JP3013866B2 - Stereo demodulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stereo receiver such as an FM receiver and a stereo demodulation circuit thereof.

[0002]

2. Description of the Related Art In the current FM stereo broadcast, FIG.
As shown in 3, SST = (L + R ) + (L-R) cos (2πfsct) + S SP ····· (1) L: audio signal of the stereo left channel R: stereo right channel audio signal S SP : pilot signal. S SP = pcos (πfsct) fsc: subcarrier frequency. A stereo multiplexed signal SST indicated by fsc = 38 kHz p = 10% is transmitted.

That is, the sum signal (L + R) of the signals L and R
Is the main channel signal, and the difference signal (L−
R), the signals obtained by balanced modulation of the subcarrier signals are used as sub-channel signals, and these signals are used as pilot signals SSP.
Multiplexed and transmitted.

For this reason, there are two methods for demodulating the signals L and R from the stereo multiplexed signal SST. There are a switching system and a matrix system. A stereo demodulation circuit is employed.

FIG. 4 shows a stereo demodulation circuit of the switching system. That is, the stereo multiplexed signal SST
Is supplied to the switching circuit 2 through the terminal 1 and also to the switching signal forming circuit 6 to form a pulse PSW synchronized with the pilot signal SSP at a frequency fsc (= 38 kHz). The switching circuit 2 is switched according to the pulse PSW.

[0006] In this case, if the pilot signal SSP is ignored for simplicity in the equation (1), the equation (1) is as follows: SST = (L + R) + (LR) cos (2πfsct) (2) where, in the equation (2), at the time when cos (2πfsct) = 1 (3), SST = (L + R) + (LR) × 1 = 2L.

In the equation (2), at the time when cos (2πfsct) = − 1 (4), SST = (L + R) + (LR) × (−1) = 2R Becomes

Therefore, when the switching circuit 2 is switched by the pulse PSW, a signal L is taken out from one output terminal of the switching circuit 2 every time the equation (3) is satisfied, and from the other output terminal, The signal R is extracted every time the equation (4) holds. That is, the audio signals L and R are demodulated and extracted from the switching circuit 2.

However, in this case, the actual switching circuit 2 is constituted by a double balance type multiplication circuit as shown in FIG. 5 , for example, to facilitate signal processing and IC integration. Then, the balanced stereo multiplexed signal SST is supplied to the transistors Q1 and Q2, and the balanced pulse PSW is supplied to the transistors Q3 to Q6 to extract the signals L and R.

Accordingly, the pulse PSW for switching at this time, as shown in FIG. 6, the duty ratio is 50%. When the stereo multiplexed signal SST is switched by such a pulse PSW, (3)
At a time other than the time when the expression or the expression (4) is satisfied, the signal SST
Is extracted as a demodulated output, and thus the demodulated signals L and R include crosstalk components ΔR and ΔL.

Therefore, the signals L and R including the crosstalk components ΔR and ΔL from the switching circuit 2 are supplied to the crosstalk cancel circuits 3L and 3R, and the signals L and R
Are removed.

The crosstalk components ΔR, ΔL
Are removed and supplied to the filters 4L and 4R, where the signal components of the pulse PSW are removed, and de-emphasis is performed to obtain the original audio signals L and R. R is taken out to terminals 5L and 5R.

[0013]

Generally, a transistor cannot operate as a complete switching element and exhibits non-linear characteristics. Therefore, the above-described switching circuit 2 also exhibits nonlinear characteristics with respect to the stereo multiplexed signal SST, and various demodulations are generated in the demodulated signals L and R due to the nonlinear characteristics.

Further, filters 4L and 4R are required to remove the signal components of the pulse PSW. However, if the filters 4L and 4R are provided, the signals L and R may be degraded around the phase or the frequency characteristics. Would.

An object of the present invention is to solve such a problem and to provide a stereo demodulation circuit corresponding to digitization of audio equipment.

[0016]

For this reason, in the present invention, when the reference numerals of the respective parts correspond to the embodiments described later, the sum signal (L + R) of the left and right channel audio signals L and R is the main channel. A stereo demodulation circuit of a receiver for receiving a stereo multiplexed signal SST in which a subcarrier signal is a sub-channel signal obtained by balanced-modulating a subcarrier signal with a difference signal (LR) between the audio signals L and R. first and second a / D converter 14L which multiplexed signal SST is supplied, and 14R, a pulse width
And a clock pulse forming circuit 17 for forming first and second clock pulses PCL and PCR having phases sufficiently different from each other and having a frequency fsc of the subcarrier signal different from each other. Clock pulse PC
L, PCR, the first and second A / D converters 14L,
14R are supplied as clock pulses thereof, so that a left channel digital audio signal L and a right channel digital audio signal R are obtained from the first and second A / D converters 14L and 14R. .

