JPS6239576B2 - - Google Patents

Description

[Detailed Description of the Invention] Signal that is the sum of left side signal L and right side signal R (L+R)
The signal is amplitude modulated, and the difference signal (L-R) and the pilot signal P are angularly modulated, for example, frequency modulated.
To receive AM stereo signals, the receiver may be configured as shown in FIG.

In the figure, 1 is a high frequency amplification circuit, 2 is an intermediate frequency amplification circuit, and 3 is an AM detector, which detects a sum signal (L+R) which is a main channel signal. Also,
A part of the intermediate frequency signal is supplied to the FM demodulator 5 through the limiter 4, and a difference signal (LR), which is a sub-channel signal, is demodulated. The sum signal (L+R) and the difference signal (L-R) are matrixed in a matrix circuit 6, and a left side signal L is output from one side.
A right side signal R is output from the other side. 7 and 8 are amplifiers.

Reference numeral 9 denotes a detection circuit for the pilot signal P, whose detection output controls a switching circuit 10, and supplies a difference signal (L-R) to the matrix circuit 6 only during stereo reception.

Now, in such an AM stereo receiver, when the reception frequency is shifted higher or lower than the channel frequency during stereo reception, it is input to the limiter 4. Since the level of the intermediate frequency signal decreases, if the level at that time is less than the limiter level, an AM component will be included in the limiter output, and when this AM component is demodulated, it will become burst noise. If the detection level change of the intermediate frequency signal is as shown in Fig. 2 with respect to the channel frequency f ₀ , the burst noise will be generated in the sections Ta and Tb.
Occurs where the level changes suddenly, such as in The interval To indicates the pulse width of the pilot detection output.

When controlling the demodulation output transmission line using the detection output of the pilot signal, for example, when sweeping the receiving frequency from lower than the channel frequency,
If this sweep is performed at a relatively fast speed, the receiving frequency will pass through the section Ta shown in Fig. 2 a moment earlier than the timing at which the pilot signal is detected and the demodulated output transmission line is closed using the detected output. The burst noise that occurs in section Ta is subjected to a kind of muting in the pilot detection output.
This burst noise can be removed without using any special means.

However, the frequency of the pilot signal P is 5 to 25Hz.
Because of this low frequency, the detection time of the pilot signal P takes several times as long as the detection time of audio information, so when the reception frequency sweep speed is fast, the burst noise generated in the section Tb during detuning is not muted.

Even if the pulse width of the pilot detection output is selected to be narrow, for example, a pulse width corresponding to the interval Tc, the burst noise at detuning cannot be muted because the responsiveness of the pilot detection output is poor.

When the reception frequency is swept from higher than the channel frequency, the burst noise in the section Ta on the detuning side is not muted as well.

Therefore, the present invention creates a muting signal for burst noise from a narrow pulse output centered on the tuning frequency fo formed based on the FM demodulation output and the pilot detection output, thereby suppressing the burst noise that occurs during detuning. This makes it possible to mute burst noise.

FIG. 3 is a system diagram showing an example of the present invention, and 20 is a muting circuit for burst noise. 21 is a circuit for forming the pulse output P _N , and the FM demodulation output (S
It is composed of a waveform shaping circuit 22 and an AND circuit 23, which are supplied with _a signal _S (see FIG. 6) is obtained. The pulse width Wa is selected within a range in which burst noise does not occur, and therefore corresponds to the section Tc in Fig. 4G, and the pilot detection output P _P
(Fig. 4E) is naturally narrower than the pulse width Wc.

The AND circuit 23 has an output for a signal meter (S meter) outputted from the detection circuit 3;
That is, the detected output S _I (FIG. 6G) is supplied to form the pulse output P _N shown in FIG. 6H. This pulse output P _N is supplied to the NAND circuit 24 together with the pilot detection output P _P to form a muting signal S _M shown in FIG. 4H.

Therefore, if the transistor Q _M as a muting element is controlled by this muting signal S _M , the demodulated output transmission path is closed only in the section Tc that includes the channel frequency fo, and the other sections Ta and Tb are opened, and the signal is muted. will be carried out. That is, the sub-channel signal (LR) is only supplied to the matrix circuit 6 when the pilot signal P _P indicating stereo broadcasting is present and the FM demodulated output S _S is obtained. Therefore, burst noise can be completely muted when detuning.

In other words, the pulse output P _N is obtained according to the slowness of the frequency sweep speed, so if the received frequency falls within the interval Tb at the time of detuning, the pulse output P _N immediately falls and enters the mutating mode. It is from. Therefore, the left side signal L or the right side signal R after the matrix does not include the bursty noise S _N as shown in FIG. 4F. The output is as shown by G in the figure.

