not shown).

  Through the operation described above, the video / audio demodulation circuit shown in FIG. 1 can demodulate the video signal and the audio signal, respectively.

Here, the influence on the demodulation operation when the oscillation frequency f _{VCO of the VCO} 19 of the video receiver 10 is shifted by Δf _VCO will be considered.

First, the AM detection circuit 16 outputs a video signal and an audio signal including the variation Δf _VCO in response to the oscillation frequency of the _VCO 19 varying to f _VCO + Δf _VCO . Specifically, the video signal after AM detection has a frequency of Δf _VCO as described in the above-described equation (2). On the other hand, the audio signal is a signal having a frequency of (f _S + Δf _VCO ) obtained by superimposing the variation Δf _VCO on the intermediate frequency f _S as shown in Expression (3).

At this time, the signal of the oscillation frequency (f _VCO + Δf _VCO ) is also output from the _VCO 19 of the video receiving unit 10 to the multiplier 41 of the phase locked loop unit 40.

In addition to the output signal of the _VCO 19 having the oscillation frequency (f _VCO + Δf _VCO ), the multiplier 41 receives a signal having the oscillation frequency f _VCOA = (f _P −f _S −f _C ) from the VCO 55 of the PLL circuit 50. The The multiplier 41 outputs a signal corresponding to the difference between these frequencies and a signal corresponding to the sum. The signal corresponding to the frequency sum of the two signals is removed by the LPF 61. Therefore, the phase comparator 62 of the PLL circuit 60 includes
_{(F VCO + Δf VCO) -} (f P -f S -f C) = f S + f C + Δf VCO (9)
The signal of the frequency of is given. Comparing the equations (9) and (7), it can be seen that the fluctuation amount Δf _VCO of the oscillation frequency of the _VCO 19 is also transmitted to the phase locked loop unit 40.

The PLL circuit 60 performs phase control using an input signal of this frequency (f _S + f _C + Δf _VCO ) as a reference oscillation signal. As a result, the VCO 63 outputs a signal having an oscillation frequency f _VCOB = (f _S + f _C + Δf _VCO ).

The frequency converter 32 receives the output signal of the AM detection circuit 16 input via the HPF 31, that is, the audio signal of the frequency (f _S + Δf _VCO ) and the signal of the oscillation frequency (f _S + f _C + Δf _VCO ) from the _VCO 63. Is given.

Thereby, from the frequency converter 32, it corresponds to the frequency difference between the two input signals.
(F _S + f _C + Δf _VCO ) − (f _S + Δf _VCO ) = f _C (10)
An audio signal converted into a frequency to be output is output. This output signal is extracted by the BPF 33, demodulated by the FM detection circuit 34, and output as an audio output.

Thus, since the audio signal output from the frequency converter 32 does not include the variation Δf _VCO of the oscillation frequency of the _VCO 19, the buzz is completely removed from the demodulated audio output. .

  As described above, according to the first embodiment of the present invention, the frequency variation component of the VCO superimposed on the audio signal is obtained by adopting the configuration in which the audio signal is frequency-converted using the signal generated by the phase locked loop unit. Canceled, the occurrence of buzz can be completely suppressed.

Embodiment 2. FIG.
FIG. 2 is a block diagram showing a configuration of a video / audio demodulation circuit according to the second embodiment of the present invention.

  Referring to FIG. 2, the video / audio demodulation circuit includes a video reception unit 10, an audio reception unit 30, and a phase locked loop unit (PLL) 40.

  The video / audio demodulation circuit according to the present embodiment differs from the video / audio demodulation circuit according to the first embodiment shown in FIG. 1 only in that the phase-locked loop unit 40 is configured by a single PLL circuit 50. Therefore, detailed description of other common parts is omitted.

  The phase-locked loop unit 40 includes a PLL circuit 50, a multiplier 41, and an LPF 61. Since the phase-locked loop unit 40 does not include the PLL circuit 60 in FIG. 1, the output signal of the multiplier 41 is directly supplied to the frequency converter 32 of the voice receiving unit 30 after the harmonic component is removed by the LPF 61. Will be.

