Stereophonic signal demodulation circuit

A stereophonic signal demodulation circuit receives an input signal including a signal modulated with first and second channel information to separate the first and second channel information from each other. The circuit includes first and second operational amplifiers and at least two switching devices namely, first and second switches for applying the input signal through respective resistors to one input terminal of the first operational amplifier and third and fourth switches for applying the input signal through respective resistors to one input terminal of the second operational amplifier. A control device controls the operations of the first through fourth switches and the first and second operational amplifiers outputs first and second channel information, respectively.

BACKGROUND OF THE INVENTION 
This invention relates to stereophonic signal demodulation circuits, and 
more particularly to an FM multiplex (MPX) stereophonic signal 
demodulation circuit. 
Heretofore, a double balanced type demodulation circuit comprising 
differential amplifiers has been employed as an FM MPX stereophonic signal 
demodulation circuit. In this conventional demodulation circuit, the 
differential amplifiers are cascade-connected in a two-stage state, and 
therefore the power supply utilization factor is low. It is impossible to 
increase the dynamic range. Furthermore, since the signal distortion of 
the differential transistors in the stages are superposed, it is difficult 
to reduce the distortion factor. In addition, because of the unbalance in 
characteristic of the differential transistors, distortion occurs, and the 
right and left channel signals become different in level. 
Moreover, for muting operation in selecting an FM broadcast station, it is 
necessary to provide a particular muting circuit in the rear stage of the 
double balance type demodulation circuit. 
Thus, the conventional demodulation circuit suffers from various 
difficulties as described above. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide an MPX stereophonic 
signal demodulation circuit in which the above-described drawbacks such as 
dynamic range reduction, distortion factor increase and the provision of a 
muting circuit accompanying the conventional double balance type 
demodulation circuit have been eliminated. 
A further object of this invention is to provide an MPX demodulation 
circuit where the output signal level in receiving a stereophonic signal 
is equal to that in receiving a monaural signal. 
Another object of the invention is to provide an MPX stereophonic signal 
demodulation circuit which is simple in arrangement to achieve right and 
left signal separation. 
These and other objects of this invention are achieved by a stereophonic 
signal demodulation circuit which receives an input signal including a 
signal modulated with first and second channel information to separate the 
first and second channel information from each other. The circuit 
comprises: first and second operational amplifiers; and at least two 
switching devices, namely, first and second switches for applying the 
input signal through respective resistors to one input terminal of the 
first operational amplifier. At least other two switching devices are 
provided, namely, third and fourth switches for applying the input signal 
through respective resistors to one input terminal of the second 
operational amplifier. A control device controls the operations of the 
first through fourth switches. The first and second operational amplifiers 
output first and second channel information, respectively. 
The control device comprises a circuit for generating a control signal 
according to the modulated signal and a circuit for generating a signal 
opposite in phase to the control signal. The first and second switches are 
controlled by the control signal, and the third and fourth switches means 
are controlled by the signal opposite in phase to the control signal. 
This invention will be described with reference to the accompanying drawing 
and the description of the preferred embodiment that follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a block diagram showing one embodiment of the invention. An 
intermediate frequency signal from a front end (not shown) is subjected to 
intermediate frequency amplification by an intermediate frequency (IF) 
amplifier 1 and is then subjected to amplitude limitation by a limiter 2. 
Thereafter, the signal thus treated is subjected to FM detection by an FM 
detection circuit 3, and a so-called "composite signal" is obtained. 
This composite signal includes a main signal which is the sum component of 
the left (L) channel signal and the right (R) channel signal, an auxiliary 
signal obtained by amplitude-modulating a subcarrier signal (38 KHz) with 
the difference component of the right and left channel signals, and a 
pilot signal 19 KHz. The composite signal is demodulated into a left (L) 
signal and a right (R) signal by an MPX (multiplex) stereophonic signal 
demodulation circuit 10. 
In the demodulation circuit 10 of the invention, the composite signal is 
applied through a buffer amplifier 4 comprising, for instance, an 
operational amplifier to switching means, namely, analog switches G1 
through G5. The analog switches G1 through G5 may be for example CMOS 
transistors. However, it is apparent that they can be made up of other 
switching elements. The outputs of the switches G1 and G2 are applied 
respectively through resistors R1 and R2 to the inversion input terminal 
of an operational amplifier OP1, the non-inversion input terminal of which 
is grounded through a resistor R10. A feedback resistor R5 is connected 
between the inversion input terminal and the output terminal of the 
operational amplifier OP1, to provide an inversion amplifier. 
The outputs of the switches G3 and G4 are applied respectively through 
resistors R3 and R4 to the inversion input terminal of an operational 
amplifier OP2, the non-inversion input terminal of which is grounded 
through a resistor R7. A feedback resistor R6 is connected between the 
output terminal and the inversion input terminal of the operational 
amplifier OP2, to form an inversion amplifier. 
The output of the switch G5 is applied through variable resistors R8 and R9 
to the non-inversion input terminals of the operational amplifiers OP1 and 
OP2, respectively. 
