JPH0320174B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM multiplex demodulator;
In particular, it relates to an FM multiplex demodulator that reduces noise that occurs during stereo broadcast or multi-channel broadcast demodulation, and is an FM that is suitable for integrated circuit implementation.
It concerns multiplex demodulators.

Conventionally, FM composite signals were generally used.
Figure 1 shows the signal-to-noise (S/N) characteristics when receiving and demodulating FM stereo broadcasts. A is the signal-to-noise (S/N) characteristic in the case of stereo demodulation, B is the signal-to-noise characteristic (S/N) in the case of monaural demodulation, and C is the channel separation characteristic in the case of stereo demodulation. In Figure 1, when the input field is below 50 dB, the S/N during stereo demodulation is approximately
20dB worse. This deterioration of the S/N ratio during stereo demodulation is unavoidable, but the noise during FM reception is extremely annoying, and various methods have been proposed to remove it. The second method is
As shown in the figure, a method is known in which a left channel output 100L and a right channel output 100R of an FM multiplex demodulation circuit 100 are coupled by a capacitor 100'. This is done by focusing on the fact that the noise that increases with stereo demodulation is 180° different in phase between the left and right channels, and that the noise that is easily noticeable to the human eye is distributed in the high range of the audible frequency band. Cancel noise by making only the range monaural,
In addition, this method maintains the separation between left and right in the low range.

This method is a precautionary measure before converting to monaural, and has the drawback that it does not improve the S/N ratio as much as monaural, and also has the drawback that this effect acts even in the case of a strong electric field, causing deterioration of separation.

The purpose of the present invention is to solve the above-mentioned drawbacks and further improve the S/N more than monaural when the electric field is weak.
The purpose is to provide an FM multiplex demodulator.

The FM multiplex demodulation circuit according to the present invention includes means 1 for obtaining left and right channel signal currents from an FM composite signal; a first current mirror circuit 122, 123, 1 having a supplied current input terminal and first and second current output terminals;
24, 201, 207, 401, 411, a second current mirror 125, 126, 127, 202, having a current input terminal to which the right channel signal current is supplied, and third and fourth current output terminals;
208, 501, 511, a third current mirror circuit 206, 211, 402, 410, 5 having a current input terminal and fifth and sixth current output terminals
02,510, means 203, 204 for commonly connecting said first and third current output terminals and coupling them to a current input terminal of said third current mirror circuit;
205, 209, 210, first and second differential pair transistors 40 driven by currents obtained from the second and fourth current output terminals, respectively;
3,404 and 503,504, third and fourth differential pair transistors 4 driven by currents obtained from the fifth and sixth current output terminals, respectively;
05,406 and 505,506, one transistor 4 of the first to fourth differential pair transistors
04, 504, 405, 505 and the other transistor 403, 503, 406, 50 of the first to fourth differential pair transistors.
means 6 for applying a signal indicative of the received field strength between the base common connection point of the transistors 6, the left and right channel output terminals 7, 9, the collector of the one transistor 404 of the first differential pair transistors and the means 407, 408 for commonly connecting the collectors of the other transistors 406 of the third differential pair of transistors and coupling them to the left channel output terminal 7;
409, 412, 413, and the collector of the one transistor 504 of the second differential pair transistors and the collector of the other transistor 506 of the fourth differential pair transistors are commonly connected to connect the right channel output terminal 9. means 507 for coupling to;
It has 508, 509, 512, and 513.

Hereinafter, the present invention will be explained in more detail using the drawings.

FIG. 3 is a block diagram showing the principle of the invention.

The FM multiplex demodulation circuit 1 has two stereo outputs: a left channel output L, which is the first demodulated signal, and a right channel output R, which is the second demodulated signal, and these two stereo outputs are added to the adder circuit 2.
Add with . At the output end of the adder circuit 2, the stereo demodulated L and R signal outputs are summed and output as a monaural signal. The monaural signal output from the adder circuit 2 is passed through a low-pass filter 3 (hereinafter referred to as LPF), and a monaural signal L+R, which is a third demodulated signal in which high frequencies are cut off, is output. In addition, the left channel output L which is the output of the FM multiplex demodulation circuit 1 and the signal L+ passed through the LPF 3
R is input to the first mixing circuit 4, and the mixing ratio of these two signals is controlled by a control signal from the control circuit 6. The two signals are mixed at a mixing ratio controlled by this control signal and output to the output terminal 7 of the first mixing circuit 4. Furthermore, the monaural signal L+R passed through the LPF 3 and the right channel output R of the FM multiplex demodulation circuit 1 are inputted to the second mixing circuit 5, and the mixing ratio controlled by the control signal from the control circuit 6 is adjusted to the right channel. The channel output R and the monaural signal L+R passed through the LPF 3 are mixed and output to the output terminal 9 of the second mixing circuit 5.