  The reason for this configuration is as follows. In the first embodiment, the PLL circuit 60 controls the oscillation frequency of the VCO 63 by applying a phase-locked loop again with the signal obtained by removing the harmonic component from the output signal of the multiplier 41 by the LPF 61 as a reference. The output signal of the phase-locked loop 40 is adjusted so that it does not contain any harmonic components.

  However, if the LPF 61 has a sufficient cutoff characteristic for harmonic components, the PLL circuit 60 for performing phase synchronization control can be omitted.

  Therefore, in this embodiment, the phase-locked loop unit 40 has a simple configuration excluding the PLL circuit 60, suppresses an increase in circuit scale accompanying the addition of the phase-locked loop unit 40, and simplifies the circuit configuration. It is intended.

According to FIG. 2, in the phase-locked loop unit 40, the output signal (see Expression (7)) of the frequency (f _S + f _C ) of the multiplier 41 is converted into the frequency converter 32 after the harmonic component is removed by the LPF 61. Is input. The frequency converter 32 outputs the audio signal converted into the frequency component f _C of the difference between the signal of the frequency (f _S + f _C ) and the audio signal of the intermediate frequency f _S. This audio signal is FM detected via the BPF 33 and output as an audio output.

Further, consider a case where the oscillation frequency VCO of the VCO 19 varies. In this case, referring to the equations (3) and (9), the frequency converter 32 receives the audio signal of the frequency (f _S + Δf _VCO ) after AM detection and the frequency (f _S + f _C + Δf) from the LPF 61. _VCO ) signal is input. In this case, similarly to the first embodiment, the frequency converter 32 cancels the component of the variation Δf _VCO , and only the audio signal having the difference frequency component f _C can be obtained.

  As described above, according to the second embodiment of the present invention, an audio output from which buzz has been completely removed can be obtained, and the configuration of the phase locked loop unit can be simplified to suppress an increase in circuit scale. it can.

Embodiment 3 FIG.
FIG. 3 is a block diagram showing a configuration of a video / audio demodulation circuit according to the third embodiment of the present invention.

  Referring to FIG. 3, the video / audio demodulation circuit includes a video reception unit 10, an audio reception unit 30, and a phase locked loop unit 40.

  The video receiving unit 10 includes antennas 11 and 12, a tuner circuit 13, a variable gain amplifier 14, an AGC detection circuit 15 that is an automatic gain control unit GC, and an audio removal filter (STF) 21.

  The video receiver 10 further includes an APC detection circuit 17, a −45 ° phase shifter 18, a VCO 19, and a + 45 ° phase shifter 20 as an automatic phase control unit PC.

The configuration of the video receiver 10 in the present embodiment is the same as the configuration of the video receiver 10 of the first embodiment shown in FIG. Therefore, the video intermediate frequency signal (carrier frequency f _P ) and audio intermediate frequency signal (carrier frequency f _P −f _S ) output from the tuner circuit 13 are amplified by the variable gain amplifier 14 and then the AM detection circuit 16. Are demodulated in step _S1 and output as a video signal and an audio signal having an intermediate frequency fS. Detailed description of each part will not be repeated.

  The voice receiving unit 30 includes an HPF 31, a frequency converter 32, a BPF 33, and an FM detection circuit 34.

The voice receiving unit 30 in the present embodiment also has the same configuration as that of the voice receiving unit 30 in FIG. 1. As described above, the voice signal of the intermediate frequency f _S is converted into the frequency f _C and then the voice is detected by FM detection. The output is extracted.

  Phase locked loop unit 40 includes two PLL circuits (PLL circuit 70 and PLL circuit 80) and multiplier 41.

  As is apparent from FIG. 3, the video / audio demodulation circuit according to the present embodiment is different from that in the video / audio demodulation circuit according to the first embodiment in that the two PLL circuits included in the phase-locked loop unit 40 are different. ing. As described below, the present embodiment is characterized in that a PLL circuit used for another purpose is used in combination with the phase-locked loop unit 40.

