JPS581350A - Fm stereophonic demodulator - Google Patents

Abstract

PURPOSE:To eliminate the influence of noise by performing multiplication by using a switching signal which is close to a composite signal frequency range and has not any unnecessary frequency component, and thus performing stereophonic demodulation. CONSTITUTION:The detection output of an FM detector 1 is supplied to a multiplier 3 through an LPF2. The detection output is also supplied to a subcarrier signal generator 7 which generates a 38KHz sine-wave subcarrier, thereby generating the 38KHz sine-wave signal which synchronizes with a pilot signal. Once inputting this subcarrier signal, a PPM (pulse-position modulation) circuit 8 imposes pulse-position modulation upon a >= about 500KHz high-frequency clock pulse signal by the sine-wave subcarrier signal to obtain a PPM signal. This PPM signal output is led to the other input terminal of the multiplier 3, where it is multiplied by the FM detection output. Audio components of this multiplication output are extracted through LPF (low-pass filter) 5 and 6 and then separated and demodulated into a left and a right channel signal.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM stereo demodulation device, and more particularly to an FM stereo demodulation device that multiplies a subcarrier signal and a composite signal when demodulating a subsignal.

There is a circuit system that separates left and right channel signals by switching a composite signal using a 38KH2 rectangular subcarrier signal when demodulating an FM stereo signal. FIG. 1 is a block diagram of such a demodulation method,
The FM-IF (intermediate frequency) signal is converted into a composite signal by an FM detector 1, and an LPF removes unnecessary components.
It is applied to the switching circuit 3 via the (low-pass filter) 2. PLL (phase locked loop) the 19KHz pilot signal contained in the output of LPF2
A 38KH2 rectangular wave subcarrier signal extracted in the circuit 4 and phase-synchronized with this pilot signal is used as a switching signal in the switching circuit 3 described above. Left and right channel signals, which are audio components, are separated and derived from this switching output, and LPFs 5 and 6 are provided for this purpose.

Here, since the subcarrier signal of 38KH2 which is the switching signal is a rectangular wave as shown in Fig. 2 (4), if this is expanded into a Fourier series, F(t) -7d
nω8t+3. gl13ωst+g blood5ω8t+...
...It is expressed as (1). Here, ω8 is the angular frequency of the subcarrier signal. In this way, the frequency spectrum of s'p'(t) is as shown in Figure 2 (B) (38KH
In addition to the fundamental wave of z, it includes odd harmonics such as 114KHz, 190KH2, etc.

A switching signal F having such a frequency spectrum
(i) If the j-koyoshi FM detection output is switched, both signals will be multiplied, but the LPF of the output section
If the passbands of 5 and 6 are 0 to 15 KHz, the detector output that appears in the stereo output by this multiplication is shown in Figure 2 (
0). Main signal (0~15KHz)
) and sub signal (38±15KH2), 114±
Signals at 15KHz, 190±15KH2, -'' (shoe sounds, proximity interference waves, etc.) are also demodulated and output.

In order to prevent this drawback, the output of the FM detector 1 is set to 114KHz and 190KH as shown in FIG. 2(D).
It becomes necessary to add an LPF with a large amount of attenuation near the z+'' song.

However, since 114KH2 is close to the composite signal component, the delay characteristics of the composite signal are not flat as shown in Figure 2 (E) due to the LPF.
The amplitude characteristics are no longer flat, and the distortion and separation characteristics of the stereo demodulated output deteriorate.

An object of the present invention is to eliminate the above-mentioned drawbacks and provide a stereo demodulation device with good characteristics.

The FM stereo demodulator according to the present invention includes means for generating a sinusoidal subcarrier signal synchronized with a stereo pilot signal included in an spM detection output, and a pulse train signal obtained by pulse position modulating a high frequency pulse signal with a sinusoidal subcarrier signal. The present invention is characterized in that it includes modulation means for generating a pulse train signal and means for multiplying this pulse train signal by an FM detection output, and left and right channel signals are separately derived from this multiplication method.

The present invention will be explained below using the drawings.

