JPH0220981A - Sound multiplex demodulating circuit - Google Patents

Abstract

PURPOSE:To remove a buzz without spoiling a stereophonic feeling by providing an L-R signal side demodulating circuit with a high-pass filter(HPF) which removes noises in a low frequency range and a phase compensating circuit which corrects its phase rotation. CONSTITUTION:The L-R signal of a sound signal inputted to an input terminal 1 is passed through a BPF 4 and FM-detected by an FM detecting circuit 5 to become a sound signal in the audio band. This sound signal has frequency characteristics corrected by a de-emphasis circuit 6 and an HPF 7 remove the buzz in a low frequency range. The output signal of the HPF 7 is corrected by the phase compensating circuit 8 to correct the rotation of the phase by the HPF 7 and then inputted to a matrix circuit 10. The matrix circuit 10 adds and subtructs the input L+R and L-R signals to extract an L-channel sound signal and an R-channel sound signal independently and outputs them from output terminals 11 and 12 respectively. Consequently, a stereophonic playback sound of good quality is obtained without spoiling the stereophonic feeling.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an audio multiplex ffi demodulation circuit that demodulates an audio signal of an audio multiplexed signal in which a video signal and an audio signal are multiplexed.

[Prior Art] An audio multiplexed signal is a signal that is the sum of an audio signal of a left channel (hereinafter abbreviated as "L channel") and an audio signal of a right channel (hereinafter abbreviated as "right channel") of a stereo audio signal. It is formed by multiplexing a video signal with a difference signal (hereinafter abbreviated as "rL+R signal") and a difference signal (hereinafter abbreviated as "rL-R signal").

Then, the LR signal modulates the subcarrier and forms an audio signal together with the L+R signal in the audio band. This L
- There are differences in the modulation method of the R signal between Japan and the United States, with Japan adopting the FM-FM method of FM modulation, and the United States adopting the AM-PM method of AM modulation.

A conventional audio multiplexing demodulation circuit for demodulating such audio multiplexed signals includes a low-pass filter (hereinafter abbreviated as "LPF") 2, as shown in FIG. 3, which is a circuit block diagram of an example of the FM-FM system. and a band-pass filter (hereinafter abbreviated as rBPFJ) 4 to obtain the L+R signal and the FM-modulated L signal, respectively.
- Take out the R signal, and the L+R signal is the S/N of the recording/playback system.
Through the de-emphasis circuit 3 for improving the L
After the R-multiple signal is FM-detected by the FM detection circuit 5, it is input to the matrix circuit 10 via the de-emphasis circuit 6, and the matrix circuit 10 performs addition/subtraction operations to separate the L-channel audio signal and the R-channel audio signal, respectively. I try to take it out independently.

Note that the stereo detection circuit 9 detects whether or not the signal input to the input terminal 1 is a stereo signal, and sends a signal that changes the calculation of the matrix circuit 10 to the matrix circuit 1.
This is a circuit that inputs to IB10.

In addition, in the AM-FM system, as shown in the circuit block diagram of FIG. 5, an AM detection circuit 13 is used as a detection circuit to perform AM detection, and the output signal is detected at a specific frequency 'i! Lf
The signal is corrected by an expander circuit 14 that expands the signal in the i4 range.

[Problem H to be solved by the invention] In the conventional audio multiplexing demodulation circuit described above, the L-R signal is easily affected by crosstalk noise from the video signal called buzz, and in particular, in the AM-FM system, this problem occurs. The drawback was that it was very susceptible to the influence of buzz.

The present invention has been made in order to eliminate these drawbacks of conventional audio multiplexing and demodulation circuits, and an object of the present invention is to provide an audio multiplexing and demodulating circuit that can eliminate buzz without impairing the stereo effect.

[Means for Solving the Problems] In order to achieve the above-mentioned object, the audio multiplex demodulation circuit of the present invention includes a high-pass filter (hereinafter referred to as (abbreviated as rHPF)
A phase compensation circuit is provided to correct the phase rotation.

That is, the audio multiplex demodulation circuit of the present invention has Lp and F7 for extracting L1R signals.

A BPF that extracts the L-R signal, a detection circuit that detects the output signal of the BPF, and the L+
In an audio multiplexing and demodulating circuit, the audio multiplexing and demodulating circuit includes a matrix circuit that adds and subtracts the R signal and the L-R signal to independently extract an L channel audio signal and an R channel audio signal, wherein the low frequency of the output signal of the detection circuit is The present invention is characterized in that the demodulation circuit on the L-R signal side is provided with an HPF that removes area components and a phase compensation circuit that corrects the phase characteristics of the output signal of the HPF.

