JPS6342455B2 - - 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 antibirdy filter, which is a high-cut filter, at 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 separation deterioration 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 drawbacks can be removed 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 means for generating positive-phase and negative-phase signals of a pulse train signal having a stereo composite signal frequency spectrum component of an FM signal, each including a predetermined DC component, and a sine demodulator synchronized with a stereo pilot signal. means for generating positive and negative signals of a wave-like subcarrier signal, generating respective product outputs of positive phase and negative phase pulse train signals containing DC components and negative phase and positive phase subcarrier signals; a first mixing means for mixing a positive-phase pulse train signal containing a DC component; and generating outputs of multiplication of the positive-phase and negative-phase pulse train signals containing a DC component and positive-phase and negative-phase subcarrier signals, respectively. and a second mixing means for mixing these multiplier outputs and a positive-phase pulse train signal containing a DC component, and stereo audio signals are obtained from the outputs of the first and second mixing means, respectively. It is characterized by:

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 directly (in positive phase) used as the other input to the multipliers 3 and 5, and is reversed in phase by the inverter 8 and becomes the other input to the multipliers 2 and 6. An adder 9 is provided to add the multiplication outputs of the multipliers 2 and 5 and the positive-phase pulse train signal containing the DC component, and also adds the multiplication outputs of the multipliers 3 and 6 and the positive phase pulse train signal containing the DC component. An adder 10 is provided to perform addition with the positive phase pulse train signal. LPFs 14 and 15 are provided to extract only the audio components of the outputs of these adders 9 and 10, 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 τ ₀ in that frequency range. Such a 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 carrier, the operating waveforms shown in FIGS. 5A to 5C will be exhibited. In figures A to C, both a
is the FM-IF signal 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. FIG. B shows a 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. Diagram C
is a case where the frequency is lower than the center frequency, so the DC component is, for example, 1/3, and the DC component is reduced by 1/6 with respect to the center frequency.

Therefore, the output of this comparator 12 is a PPM (pulse position modulation) signal corresponding to the FM signal frequency, and if this pulse is integrated, an FM detection output can be obtained, but in this example, this pulse train signal is a stereo composite signal. In view of the fact that it contains signal components, it is used as a switching signal for multipliers 2, 3, 5 and 6 without being integrated. 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 _p + α {M(t) + S(t) sinω _S t + Psinω _S /2t}...(1 ) becomes. Here, V _p 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 As explained using the waveform in FIG. 5, V _p becomes 1/2. Also, α is proportional to the delay time (proportional to the phase slope),
Corresponds to detection efficiency. ω _S is the angular frequency of the subcarrier signal, M(t) is the main signal and L(t)
+R(t) and S(t) is the sub signal L(t)−
Indicate R(t). P is a constant.

The negative phase signal () from the inverter 4 is expressed as ()=(1- _Vp )-α{M(t)+S(t) _sinωSt + _PsinωS /2t}...(2). Therefore, the multiplication output a(t) of multiplier 2
is a(t)=−sinω _S t・[V _p +α{M(t)+S(t) sinω _S t +Psinω _S /2t}]...(3), and the output b(t) of the multiplier 5 is b(t)=sinω _S t [(1−V _p ) −α{M(t)+S(t) sinω _S t +P sinω _S /2t}] (4). Therefore, the output of adder 9 is (1), (3) and
Rearranging using equation (4), a(t)+b(t)+e(t)=V _p αM(t)+(1
−2V _p ) sinω _S t +αS (t) sinω _S t−2αM (t) sinω _S t−2αS
(t) sin ² ω _S t+αP sin ω _S t/2 −2αP sin ω _S t·sin ω _S /2t (5). Here, −2αS(t) sin ² ω _S t = −2αS(t)・1−cos2ω _S t/2 = −αS(t) + αS(t) cos2ω _S t …(6) and tuning Assuming that the point is at the center frequency, V _p = 1/2, so (1-2V _p ) sinω _S t =
It becomes 0. In other words, the steady 38KHz subcarrier component is canceled, one of the unnecessary frequency components is canceled by the above operation, and it is convenient, and the output of the LPF 14, which passes only the audio component, is R' (t)=V _p +αM(t)−αS(t)=V _p +2αR(t) (7) Only the right channel signal is derived. Similarly, it can be seen that only the left channel signal is derived from the output of the LPF 15, making stereo demodulation possible.

FIG. 6 shows a specific example of the multipliers 2, 3, 5, and 6, which are simply configured with a transistor Q ₁ and resistors R ₁ and R ₂ , and the transistor Q ₁ is connected to the pulse train signal e(t) which is the detected output. ), (t) allows 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. VCO2
0 generates a 76 KHz pulse train signal with a duty of 50%, and the 38 KHz signal that is the output of the 1/2 frequency divider 21 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.

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. In addition, since the stereo demodulation operation is based on switching, an analog multiplier is not required, and S/
Demodulation with excellent N and distortion becomes possible. Furthermore, unnecessary subcarrier components are completely canceled in the addition (mixing) stage for demodulation, making it easier to design the LPF in the output stage.

[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. 2 is a block diagram showing a specific example of a signal generator. 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 Means for generating positive-phase and negative-phase signals of a pulse train signal having a composite signal frequency spectrum component of an FM signal, each including a predetermined DC component, and positive-phase and negative-phase signals of a sinusoidal subcarrier signal synchronized with a stereo pilot signal. means for generating a signal, generating respective multiplication outputs of the positive-phase and negative-phase pulse train signals containing the DC component and the negative-phase and positive-phase subcarrier signals, and generating the multiplication outputs of the positive-phase and negative-phase pulse train signals containing the DC component, respectively, and including the multiplication outputs and the DC component; a first mixing means for mixing a positive-phase pulse train signal; and a first mixing means for generating a product output of each of the positive-phase and negative-phase pulse train signals including the DC component and the positive-phase and negative-phase subcarrier signals; a second mixing means for mixing the multiplication output and the positive-phase pulse train signal containing the DC component, and stereo audio signals are obtained from the outputs of the first and second mixing means, respectively. Features of FM stereo demodulator.