JPS5920211B2 - AM stereo receiver muting device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a muting device for a stereo receiver, and more particularly to a muting device for a receiver that receives AM-PM stereo AM broadcasting.

In the AM stereo system, the AM-PM system phase-modulates a carrier wave using a sub-signal that is a difference signal between left and right channel signals, and then amplitude-modulates it using a main signal that is a sum signal of both channel signals.

In such an A-PM system receiver, the main signal is detected by an envelope frequency detector that extracts the amplitude fluctuation component of the carrier signal, and the amplitude fluctuation component of the carrier J transmission signal is removed by a limiter. The sub-signal is detected by a post-PM detector, and the main and sub-signals are matrixed and separated and derived as left and right channel signals.
There is a phase detection method using a so-called PLLj (phase locked loop) circuit as a sub-signal PM detector.

In such a system, when the input signal level falls below a specified value and the PLL circuit is in an unlocked state, the PLL
If the free-running frequency of the circuit's VCO (voltage controlled oscillator) and the input signal frequency deviate by more than a predetermined value and the PLL circuit is locked, or if there is no stereo pilot signal and monaural broadcasting, etc., the sub signal A lot of noise is generated in the detection output line, which results in unpleasant noise.
SUMMARY OF THE INVENTION An object of the present invention is to provide a muting device for an AM stereo receiver that can eliminate unpleasant sounds generated in each of the above cases and generate good reception output.

The muting device in the AM stereo receiver of the AM-PM system according to the present invention generates a first detection signal when the AM stereo signal level is below a predetermined value, and
A second detection signal is generated when the circuit is unlocked, and the PL
When the free-running oscillation frequency of the VCO of the L circuit and the AM stereo signal frequency deviate by more than a certain value, a third detection signal is generated, and furthermore, the stereo pilot signal in the AM stereo signal is detected and this stereo pilot signal is detected. The fourth detection signal is generated when there is no signal, and the detection output is muted in response to the generation of at least one of the first to fourth detection signals.

The present invention will be explained below based on the drawings.

FIG. 1 shows a circuit block according to an embodiment of the present invention.
- The AM stereo signal input after PM is amplitude limited by limiter 1, amplitude fluctuations are removed from the main signal, and the signal is introduced into a phase detector having a PLL circuit configuration shown in 2.

The PLL circuit 2 has a well-known configuration and includes a 900 phase comparator 22 that compares the frequency and phase of the input of the limiter 1 and the oscillation signal of the VCO 2l, and the comparison output is sent to the i-loop filter 23 and the DC amplifier 24. The frequency and phase of the VCO 2l are controlled through the . This
' AM-PM stereo signal e is expressed by the following formula.

To this, L. R is the left and right channel signal, k1 is the AM modulation degree, K2 is the PM modulation degree, and ω.

is the angular frequency of the carrier wave, P is the angular frequency of the pilot signal (
2πX5), and A indicates the amplitude of the pilot signal. Therefore, the output e'c of limiter 1 is given by the following equation. And the oscillation signal of VCO2l is SinO).

If t, the output of the 90V phase comparator 22 will provide a signal corresponding to the phase difference between the two input signals, that is, K2(L-R)+ACOSPt. Since this output indicates the sub-signal, the output signal of the phase comparator 22 is derived as the sub-signal detection output. By detecting the output level of the comparator 22, that is, the output level of the sub signal, it is possible to determine whether the PLL circuit is locked or not. If not, a non-lock detection signal is generated and applied to the mutating TOl leg circuit 4.

By detecting the DC component according to the phase difference by the rube filter 23' of the PLL circuit 2, the deviation △f between the limiter output frequency and the free-running oscillation frequency of the VCO 2l can be detected. It is provided as an input to the muting control circuit 4. FIG. 2 shows the relationship between the output DC level of the filter 23 and the disc number deviation Δf. Since both are in a proportional relationship, it is determined whether this DC level is within a predetermined level range or not. It becomes possible to judge whether the PLL circuit is locked within the frequency range of . Therefore, the frequency shift detector 5 has a so-called window comparator configuration. A level detector 6 is also provided to detect the input signal level, and when the input signal level decreases below a certain value, a detection signal is generated and applied to the muting control circuit 4.

