JPH0313771B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM stereo demodulation circuit, and more particularly to an FM stereo demodulation circuit 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 38KHz rectangular subcarrier signal when demodulating an FM stereo signal. Figure 1 is a block diagram of such a demodulation system, in which an FM-IF (intermediate frequency) signal is converted into a composite signal by an FM detector 1, and then switched through an LPE (low-pass filter) 2 that removes unnecessary components. applied to circuit 3. The 19KHz pilot signal contained in the output of LPF2 is transferred to PLL (phase locked loop) circuit 4.
A 38 KHz rectangular wave subcarrier signal extracted in the pilot signal 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 38KHz subcarrier signal which is the switching signal is a rectangular wave as shown in Figure 2A, if this is expanded into a Fourier series, F(t) = 4/πsinωSt+4/3πsin3ωSt+
It is expressed as 4/5πsin5ωSt+...(1). Here, WS is the angular frequency of the subcarrier signal. In this way, the frequency spectrum of F(t) includes odd harmonics such as 114 KHz, 190 KHz, . . . in addition to the fundamental wave of 38 KHz, as shown in FIG. 2B.

If the FM detection output is switched by the switching signal F(t) having such a frequency spectrum, both signals will be multiplied, but the passband of LPF 5 and 6 of the output section will be changed from 0 to 15K.
Hz, the detector output appearing in the stereo output by this multiplication becomes as shown in FIG. 2C. In other words,
Main signal (0~15KHz) and sub signal (38±15K
Hz), signals at 114±15KHz, 190±15KHz, etc. (noise, nearby interference waves, etc.) are also demodulated and output.

In order to prevent such defects, the output of the FM detector 1 is set to 114KHz, 190KHz,... as shown in Figure 2D.
It becomes necessary to add an LPF with large attenuation nearby. However, since 114KHz is close to the composite signal component, this LPF causes the delay characteristics of the composite signal to become uneven, as shown in Figure 2E, and the amplitude characteristics to become uneven, resulting in the stereo demodulated output This results in deterioration of distortion and separation characteristics.

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

The FM stereo demodulation circuit according to the present invention includes means for generating a pulse train signal having a composite signal frequency spectrum component of an FM signal, means for generating a sine wave subcarrier signal synchronized with a stereo pilot signal, and first to fourth signals. It has a transmission path and first to fourth switching elements inserted in series in the first to fourth signal transmission paths, respectively, and the reverse phase signal of the pulse train signal is transmitted to the first and fourth switching elements. As a control signal,
A positive phase signal of the pulse train signal is used as a control signal for the second and third switching elements, a positive phase signal of the subcarrier signal is inputted to the first and second signal transmission paths,
The reverse phase signal of the subcarrier signal is input to the third and fourth signal transmission paths, the audio components of the outputs of the first and third signal transmission paths and the composite signal are added, and the second and fourth signals are added. It is characterized in that the audio components of the output of the transmission line and the composite signal are added, and the respective summed outputs are made into left and right channel signals.

The present invention will be explained below with reference to the drawings.

FIG. 3 is a circuit diagram of an embodiment of the present invention, in which a pulse count detector 1 is used as a means for generating a pulse train signal having a composite signal frequency spectrum.
As is well known, this detector 10 has a configuration that triggers a monostable multivibrator at the rising transition timing of the limiter output of the FM-IF signal, and thus detects the FM received signal according to each instantaneous frequency. Outputs a position modulated pulse train signal with a constant width, that is, a PPM signal. Generally, this
This is because the PPM signal contains the frequency spectrum of the stereo composite signal.
The PPM signal is passed through an LPF to produce an FM detection output, but in the present invention, this PPM signal is used as a switching signal for direct demodulation.

On the other hand, the 19KHz stereo pilot signal included in this detection output is extracted and the 38K signal synchronized with this is extracted.
To obtain a Hz sinusoidal subcarrier signal, e.g.
A subcarrier signal generator 11 having a PLL circuit configuration is provided. By switching this sine wave subcarrier signal with the previous PPM signal, a multiplication output of both signals is obtained, and left and right channel signals are separated and derived, respectively.