[0017]

In the A / D converters 14L and 14R, audio signals L and R are stereo-demodulated from the stereo multiplexed signal SST and A / D-converted at the same time, and digital audio signals L and R are output.

[0018]

In FIG. 1, reference numerals 11 to 13 denote FM tuner circuits, 11 denotes an FM front-end circuit thereof, 12 denotes an FM intermediate frequency amplifier, and 13 denotes an FM demodulation circuit. The signal SST is output.

The stereo multiplexed signal SST is
The clock pulses PCL and PCR are supplied to the A / D converters 14L and 14R, and are also supplied to a clock pulse forming circuit 17 composed of, for example, a PLL, as shown in FIG. That is, these clock pulses PCL and PCR are pulses having a frequency fsc (= 38 kHz), synchronized with the pilot signal SSP, and having sufficiently narrow pulse widths. Have been. That is, the pulse PCL is
The pulse P is formed every time the above equation (3) holds,
CR is formed every time the equation (4) is satisfied.

The clock pulses PCL and PCR are supplied to the A / D converters 14L and 14R as sampling and holding clocks and A / D conversion clocks.

Therefore, in the A / D converter 14L, the signal SST is sampled and held by the clock pulse PCL and A / D converted every time the equation (3) is satisfied. The A / D converted digital audio signal L is output from the D converter 14L.

In the A / D converter 14R, the signal SST is sampled and held by the clock pulse PCR and A / D converted every time the equation (4) is satisfied. D converter 1
A / D converted digital audio signal R from 4R
Is output.

That is, the A / D converters 14L and 14L
In R, the audio signals L and R are demodulated from the stereo multiplexed signal SST, and the signals L and R are converted to A / R.
D conversion is performed.

Thus, the A / D converters 14L and 14L
From R, for example, digital audio signals L and R of 16 bits / 1 sample are extracted at a sampling rate of a frequency of 38 kHz (= fsc).

In this case, the clock pulses PCL, PCL
The pulse width of CR, that is, A / D converter 14L, 1
The time required for sampling and holding the signal SST in the 4R can be made sufficiently smaller than the period of the pulses PCL and PCR, so that the digital audio signals L and R from the A / D converters 14L and 14R can be obtained. , The crosstalk components ΔR and ΔL are hardly included, or even if they are included, they are negligibly small.
By the way, when the pulse width of the pulses PCL and PCR is 1/12 of the cycle, that is, when the pulse width is 30 ° with respect to the cycle of 360 °, the crosstalk is −45 dB.

The A / D converters 14L, 1L,
The digital audio signals L and R from the 4R are supplied to the digital filters 15L and 15R to remove unnecessary signal components, to perform de-emphasis, and then to the terminals 16L and 16R.

Although not shown, the digital audio signals L and R taken out from the terminals 16L and 16R together with the digital audio signals from a CD player, DAT, DAB tuner (digital audio broadcasting tuner), etc. The digital audio signal is supplied to the D / A converter device and selected (this selection is made by a user's switching operation).
A-converted and taken out. Alternatively, the analog audio signals L and R are supplied to the D / A
Is converted.

Thus, according to the present invention, the stereo multiplexed signal SST is supplied to the A / D converters 14L and 14R, and the A / D conversion is performed at the subcarrier frequency fsc of the stereo multiplexed signal SST. In the A / D converters 14L and 14R, the signal S
The audio signals L and R can be demodulated from the ST and the digitized audio signals L and R can be obtained at the same time.

In this case, in particular, according to the present invention, since the stereo multiplexed signal SST is directly subjected to A / D conversion, there is no need to perform switching, so that various distortions due to the switching may occur. Absent.

Moreover, the digital filters 15L, 15R
Since the audio signals L and R are digitally processed, when the audio signals L and R are D / A-converted in a subsequent stage, the characteristics can be made such that the phase rotation and the frequency characteristics are not deteriorated. Obtainable.

Further, A / D converters 14L and 14R
The pulse width of the clock pulses PCL and PCR supplied to the
Since the digital audio signals L and R from the A / D converters 14L and 14R contain little crosstalk components ΔR and ΔL, since they can be made sufficiently smaller than the periods of the pulses PCL and PCR, Even if it is included, it becomes negligibly small, and the crosstalk canceling circuit can be omitted.

Also, since the audio signals L and R from the FM tuner can be obtained as digital signals,
Even in a single D / A converter device which selects a digital audio signal from a CD player, DAT, DAB tuner and the like and converts the digital audio signal into an analog audio signal, the audio signals L and R from the FM tuner are also used.
Can also be handled.