The control circuit 6 controls the first and second mixing circuits 4,
A control signal is generated to control the mixing ratio of 5. When the electric field is strong, the first mixing circuit 4 outputs only the left channel signal L of the left channel signal L of the input signal and the monaural signal L+R passed through the LPF 3, depending on the applied control signal. When the electric field is weak, the mixing ratio of the left channel signal L and the monaural signal L+R passed through the LPF 3 is changed, and when the electric field is weak, only the monaural signal L+R passed through the LPF 3 is output.

Similarly, in the case of a strong electric field, the second mixing circuit 5 outputs the right channel signal R of the input signal and the right channel signal R of the monaural signal L+R passed through the LPF 3.
When the electric field is medium to weak, the right channel signal R and the monaural signal L+ are output through LPF3.
The control circuit 6 changes the mixing ratio with R and outputs only the monaural signal L+R passed through the LPF 3 when the electric field is weak.

Therefore, if a signal corresponding to the electric field strength, for example, a drive signal for a signal strength indicator (hereinafter referred to as a signal meter) of an FM receiver, is input to the input terminal 8 of the control circuit 6, it will be possible to detect the signal during strong electric field, that is, during stereo demodulation. The first mixing circuit 4 is in the range where the S/N condition is good.
A stereo signal of the left channel signal L is output to the output terminal 7 of the second mixing circuit 5, and the right channel signal R is output to the output terminal 9 of the second mixing circuit 5 without deteriorating the left/right channel separation. In the range where the S/N during stereo demodulation is worse than the S/N during monaural demodulation, the first and second mixing circuits 4 and 5 have a control system that changes the mixing ratio of the stereo signal and monaural signal depending on the electric field strength. The deterioration of S/N during stereo demodulation is improved by gradually transitioning to a monaural signal, and the amount of separation between the left and right channels is also gradually transitioning to monaural.

That is, in the range from a medium electric field to a weak electric field, a mixed signal of the left channel signal L and the monaural signal L+R whose mixing ratio changes depending on the electric field strength is output to the output terminal 7 of the first mixing circuit 4, and the second The output terminal 9 of the mixing circuit 5 receives a right channel signal R and a monaural signal L+R whose mixing ratio has changed depending on the electric field strength.
A mixed signal is output. Furthermore, when the electric field is weak, the first
The output terminal 7 of the second mixing circuit 4 and the output terminal 9 of the second mixing circuit 5 both output a monaural signal L+R. This monaural signal L+R is outputted to the output terminal 7 of the first mixing circuit 4 and the output terminal 9 of the second mixing circuit 5 through the LPF 3, which acts as a noise filter.The monaural signal L+R has high frequency noise removed by the LPF 3. The S/N ratio can be further improved by converting the signal into a monaural signal.

FIG. 4 shows an embodiment of the present invention. In the same figure, 1
is an FM multiplex demodulation circuit, 2 is an adder circuit, 3 is a low-pass filter, 4 is a first mixing circuit, 5 is a second mixing circuit, 6 is a control circuit, 7 is an output terminal of the first mixing circuit, 8 is an input terminal of the control circuit 6, and 9 is an output terminal of the second mixing circuit, which correspond to FIG. 3, respectively. Further, 10 is a power supply line.

In the FM multiplex demodulation circuit 1, a stereo composite signal is input to the base of the transistor 101, and the 38kHz subcarrier is used to input the stereo composite signal to the base of the transistor 101.
2, 103, 104, 105, transistors 107, 110, 111, resistor 11
The right channel signal R is applied to the collector of the transistor 107 of the current mirror circuit composed of the transistors 106, 114, and 113.
5. Output the left channel signal L to the collector of the transistor 106 of the current mirror circuit composed of resistors 116 and 117. transistor 10
8, 109 and resistors 120, 121 are circuits for canceling leakage components of the left and right channels.

