JPS5966239A - Stereophonic demodulating circuit - Google Patents

Abstract

PURPOSE:To eliminate a beat generated at a stereo demodulating circuit, by modulating in terms of pulse width an input composite signal with a regenerated subcarrier of a sinusoidal wave synchronized in phase with a pilot signal in the input composite signal and switching the input signal with this modulated output. CONSTITUTION:The composite signal supplied to an input terminal IN is supplied to a subcarrier generating circuit 7, and a square subcarrier wave synchronized in phase with the pilot signal is outputted from the circuit 7. The output of the circuit 7 is formed into a sinusoidal wave via a BPF9, and this sinusoidal wave modulates in terms of pulse width a carrier of a pulse width modulating circuit 10. The output of the circuit 10 becomes a pulse output where the width of the carrier is compressed or expanded corresponding to the sinusoidal wave output from the BPF9, this pulse width-modulated signal is inputted to a switching demodulating circuit 8, where an input composite signal is demodulated and L, R stereophonic demodulating signals having less distortion are outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stereo demodulation circuit for an FM stereo tuner.

(Prior art)

A conventional stereo demodulation circuit includes, for example, a phase comparator 1, a RONO!, as shown in FIG. filter 2, DC amplifier 3
, a voltage controlled oscillator 4 with a free-running frequency of 76 kHz, z, and frequency dividers 5 and 6 each having a free-running frequency of 76 kHz. .

Droplet carrier wave generation circuit 7 Kidney/Human, consisting of a PLL circuit.

power terminal IN. Feed composite signal i, □
Subcarrier frequency [q, I;, '7. From °,, to the pilot ship number in the y position I, the rod phase and synchronized re-transfer conveyance output were obtained, and this iii! 1 carrier wave, out, 7J, t,
Use it as a switch signal □7. I'chi'-! control circuit, 8
Supply to ;x,'ry chip demodulation, circuit! By switching the positive signal, you can create stereo tone! Ru0.

However, since the reproduced subcarrier output from the subcarrier generation circuit 7 has a rectangular waveform, - in the raw subcarrier;
,7. contains all odd harmonic components of subcarrier frequency q (38kHz2)! I'm reading.

On the other hand, when the composite signal contains components near odd-numbered harmonics of the subcarrier frequency, the composite signal is subcarried, and the odd-numbered harmonics of the transmitted frequency are generated. When switching is performed using the regenerated subcarrier output, the signal in the composite signal.

component and the reproduced subcarrier output! −th bulk harmonic component.

There is a drawback that the signal components in the composite signal are stereo demodulated due to multiplication by pruning switching.

In order to avoid this problem, all low-pass filters are inserted at point A in FIG. 1 to remove all signal components in the vicinity of odd multiples of the subcarrier frequency in the composite signal. However, since the third harmonic of the subcarrier has a frequency of 114 kHz, the cutoff frequency of the low-pass filter described above must be selected to a frequency below 100 kHz.By inserting all such low-pass filters, the composite A disadvantage is that some of the sub-signals in the signal are attenuated or phase shifted, resulting in poor separation.

(Object of the Invention) The present invention has been made in view of the above, and eliminates all the above-mentioned drawbacks by eliminating stereo demodulation ability for components near odd multiples of the subcarrier frequency in a composite signal. Moreover, even when all input stage ICo-has filters are inserted, the cutoff frequency of the low 1' filter can be set high.
The aim is to provide all A routes.

(Structure of the Invention) The present invention provides subcarrier generation means for generating all reproduced subcarriers of a sinusoidal waveform phase-synchronized with GYROT NU in the input composite signal when an input composite signal is supplied; The present invention is characterized by comprising: a pulse width modulation circuit that performs pulse width modulation as an output full modulation signal; and a switching demodulation circuit that performs switching using the output of the pulse width modulation circuit for the input composite signal. .

The present invention will be explained below with reference to Examples.

FIG. 2 is a block diagram showing one embodiment of the present invention.

In FIG. 2, the same components as those shown in FIG. 1 are designated by the same reference numerals.

One embodiment of the present invention includes a subcarrier generation circuit 7 and a switching demodulation circuit 8 in the stereo tuning circuit shown in FIG.
A pantone noise filter 9 consisting of a tuning circuit, for example, and a pandox filter 9 are provided with a rectangular waveform reproduced subcarrier output from the subcarrier generating circuit 7 to obtain a sine wave output at the subcarrier frequency. A pulse width modulation circuit 10 is provided, which is supplied with the output of the bandpass filter 9 and outputs the entire pulse width modulated signal modulated by the output of the bandpass filter 9. Here, the subcarrier generation circuit 7 and the bandpass filter 9
The subcarrier generating means for generating a reproduced subcarrier output having a sinusoidal waveform phase-synchronized with the pilot signal is constituted entirely.

The Nockles biased modulated signal output from the 79 pulse width modulation circuit 10 is supplied to the switching demodulation circuit 8 as a switching signal, and the switching demodulation circuit 8 causes the component not signal to be switched.

(Operation of the Invention) In one embodiment of the present invention configured as described above, the subcarrier generation circuit 7 is supplied with the composite signal supplied to the input terminal IN, and is phase synchronized with the pilot signal from the subcarrier generation circuit 7. A square wave reproduced subcarrier output shown in FIG. 3 (,) is output.