For this purpose, first to fourth signal transmission paths consisting of resistance elements R ₁ to R ₄ are provided, and a positive phase subcarrier signal is sent to the resistances R ₁ and R ₂ , which are the first and second signal transmission paths. , and anti-phase subcarrier signals are applied to resistors R ₃ and R ₄ , which are the third and fourth signal transmission paths, respectively. First to fourth analog switching elements SW ₁ to _{SW 4} are provided in series on these first to fourth signal transmission paths, respectively. The first and fourth switching elements SW ₁ and SW ₄ are on/off controlled by PPM signals of opposite phases, and the second and third switching elements SW ₂ and
SW ₃ is controlled on/off by the positive phase PPM signal.

The outputs of the first and third signal transmission paths are common, and the PPM output is commonly applied via the resistor R ₅ , so that these signal outputs are added and input to the amplifier 12 . Note that the parallel circuit of resistor R ₇ and capacitor C ₁ serves as a negative feedback circuit for amplifier 12, and a right channel signal is obtained from the output of this amplifier. Furthermore, the outputs of the second and fourth signal transmission paths are common, and the PPM output is also commonly applied via the resistor R ₆ , so that these signal outputs are added and input to the amplifier 13 . A parallel circuit of resistor R ₈ and capacitor C ₂ serves as a negative feedback circuit for amplifier 13, and the output of this amplifier becomes the left channel signal.

Figure 4 is a diagram of the operation waveforms of each part in Figure 3, where A is the PPM signal by pulse count detection, B and C.
is a positive and negative phase signal obtained by converting the signal shown in A into a rectangular pulse for controlling the switching element, D is a stereo pilot signal, E and F are positive and negative phase signals of subcarrier signals, and G is a switching waveform by the third switching element _SW3. , H is the first switching element
I shows the switching waveform by _SW1 , I shows the switching waveform by the second switching element _SW2 , and J shows the switching waveform by the fourth switching element _SW4 .

Here, the FM stereo composite signal is defined as C(t)=M(t)+S(t)sinωSt...(1). Also, the left and right channel signals are L(t), R
(t), M(t) = L(t) + R(t) is the main signal, S(t) = L(t) - R(t) is the sub signal, and sinωSt is the subcarrier. show. still,
Pilot signals are omitted. The detection output from the detector 10 has a composite signal component, and the carrier frequency of this rectangular wave, that is, FM-
It is assumed that the IF frequency is extremely high compared to the composite signal frequency. As shown in Figure 4A, if the amplitude of the PPM signal is V ₁ , then the PPM signal A
is V _M (t) = V ₁ · K · C (t) + V ₁ · (t) ... (2). K is a constant determined by FM detection efficiency,
(t) is a signal existing at frequencies near and above the carrier frequency of the pulse.

Figures B and C show the signals of A as positive and negative phase switching signals of 1 and 0, VS ₁ = 1/2 (1 + K・C (t) + (t)) ...(3) VS ₂ =1/2(1-K·C(t)-(t))...(4) It is expressed as follows. Let the subcarrier signals shown in FIG. 4E and F be S ₁ (t)=V ₂ sinωSt …(5) S ₂ (t)=−V ₂ sinωSt …(6), and in the circuit of FIG. 3, R ₅ = R ₆ = R _a , R ₁
= R ₂ = R ₃ = R ₄ = R _b , R ₇ = R ₈ = R _c , C ₁ = C ₂ = C ₀
shall be.

Here, in the third signal transmission path, since the negative phase subcarrier is switched with the positive phase PPM, the switching waveform becomes as shown in Fig. 4G, and the subcarrier is transmitted when switching element SW ₃ is on. , no longer transmitted when turned off. As a result, the output current i ₃ (t) of switching element SW ₃
is, i ₃ (t)=VS ₁ (t)・S ₂ (t)/R _b =V ₂ /2R _b {-1
/2K・S(t)−K・M(t)sinωSt +1/2K・S(t)cos2ωSt−(t)sinωSt
−sinωSt}…(7). The first term in equation (7) is the sub-signal S(t) at 23 to 53 KHz that is band-converted to the audio component range from 0 to 15 KHz by multiplying the composite signal and the subcarrier signal, and this is used for stereo demodulation. It becomes a sub signal. The second and third terms are also caused by the above multiplication, and the fourth term is a multiplication of the subcarrier and the signal (t) which is in a much higher frequency range than the composite signal, and has an extremely high frequency.
The fifth term shows that the subcarrier appears as is.