[Function] Low-frequency buzz is mainly caused by harmonics of the vertical synchronization signal fv of the video signal and harmonics of the horizontal synchronization signal fH, and buzz caused by the vertical synchronization signal fV is especially audible. Therefore, the harmonics of the vertical synchronization signal fv up to the fifth order (if fv = 60H2, 300Hz
) By removing it with HPF, it is possible to make the buzz hardly perceptible to the auditory sense. In addition, if the phase rotation of the L-R signal caused by this HPF is corrected with a phase compensation circuit, the stereo feeling will not be impaired. However, in this case, the separation between channels in the low frequency region will decrease somewhat; As with the 3D separation, it is difficult to feel the directionality, so it has almost no effect on the auditory sense.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit block diagram of an embodiment of an FM-FM audio multiplexing and demodulating circuit according to the present invention. Note that circuit blocks that are the same as those in the conventional example shown in FIG. 3 are given the same numbers, and their explanations will be omitted.

In FIG. 1, an FM detected audio signal is input to an input terminal 1. The input terminal 1 has an LPF 2. for separating L+R signals. BPF for separating L-R signals
4 and a stereo detection circuit 9 are connected thereto.

The output signal of the LPF 2 is input to the de-emphasis circuit n3, and the output signal of the de-emphasis circuit n3 is input to the matrix circuit 10.

The output signal of the BPF 4 is input to the FM detection circuit 5, and the FM
The output signal of the detection circuit 5 is input to a de-emphasis circuit 6. The output signal of the de-emphasis circuit 6 is input to an HPF 7 for removing noise in a low frequency region. H
PF7 has a cutoff frequency of 300Hz and a slope of +2dB1.
It is 0ct.

The output signal of the HPF 7 is input to a phase compensation circuit 8 for compensating for rotation of the phase of the output signal by the HPF 7, and the output signal of the phase compensation circuit 8 is input to a matrix circuit 10.

In the embodiment shown in FIG. 1, the audio signal input to the input terminal 1 is converted into an audio band audio signal L+ by the LPF 2.
The R signal is taken out and input to the de-emphasis circuit 3. The L+R signal whose frequency characteristics have been corrected by the de-emphasis circuit 3 is input to the matrix circuit 10.

On the other hand, the R signal of the audio signal input to the input terminal 1 is subjected to FM detection by the FM detection circuit 5 via the BPF'4, and becomes an audio signal in the audio band.

After the frequency characteristics of this audio signal are corrected in a de-emphasis circuit 6, buzz in the low frequency region is removed by an HPF 7. The output signal of HPF7 is converted to HP by phase compensation circuit 8.
After correcting the phase rotation caused by F7, the signal is input to the matrix circuit 10.

In the matrix circuit 10, the input L+R signal and the L
- Add/subtract the R signal and add/subtract the L channel audio signal and R signal.
The audio signals of the channels are taken out independently, and the respective output terminals 11. It is output from the output terminal 12.

Note that the stereo detection circuit 9 detects whether the audio signal input to the input terminal 1 is a stereo signal or not, and if it is a stereo signal, outputs a command signal that causes the matrix circuit 10 to perform an addition/subtraction operation. , if the signal is not a stereo signal, the L+R signal is sent to each output terminal 11 of the matrix circuit 10.
．． A command signal to be output from 12 is output.

FIG. 2 is a circuit diagram showing an example of a specific circuit configuration of the HPF 7 and phase compensation circuit 8 shown in FIG. 1.

In FIG. 2, the circuit block 23 is a buffer circuit, the concave block 24 is a phase compensation circuit, and the circuit block 25 is an H
PF, the circuit block 26 is a phase inversion circuit.

The buffer circuit 23 is composed of an OP amplifier A1, a resistor R1 connected to the input terminal 21 is connected to a non-inverting input terminal of the OP amplifier A1, and a resistor R2 is grounded from this non-inverting input terminal. The inverting input terminal of OP amplifier A1 is connected to the output terminal.

The phase compensation circuit 24 is composed of resistors R4 and R5 connected to the non-inverting input terminal of the OP amplifier A2, and resistor R3 and capacitor C1 connected to the inverting input terminal of the OP amplifier A2. Resistor R6 is a feedback resistor.