The left channel signal L from the FM multiplex demodulation circuit 1 is transmitted to the transistor 12 in the same circuit.
2, 123, resistor 124, transistor 201 and resistor 207 in adder circuit 2, and transistor 401 and resistor 411 in first mixing circuit 4. As a result, transistors 201, 401 A left channel signal L appears at the collector of . On the other hand, the right channel signal R includes transistors 125 and 126, a resistor 127, a transistor 202 in the adder circuit 2,
The resistor 208 is supplied to the current mirror circuit composed of the transistor 501 and the resistor 511 in the second mixing circuit 5, and as a result, the right channel signal R is supplied to the collectors of the transistors 202 and 501.
appears. Since the collectors of transistors 201 and 202 are commonly connected, the left and right channel signals are added together, and this added signal is filtered through transistors 203-205, resistors 209 and 210, and further filtered out by high-frequency noise by capacitor 301 of LPF3. Transistors 206, 402, 502 and resistor 2
It is supplied to a current mirror circuit composed of 11, 410, and 510. Transistors 402, 50
As a result, a monaural signal appears at the collector of 2.

The first mixing circuit 4 includes transistors 403 and 40
The left channel is input to the transistor 401 by the difference between the base voltages of the transistors 403 and 406 and the base voltages of the transistors 404 and 405, that is, the control voltage from the control circuit 6. The mixing ratio of the signal L and the monaural signal L+R input to the transistor 402 is controlled.

Similarly, the second mixing circuit 5 also has a transistor 50.
It has a double differential amplifier configuration of 3,504,505,506, and the mixing ratio of the right channel signal R input to the transistor 501 and the monaural signal L+R input to the transistor 502 is determined by the control voltage from the control circuit 6. is controlled. The control circuit 6 includes transistors 601, 6
The control voltage that controls the mixing ratio of the first mixing circuit 4 and the second mixing circuit 5 is provided by transistors 601 and 6.
This is generated using the transfer characteristics of the differential amplifier 600 of 02. The voltage of a reference voltage source 613 is level-shifted and applied to the base of the transistor 602 by a transistor 605 and a diode 606, and the signal meter drive voltage obtained after intermediate frequency amplification of the receiver is applied to the input terminal 9 of the control circuit 6. This voltage is level-shifted by a transistor 603 and a diode 604 and applied to the base of the transistor 601. The output voltage of the differential amplifier 600 is connected to the transistor 607 and the diodes 608 and 60.
9, transistor 610, diode 608, 6
09, transistor 610, diode 611,
The level is shifted at 612 and becomes a control voltage for controlling the first and second mixing circuits 4 and 5.

For example, in the case of a strong electric field, the input terminal 8 of the control circuit 6
the voltage applied to is higher than the reference voltage source 613;
Therefore, transistor 601 is turned on and transistor 602 is turned off. At this time, the voltage at the cathode of diode 609 is higher than that at the cathode of diode 609.
It is lower than the voltage at the cathode of No. 12. Therefore, in the first mixing circuit 4, the base voltages of the transistors 403 and 406 are lower than the base voltages of the transistors 404 and 405, so the transistors 403 and 406 are turned off and the transistors 404 and 405 are turned on. At this time, only the left channel signal L input to the transistor 401 is output from the output circuit composed of transistors 407, 408, and 409 to the output terminal 7 of the first mixing circuit, and the monaural signal L+R input to the transistor 402 is not output. .

Similarly, in the second mixing circuit 5, transistors 503 and 506 are turned off, transistors 504 and 505 are turned on, and transistor 501 is turned off.
Only the left and right channel signals R input to the transistors 507, 508, and 509 are output to the input terminal 9 of the second mixing circuit 5, and the input monaural signal L+R of the transistor 502 is not output. . In addition, when the electric field is weak, the voltage applied to the input terminal 8 of the control circuit 6 is lower than the voltage value of the reference voltage source 613, the transistor 601 is cut off, the transistor 602 is turned on, and the cathode voltage of the diode 609 is equal to the cathode voltage of the diode 612. Becomes higher. Therefore, in the first mixed circuit, the base voltages of transistors 403 and 406 are higher than the base voltages of transistors 404 and 405.
is conductive, transistors 404 and 405 are turned off, and at this time, only the monaural signal L+R input to transistor 402 is output to the output terminal 7 of the first mixing circuit 4, and the left channel signal L is not output.

Similarly, in the second mixing circuit 5, the transistors 503 and 506 are conductive, and the transistor 50
4,505 is suddenly disconnected, and only the monaural signal L+R input to the transistor 502 is output to the input terminal 9 of the second mixing circuit 5, and the right channel signal R is not output.