Similarly, the switching output currents i ₁ (t), i ₂ (t), i ₄ (t) of the first, second and fourth signal transmission paths
is, i ₁ (t)=V ₂ /2R _b {-1/2K・S(t)−K・M
(t) sinωSt + 1/2K・S (t) cos2ωSt − (t) sinωS
ttsinωSt}...(8) i ₂ (t)=V ₂ /2R _b {1/2K・S(t)+K・M(
t) sinωSt−1/2K・S(t) cos2ωSt+(t)sinωSt+sinωSt} …(9) i ₄ (t)=V ₂ /2R _M {1/2K・S(t)+K・M(
t) sinωSt−1/2K·S(t) cos2ωSt+(t) sinωSt−sinωSt}...(10). Here, the current flowing through resistors R ₅ and R ₆ is i ₅
(t), i ₆ (t), then i ₅ (t)=i ₆ (t)=1/R _a V _M (t)=V ₁ /R _a {K・
M(t)+K·S(t)sinωSt+(t)}...(11)

Therefore, the input current i ₇ (t) to the amplifier 12 is i ₇ (t) = i ₁ (t) + i ₃ (t) + i ₅ (t) = K{V ₁ /
R _a M(t)−V ₂ /2R _b S(t)} +K{V ₁ /R _a S(t)−V ₂ /2R _b M(t)}s
inωSt＋V ₂ /2R _b K・S(t)cos2ωSt +V ₁ /R _a (t)−V ₂ /R _b (t)×sinωSt…(12
), and the input current i ₈ (t) to the amplifier 13 is i ₈ (t)=K{V ₁ /R _a M(t)+V ₂ /2R _b S(t)}
+K{V ₁ /R _a M(t) +V ₂ /R _b M(t)}sinωSt −V ₂ /2R _b K・S(t)cos2ωSt+V ₁ /R _a (t)
+V ₂ /R _b (t) sinωSt...(13). Here, assuming V ₁ /R _a =V ₂ /R _b , and considering only the audio components of i ₇ (t) and i ₈ (t) from 0 to 15KHz, i ₇ (t) = KV ₁ /R _a {M(t)−S(t)}=KV ₁ /R _a R(t)…(1
4) i ₈ (t)=KV ₁ /R _a L(t) (15) The left and right channel signals are separated. Then, each output from the amplifiers 12 and 13 is VR(t)=-1+SC ₀ R _c /SC ₀・KV ₁ /R _a L(t)...(16
) VL(t)=-1+SC ₀ R _c /SC ₀・KV ₁ /R _a R(t)…(17
), and de-emphasis is applied with a time constant of C ₀ R _c .

FIG. 5 is a circuit diagram of another embodiment of the present invention,
Parts equivalent to those in FIG. 3 are designated by the same reference numerals. In this example, the first and third switching elements become the first and third signal transmission paths, respectively,
These two outputs are made common by a common output resistor R ₉ and are input to the amplifier 12 . Further, the second and fourth switching elements become second and fourth signal transmission paths, respectively, and their outputs are shared by a common output resistor R ₁₀ and are input to the amplifier 13. Here, by setting R ₉ =R ₁₀ =R _b , the right and left channel signals are separated from each output of the amplifiers 12 and 13, similar to the example shown in FIG.

FIG. 6 is a circuit diagram of still another embodiment of the present invention, in which parts equivalent to those in FIG. 3 are designated by the same reference numerals. In this example, all of the resistive elements R ₁ to _{R 4} of the signal transmission path in FIG. 3 are omitted, and the circuit configuration is simplified by driving the sinusoidal subcarrier signal source as a current source. Therefore, each signal transmission path is constituted by a switching element. Since the drive source is a current source, the output impedance is large, and therefore the current when each switching element is turned on has the advantage of being less affected by the on-resistance of these switching elements.

Current output iS ₁ for positive and negative phase subcarrier signals
(t), iS ₂ (t) are iS ₁ (t)=I ₀ sinωSt …(18) iS ₂ (t)=−I ₀ sinωSt …(19), and I ₀ =V ₂ /R _b. Then, the left and right channel signals are demodulated as in the example of FIG.

