JPH033975B2 - - Google Patents

Description

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

There is a switching type demodulator that switches a composite signal using a 38KHz square wave subcarrier signal when demodulating an FM stereo signal, but because the square wave subcarrier signal for switching has a 38KHz odd harmonic component. , this odd harmonic (38×3=114KHz, 38
×5=190KHz) causes beat interference with adjacent stations and has an undesirable effect. To prevent this
It is necessary to insert a so-called anti-birdy filter, which is a high-cut filter, into the input of the FM stereo demodulation stage, but the frequency characteristics and delay characteristics of the composite frequency component of this filter are no longer flat, causing distortion and deterioration of the parity in the stereo demodulation output. is occurring. Furthermore, deterioration in sound quality due to distortion of the core material of the filter cannot be avoided.

For this purpose, the above disadvantages can be eliminated by adopting an analog multiplication method using a sine wave subcarrier signal, but it is extremely difficult to construct a linear multiplication circuit with excellent S/N and distortion. .

SUMMARY OF THE INVENTION An object of the present invention is to provide an FM stereo demodulator capable of eliminating the above drawbacks and separating FM stereo signals with good characteristics.

The FM stereo demodulator according to the present invention includes a detection means for generating positive and negative phase signals of a pulse train signal having a composite signal frequency spectrum component of an FM signal, and a detection means for generating positive and negative phase signals of a sinusoidal subcarrier signal synchronized with a stereo pilot signal. sine wave signal generating means for adding and outputting a predetermined DC component (including zero) to a negative phase signal; a first multiplier that generates multiplication outputs of the negative phase and positive phase subcarrier signals containing (including) a negative phase subcarrier signal, a first mixing device that mixes these multiplication outputs, and a negative phase pulse train of the detection means; a second multiplication means for generating respective multiplication outputs of the signal and positive phase and negative phase subcarrier signals containing the predetermined DC component (including zero) of the output of the sine wave signal generation means; and second mixing means for mixing, and the audio frequency components of the left and right channels can be obtained from the outputs of the first and second mixing means.

The present invention will be explained below based on the drawings.

FIG. 1 is a circuit block diagram of an embodiment of the present invention, in which an FM signal is applied to a pulse detector 1. In FIG. The detector 1 may be any type of detector that outputs a pulse train signal containing a composite signal frequency spectrum component of an FM stereo signal, and may have a configuration equivalent to a quadrature detector circuit as shown in FIG. 2, for example. . This is a well-known configuration, and includes a limiter 11, a phase comparator 12 which takes the limiter output as one input, and a phase shifter 13 which shifts the phase of the limiter output according to the frequency. The output of the comparator 13 is the other input of the comparator 12, and the comparison output becomes a pulse train signal containing a predetermined DC component. The characteristics and operation of this detector 1 will be described later.

This pulse train signal including the DC component becomes one input of the multipliers 2 and 3 as it is (positive phase), and also becomes the opposite phase via the inverter 4 to the multipliers 5 and 6.
It is used as a signal for switching.

On the other hand, this pulse train signal contains a stereo composite signal component of the FM stereo signal,
A signal generator 7 is provided which extracts the stereo pilot signal therein and generates a sinusoidal 38KHz subcarrier signal. This sine wave subcarrier signal is used as the other input to the multiplier 5 as it is (in positive phase), and is inverted in phase by the inverter 8 and becomes the other input to the multiplier 6. A DC component V is added to this negative phase output, which serves as another input to the multiplier 2, and a DC component V is similarly added to the positive phase output, which serves as another input to the multiplier 3.

Addition of the outputs of multipliers 2 and 5, as well as multiplier 3
Adders 9 and 10 are provided to add the outputs of 6 and 6, respectively.
Extract only the audio frequency components of each output of
LPFs 14 and 15 are provided, and right and left channel signals are demodulated from both outputs, respectively.