In this case, the D / A converter device only needs to select the input digital signal according to the operation of the user, and then perform D / A conversion and output the digital signal. The processing unit can be clearly separated, and an extremely high quality analog audio signal can be output.

[0034]

[0035]

[0036]

[0037]

[0038]

[0039]

[0040]

[0041]

[0042]

[0043]

[0044]

[0045]

[0046]

[0047]

[0048]

[0049]

According to the present invention, the stereo multiplexed signal SST is supplied to the A / D converters 14L and 14R, and the subcarrier frequency f of the stereo multiplexed signal SST is supplied.
A / D conversion is performed by sc, so A / D
In the converters 14L and 14R, the audio signals L and R can be demodulated from the signal SST, and at the same time, the digitized audio signals L and R can be obtained.

In this case, in particular, according to the present invention, since the stereo multiplexed signal SST is directly subjected to A / D conversion, there is no need to perform switching, so that various distortions due to the switching may occur. Absent.

Moreover, the digital filters 15L, 15R
Since the audio signals L and R are digitally processed, when the audio signals L and R are D / A-converted in a subsequent stage, the characteristics can be made such that the phase rotation and the frequency characteristics are not deteriorated. Obtainable.

Further, A / D converters 14L and 14R
The pulse width of the clock pulses PCL and PCR supplied to the
Since the digital audio signals L and R from the A / D converters 14L and 14R contain little crosstalk components ΔR and ΔL, since they can be made sufficiently smaller than the periods of the pulses PCL and PCR, Even if it is included, it becomes negligibly small, and the crosstalk canceling circuit can be omitted.

Since the audio signals L and R from the FM tuner can be obtained as digital signals,
Even in a single D / A converter device which selects a digital audio signal from a CD player, DAT, DAB tuner and the like and converts the digital audio signal into an analog audio signal, the audio signals L and R from the FM tuner are also used.
Can also be handled.

In this case, the D / A converter device only needs to select the input digital signal according to the operation of the user, and then perform D / A conversion and output the digital signal. The processing unit can be clearly separated, and an extremely high quality analog audio signal can be output.

[0055]

[0056]

[Brief description of the drawings]

FIG. 1 is a system diagram showing an example of the present invention.

FIG. 2 is a waveform chart for explaining the operation of the circuit of FIG. 1;

FIG. 3 is a frequency spectrum diagram of a stereo multiplexed signal .

FIG. 4 is a system diagram for explaining a conventional example .

FIG. 5 is a connection diagram illustrating an example of a part of the circuit in FIG . 4 ;

FIG. 6 is a waveform chart for explaining the operation of the circuit of FIG. 5 ;

[Explanation of symbols]

11 front-end circuit 13 FM demodulation circuit 14L, 14R A / D converter 15L, 15R Digital filter 17 clock pulse forming circuit 18 Synthesis circuits

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04H 5/00

Claims (2)

(57) [Claims] An audio signal L for left and right channels,
In a stereo demodulation circuit of a receiver for receiving a stereo multiplexed signal in which a sum signal of R is a main channel signal and a signal obtained by balancedly modulating a subcarrier signal by the difference signal of the audio signals L and R is a subchannel signal. The first and second A to which the stereo multiplexed signal is supplied
/ D converter, and a clock pulse forming circuit that forms first and second clock pulses whose pulse width is sufficiently smaller than the period and whose phases are different from each other at the frequency of the subcarrier signal. Providing first and second clock pulses to the first and second A / D converters, respectively, as the clock pulses; from the first and second A / D converters, a digital signal of the left channel; A stereo demodulation circuit for obtaining an audio signal L and the right-channel digital audio signal R. 2. The left and right channel audio signals L,
The sum signal of R is used as the main channel signal, the signal obtained by balanced-modulating the subcarrier signal by the difference signal between the audio signals L and R is used as the subchannel signal, and these signals are multiplexed with the pilot signal SSP. A stereo demodulation circuit of a receiver for receiving a stereo multiplexed signal, wherein the stereo multiplexed signal is supplied to the first and second A
And a first and second clock pulses having a pulse width sufficiently smaller than a period, synchronizing with the pilot signal, and being 180 ° out of phase with each other at the frequency of the subcarrier signal. A clock pulse forming circuit that supplies the first and second clock pulses to the first and second A / D converters as those clock pulses, respectively. The A / D converter outputs the A / D converted audio signal L and A / D of the left channel.
A stereo demodulation circuit for obtaining the converted right channel audio signal R.