JPH0261825B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stereo demodulator, and particularly to stereo separation adjustment thereof.

Generally, in the detection output output from the FM detection circuit of an FM tuner, the sub signal component, which is a high frequency component, is attenuated from the main signal component due to the phase characteristics of the FM detection circuit, resulting in poor stereo separation in the stereo demodulation circuit. It is known to do.

Conventionally, in order to correct deterioration of stereo separation, gain adjustment of a sub signal and a main signal is performed in a stereo demodulation circuit. 1st
The figure shows a stereo decoder of a stereo multiplex demodulation circuit using this gain adjustment method.
This stereo decoder 2 includes transistors 4, 6,
8, 10, 12, 14, 16, 18, 20, 2
2, 24, 26, resistance 28, 30, variable resistance 32
and a constant current source 34. A 19KHz switching signal is applied from the stereo switch circuit 36 to the bases of the transistors 12, 14, 16, and 18, and the transistors 20, 22, 24, and
A stereo detection output is given to the base of 26 from an amplifier 38. The power supply terminal 40 is connected to the power supply Vcc, and the R signal output terminal 42 and the L signal output terminal 4
A right signal and a left signal are respectively taken out from 4. The terminal 46 is a terminal for adjusting stereo separation, and the variable resistor 32 connected to this terminal 46
is added to adjust the main signal gain.
In the stereo demodulation circuit including the stereo decoder 2 configured as described above, each terminal 4...34 is configured with a semiconductor integrated circuit (IC) except for the variable resistor 32 externally attached to the terminal 46. The main signal gain adjustment for the main signal gain adjustment is performed by adjusting the resistance ratio of resistors 28 and 30, which are monolithic resistors inside the IC, and a variable resistor 32 outside the IC, which have different characteristics. For this reason, the resistance 28 due to non-uniform characteristics of the IC,
It is difficult to adjust the gain of the sub signal and the main signal due to the variation in the value of 30, and this gain adjustment type stereo demodulation circuit has the disadvantage that stable separation cannot be obtained.

Furthermore, conventionally, there is a method of performing high frequency correction on the input signal of the stereo demodulation circuit. FIG. 2 shows this method, in which a high-frequency filter 56 consisting of a resistor 52 and a capacitor 54 is inserted between the FM detection circuit 48 and the stereo demodulation circuit 50 to perform high-frequency correction. Compensates for attenuation of signal components. According to this method, while theoretically it is possible to compensate for the attenuation of sub-signal components, the low-frequency signal gain is determined by the ratio of the external resistor 52 and the input impedance of the stereo demodulation circuit 50; If you try to adjust the stereo separation by adjusting the resistor 52 to vary the characteristics of the high-pass filter, the input level of the stereo demodulation circuit will change, and in the end you will not be able to achieve stable stereo separation. I can't.

An object of the present invention is to provide a stereo demodulation device that can obtain good stereo separation through simple adjustment, and the sub-signal demodulation gain and main signal demodulation gain of the stereo demodulation circuit are set to constant gains. This is characterized in that the time constant of the low-pass filter provided in the preceding stage is variable.

Embodiments of the present invention shown in the drawings will be described below.

FIG. 3 shows a preferred embodiment of the stereo demodulator of the present invention. In the figure, input terminal 60
An intermediate frequency output is input from an intermediate frequency amplifier circuit (not shown), and the detection output of the FM detection circuit 62 is sent to the stereo demodulation circuit 6 via a low-pass filter 64.
6 is input. In this stereo demodulation circuit 66, the right signal and left signal obtained by demodulation are individually output from a right signal output terminal 68R and a left signal output terminal 68L.

The low-pass filter 64 is inserted to correct the high-frequency phase of the FM detection output and improve the stereo separation of the stereo demodulation output. In this embodiment, the low pass filter 6
4 is composed of a variable resistor 70 and a capacitor 72, and the time constant of the low-pass filter 64 is set by the value of the variable resistor 70. That is, by changing the time constant using the variable resistor 70, the stereo separation is set to the highest state.