  The PLL circuit 70 includes a low-pass filter (LPF) 61 for removing high-frequency components from the output signal of the multiplier 41, and a 1 / l divider 64 that divides the output signal of the LPF 61 by l (l is a natural number). A phase comparator 62 that detects a phase difference between the output signal that has been frequency-divided by 1 and the signal that has been fed back via the 1 / k frequency divider 65, and a frequency that corresponds to the comparison result of the phase comparator 62. And a 1 / k frequency divider 65 that divides the frequency of the output signal of the VCO 63 by k (k is a natural number).

  The PLL circuit 80 is a part that is originally used for automatic frequency control (AFT) in the video / audio demodulation circuit. In the present embodiment, the PLL circuit 80 is also used as the PLL circuit of the phase-locked loop unit 40.

The automatic frequency control unit 90 has a function of correcting an error in units of carrier intervals between the transmission carrier frequency and the reception carrier frequency, and the video intermediate frequency signal given by being multiplied by the output signal of the PLL circuit 80 in the multiplier 41 is obtained. 1/2 frequency divider 92 that divides the frequency by 2, and a frequency for detecting a frequency difference between the output signal of 1/2 frequency divider 92 and the output signal of crystal oscillator 81 having a predetermined local oscillation frequency (oscillation frequency f _X ) It includes a comparator 91, a PLL circuit 80 for locking the self-oscillation in a local oscillation frequency _{f X,} an AFT output circuit 93 for generating an AFT output voltage for exerts an AFT function from the comparison result of the frequency comparator 91.

PLL circuit 80 includes a crystal oscillator 81 which oscillates at a predetermined local oscillation frequency _{f X,} the 1 / n frequency divider 82 for frequency division (natural number n) the frequency n of the output signal of the crystal oscillator 81, n divider A phase comparator 83 for comparing the phase of the output signal and the signal fed back via the 1 / m (m is a natural number) frequency divider 84, and a signal having a frequency corresponding to the comparison result of the phase comparator 83 And a 1 / m frequency divider 84 that divides the frequency of the output signal of the VCO 85 by m.

  The AFT operation in the automatic frequency control unit 90 will be described below.

Crystal oscillator 81 generates a reference oscillation signal of local oscillation frequency f _X. Reference oscillation signal of the local frequency f _X, after being divided in the 1 / n frequency divider 82 to a frequency of f _{X /} n, given to the phase comparator 83.

The VCO 85 generates a signal having a frequency corresponding to the phase comparison result (hereinafter referred to as an oscillation frequency f _VCOC ). The output signal of the frequency _{f VCOC,} after being divided to the frequency of _{f VCOC} / m at 1 / m frequency divider 84 is supplied to a phase comparator 83.

The phase comparator 83 compares the phase of the reference oscillation signal having the frequency f _X / n and the signal having the frequency f _VCOC / m, and adjusts the oscillation frequency f _VCOC so that the phase difference between the two becomes zero. Is output.

The VCO ₈₅ increases or decreases its own oscillation frequency f _VCOC according to the control signal. Such a series of operations is repeated until the frequency and phase of both signals coincide.

When the phases of both signals finally match, the frequency f _X and the oscillation frequency f _VCOC are
f _X / n = f _VCOC / m (11)
It becomes the relationship. In this embodiment, setting the local oscillation frequency f _X and the frequency division ratio n of the crystal oscillator 81, the m as established the following relationship.

_{f X / n = (f P} -2f X) / m (12)
Here, f _P is the carrier frequency of the video intermediate frequency signal. The VCO 85 operates so that the oscillation frequency f _VCOC is (f _P −2f _X ).

The multiplier 41, the signal of the oscillation frequency _{f VCO} from the VCO 41, given a signal of the oscillation frequency _{f VCOC} from VCO85, the frequency components of both of the difference,
_{f VCO - (f P -2f X} ) = f VCO -f P + 2f X (13)
Are output as multiplication results.