The resistance values of resistor R3, resistor R4 and resistor R5 are R1
The resistance value of resistor R6 is 2R, and the capacitance of capacitor C1 is C.
If the input voltage of the 1-phase compensation circuit 24 is Vl and the output voltage is v2, then the number T of transmissions of the phase compensation circuit 24 is s
= Jω, T (s) = V2 /V1 = ts-1/(RC)l/2ts+1/(RC)I V2 /Vl =-1/2, and the 1 phase characteristic θ(ω) is θ((+3 ) = arc tan (-ω/ (1/
RC) ) −arc tan Iω/ (1/RC
) 1=arc tan (-ωRC) -arc t
an (ωRC)=-2arc tan ((1)
RC).

HP F 25c consists of resistors R7, R11, near 7 capacitors c2, C3, capacitors C2, C3 and resistor R8 are connected in series, resistor R7 is connected to capacitor C2,
It is grounded from the connection point of C3.

Set the resistance values of resistors R7 and R8 to R, and capacitors C2 and C3.
If the capacity of is C, then the transfer function 81(s) of the front stage of HP F 25 is: H4(s) = R/ iR+1/ (sC)
1=sCR/ (1+5CR), and the phase characteristic θ1(ω) is θ1(ω)=arCtan l-1/ (RωC)
1=-art: tan (1/RωC).

The number of transmission intervals H2 (s) of all stages of HP F 25 is H2fs
)=Hl(s)·H1(s), and the phase characteristic θ2(ω) becomes θ2(ω)=2θ1(ω)=−2arCtan(1/RωC).

The phase inversion circuit 26 is composed of an OP amplifier A3 and its feedback resistor R9, and simultaneously inverts the phase and serves as a buffer amplifier for the HP F 25.

From the above, if the phase rotation of the HPF 25 is θh, the phase correction amount of the phase compensation circuit 24 is θp, and the phase inversion amount (-180°) of the phase inversion circuit 26 is θt, then the phase rotation of the entire circuit in FIG. The quantity θ is θ = θ h 10 θρ 10 [=-2arc
tan (1/RωC) + 1-2arct
an ((1)RC) l + (-180°) = -2
1 arc tan (1/RωC) + arc
tan (RC off) l + (-1800) =
-2(soo)+(-1aO°)=-360'=00, which means that the phase rotation of the HP F 25 has been corrected by the phase compensation circuit 24.

It goes without saying that the HPF and phase compensation circuit of the present invention are not limited to the circuit shown in FIG.

FIG. 4 is a circuit block diagram showing an embodiment of an AM-FM audio multiplexing and demodulating circuit according to the present invention.

In this embodiment, in response to the fact that the LR signal is AM modulated, the FM detection circuit 5 in the embodiment shown in FIG. other circuit building blocks,
Since the operation is the same, the explanation thereof will be omitted.

[Effects of the Invention] In the audio multiplexing demodulation circuit of the present invention, an HPF is inserted in the demodulation circuit on the L-R signal side, which is easily affected by buzz, to remove buzz in the low frequency range that is most noticeable to the ear. Since a phase compensation circuit is provided to correct the phase rotation caused by this HPF, it is possible to reduce the buzz to the extent that it does not cause any audible problem, and at the same time, it does not impair the stereo feeling.
It is possible to obtain extremely high quality stereo playback sound.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram of an embodiment of the FM-FM audio multiplexing and demodulation circuit of the present invention, and FIG. 2 is a circuit diagram showing an example of a specific circuit of the HPF and phase compensation circuit of the embodiment of FIG. 1. 3 is a circuit block diagram showing an example of a conventional FM-FM audio multiplexing and demodulation circuit, and FIG.
FIG. 5 is a circuit block diagram showing an example of an audio multiplexing demodulation circuit of the M system. FIG. 5 is a circuit block diagram showing an example of a conventional audio multiplexing demodulation circuit of the AM-FM system.

Claims (1)

[Claims] A low-pass filter that extracts a signal that is the sum of a left channel audio signal and a right channel audio signal, and a band pass filter that extracts a signal that is the difference between a left channel audio signal and a right channel audio signal. , a detection circuit that detects the output signal of the bandpass filter, and a signal that is the sum of the left channel audio signal and the right channel audio signal, and a signal that is the difference between the left channel audio signal and the right channel audio signal. An audio multiplexing demodulation circuit having a matrix circuit that performs addition/subtraction operations and independently extracts a left channel audio signal and a right channel audio signal, the audio multiplexing demodulation circuit comprising: a high-pass filter that removes a low frequency region component of the output signal of the detection circuit; , a phase compensation circuit for correcting the phase characteristic of the output signal of the high-pass filter is provided in a demodulation circuit on the signal side of the difference between the left channel audio signal and the right channel audio signal. Multiplex demodulation circuit.