JP3109531B2 - FM demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulator.
More particularly, the present invention relates to a circuit that reduces switching noise of an audio signal caused by the presence or absence of reception.

[0002]

2. Description of the Related Art With respect to a conventional FM demodulator, a description will be given of a U.S. voice multiplexing system in which FM signals are intermittently transmitted.

FIG. 3 shows an SAP in the US voice multiplex system.
(Second Audio Program) It is a block diagram of an example of an audio demodulation device. Frequency spectrum of the US audio multiplexing method, as shown in FIG. 4, ± SAP speech about the five times the horizontal synchronizing signal frequency f _H 10KH
This is an FM signal having a frequency deviation of z. The demodulation characteristic in this case is a so-called S curve shown in FIG.

In this configuration, F from input terminal 21
The M input signal passes through a BPF 1 (bandpass filter), is demodulated by a PLL detector 6, and a signal is detected by a synchronous detector 7. The output matrix unit 10 is a block for switching an output signal, and includes a monaural, stereo, S
A signal from an AP or the like is selected and output. Control of these output signals can be performed from the manual switch (SW) unit 9, but there is no stereo signal depending on the reception state.
When there is no P signal or when the noise is large due to a weak electric field in the case of an antenna signal, the SAP is automatically switched from stereo to stereo and from SAP to monaural. It is also possible to select a mute of voice (unvoiced state).

[0005]

In this conventional demodulator, switching noise always occurs at the time of the mode switching described above.
In particular, when the SAP signal is suddenly sent in the SW state in which the SAP signal is selected, a large noise occurs. This SAP signal is annoying because it is not a broadcast that is always transmitted, even if it is a program or CM.

[0006] This noise can be expressed by S 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 VCO 4 and is different from K given by 5 · f _H (= 78.7 KHz), which locks upon reception of the SAP signal. This difference is output as switching noise.

This increases as the deviation of the free-run frequency increases. To prevent this frequency shift, VC
There is also a method to adjust the free-run frequency of O4,
For that purpose, an external variable resistor is required, and it takes time and effort to adjust the variable resistor, which is disadvantageous for downsizing.

Normally, these adjusting devices have been integrated into an integrated circuit. In particular, in an integrated circuit incorporating a filter, there is a configuration in which the free-run frequency of the VCO is adjusted in conjunction with the filter adjustment. Is determined by the relative accuracy inside the integrated circuit, and an error of about ± 3% occurs. Usually, the demodulated output is 500V _rms at 100% (1.4Vp-
p) is required and Δf is 10 KHz (corresponding to 0.7 V)
Is 100%, so 70m
It becomes V noise. In addition, the frequency fluctuates by 3%,
The noise becomes 8.7 × 0.03 × 70 mV / KHz = 165 mV. This is a noticeable sound.

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

[0010]

According to the present invention, an FM receiver comprises:
An FM demodulator for demodulating (frequency-modulated) audio signals, a carrier signal generating circuit for generating only the same FM carrier signal as the FM signal,
An input detection circuit for detecting the presence or absence of the M signal; and the FM detection circuit when the input detection circuit detects the input of the FM signal.
In a state where the signal demodulation is performed and the input of the FM signal is not detected, the signal from the carrier signal generation circuit is transmitted to the FM signal.
A switching circuit for inputting the signal to a signal demodulation unit.

[0011]

FIG. 1 is a block diagram of an FM demodulator according to one embodiment of the present invention. In this embodiment, the input terminal 2 of the FM signal
1 other input terminal 26 of the f _H signal (= 15.73KHz) is provided. Free-running frequency of the SAP is 5f _H
After forming the waveform with the limiter amplifier 11, 5f
The BPF12 of _H, is taken out signals 5f _H (78.7KHz).

When 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 switches SW
13 is connected to the H side, PLL detection unit 6 moves in synchronism with the signals 5f _H, Therefore, always the lock state is maintained, the output DC of SAP demodulated output terminal 26 5
Is obtained at the point K. Therefore, there is almost no switching noise.

[0013] Usually, since the signal f _H is easily obtained by the oscillation circuit, no problem. Although circuit blocks for constructing these components are added, if they are integrated, they do not hinder miniaturization.

[0014]

As described above, the present invention has an effect that the switching noise generated when the mode is switched according to the reception state can be reduced.

[Brief description of the drawings]

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

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

FIG. 3 is a block diagram of a conventional FM demodulation device.

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

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

[Explanation of symbols]

Reference Signs List 1 5 · f _H BPF 2 Limiter amplifier 3 Phase comparator 4 VCO 5 LPF 6 PLL detector 7 Synchronous detector 8 Logic unit 9 External SW 10 Output matrix unit 11 Limiter 12 5 · f _H BPF 13 Switching SW 14 FM carrier signal generator 21 FM signal input terminal 22 L + R (mono) input terminal 23 L-R (sub signal) input terminals 24 and 25 an output terminal 26 f _H input terminal

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 1/10-1/14 H04H 5/00 302 H04N 5/60 102 H03D 3/02

Claims (1)

(57) [Claims] 1. An FM demodulator for demodulating an FM (frequency modulated) audio signal, comprising:
A carrier signal generation circuit for generating only an M carrier signal, an input detection circuit for detecting the presence or absence of the input FM signal, and demodulation of the FM signal when the input detection circuit detects the input of the FM signal A switching circuit for inputting a signal from the carrier signal generation circuit to a demodulation unit of the FM signal when the input of the FM signal is not detected.