JPH031855B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stereo demodulation circuit, and more particularly to a multiplex stereo demodulation circuit using a switching method.

A multiplex demodulation circuit that separates and demodulates a composite signal, which is an FM detection signal, into left and right channel signals has a method of demodulating by multiplying the composite signal by a 38KHz subcarrier signal. FIG. 1 is a block diagram illustrating the principle of a demodulation circuit using such a multiplication method. A composite signal V(t) is input to multipliers 1 and 2, respectively, and multiplied by the positive and negative phase signals of the subcarrier signal. be done.
These multiplication outputs are input to adders 3 and 4, respectively, while composite signals are input via resistors 5 and 6, respectively. Therefore, in each of the adders 3 and 4, the respective multiplication outputs V ₁ (t) and V ₂ (t) are added to the main signal component in the composite signal, and the left and right channel signal components are separated. ing.

In such a configuration, the composite signal V(t)
is expressed as: V(t)=L(t)+R(t)+ {L(t)−R(t)}cosω _S t (1). Here, L(t) and R(t) are left and right channel signals, and ω _S is the angular frequency of the subcarrier signal. The product output V ₁ of this signal V(t) and the positive/inverse signal ±cosω _S t of the subcarrier signal is
(t), V ₂ (t) are arranged as M(t) for the main signal and S(t) for the sub signal, then V ₁ (t) = 1/2S(t) + M(t) cosω _S t +1/2S(t)cos2ωS _t ...(2) V ₂ (t)=-1/2S(t)-M(t) _cosωS t -1/2S(t)cos2ωS _t ...(3) becomes. Therefore, if the composite signal V(t) is attenuated to 1/2 by resistors 5 and 6 and added to the multiplication outputs shown by equations (2) and (3) by adders 3 and 4, each The low frequency components (audio components) V _A1 (t) and V _A2 (t) in the outputs of adders 3 and 4 are as follows: V _A1 (t) = 1/2S (t) + 1/2M (t) = L(
t) ...(4) V _A2 (t)=-1/2S(t)+1/2M(t)=R
(t) ...(5) Thus, the left and right channel signals are separated.

Figures 2 and 3 are circuit diagrams showing specific examples of the demodulation circuit using the multiplication method described above, and both are 38KHz.
This is an example of a multiplex stereo demodulation circuit using a so-called switching method in which a sinusoidal subcarrier signal is switched and multiplied by a pulse train signal having a frequency spectrum of a composite signal expressed by equation (1). In both figures, the first
Parts equivalent to those in the figures are designated by the same reference numerals.

First, referring to FIG. 2, a pulse train signal obtained by detecting an FM signal using a pulse count detection method is used as a composite signal, and this becomes a control signal for controlling the analog switching elements 7 and 8 on and off. ing. Further, the reverse phase signal of this pulse train signal controls on/off of the analog switching elements 9 and 10. The positive phase signal of the subcarrier signal is passed through resistors 11 and 12 and further through switching elements 7 and 10 to the adder 3,
It is one input of 4. On the other hand, the negative phase signal of the subcarrier signal becomes one input of adders 3 and 4 via resistors 13 and 14 and further via switching elements 9 and 8.

If each switching element 7 to 10 is set to be on when the control input is high level and off when the control input is low level, the composite signal of the outputs of switching elements 7 and 9, that is, the output of multiplier 1 will be (2). As shown in the equation, the positive phase component of the sub-signal S(t) is obtained.
Further, as a composite signal of the outputs of the switching elements 8 and 10, that is, an output of the multiplier 2, an opposite phase component of the sub-signal is obtained as shown in equation (3).

Adders 3 and 4 consist of operational amplifiers 15 and 16 and resistors 17 and 18, and each multiplication output ±S(t)
and the main signal component M(t) of the composite signal are added to obtain left and right channel signals, respectively.

In such a circuit, since there is no subcarrier signal when monaural, the subcarrier signal terminal is in a grounded state, but each switching element 7 to 1
0 is turned on and off by a composite signal (in this case, since it is monaural, it is only the main signal). Therefore, when each switching element is in the ON state, each of the resistors 11 to 14Rg is equivalently inserted between the input of each adder and the ground, as shown in FIG. In general, a signal source resistance Rg equivalently inserted between the input and ground in an amplifier
Considering the fact that the larger the value, the better the noise figure of the amplifier is, in the circuit shown in Figure 2, the noise due to the signal source resistance when the switching element is turned on during monaural operation becomes large, leading to deterioration of the S/N ratio. do.
In FIG. 4, OP indicates an operational amplifier of the adder, and Rf indicates a feedback resistor.

In the circuit of FIG. 3, the positive phase component of the subcarrier signal is connected to resistors 13a, 13b and 14a, 14.
One input each of adders 3 and 4 is provided through the adders 3 and 4, and switching elements 9 and 8 are inserted between the connection points of the resistors 13a, 13b and 14a, 14b and the ground, respectively. On/off control is performed by the positive and negative phase signals of the pulse train signal obtained by the pulse count detection method. Further, the opposite phase component of the subcarrier signal is input to each of adders 3 and 4 via resistors 11a, 11b and 12a, 12b, and resistors 11a, 11
Switching elements 7 and 10 are respectively inserted between the connection points of 12a and 12b and the ground, and these switching elements 7 and 10 are controlled on and off by the positive and negative phase signals of the pulse train signal, respectively. The other configuration is the same as that of the circuit shown in FIG.