A control signal generating circuit 20 is provided to control the 
conductive and non-conductive states of the switches G1 through G5. In the 
control signal generating circuit 20, the 19 KHz pilot signal is detected 
from the composite signal by a 19 KHz detection circuit 5, and this 19 KHz 
signal is modulated into a 38 KHz signal A in phase with the subcarrier 
signal and a signal B opposite in phase to the signal A, both being 
provided by a 38 KHz signal generating circuit 6. The 19 KHz detection 
circuit 5 is designed so that it outputs a high level signal C and a low 
level signal D when no pilot signal is available, i.e. when a monaural 
signal is received. It outputs high level signals C and D when a 
stereophonic signal is received. The control signal generating circuit 20 
further comprises a mute signal generating circuit 7 which detects the 
signal of the IF amplifier 1 to generate a mute signal E. The mute signal 
generating circuit 7 detects the fact that the IF signal includes no 
stereophonic signal, to provide the low level mute control signal E. 
In the circuitry thus organized, first in receiving a stereophonic signal 
the switches G1 and G2 are controlled by the signal A (FIG. 2, (a) and 
(b)) in phase with the subcarrier signal, while the switches G3 and G4 are 
controlled by the opposite phase signal B (FIG. 2, (c) and (d)). If the 
switch G5 is designed to be controlled by the high level signal C (FIG. 2, 
(e)) from the 19 KHz detection circuit, and all of the switches are 
designed to be rendered conductive (on) by a high level control signal, 
then when the subcarrier is at the high level, the switches G1 and G2 are 
turned on, while the switches G3 and G4 are turned off. During this 
period, the composite signal is subjected to inversion and amplification 
and produced by the operational amplifier OP1. For the period in which the 
composite signal is in phase with the subcarrier signal, the composite 
signal includes the L channel component, and therefore the output of the 
operational amplifier OP1 is the L channel information. In this case, the 
amplification gain of the operational amplifier OP1 is represented by 
R5/(R1//R2). Therefore, if R1=R2=R, then the amplification gain is 
2.multidot.R5/R. (R1//R2 is the parallel combined resistance of R1 and 
R2). 
It is obvious that when the subcarrier is at the low level, the composite 
signal is subjected to inversion and amplification and outputted by the 
other operational amplifier OP2. During this period, the composite signal 
includes the R channel information component, and therefore the output of 
the operational amplifier OP2 is the R channel information. In this case, 
the gain of the operational amplifier is represented by R6/(R3//R4). 
Therefore, if R3=R4=R, then the gain is 2.multidot.R6/R. Accordingly, if 
R5=R6, then the R channel signal can be outputted with the same level. 
At the time of receiving a stereophonic signal, the switch G5 is turned on 
by the control signal C. Therefore, the amplitude of the composite signal 
is adjusted by the variable resistors R8 and R9 and the composite signal 
thus treated is applied to the non-inversion input terminals of the 
operational amplifiers OP1 and OP2. As a result, the composite signal and 
the L and R channel signals are subjected to subtraction in the 
operational amplifiers OP1 and OP2, respectively. Therefore right and left 
channel separation can be achieved by adjusting the resistance of the 
variable resistors R8 and R9. 
In the case where the right and left channel signals are equal, i.e. in the 
case of monaural signal reception, the high level control signal C is 
applied to the switches G1, G3 and G5, while the low level control signal 
D is applied to the switches G2 and G4 (cf. FIG. 2). Accordingly, the 
amplification gains of the inversion amplifiers OP1 and OP2 are R5/R, 
R6/R, respectively. That is, each of the gains is one half (1/2) of the 
gain obtained in the case of stereophonic signal reception. However, 
during the stereophonic signal reception the demodulation efficiency is 
set to 1/2 by the switching operations of the switches G1 through G4 and 
therefore the output level at the time of receiving a stereophonic signal 
is equal to the output level at the time of receiving a monaural signal. 
During muting operation, the low level mute control signal E is applied to 
all of the switches G1 through G5 and therefore transmission of the signal 
to the output section is blocked (cf. FIG. 2). Thus, the muting operation 
can be achieved simply but completely. 
In the above-described embodiment of the invention, a first set of two 
parallel-connected switches G1 and G2 and a second set of two 
parallel-connected switches G3 and G4 are connected to the inversion input 
terminals of the operational amplifiers OP1 and OP2, respectively, and the 
series-connected resistors are equal in resistance. However, it is 
apparent that the same effect can be obtained by connecting at least three 
parallel-connected switches to the inversion input terminal of each 
operational amplifier and by suitably selecting the resistances of the 
series-connected resistors. 
As is apparent from the above description, the demodulation circuit 
according to the invention is configured merely by inversion amplifiers 
and the switches. Therefore, the demodulation circuit according to the 
invention, unlike the conventional double balance type demodulation 
circuit, is free from drawbacks such as distortion factor increase, 
dynamic range decrease and the additional provision of a muting circuit. 
Thus, the demodulation circuit is high in performance. 
Furthermore, the demodulation circuit of the invention is advantageous in 
that the signal level at the time of receiving a stereophonic signal is 
equal to the signal level at the time of receiving a monaural signal. In 
addition, with the simple circuit, the separation adjustment can be 
conducted separately for the L channel and the R channel.