The 1/2 divider 92 further divides the multiplication result represented by the equation (13) by 2, and outputs a signal having a frequency of (f _VCO −f _P + 2f _X ) / 2.

Frequency comparator 91 detects a frequency difference between the output signal of the oscillation frequency f _X of the output signal of the 1/2-frequency divider 92 and the crystal oscillator 81.

Here, if the automatic phase control unit PC operates normally, the oscillation frequency f _VCO of the _VCO 19 becomes equal to the carrier frequency f _P of the video intermediate frequency signal. Therefore, 1 / frequency of the output signal of the 1/2 frequency divider 92 is f _X becomes, the same frequency as the output signal of the crystal oscillator 81. As a result, the output voltage of the AFT output circuit 93 takes a median value.

On the other hand, when the carrier frequency of the video intermediate frequency signal, which is an input signal of the automatic phase control unit PC, is shifted to f _P + Δf _P , the oscillation frequency f _VCO of the _VCO 19 follows this,
f _VCO = f _P + Δf _P (14)
It becomes. The multiplier 41 is a frequency component of a difference between the output signal of the VCO 19 having the frequency represented by the expression (14) and the output signal of the oscillation frequency f _VCOC from the VCO ₈₅ .
_{f VCO -f VCOC = (f P} + Δf P) - (f P -2f X)
= Δf _P + 2f _X (15)
The result of multiplication is output. The multiplication result of Expression (15) is divided into (Δf _P + 2f _X ) / 2 by the 1/2 frequency divider 92. Therefore, a frequency shift of Δf _P / 2 occurs between the two signals input to the frequency comparator 91, and an AFT output voltage corresponding to this shift is output from the AFT output circuit 93.

  As described above, the automatic frequency control unit 80 detects the frequency shift between the received video intermediate frequency signal and the local oscillation signal, and corrects this shift according to the detection result. In the present embodiment, as described above, the automatic frequency control unit 80 is used to cancel the buzz component included in the audio signal. Details of the operation will be described below.

As shown in FIG. 3, the output signal of the multiplier 41 is given to the 1/2 frequency divider 92 and also to the LPF 61 inside the PLL circuit 70. The frequency of this output signal is (f _VCO −f _P + 2f _X ) from the equation (13). Usually, the oscillation frequency f _VCO of the _VCO 19 is locked to the carrier frequency f _P of the video intermediate frequency. _X. PLL circuit 70 as a reference oscillation signal the output signal of the multiplier 41 of the frequency 2f _X, performing phase synchronization control.

In particular, the output signal of the frequency 2f _X is input to the 1 / l divider 64 through the LPF 61, is l division is input to the phase comparator 62. When the feedback signal from the 1 / k frequency divider 65 is further input to the phase comparator 62, the phase comparison between the two is performed.

The VCO 63 generates a signal having a frequency corresponding to the phase comparison result (hereinafter referred to as an oscillation frequency f _VCOD ). The output signal of the frequency _{f VCOD,} after being divided to the frequency of _{f VCOD} / k in 1 / k frequency divider 65 is supplied to a phase comparator 62.

The phase comparator 62 compares the phase of the reference oscillation signal having the frequency 2f _X / l and the signal having the frequency f _VCOD / k, and adjusts the oscillation frequency f _VCOD so that the phase difference between the two becomes zero. Is output. When the phases of both coincide, the frequency f _X and the oscillation frequency f _VCOD are
2f _X / l = f _VCOD / k (16)
It becomes the relationship. In the present embodiment, the oscillation frequency f _VCOD of the VCO ₆₃ is set to the frequency (f _S + f _C ) as in the first embodiment. That is, the local oscillation frequency f _X and the division ratio k, a l is set as true the following relationship.

2f _X / l = (f _S + f _C ) / k (17)
Here, it is assumed that the oscillation frequency f _{VCO of the VCO} 19 of the automatic phase control unit PC has fluctuated to f _VCO + Δf _VCO .