The square-wave regenerated subcarrier output shown in FIG. 3(a) is supplied to the non-nand pass filter 9, and the square-wave regenerated subcarrier output shown in FIG.
) A reproduced subcarrier output (38 kHz) having a sine waveform is output.

On the other hand, the carrier wave of the pulse width modulation circuit 10 has a sufficiently large value with respect to the subcarrier frequency, for example, 500 to 60 times the subcarrier frequency.
The frequency is set to about 0 kHz.

This carrier wave is pulse width modulated by the output of the bandpass filter 9 shown in FIG. 3(b). Therefore 7
As schematically shown in FIG. 4, the output of the signal width modulation circuit 10 becomes a Nockles output in which the /j signal width of the carrier wave is expanded or contracted corresponding to the sine wave output shown in FIG. 3(b). The pulse width modulated signal output from the pulse width modulation circuit 10 has a square waveform containing a subcarrier frequency (38 kHz) component, and its frequency spectrum is as shown by the solid line in FIG. FIG. 5 shows an example of the case where the carrier wave of the pulse width modulation circuit 10 is set to 500 kHz. In FIG. Svector kz D and E are 1st lower 1! ll: indicates the spectrum of the band wave and the first upper side band wave. In addition to the above, the pulse width modulation signal outputted from the pulse width modulation circuit 10 has second or higher upper and lower sideband waves, but the level thereof is so small as to be reasonably low. .

The pulse width modulated signal having the frequency spectrum shown in FIG. 5 is supplied as a switching signal to the switching demodulation circuit 8, and the composite signal is stereo demodulated to switch the conduit signal. However, from the frequency spectrum shown in Figure 5, it is clear that the subcarrier frequency (
:3.8. There is no component in the frequency range exceeding kI(z)k to about 400 kHz.Therefore, there is no component in the frequency range exceeding kI(z)k to about 400 kHz.Therefore, the third and third subcarrier frequencies that existed in the switching signal in the conventional 3-tea harmonic circuit are 5th order,
...<The harmonic components of the key 4001dfz and the composite signal are no longer multiplied by the switching. As a result, even if signal components near odd multiples of the subcarrier frequency are present in the composite signal, the above signal components in the composite signal will not be stereo demodulated.

However, in one embodiment of the present invention, the demodulation circuit provided before the stereo demodulation circuit had the ability to demodulate up to the carrier frequency of the pulse width modulation circuit 10, or due to other reasons, the Noculus width modulation If a signal component near the carrier frequency of circuit 0 exists in the composite signal, the pulse width modulation circuit 10 in the composite signal
Signal components near the carrier frequency will be stereo demodulated. In this case, insert a rotor or filter at point A in Fig. 2 to obtain t4 in the composite signal.
It is sufficient to remove signal components near the carrier wave frequency row of the pulse width modulation circuit 10. In this case, the frequency characteristics of the low/4' filter inserted at point A in Fig. 2 are as shown by the lower dot chain in Fig. 5. It can be set to a certain degree. This exceeds the subcarrier frequency by 400
This is because there is no switching signal component in the frequency range up to kHz. In this way, since the cutoff frequency of the low-pass filter inserted at point A can be set high, sub-signals in the composite signal will not be adversely affected by inserting the low-pass filter at point A, and the degree of separation will be improved. It won't make things worse.

It is also known that as the switching signal supplied to the switching demodulation circuit 8 shifts from a square waveform to a sine waveform, the distortion of the stereo demodulated output signal increases. However, since the pulse width modulated signal outputted from the pulse width modulation circuit 10 has a square waveform, the distortion of the stereo demodulated output signal may increase.

(Effects of the Invention) As described above, according to the present invention, components in the vicinity of frequencies that are odd multiples of the subcarrier frequency in a composite signal are no longer subjected to stereo demodulation. For this reason, FM
1 caused by interference etc. in a stereo tuner
There is no stereo demodulation capability for beats around 00 kHz, and the beats generated by conventional stereo demodulation circuits no longer occur.

Further, even when a low-pass filter is inserted in the input stage of the stereo demodulation circuit, the degree of separation will not be deteriorated.

Furthermore, the distortion of the stereo demodulated output signal does not increase.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional stereo demodulation circuit. FIG. 2 is a block diagram showing one embodiment of the present invention. FIGS. 3(a) and 3(b) show the output waveforms of the subcarrier generation circuit and the output waveforms of the pantone 4 filter in one embodiment of the present invention. FIG. 4 shows A? in one embodiment of the present invention. Pulse width modulation circuit output waveform. FIG. 5 is a frequency spectrum of a pulse width modulated output for explaining the operation of an embodiment of the present invention. 7... Subcarrier generation circuit, 8... Switching tM
FA circuit, 9...Pantono and filter, 10...
Pulse width modulation circuit. Figure 1 Figure 2 Figure 6 t □ Figure 4 Figure 5 -1 38KH2

Claims (1)

[Claims] a subcarrier generating means that is supplied with an input composite signal and generates a regenerated subcarrier of a sinusoidal waveform that is phase-synchronized with the pilot signal contained in the input composite signal; It is characterized by comprising a pulse width acquisition circuit that modulates the width of 1,000 pulses as a modulation signal, and an internal demodulation circuit that switches the input composite signal with the output of the knob-width modulation circuit. Stereo demodulation circuit 0