Further, the output terminal 7 of the first mixing circuit 4 and the second
Since the monaural signal L+R outputted to the input terminal 9 of the mixing circuit 5 is the monaural signal L+R with high-frequency noise cut through the LPF 3, the S/N ratio as a monaural signal can be improved. Furthermore, by selecting the capacitance value of the capacitor 301 of the LPF 3, the amount of attenuation of high frequency noise can be selected. In addition, in the medium electric field to weak electric field to weak electric field region, the control voltage from the control circuit 6 is applied to the input terminal 8 of the control circuit 6.
Therefore, the difference between the base voltages of transistors 403, 406 and transistors 404, 405 of first mixing circuit 4 also changes depending on the input voltage of input terminal 8 of control circuit 6.
Therefore, a signal obtained by mixing the left channel signal L input to the transistor 401 and the monaural signal L+R input to the transistor 402 is output to the output terminal 7 of the first mixing circuit 4, and the left channel signal L and the monaural signal L+R are output to the output terminal 7 of the first mixing circuit 4. The mixing ratio changes depending on the input voltage at the input terminal 8 of the control circuit 6.

Similarly, the input terminal 9 of the second mixing circuit 5 receives the right channel signal R input to the transistor 501.
and the monaural signal L+R input to the transistor 502 are output, and the mixing ratio changes depending on the input voltage at the input terminal 8 of the control circuit 6. Further, by appropriately selecting the mutual conductance of the differential amplifier 600, the rate of change of the mixing ratio can be selected.

Therefore, if a signal corresponding to the electric field strength, for example, the drive voltage of the signal meter of an FM receiver, is input to the input terminal 8 of the control circuit 6, the left channel signal L will be output even during stereo demodulation in the case of a strong electric field with good S/N condition. , outputs the stereo signal of the right channel signal R without degrading the separation of the left and right channels, and improves the S/R during stereo demodulation in medium to weak electric fields.
In the range where the N ratio deteriorates by approximately 20 dB, the stereo signal output gradually shifts to the monaural signal output, that is, the left and right channel separation gradually shifts to monaural to reduce the deterioration of the S/N ratio during stereo demodulation. Make improvements.

In the case of a weak electric field where the S/N ratio during monaural demodulation deteriorates, the S/N ratio can be improved by using the LPF to cut out high-frequency noise as a monaural signal.

An example of a characteristic diagram when the present invention is used is shown in FIG. 1 by solid lines A, D, and E. That is, the solid line D shows the channel separation characteristic according to this embodiment, and the solid line E shows the S/N characteristic according to this embodiment.

As described above, according to the characteristic curve D, it can be seen that there is no deterioration of the separation in the case of a strong electric field, and that it gradually shifts to cenoral in the range of medium to weak electric fields, and becomes monaural in a weak electric field. Similarly, according to characteristic curve E, in the range from medium electric field to weak electric field, the S/N characteristic shifts to the S/N characteristic of A when demodulating stainless steel, and furthermore, in a weak electric field, the S/N characteristic of B when demodulating monaural changes to a more improved S/N characteristic. I understand that it is familiar.

[Brief explanation of the drawing]

Figure 1 shows the S/N of the FM multiplex demodulator.
A graph showing a characteristic curve and a channel separation characteristic curve, Fig. 2 is a diagram showing a conventional noise removal method during FM reception, Fig. 3 is a block diagram showing the principle of the present invention, and Fig. 4 is a diagram showing the conventional noise removal method during FM reception. FIG. 2 is a circuit diagram implementing an embodiment.

Claims (1)

[Claims] 1 means for obtaining left and right channel signal currents from an FM composite signal, a current input terminal to which said left channel signal current is supplied, and a first and second
a first current mirror circuit having a current output terminal; a second current mirror circuit having a current input terminal to which the right channel signal current is supplied; and third and fourth current output terminals; and a third current mirror circuit having a sixth current output terminal, means for commonly connecting the first and third current output terminals to couple to the current input terminal of the third current mirror circuit, and the second current mirror circuit having a sixth current output terminal. and first and second differential pair transistors each driven by a current obtained from the fourth current output terminal, and third and second differential pair transistors each driven by current obtained from the fifth and sixth current output terminals. 4 differential pair transistors, a base common connection point of one transistor of the first to fourth differential pair transistors and a base common connection point of the other transistor of the first to fourth differential pair transistors; means for applying a signal indicative of received field strength between the left and right channel output terminals, the collector of the one transistor of the first differential pair transistors and the other transistor of the third differential pair transistors; means for commonly connecting the collectors and coupling them to the left channel output terminal;
An FM multiplex demodulation circuit comprising means for commonly connecting the collectors of the one transistor of the differential pair transistors and the collectors of the other transistor of the fourth differential pair transistors to the right channel output terminal.