JPH04304033A - Fm demodulator - Google Patents

Abstract

PURPOSE:To reduce a switching noise caused at FM reception by applying a signal the same as an FM carrier signal to a demodulation section in the absence of an input FM signal. CONSTITUTION:A carrier signal generating circuit 14 generates a same carrier signal as that of an FM signal. An input detection circuit 7 detects the presence of the inputted FM signal. A changeover circuit 13 demodulates the input FM signal when the input detection circuit 7 detects an input and inputs a signal of a carrier signal generating circuit 14 to an FM demodulation section when the input detection circuit 7 does not detect an input.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an FM demodulator.
In particular, the present invention relates to a circuit that reduces switching noise of audio signals caused by the presence or absence of reception.

0002

2. Description of the Related Art Regarding a conventional FM demodulator, an explanation will be given of the American audio multiplexing system when FM signals are sent intermittently.

FIG. 3 shows SAP (
FIG. 2 is a block diagram of an example of a second audio program (second audio program) audio demodulation device. The frequency spectrum of the US audio multiplexing system is as shown in Figure 4.
The audio is ±10 around 5 times the horizontal synchronization signal frequency fH.
It is an FM signal with a frequency deviation of KHz. The demodulation characteristic in this case is a so-called S curve shown in FIG.

In this configuration, F from the input terminal 21
The M input signal passes through a BPF 1 (band pass filter), is demodulated by a PLL detector 6, and is detected by a synchronous detection section 7. The output matrix section 10 is a block for switching output signals, and is a block that switches output signals, and outputs monaural, stereo, S
Selects and outputs signals such as AP. Control of these output signals can be performed from the manual switch (SW) section 9, but depending on the reception condition, there may be no stereo signal.
When there is no P signal, or when the antenna signal has a weak electric field and a large amount of noise, the SAP is automatically switched to stereo, or from SAP to monaural. It is also possible to select mute (silent state) of the audio.

[0005]

SUMMARY OF THE INVENTION In this conventional demodulator, switching noise always occurs during the mode switching described above. Particularly, when the SAP signal is suddenly sent in the SW state where the SAP signal is selected, a large noise occurs. Since this SAP signal is not constantly transmitted even in the case of programs and commercials, it becomes annoying to the ears.

[0006] This noise, in the absence of the SAP signal,
This is because the output of the AP is determined by the DC voltage L determined by the free run frequency of the VCO4, which is different from the K given by 5 fH (=78.7 KHz) that locks when receiving the SAP signal. The difference is output as switching noise.

[0007] This becomes larger as the free run frequency deviation becomes larger. To prevent this frequency shift, VC
There is also a way to adjust the free run frequency of O4,
For this purpose, an external variable resistor is required, and its adjustment takes time and effort, which is disadvantageous for miniaturization.

[0008] Normally, these adjustment devices are increasingly integrated into integrated circuits, and in particular, in integrated circuits that also incorporate filters, there are some that are configured to adjust the free run frequency of the VCO in conjunction with this filter adjustment. , is determined by the relative accuracy within the integrated circuit, and an error of about ±3% occurs. Normally, the demodulation output is 500Vrms at 100% (1.4V
p-p) is required, and 10KHz (equivalent to 0.7V) of △f is 100%, so with a frequency deviation of 1KHz, 7
It becomes 0mV noise. Also, the frequency varies by 3%,
The noise is 78.7×0.03×70mV/KHz=165mV. This is a very audible sound.

[0009] The purpose of the present invention is to eliminate such drawbacks,
An object of the present invention is to provide an FM demodulator with reduced switching noise.

0010

[Means for Solving the Problems] The configuration of the present invention is based on FM (
An FM demodulator that demodulates a frequency-modulated audio signal includes a carrier signal generation circuit that generates only an FM carrier signal that is the same as the FM signal;
an input detection circuit that detects the presence or absence of a signal, and when the input detection circuit detects the input of the FM signal, demodulates the FM signal; when the input detection circuit does not detect the input of the FM signal, it demodulates the signal from the carrier signal generation circuit; and a switching circuit that inputs the signal to the FM signal demodulation section.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of an FM demodulator according to an embodiment of the present invention. In this embodiment, the FM signal input terminal 2
1, an input terminal 26 for an fH signal (=15.73 KHz) is provided. SAP free run frequency is 5f
H, and after waveform shaping by the limiter amplifier 11,
With BPF12 of 5fH, 5fH (78.7KH
z) signal is extracted.

[0012] While the SAP signal is being sent to the input terminal 21, the switch (SW) 13 is closed to the I side. If there is no SAP signal, the logic circuit 8
13 is connected to the H side, and the PLL detection unit 6 moves in synchronization with the 5fH signal. Therefore, the locked state is always maintained, and the output DC of the SAP demodulation output terminal 26 is at point K in FIG. The value of is obtained. Therefore, switching noise is almost eliminated.

Normally, the fH signal is easily obtained by an oscillator circuit, so it does not pose a problem. Additional circuit blocks will be added to configure these, but
If it is integrated into an integrated circuit, it will not be a hindrance to miniaturization.

[0014]

As explained above, the present invention has the effect of reducing the switching noise that occurs when the mode is switched depending on the receiving condition.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a signal waveform diagram of each part shown in FIG. 1.

FIG. 3 is a block diagram of a conventional FM demodulator.

FIG. 4 is a baseband spectrum diagram of US audio multiplex broadcasting.

FIG. 5 is a demodulation characteristic diagram of the FM demodulator.

[Explanation of symbols]

1 5.fH BPF 2 Limiter amplifier 3 Phase comparator 4 VCO 5 LPF 6 PLL detection section 7 Synchronous detection section 8 Logic section 9 External SW 10 Output matrix section 11 Limiter 12 5.fH BPF 13 Switching SW 14 FM carrier signal generator 21F
M signal input terminal 22 L+R (monaural) input terminal 23
LR (sub signal) input terminals 24, 25 Output terminal 26 fH input terminal

Claims (1)

[Claims] 1. An FM demodulator that demodulates an FM (frequency modulated) audio signal, comprising: a carrier signal generation circuit that generates only an FM carrier signal that is the same as the FM signal; An input detection circuit for detecting, and when the input detection circuit detects the input of the FM signal, demodulates the FM signal, and when the input detection circuit does not detect the input of the FM signal, the input detection circuit demodulates the signal from the carrier signal generation circuit. An FM demodulator comprising: a switching circuit for inputting an FM signal to a demodulator.