By this muting operation, the interval Ta.Tb
, the mode is forcibly switched to monaural mode, and the
Only Tc is in stereo mode.

In FIG. 3, 30 is a tuning meter, 31 is an S meter, and 32 is a drive circuit for the S meter 31. Further, 33 is a band pass filter for extracting the pilot signal _P , and 35 is a stereo display circuit having a display lamp 36 and its control transistor Q _{P. P}
is controlled.

40 is a muting signal forming circuit for mutating the audio signal after matrixing, which includes a waveform shaping circuit 41 and an AND circuit 42.
The waveform shaping circuit 41 has a shaped output P _A
A shaped output P _B (Fig. 6 I) having a waveform similar to that is output, and this shaped output P _B and the detection output S _I
The AND output P _W is used as a muting signal. SW is a muting release switch and is operated manually.

FIG. 5 shows a specific example of the waveform shaping circuit 22.
The FM demodulated output S _S is supplied to a transistor Qa constituting a slicing circuit 44 for slicing the positive output, and the output above the slicing level la (FIG. 6A) determined by resistors 45a and 45b is sliced. , the slice output Sa shown in FIG. 6B is obtained at the collector of the transistor Qa. The FM demodulation output S _S is further processed by a transistor Qb that constitutes a slice circuit 46 that slices the output on the negative side.
The output below the slice level lb (FIG. 6A) determined by the resistors 47a and 47b is sliced, and the sliced output Sb shown in FIG. 6C is obtained at the collector of the transistor Qb.

Reference level V of slice output Sb on the negative side
– is the reference level of the slice output Sa on the positive side V ₊
, the slice output Sb is further supplied to a transistor Qc to form a slice output S'b (D in the same figure) whose reference level is level-shifted to 0 volts, and this is further supplied to a variable resistor for level shifting. 48 to form a slice output S″b level-shifted to V ₊ .

This slice output S″b is the other slice output Sa
and is supplied to the combiner 49, so that the transistor Qd
A composite output shown in FIG. 6F, that is, a shaped output P _A is obtained from the emitter. Therefore, by ANDing the shaped output P _A and the detected output S _I , the desired pulse output P _N can be generated.

As is clear from this, the slice level
Since the pulse width Wa of the pulse output P _N is determined by la and lb, the slice levels la and lb are determined so that this pulse width Wa coincides with the interval Tc.

Therefore, by configuring the other waveform shaping circuit 41 as shown in FIG. 5 and slightly increasing the absolute values of the slice levels la and lb, the shaping output _P to obtain the muting signal P W having a predetermined pulse width Wb can be obtained. _B can be formed.

As explained above, according to the present invention, it is possible to reliably mute burst noise during stereo reception, so that unpleasant sounds are not emitted during detuning. In addition, as in this embodiment, the detection output S
When forming the pulse output P _N using _I ,
Since the pulse output P _N cannot be obtained when the input electric field strength becomes low, in this case even if the mode is stereo mode, the mode is forcibly switched to monaural mode, so that high-quality stereo broadcasting can always be heard.

Note that in the above embodiment, an intermediate frequency signal may be used instead of the detection output S _I. In addition, in the above embodiment, the difference signal (L-R) is FM-modulated AM stereo signal, but PM-modulated AM stereo signal
The present invention can also be applied to a stereo signal receiver.

[Brief explanation of the drawing]

Fig. 1 is a schematic system diagram of an AM stereo receiver, Fig. 2 is a diagram for explaining the operation, Fig. 3 is a system diagram showing an example of the AM stereo receiver according to the present invention, and Fig. 4 is its operation. Waveform diagram for explanation, 5th
The figure is a connection diagram showing an example of a waveform shaping circuit, and FIG. 6 is a waveform diagram for explaining its operation. 3 is a detector, 4 is a limiter, 5 is an FM demodulator,
9 is a pilot signal detection circuit, 10 is a switching circuit, 20 and 40 are muting circuits, 2
1 is a pulse output P _N forming circuit, S _S is an FM demodulation output, S _I is a detection output, and S _M and P _W are mutating signals.

Claims (1)

[Claims] 1. In a receiver that receives an AM stereo signal that is amplitude-modulated with the sum signal of the left side signal and the right side signal, and angle-modulated with the difference signal thereof and a pilot signal consisting of a low frequency of several tens of Hz, A muting element is connected to the signal transmission path,
A detection output of a pilot signal obtained by passing the output of the demodulator through a bandpass filter, a pulse output narrower than the detection output obtained by obtaining the output of the demodulator through a waveform shaping circuit, and a detection output of the sum signal. An AM stereo receiver in which a muting signal is formed and the muting element is controlled by the muting signal.