FIG _. 7 shows an _example of _the current source circuit for the 3KHz _subcarrier signal in FIG. ₁₁ ～
It is output by a positive sequence current generation circuit consisting of R ₁₆ , R ₂₃ , R ₂₄ , and R ₂₇ . Further, the negative phase subcarrier signal is transmitted through the input amplifier 15, transistors Tr ₅ to _{Tr 8} , diodes D ₃ to D ₄ and resistors R ₁₇ to R ₂₂ , F ₂₅ , R ₂₆ ,
It is output by a negative phase current generation circuit consisting of R ₂₈ and R ₂₉ . Both circuits have a so-called single-ended push-pull configuration in which complementary transistors share a common collector output. By providing negative feedback in the amplifiers 14 and 15, a sine wave current with less distortion can be obtained.

In the figure, R ₁₁ = R ₁₂ = R ₁₃ = R ₁₄ = R ₁₅ = R ₁₆
= R ₁₇ = R ₁₈ = R ₁₉ = R ₂₀ = R ₂₁ = R ₂₂ , R ₂₃ = R ₂₄ =
If R ₂₅ = R ₂₆ , R ₂₇ = R ₂₈ = R ₂₉ = R _d , and the sinusoidal subcarrier input is V ₂ sinωSt, the output current is i ₀ (t) = ±V ₂ /R _d sinωSt …( 20).

FIG. 8 is a circuit diagram of another embodiment of the present invention,
The common output resistors R ₉ and R ₁₀ in the example in Figure 5 are placed on the input side, and the positive-phase subcarrier signals of the first and second signal transmission paths, which are common inputs, are connected to the resistors.
_R11 to each signal path, and the reverse phase subcarrier signals of the third and fourth signal transmission paths are connected to the resistor.
It is introduced into each signal path via _R12 . In this example as well, it is possible to demodulate the left and right channel signals.

As described above, according to the present invention, since the sine wave subcarrier is used as a multiplication signal, unnecessary harmonics are not included, and therefore there is no beat interference demodulated by multiplication. Since there is no need to pass it through, there is no distortion. Furthermore, the main signal component M(t)=L(t)+S(t) is not affected by switching because it is input to the output amplifier only through the resistor without passing through the switching circuit. Furthermore, switching elements SW ₁ , SW ₂ and
Since SW ₃ and SW ₄ are controlled on and off in opposite phases to each other, the impedance seen from the input end of the positive and negative phase subcarrier signals is always constant, making driving easier, and unnecessary subcarrier signals are canceled. This is convenient.

[Brief explanation of the drawing]

Figure 1 is a block diagram of a conventional stereo demodulation circuit, Figure 2 is a diagram explaining the characteristics of the circuit in Figure 1,
3 is a circuit diagram of one embodiment of the present invention, FIG. 4 is a waveform diagram of each part of the circuit of FIG. 3, FIGS. 5 and 6 are circuit diagrams of other embodiments of the present invention, and FIG. This figure is a circuit diagram showing a partial example of a subcarrier signal generating circuit used in the circuit of FIG. 6, and FIG. 8 is a circuit diagram of still another embodiment of the present invention. Explanation of symbols of main parts, 10...Pulse count detector, 11...Subcarrier signal generator, SW ₁ ~
SW ₄ ...Switching element.

Claims (1)

[Claims] 1 means for generating a pulse train signal having a frequency spectrum component of a stereo composite signal which is an FM detection signal; means for generating a sine wave subcarrier signal synchronized with a stereo pilot signal in the stereo composite signal; a fourth signal transmission path; and first to fourth switching elements inserted in series in the first to fourth signal transmission paths, respectively, and a reverse phase signal of the pulse train signal is transmitted to the first to fourth signal transmission paths. and a fourth switching element as a control signal, a positive phase signal of the pulse train signal as a control signal of the second and third switching elements, and a positive phase signal of the subcarrier signal as a control signal of the first and second signal transmission. input the reverse phase signal of the subcarrier signal to the third and fourth subcarrier signals.
, and add the outputs of the first and third signal transmission paths and the pulse train signal, and add the outputs of the second and fourth signal transmission paths and the pulse train signal. An FM stereo demodulation circuit characterized in that each of these addition outputs is made into left and right channel signals.