The output of the multiplier 41 is
(F _VCO + Δf _VCO ) − (f _P −2f _X ) = Δf _VCO + 2f _X (18)
It becomes the frequency of. Since the PLL circuit 70 uses the output of the multiplier 41 as a reference oscillation signal, the oscillation frequency f _VCOD of the VCO ₆₃ is
(Δf _VCO + 2f _X ) / l = f _VCOD / k (19)
By substituting the relationship of equation (17) into this,
(Δf _VCO + 2f _X ) × k / l = (f _S + f _C ) + {(f _S + f _C ) / 2f _X } × Δf _VCO (20)
Given in. The output signal of the VCO 63 represented by the equation (20) is input to the frequency converter 32 of the voice receiving unit 30.

On the other hand, the HPF 31 of the voice receiving unit 30 includes a variation Δf _VCO of the oscillation frequency of the _VCO 19,
(F _VCO + Δf _VCO ) − (f _P −f _S ) = f _S + Δf _VCO (21)
An audio signal having a frequency of 1 is input to the frequency converter 32.

Therefore, the output signal of the frequency converter 32 becomes a signal of the frequency component of the difference between the equations (20) and (21),
[(F _S + f _C ) + {(f _S + f _C ) / 2f _X } × Δf _VCO ] − (f _S + Δf _VCO ) = f _C + {(f _S + f _C ) / 2f _X −1} × Δf _VCO (22)
It becomes. From equation (22), it can be seen that the buzz component included in the audio output is reduced by {(f _S + f _C ) / 2f _X −1} times compared to the conventional one.

Furthermore, if f _C is selected so that (f _S + f _C ) / 2f _X = 1, the coefficient of Δf _VCO becomes 0, and the buzz component can be completely removed.

  As described above, according to the third embodiment of the present invention, the buzz component included in the audio output can be reduced even when the PLL circuit used in the automatic frequency control unit is used in combination. The reception performance can be improved without increasing the complexity and complexity.

  Furthermore, in this configuration, if the frequency ratio is optimized, the buzz component can be completely removed as in the first embodiment.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a block diagram which shows the structure of the video / audio demodulation circuit according to Embodiment 1 of this invention. It is a block diagram which shows the structure of the video / audio demodulation circuit according to Embodiment 2 of this invention. It is a block diagram which shows the structure of the video / audio demodulation circuit according to Embodiment 3 of this invention. It is a block diagram which shows an example of a structure of the conventional video / audio demodulation circuit.

Explanation of symbols

  10, 100 Video receiver, 11, 12, 101, 102 Antenna, 13, 103 Tuner circuit, 14, 104 Variable gain amplifier, 15, 105 AGC detector circuit, 16, 106 AM detector circuit, 17, 107 APC detector circuit, 18, 108-45 ° phase shifter, 19, 55, 63, 85, 109 VCO, 20, 108 + 45 ° phase shifter, 21, 11 STF, 30, 300 voice receiver, 31 HPF, 32 frequency converter, 33, 301 BPF, 34, 302 FM detector, 40 phase locked loop unit, 41 multiplier, 50, 60, 70, 80 PLL circuit, 51, 81 crystal oscillator, 52, 82 1 / n divider, 53, 62,83 phase comparator, 54,84 1 / m divider, 61 LPF, 64 1 / l divider, 65 1 / k divider, 90 automatic frequency Number control unit, 91 frequency comparator, 92 1/2 frequency divider, 93 AFT output circuit, GC automatic gain control unit, PC automatic phase control unit.