To explain the pulse detector 1 shown in FIG. 2, it is assumed that the phase shifter 13 has frequency versus phase and delay characteristics as shown in FIG. That is, it is assumed that the phase changes in proportion to the frequency and has a constant delay time T ₀ in that frequency range. The phase shifter 13 can have a configuration as shown in FIG. 4, for example, and is equivalent to a type of delay circuit. Now, the phase characteristics of the phase shifter 13 are
If the phase is set at 90° with respect to the center frequency of the FM-IF transmission, the operating waveforms shown in FIGS. 5A to 5C will be exhibited. In figures A to C, both a
is the FM-IF signal that has passed through the limiter 11, b is the delayed output from the phase shifter 13, c is the phase comparison output,
Figure A shows the waveforms of various parts when the center frequency is equal to the center frequency, and the comparison output is a pulse train signal with a duty of 1/2, and thus its DC component is 1/2. Figure B shows the case where the frequency is higher than the center frequency, so the DC component is, for example, 2/3, which means that the DC component increases by 1/6 with respect to the center frequency. Figure C shows a case where the frequency is lower than the center frequency, and therefore the DC component is, for example, 1/3, which means that the DC component is lowered by 1/6 with respect to the center frequency.

Therefore, the output of this comparator 12 is a PPM (pulse position modulation) signal according to the FM signal frequency,
If this pulse is integrated, an FM detection output can be obtained, but in this example, since this pulse train signal contains a stereo composite signal component, the multipliers 2, 3, 5, and 6 is used as a switching signal. In this way, if the pulse train signal has the frequency spectrum components of the stereo composite signal including the DC component, it is possible to use PPM signal output using other detection methods such as quadrature detection or pulse count detection. can.

Therefore, if the low frequency component of this PPM signal, that is, the composite signal component is e(t), then e(t) = V ₀ + α {M(t) + S(t) sin ωst + P sinωs/2t} (1) Become. Here, V ₀ is a DC component that changes with tuning, and if the delay time is set so that the phase difference is 90° at the center frequency of the FM-IF signal, that is, the tuning point, then the fifth As explained using the waveform in the figure, V ₀ becomes 1/2. Also α
is proportional to the delay time (proportional to the phase slope) and corresponds to the detection efficiency. ωs is the angular frequency of the subcarrier signal, M(t) is the main signal and L(t)+
R(t) is the audio frequency component of the sub-signal, and S(t) is the audio frequency component of the sub-signal and L(t)-R(t). P is a constant.

The negative phase signal () from the inverter 4 is expressed as ()=(1- _V0 )-α{M(t)+S(t)sinωst+Psinωs/2t} (2). Therefore, the multiplication output a of multiplier 2
(t) is a(t)=(V-sin ωst)[V ₀ +α{M(t)+S
(t) sin ωst + P sinωs/2t} (3), and the output b(t) of the multiplier 5 is b(t)=sin ωst・[(1−V ₀ )−α{M(t)+
S(t) sin ωst+P sinωs/2t} (4). Therefore, the output of adder 9 can be rearranged using equations (3) and (4) as follows: a(t)+b(t)=V・V ₀・αM(t)+V・αS(t
) sin ωst＋(1−2V ₀ )sin ωst−2αM(t) sin ωs
t−
2αS (t) sin ² ωst + V・α・P sinωs/2t−2αP
It is expressed as sinωs/2t sinωst(5). Here, −2αS(t) sin ² ωst=−2αS(t) ・1−con2ωst/2=−αS(t) +αS(t)cos2ωst (6) And the tuning point is at the center frequency. Then, since V ₀ = 1/2, (1-2V ₀ ) sin ωst
= 0, the stationary subcarrier signal component is canceled, and one of the unnecessary frequency components is conveniently canceled by the above operation. On the other hand,
Since the sub signal component cannot be heard, the adder 9
Substantial audio frequency components can be heard from the output. Furthermore, the output of the LPF 14, which allows only audio frequency components to pass, is obtained as follows: R'(t)=1/2V+V·αM(t)−αS(t)(7).

Here, if the magnitude of the DC applied voltage V is 1, then R'(t) = 1/2 + αM (t) - αS (t) = 1/2 + 2αR (t) ...(8), and the right channel signal is It will be demodulated and output. The same applies to the left channel signal L(t).

FIG. 6 shows a specific example of the multipliers 2, 3, 5, and 6, with a simple _{configuration} consisting of a transistor Q ₁ and resistors R ₁ and R ₂ . , () enables multiplication by performing a switching operation.