Furthermore, a pilot signal level detector 7 is provided which detects the stereo pilot signal AcOsPt included in the output of the phase comparator 22 and generates a detection signal when this signal is absent, and this detection output is also sent to the muting control circuit 4. The applied temperature is ℃・ru. The control circuit 4 generates control signals for on/off control of the muting circuit 8 inserted in the sub-signal line in response to each of the four detection signals, and basically has an OR configuration. On the other hand, A
An envelope detector 9 inputting the M-PM stereo signal is provided to detect the main signal and apply it to the matrix circuit 10 together with the sub-signal detection output to separate left and right channel signals.

It can be seen that by doing this, all the unpleasant sounds during detuning, unpleasant sounds when the electric field strength decreases, and even unpleasant sounds during monaural broadcasting are removed.

In this case, since the output of the phase comparator 22, that is, the sub-signal detection output, is K2(LR)+AcOSPt as described above, it contains the pilot signal component (5 Hz).

Although this pilot signal component is attenuated at the output of the loop filter, it cannot be completely removed.
The frequency shift detector 5 will respond to this component.
Therefore, the input level of the frequency shift detector 5 may fluctuate by a 5 Hz component approximately centered on the reference level 1 of the window converter, and as a result, a so-called chattering phenomenon appears in the detection output, and mutating is performed accordingly. The circuit 8 will operate and generate unpleasant sounds. In order to eliminate this drawback, the input/output characteristics of the window comparator constituting the detector 5 are provided with hysteresis characteristics, resulting in detuned frequency detection characteristics as shown in FIG. The circuit no longer follows, and strange noises are prevented.

To be more specific, first, as shown in FIG. 4, the frequency deviation detector 5 is connected to a Schmitt trigger circuit 51, an inverter 52, another Schmitt trigger circuit 53 whose input is the output of the inverter 52, and the outputs of both Schmitt trigger circuits 51, 53. It is composed of AND gates 5 and 4 as inputs.

Then, the output of the loop filter 23 is applied to the Schmitt trigger circuit 51 and the inverter 52, respectively, and the gate output of the AND gate 54 is used as the frequency shift detection output. Since the input signal characteristics of this detector 5 are shown by the solid line in FIG. One threshold (reference level) V2 (one △F
2), the output of this circuit 51 inverts starting from a high level to a low level as shown in Figure b.

This state does not change even if the tuning is further detuned. Next, when the frequency increases from this state toward the center and reaches another threshold value V1 (corresponding to -Δf1) of the Schmitt trigger circuit 51, this. The output of the circuit 51 is inverted from a low level to a high level as shown in Figure e, and thereafter the high level does not change even if the frequency increases. FIG. d shows a comprehensive input/output characteristic of the Schmitt trigger circuit 51. Next, since the input signal characteristic shown by the solid line in FIG. E, f finally becomes g. still,
The two threshold values of this circuit 53 are also both V1 and V2.
Therefore, the output characteristics of the AND gate 54 are as shown in FIG. h, and the detection output characteristics shown in FIG. 3 are obtained. As described above, according to the present invention, it is possible to satisfactorily eliminate unpleasant sounds during tuning and detuning.

[Brief explanation of the drawing]

Figure 1 is a block diagram of an embodiment of the present invention, Figure 2 is a diagram showing the detuning frequency characteristics of a rube filter, Figure 3 is a diagram showing the detection characteristics of a frequency shift detector, and Figure 4 is a diagram showing frequency shift detection. FIG. 5 is a diagram illustrating the characteristics of the circuit shown in FIG. 4.

Claims (1)

[Claims] 1. A carrier wave whose phase is modulated by a sub-signal which is a difference signal between the first and second channel signals receives an AM stereo signal whose amplitude is modulated by the sum signal of the first and second channel signals. A muting device in an AM stereo receiver, wherein the lever signal detects the sub-signal using a phase-locked loop circuit,
means for generating a first detection signal when the level of the M stereo signal falls below a predetermined value; and means for detecting an unlocked state of the phase-locked loop circuit and generating a second detection signal; frequency deviation detection means for generating a third detection signal when the free-running frequency of the voltage controlled oscillator in the phase-locked loop circuit and the frequency of the AM stereo signal are separated by a predetermined value or more; means for detecting a stereo pilot signal included in the stereo signal and generating a fourth detection signal when the pilot signal is absent; and at least one of the first to fourth detection signals.
mutating means for mutating a detected output signal of only the sub-signal in response to one generated signal. 2. The frequency deviation detecting means comprises a window comparator that generates the third detection signal when the output level of the loop filter in the phase-locked loop circuit is outside a predetermined level range. equipment. 3. The device according to claim 2, wherein the input/output characteristics of the wound comparator have hysteresis.