FIG. 3 is a block diagram illustrating the present invention in detail;
The detected output from the M detector 1 is input to the multiplier 30 via the LPF 2. The detection output is also input to a subcarrier signal generator 7 which generates a 38KHz sinusoidal subcarrier, and generates a sinusoidal 38KH signal synchronized with the pilot signal.
A z signal is generated. A PPM (Pulse Position Modulation) circuit 8 is provided which receives the 38KH2 subcarrier signal as input, and a high frequency clock signal of approximately 500KHz or more is pulse-modulated by the sine wave subcarrier signal. This becomes a PPM signal. This PPM signal output becomes the input of the multiplier 3 and is multiplied by the FM detection output. The audio components of this multiplication output are respectively derived by LPFs 5 and 6 and separated and demodulated into left and right channel signals. become.

4-(F) are diagrams showing the operation and characteristics of the circuit of FIG. 3. First, (4) is a 38 KHz sine wave subcarrier signal waveform. (B) is a 9 PPM pulse train signal waveform pulse modulated by this subcarrier signal. Considering the frequency spectrum of this PPM wave, it has a 38 KHz component which is the frequency of the subcarrier signal which is the modulated wave, and also has a distribution according to the modulation degree near the carrier frequency of the PPM wave and its odd harmonics. However, if the carrier of the PPM wave is selected as a high frequency of approximately 500 KH2 or higher, frequency components other than these 38 KHz components will be as shown in Figure (0).

Therefore, among the FM detection outputs, what appears in the stereo demodulation output by multiplication are the main signal (θ~15KH2), the sub signal (23~53KH3), and the frequencies near the carrier frequency of the PPM wave and odd multiples thereof. Since interference waves and noise are limited, the frequency spectrum of the demodulated output is as shown in (D). As a result, since the characteristics of LPF 2 only need to be cut off from the vicinity of the carrier frequency of the high-frequency P-PM wave, it is possible to obtain an LPF characteristic that is flat up to the high frequency range as shown in (E), and its delay characteristic 4 (F)
It is possible to make it flat as shown in the figure. Therefore, F.M.
The detected output is demodulated with flat amplitude and delay, and appears without deterioration of distortion or separation. Also, P
If the frequency characteristics of the output of the M detector 1 do not extend to high frequencies, the LPF 2 can be omitted.

The demodulation principle in the circuit block of FIG. 3 will be briefly explained using mathematical expressions. Now, the left and right channel signals are L(t)
, R(t), the main and sub signals are M(t)-L(t)+R(t), 8(t)-L, respectively.
(t) −R(t). Therefore, if the subcarrier signal is image ω8t, the composite signal 0(t) which is the FM detection output is 0(t) −M(1+8(t)dnω8t...
......(2). Note that the pilot signal component is omitted for simplification. Then, since the main component of the PPM wave is image ω8t, the output of the PPM circuit 8 takes into account the DC component and assumes ωst, then the output of the pair of multipliers 3 is IL(') − (2+ -1ω Hataket) ・
0(t) ・Song・ (3) IFa(t) − (Tsin
(das・0(t) −distance−(4). Therefore, s
(2) t By rearranging equation (3), we get 1L(t)−
10(M(t)+8(t))+(+5(t)10M
(t)) oos2ω8t-T8(t)oos2ω8t ・--
--------... (5) IR(t) Ichiban(M(t)-
8(1) + (+8(t)-M(t))gktω8t
+78(t)cm2ω8t・・・・・・・・・・・・(
6). These fL(t) and -t) are LPF 5
Since only the audio component is derived by passing through the LPFs 5 and 6, the outputs t't,'(t
) *ν, :<*> is νL(t)−D(M(t)+
8(1) -L(t))...(7)tIR'(t)-
Ding (M(t) -8<t) ) -R(t)...(8
), and the left and right channel signals are separated and demodulated.

FIG. 5 shows an embodiment of the multiplier 3 used in the present invention, which has a double-balanced differential circuit configuration and includes a pair of differential transistors 'r, ! A composite signal, which is an FM detection output, is applied between both paces l'r and 2. Resistance R
6 and R7 are emitter resistors of both transistors, and resistor R5 is a common emitter resistor to form a matrix circuit. A base bias (1) is applied to both transistors by resistors R1 and R2.

A differential transistor 'r, 3; Ty4 is provided that uses the collector output of the transistor Tτ1 as a current source, and a differential transistor 'r, t Ty6 that uses the collector output of the transistor Tr2 as a current source is provided. There is.