Claims (6)

A video receiver that demodulates the video intermediate frequency signal and outputs a video output;
An audio receiver that demodulates an audio intermediate frequency signal and outputs an audio output;
The video receiver is
A phase control unit for generating a first oscillation signal having the same frequency as the video intermediate frequency signal and having a predetermined phase relationship;
An AM detection circuit that amplitude-demodulates the video intermediate frequency signal using the first oscillation signal,
The voice receiver is
An audio signal extraction circuit that extracts a first audio signal that is a difference in frequency between the video intermediate frequency signal and the audio intermediate frequency signal from an output of the AM detection circuit;
A frequency converter that mixes the first audio signal with a reference signal having a predetermined frequency and converts the frequency into a second audio signal that is a difference in frequency between the first audio signal and the reference signal;
An FM detector for frequency demodulating the second audio signal;
A phase locked loop unit that generates the reference signal based on the first oscillation signal and supplies the reference signal to the frequency converter;
The phase-locked loop unit is
A first phase-locked loop circuit that outputs a second oscillation signal having the same frequency as the frequency difference between the audio intermediate frequency signal and the second audio signal, based on an internal local oscillation signal;
A multiplier for multiplying the first oscillation signal by the second oscillation signal and outputting a multiplication result;
A video / audio demodulation circuit including a reference signal extraction circuit that outputs, as the reference signal, a frequency difference between the first oscillation signal and the second oscillation signal among the multiplication results. The reference signal extraction circuit generates a third oscillation signal having the same frequency as the frequency difference between the first oscillation signal and the second oscillation signal in the multiplication result. Including
The video / audio demodulation circuit according to claim 1, wherein the second phase-locked loop circuit outputs the third oscillation signal as the reference signal. A video receiver that demodulates the video intermediate frequency signal and outputs a video output;
An audio receiver that demodulates an audio intermediate frequency signal and outputs an audio output;
An automatic frequency control unit that corrects a frequency shift between the video intermediate frequency signal and a local oscillation signal having a predetermined local oscillation frequency;
The video receiver is
A phase control unit for generating a first oscillation signal having the same frequency as the video intermediate frequency signal and having a predetermined phase relationship;
An AM detection circuit that amplitude-demodulates the video intermediate frequency signal using the first oscillation signal,
The voice receiver is
An audio signal extraction circuit that extracts a first audio signal that is a difference in frequency between the video intermediate frequency signal and the audio intermediate frequency signal from an output of the AM detection circuit;
A frequency converter that mixes the first audio signal with a reference signal having a predetermined frequency and converts the frequency into a second audio signal that is a difference in frequency between the first audio signal and the reference signal;
An FM detector for frequency demodulating the second audio signal;
The automatic frequency controller is
Based on the local oscillation signal having a frequency obtained by multiplying the sum of the frequencies of the first audio signal and the second audio signal by n times (n is a natural number) as the predetermined local oscillation frequency, the video intermediate frequency A first phase-locked loop circuit that outputs a second oscillation signal having the same frequency as the frequency difference between the signal and the local oscillation signal;
A multiplier for multiplying the first oscillation signal by the second oscillation signal and outputting a multiplication result;
A frequency comparator that detects a frequency difference between the first oscillation signal and the second oscillation signal and a frequency difference between the local oscillation signal and the multiplication result;
A phase locked loop unit that generates a reference signal of the predetermined frequency based on the first oscillation signal and supplies the reference signal to the frequency converter;
The phase-locked loop unit is
The first phase-locked loop circuit;
The multiplier;
A reference signal extraction circuit that multiplies the frequency difference between the first oscillation signal and the second oscillation signal among the multiplication results output from the multiplier by 1 / n and outputs the result as a reference signal having the predetermined frequency. And a video / audio demodulation circuit.   4. The video / audio demodulation circuit according to claim 3, wherein the predetermined local oscillation frequency is set to the same frequency (n = 1) as a sum of frequencies of the first audio signal and the second audio signal. 5. The reference signal extraction circuit generates a second oscillation signal having a frequency 1 / n times the frequency difference between the first oscillation signal and the second oscillation signal among the multiplication results. Including a phase-locked loop circuit,
The video / audio demodulation circuit according to claim 3, wherein the second phase-locked loop circuit outputs the third oscillation signal as the reference signal. The reference signal extraction circuit generates a third oscillation signal having the same frequency as the frequency difference between the first oscillation signal and the second oscillation signal in the multiplication result. Including
The video / audio demodulation circuit according to claim 4, wherein the second phase-locked loop circuit outputs the third oscillation signal as the reference signal.

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