By doing this, the left and right channel signals are separated from the outputs of adders 3 and 4 and demodulated as in the example shown in FIG. Resistors 11b to 14b are inserted between the input of the device and the ground, respectively, and the signal source resistance Rg shown in FIG. 4 becomes equivalently small, resulting in a deterioration of the S/N ratio.

SUMMARY OF THE INVENTION An object of the present invention is to provide a demodulation circuit which prevents deterioration of S/N when monaural when performing stereo demodulation using a multiplication method using switching.

The stereo demodulation circuit according to the present invention switches a subcarrier signal by controlling a switching element on and off using a pulse train signal that includes the frequency spectrum of a composite signal, which is an FM detection signal, and obtains a multiplication output of the pulse train signal and the subcarrier signal. This is a stereo demodulation circuit that separates and outputs the left and right channel signals by adding the left and right channel signals at a predetermined ratio.The main feature of this circuit is that it controls the switching element to be in the OFF state when monaural. It's what I did. The present invention will be explained below with reference to the drawings.

FIG. 5 is a circuit diagram of an embodiment of the present invention, in which the present invention is applied to the conventional example shown in FIG. 2, and equivalent parts in both figures are designated by the same reference numerals. If we only mention the parts that differ from Figure 2,
A pulse train signal having a frequency spectrum of a composite signal, which is an FM detection signal, becomes a positive phase signal through two cascaded NOR gates 19 and 20, and becomes an on/off control signal for switching elements 7 and 8. Further, a negative phase signal is obtained from the output of the NOR gate 19, and this serves as an on/off control signal for the switching elements 9 and 10. The other inputs of both NOR gates 19 and 20 are monaural,
A switching output from a switch 21 that changes depending on the stereo mode is applied, and a high level signal is applied when monaural, and a low level signal is applied when stereo. This switch 21 uses the 19KHz included in the FM detection output.
This is an electronic switch that is automatically controlled by detecting the presence or absence of a stereo pilot signal. The other configurations are the same as the circuit shown in FIG.

By doing this, in the stereo mode, since the other inputs of the NOR gates 19 and 20 are both at a low level, both NOR gates operate as an inverter. Therefore, the left and right channel signals are separately derived by performing the same operation as the circuit shown in FIG.

On the other hand, in monaural mode, Noah Gate 1
Since each of the other inputs 9 and 20 will be at a high level, both gates will be closed and their outputs will both be clamped at a low level. Therefore, since all the switching elements 7 to 10 are controlled to be turned off by this low level signal, each adder 3, 4
The signal source impedance seen from the input of each multiplier 1 and 2 is approximately infinite. This is the resistance in Figure 4.
This means that Rg has become infinite, and therefore the increase in noise due to the resistors 11 to 14 during monaural recording is eliminated, and S/N deterioration is prevented.

FIG. 6 is a circuit diagram of another embodiment of the present invention,
The present invention is applied to the conventional example shown in FIG.
In FIG. 6, parts equivalent to those in FIGS. 3 and 5 are designated by the same reference numerals. To describe only the parts that are different from FIG. 3, the pulse train signal having a composite signal component becomes a positive phase signal through two cascade-connected NOR gates 19 and 20, and becomes an on/off control signal for switching elements 7 and 8. A negative phase signal is obtained from the output of the NOR gate 19, and this becomes an on/off control signal for the switching elements 9 and 10. Both Noah Gates 19, 20
Since the switching output of the switch 21 is applied to each other input of the switch 21, and a low level signal is applied to the NOR gates 19 and 20 in the stereo mode, both gates operate as an inverter, and therefore the third
The operation is similar to that shown in the figure, and left and right channels can be separated.

In monaural mode, Noah Gate 19, 20
Since each input of is high, both gates are closed and their outputs are both low. Therefore,
Since all switching elements 7 to 10 are in the off state, the inputs of each adder 3 and 4 are connected to each multiplier 1,
2, the signal source impedance is resistor 11a, 1
Sum of 1b, sum of 12a, 12b, 13a, 13b
, and the sum of 14a and 14b. This is larger than the signal source impedance due to the resistors 11b, 12b, 13b, and 14b in the case of the circuit shown in FIG. 3, and the S/N ratio can be improved.

As described above, according to the present invention, it is possible to suppress resistance noise during monaural sound with an extremely simple configuration, so that a stereo demodulation circuit with a good S/N ratio can be obtained.

[Brief explanation of the drawing]

Figure 1 is a block diagram explaining the principle of a stereo demodulation circuit using the multiplication method, Figures 2 and 3 are diagrams showing specific examples of conventional stereo demodulation circuits, and Figure 4 is a block diagram of the circuit shown in Figures 2 and 3. FIGS. 5 and 6 are equivalent circuit diagrams illustrating the signal source impedance seen from the adder input when each switching is on in monaural mode, and are circuit diagrams of each embodiment of the present invention. Explanation of the signs of the main parts, 1, 2...multiplier,
3, 4... Adder, 7 to 10... Switching element, 19, 20... Noah gate, 21... Mode switch.

Claims (1)

[Claims] 1. A switching element is controlled on/off by a pulse train signal including the frequency spectrum of a composite signal, which is an FM detection signal, to switch a subcarrier signal, and a multiplication output of the pulse train signal and the subcarrier signal is obtained, and this multiplication output and the composite signal are combined. A stereo demodulation circuit configured to separate and output left and right channel signals by adding them at a predetermined ratio, the stereo demodulation circuit comprising means for controlling the switching element to an off state when monaural.