And the positive phase P is connected to the bases of transistors T-3 and T76.
Two PM waves are applied, and a PPM wave of opposite phase is applied to the bases of transistors T, 4 and TTS, respectively, and the base bias v2 of these transistors is applied to each base by resistors R3 and R4. Transistors Tr3 and TT
The collector of transistor S is connected to a common collector resistor R8, from which a left channel signal is obtained, and the collectors of transistors Tr4 and Tr6 are connected to a common collector resistor R, from which a right channel signal is obtained.

Now, if the emitter voltage of the transistor Tr2 is V, the emitter voltage of the transistor T71 is VB10 (c
). Therefore, both transistors Tl'l + Ty
The collector currents Iox(t) and Io2(t) of 2 are:
becomes. Note that sRO"R6"R7. Then, if the high frequency component is omitted, the PPM wave for switching is T±A(i)at as mentioned above, so
(Constant determined by the modulation degree of pPPM wave when A≦1) Audio component IL of the current flowing through resistor R8+R9
(R, In(t) becomes .Here, if RO-2A-Rs/(IA), -A It, (1=page(2%++M(t)+8(t))= −
・-us) IB(1)-possible(2v11+M(t)-8(
t)) ...---(14), and left and right channel outputs are obtained.

FIG. 6 is a circuit block diagram of a specific example of the 38 KHz subcarrier signal generator 7 in FIG.
The phase is compared with the KHz square wave. This comparison output is LP
The signal is input to VOO (voltage controlled oscillator) 14 via F12 and DC amplifier 13. The VOO14 oscillates at 76 KHz, and the splitter 15 generates a rectangular wave with a duty of 50- at z.In the conventional PLL circuit for stereo demodulation, the output of the splitter 15 was used as a switching signal. , in this invention, this □ is LPF164
This is a sine wave signal of 38KH, which is)
L road, Heinka. Shi-C Yoi, together with 831. ．． At step 2.7, the signal is again converted into a 38KH2 rectangular wave and input to the branching unit 11. By doing so, a sine wave subcarrier signal synchronized with the 19KH2 pilot signal can be obtained.

FIG. 7 is a circuit block diagram showing a specific example of the PPM circuit 8 in FIG.
OolB is provided, and FM waves modulated with subcarriers are output from this VO018. This is converted into a trigger pulse by the trigger circuit 19, and becomes a trigger signal for the monostable multivibrator 20. The output of this monostable multivibrator 20 becomes a PPM signal.

Figure 8 - (This is a diagram of each operation waveform of the circuit in Figure 7.
(4) is the subcarrier signal, (B) is the output of VO018, (0) is the trigger pulse, and (the parrot is the output of the monostable multivibrator, which indicates a PPM wave.

As described above, according to the present invention, since stereo demodulation is performed by performing multiplication using a switching signal that does not have unnecessary frequency components close to the composite signal frequency range, it is not affected by noise or interference. Furthermore, since an LPF is often used that has a negative effect on the conoposite signal component, high-quality stereo demodulation with good characteristics is possible.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional PM stereo demodulation device, FIG. 2 is a diagram explaining the operating characteristics of the device in FIG. 1, FIG. 3 is a block diagram showing the principle of the present invention, and FIG. 5 is a diagram illustrating an example of the circuit of the multiplier in FIG. 3, FIG. 6 is a block diagram of the subcarrier signal generator in FIG. 3, and FIG. FIG. 3 is a block diagram of the PPM circuit, and FIG. 8 is an operating waveform diagram of the circuit of FIG. 7. Explanation of symbols of main parts 1...y detector 3...multiplier 7...3
8KHz subcarrier generation 8... PPM circuit applicant Motohiko Fujimura, agent of Pioneer Corporation, patent attorney

Claims (1)

[Claims] means for generating a sinusoidal subcarrier signal synchronized with a stereo pilot signal; modulation means for generating a pulse train signal by pulse position modulating a high frequency pulse signal with the sinusoidal subcarrier signal; What is claimed is: 1. A stereo demodulator comprising: multiplication means for performing multiplication with a component output; and left and right channel signals are separated and derived from the multiplication output.