The stereo demodulation circuit 66 is a semiconductor integrated circuit (IC)
The main signal demodulation gain and the sub signal demodulation gain are set to constant values only by the constituent elements inside the IC. FIG. 4 shows a specific circuit diagram of the stereo decoder 74 included in the stereo demodulation circuit 66. This stereo decoder 7
4 is transistor 76, 78, 80, 82, 8
4,86,88,90,92,94,96,9
8. Resistors 100, 102, 104 and constant current source 1
It consists of 06. Transistor 76...9
8 and the constant current source 106 have the same configuration as the decoder 2 shown in FIG.
4, 86, 88, and 90 are supplied with a 19KHz switching signal from the stereo switch circuit 36, and transistors 92, 94, 96, and 9
The FM detection output low-pass filter 64 of the FM detection circuit 62 is input to the base of the FM detection circuit 62 via the amplifier 38. The power supply terminal 108 is connected to the power supply Vcc, and the right signal output terminal 68R and left signal output terminal 68L correspond to those shown in FIG.
In the stereo decoder 74 configured in this way, the resistors 100, 102, and 104 are composed of monolithic resistors that are components on an integrated circuit.
The main signal demodulation gain is set to a constant value by a resistor 100, and the sub signal demodulation gain is set to a constant value by resistors 102 and 104, respectively. Regarding the magnitude relationship of each setting value, the sub signal demodulation gain is set to be a larger value than the main signal demodulation gain. That is, when a stereo signal with a normal distributed phase is applied to the input of the stereo demodulation circuit 66, the ratio between the main signal demodulation gain Gm and the sub signal demodulation gain Gs that provides the best stereo separation in the demodulated output is used as a reference. A main signal demodulation gain Gm is set, and a set value Gso of the sub signal demodulation gain is set to a value larger than the gain Gs (Gso>Gs). The best stereo separation is obtained when the main signal level and sub signal level are the same.
Therefore, theoretically, the respective gains are Gm and
It is sufficient to set it to Gs, but in reality it is difficult to obtain the same gain as the set value due to variations in manufacturing. Therefore, the present invention sets the sub signal demodulation gain setting value to Gso instead of Gs, and makes the sub signal level and main signal level the same by using a low pass filter with an adjustable time constant that can attenuate the sub signal. It is.

Note that the cutoff frequency of the low-pass filter can be set to various values as long as it can attenuate the sub-signal without affecting the audio frequency; for example, 20 KHz is one of the suitable cutoff frequencies.

With the above configuration, the sub-signal demodulation gain is corrected to a constant value by the monolithic resistors 100, 102, and 104 inside the IC, so it is stable with little variation. On the other hand, phase correction of the FM detection output is performed by a low-pass filter 64. The resistance value R of the resistor 70 constituting the low-pass filter 64 can be set sufficiently small compared to the input impedance Zin of the stereo demodulation circuit, so even if the resistor 70 is adjusted and phase correction is performed, the input level of the stereo demodulation circuit 66 remains unchanged. It doesn't change much. Therefore, phase correction of the FM detection output can be performed only by adjusting the variable resistor 70.

As explained above, according to the present invention, the sub signal demodulation gain and the main signal demodulation gain are corrected by setting the sub signal demodulation gain of the stereo demodulation circuit to a higher value than the main signal demodulation gain, and the FM detection output Since the phase correction was made by adjusting the time constant of the low-pass filter, the demodulation gain correction and the phase correction can be corrected without affecting each other, and therefore stable stereo separation can be achieved. It is possible to maintain good stereo separation and obtain good stereo separation with simple adjustment.

[Brief explanation of the drawing]

Figure 1 shows the decoder of a conventional stereo demodulation circuit.
A circuit diagram showing an IC equivalent circuit. Figure 2 is a block diagram showing a conventional stereo separation adjustment device.
FIG. 3 is a block diagram showing an embodiment of the stereo demodulation apparatus of the present invention, and FIG. 4 is a circuit diagram showing an IC equivalent circuit of a decoder of the stereo demodulation circuit. 64...Low pass filter, 66...Stereo demodulation circuit, 70...Variable resistor, 72...Capacitor, 10
0, 102, 104...resistance.

Claims (1)

[Claims] 1 A stereo demodulation circuit in which the sub-signal demodulation gain is set to a constant value higher than the main signal demodulation gain, and a sub-signal demodulation gain and main signal demodulation gain of the demodulation circuit installed in the preceding stage of this stereo demodulation circuit. 1. A stereo demodulator comprising: a low-pass filter with an adjustable time constant capable of attenuating a sub-signal with respect to a main signal based on the difference between the sub-signals and the main signal.