JPS5929415Y2 - automatic switching circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic stereo/monaural switching circuit in an FM stereo receiver.

This type of conventional automatic switching circuit detects the level during tuning and switches the FM receiver to stereo when the level is above a predetermined level, and to monaural when the level is below a predetermined level. Therefore, even when detuned, if there is a strong input, a stereo state occurs, causing a problem in S/N.

Therefore, the present invention attempts to solve the above-mentioned problems by detecting not only the level but also the degree of tuning, and outputting a monaural switching signal when both conditions are not met. It is.

Below, questions will be asked regarding one embodiment of the present invention with reference to the figures.

In FIG. 2, the voltage Vf from an FM detector (not shown) having the S-curve characteristic shown in FIG. 1 is input to the inverting input terminal of one differential amplifier 1, and the non-inverting input terminal increases from the center frequency f! A voltage Vr1 corresponding to the detector output voltage when shifted by jfo is input as a comparison voltage.

Further, the above-mentioned voltage Vf is input to the non-inverting input terminal of the other differential amplifier 2, and the center frequency f is input to the inverting input terminal.

A voltage r2 corresponding to the output voltage of the detector when shifted by Ifo in the decreasing direction is applied as a comparison voltage.

This voltage ■r2 is the same as the voltage ■r1.

One of the differential amplifiers 1 has a diode D4 so that only a negative voltage output is taken out, and the other differential amplifier 2 has a diode D5 so that only a negative potential output can be taken out.

That is, the diode D4 becomes conductive when Vf>Vrl, lowering the potential at point A, and the light emitting diode D also lights up at that time.

Diode D5 becomes conductive when V f < -V r2 and lowers the potential at point A, and light emitting diode D2 also lights up at that time.

R1-R3 are protective resistors.

As a result of the above, the potential at point A is the center wave number f. A high potential is maintained only when the voltage is within the range of ±Afo.

Transistor Q1 receives an intermediate frequency (IF) signal voltage VL, and when the IF signal voltage VL is above a predetermined level, it enters an active region and can make the next stage transistor q active.

This transistor Q2 changes the potential at point A to light emitting diode D.
Therefore, this transistor Q2 becomes active and generates a predetermined voltage in the collector resistor R4 only when the IF signal vL is higher than a predetermined value and the potential at point A is high.

Transistor Q3 is given a predetermined bias by resistor R5, and becomes conductive when a predetermined reverse bias voltage is not generated across resistor R4.

Then, this transistor Q3 becomes MP due to its conduction.
Switch X to monaural state, and switch to stereo state by blocking.

In the above, when the reception state is out of tune by +If or more, as is clear from FIG. 1, Vf>Vr.

Therefore, the output of the differential amplifier 1 becomes negative, and the light emitting diode D1 lights up to indicate this state.

Also, at this time, the diode D4 becomes conductive, so the potential at point A decreases.

Therefore, even if transistor Q1 becomes active, transistor Q2 does not become active, and a predetermined reverse bias voltage is not generated across resistor R4, so transistor Q3 is not cut off.
MPX does not change from the monaural state.

That is, since it is in a detuned state, ■F signal voltage vL is normally at a low level, but even if there is some strong input and the voltage V□ reaches a predetermined value and transistor Q1 becomes active. Transistor Q2 is not active, transistor Q3 is not cut off, and MPX is not switched to stereo.

Furthermore, the light emitting diode D3 does not light up.

Next, when in the tuned state, V r 1>Vf>-V
r2, the outputs of both differential amplifiers 1 and 2 are positive, and therefore the light emitting diodes D1. Neither D2 lights up, and diode D4. D is not conductive, and the potential at point A becomes high.

Therefore, at this time, when the IF signal voltage (L) becomes large, both transistors Qi and Q2 become active, and the light emitting diode D3 is sufficiently illuminated by the current flowing through the transistor Q2, indicating that the light emitting diode D3 is in the tuned state.

Further, a predetermined reverse bias voltage is generated in the resistor R4 to cut off the transistor Q3.

Therefore, MPX is switched to stereo.

Next, when the receiving state is out of tune by -If or more, Vf
〈−■“2, so the output of the other differential amplifier 2 becomes negative.

For this reason, the light emitting diode D2 lights up to indicate that it is in a detuned state.

Further, the diode D5 becomes conductive and lowers the potential at the point A, so that the transistor Q3 is not cut off and MPX is not switched to stereo mode as in the detuned state described above.

From the above, according to the present invention, the MPX switches to the stereo state only when the input is at a predetermined level or higher during tuning,
When detuning, even if the input level is above a predetermined value, the MP
X no longer changes from its monaural state.

Therefore, the receiver is less likely to be in a noisy reception state, and can always obtain a good reception state.

In addition, in the present invention, in particular, FM having S-curve characteristics
The detected output from the detector is compared with a first comparison signal having a level corresponding to the detector output when a predetermined frequency deviates from the center frequency of the detector in the positive direction, and the detector output is compared with the first comparison signal. a first comparison means that generates a first signal when the signal level is equal to or lower than the signal level; and a first comparison means that generates a first signal when the detected output is equal to or lower than the signal level; a second comparing means which generates a second signal when the detected output is equal to or higher than the level of the second comparison signal; and means for generating a third signal indicative of a state of entrainment when both signals are present, and by simply setting the levels of the first and second comparison signals in the first and second comparison means. Since the key is detected, the specific frequency component is extracted using a filter that requires troublesome adjustment of circuit constants in order to select the specific frequency component, and the tuning is performed depending on whether the level exceeds a predetermined value. - Not only is the circuit configuration simpler than those that detect detuning, but it is also easier to adjust.

Furthermore, in the present invention, the operation of the first and second display elements for detuning display is controlled by each of the first and second signals, and the third and fourth signals are ANDed. , MPX is switched to a monaural state when either signal is not present, and MPX is switched to a stereo state when both signals are present, so tuning and separation are determined by the states of the first and second display elements. Many excellent practical effects can be obtained, such as being able to know the key, and even knowing the direction of detuning when the tune is out of tune.

[Brief explanation of the drawing]

FIG. 1 is an FM detection characteristic diagram, and FIG. 2 is a circuit diagram of an embodiment of the present invention. 1.2...Differential amplifier, V r 1* V r
2...Comparison voltage.

Claims (1)

[Scope of utility model registration request] In an FM stereo receiver, F with S curve characteristics
The detected output from the M detector is compared with a first comparison signal at a level corresponding to the detector output when a predetermined frequency deviates from the center frequency of the detector in the positive direction, and the detector output is compared with the first comparison signal. a first comparing means that generates a first signal when the level is equal to or lower than the level of the comparison signal; a second comparison means for comparing with a second comparison signal and generating a second signal when the detected output is equal to or higher than the level of the second comparison signal, and ANDing the first and second signals; means for generating a third signal indicating a synchronized state when both of the signals are present, and circuit means for generating a fourth signal when the IP signal from the IF circuit exceeds a predetermined level; and a second signal to respectively control the operation of the first and second display elements for detuning display, AND the third and fourth signals, and when one of the signals is absent, An automatic switching circuit that switches the MPX to the monaural state and switches ■■ to the stereo/off state when both signals are present.