FIG. 7 shows a specific example of a 38 KHz sine wave subcarrier signal generator 7, which has a PLL (phase locked loop) configuration. The phase of the 19KHz stereo pilot signal is compared with the output of the 1/2 frequency divider 17 in the phase comparator 16, and the comparison output is
It becomes a control input for a VCO (voltage controlled oscillator) 20 via an LPF 18 and a DC amplifier 19. VCO20
generates a 76KHz pulse train signal with a duty of 50%, and is the output of the 1/2 frequency divider 21. The 38KHz signal is converted into a sine wave by the LPF 22 and used as a subcarrier signal. this
The LPF output is again made into a pulse by the limiter 23,
This pulse train signal is further divided by 1/2 frequency divider 17.
19KHz is used as one input of the comparator 16. By doing this, you can synchronize with the pilot signal.
A 38KHz sine wave signal can be obtained accurately.

FIG. 8 is a block diagram of another embodiment of the present invention, in which parts equivalent to those in FIG. 1 are designated by the same reference numerals. In the figure, a signal obtained by adding a negative DC component -V to the positive phase subcarrier signal is used as the other input to the multiplier 5, and a signal obtained by adding the negative DC component -V to the negative phase subcarrier signal is used as the other input to the multiplier 6. The other configuration is the same as that shown in FIG. 1.

The output b'(t) of the multiplier 5 is b'(t)=(sin ωst−V)・[(1 −V ₀ )−α{M(t)+S(t) sin ωst +P sinωs/2t} ] (9) Therefore, the output of adder 9 is a(t)+b'(t)=V(2V ₀ -1)+2V・αM(t
)
+2α・VS (t) sin ωst + (1−2V ₀ ) sin ωst−2αM (t) sin ωst −2αS (t) sin ² ωst+2α・VP sinωs/2t −2αP sinωs/2t sin ωst (10). When V ₀ = 1/2, the DC component becomes zero,
Moreover, subcarrier signals can also be eliminated. Therefore, the output of the LPF 14 is
Then, R′(t)=αM(t)−αS(t)=2αR(t)…(11
) and the right channel is demodulated. The same applies to the left channel.

Note that the audio frequency component refers to a frequency component in the audible band, and therefore frequency components other than the audible band in equations (5), (10), etc. are not included.

As described above, according to the present invention, there is no need for a so-called anti-birdie filter for removing beat interference waves from adjacent stations, so there is no signal deterioration. Further, since the stereo demodulation operation is performed by switching, an analog multiplier is not required, and demodulation with excellent S/N and distortion can be achieved with a simple circuit. Furthermore, since unnecessary subcarrier components are completely canceled in the addition (mixing) stage for demodulation, the output stage
LPF design becomes easier.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a block diagram of an example of a detector that generates a PPM signal, Fig. 3 is a characteristic diagram of the circuit shown in Fig. 2, and Fig. 4 is a block diagram of an example of a detector that generates a PPM signal. Figure 5 is a waveform diagram explaining the circuit operation of Figure 2, Figure 6 is a diagram showing a concrete example of a multiplier, Figure 7 is a 38KHz subcarrier. FIG. 8 is a block diagram showing a specific example of a signal generator. FIG. 8 is a block diagram of another embodiment of the present invention. Explanation of symbols of main parts, 1...Pulse detector,
2, 3, 5, 6... Multiplier, 7... Sine wave subcarrier signal generator, 9, 10... Adder.

Claims (1)

[Claims] 1 A detection means for generating positive and negative phase signals of a pulse train signal having a composite signal frequency spectrum component of an FM signal, and a predetermined DC component ( sine wave signal generation means for adding and outputting the positive phase pulse train signal of the detection means and the predetermined DC portion (including zero) of the output of the sine wave signal generation means; and a positive phase subcarrier signal, a first mixing means for mixing these multiplication outputs, and a pulse train signal of the opposite phase of the detection means and the sine wave signal generation means. a second multiplier for generating respective multiplication outputs of the output of the normal phase and anti-phase subcarrier signals containing the predetermined DC component (including zero); and a second mixing means for mixing these multiplication outputs. and the first
and an FM stereo demodulator characterized in that audio frequency components of left and right channels can be obtained from the output of the second mixing means.