JPH11274858A - Fm demodulation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an FM demodulation circuit capable of dealing with plural audio systems while keeping a high S/N in simple configuration. SOLUTION: An SIF signal is inputted to a BPF 1, an audio sub carrier is passed through the BPF 1, and a signal component outside an SIF band is suppressed. Then, the SIF signal passed through the BPF 1 is inputted to first and second FN detectors 2 and 4. At the second FM detector 4, the phases of the inputted SIF signal and a reference signal from a VCO 42 are detected and supplied to the VC 42 after the integration of detected outputs at an LPF 5, and the oscillation frequency of the VCO 42 is controlled. The signal from this LPF 5 is inputted to the first FM detection circuit 2 as well, the free run frequency of the VCO consisting of the first FM detection circuit 2 is varied based on this detected output, and a detection enable frequency range is shifted from side to side. Then, the band of the detected output from the first FM detection circuit 2 is limited by the LPF 3 and an audio signal is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulation circuit for receiving a television signal and demodulating an SIF signal in the television signal.

[0002]

2. Description of the Related Art In general, there are M, B, I and D types of audio systems in a television broadcasting system depending on the subcarrier frequency. The voice subcarriers of these schemes are 4.5 MH each.
z, 5.5 MHz, 6.0 MHz, and 6.5 MHz, and are FM-modulated within the audio signal band.

[0003] In order to detect these signals by PLL detection and to detect audio signals of all types, from 4.5 MHz to 6.5 MHz.
It is necessary to detect the frequency up to z.

FIG. 2 shows a conventional FM detection circuit. FIG. 3 shows the input / output characteristics of the FM detection circuit in FIG.

In FIG. 2, an SIF signal in a television signal is used to avoid the influence of a harmonic beat of a video signal or the like.
The BPF removes signals other than the above-mentioned subcarrier. The SIF signal that has passed through the BPF is subjected to FM detection by the first FM detector, and the detected output signal is converted into a baseband signal from which sub-carrier components have been removed. Limited and output.

[0006]

By the way, if the first FM detector capable of detecting a signal in the range from 4.5 MHz to 6.5 MHz as in the prior art is designed, a wide band BPF is required. The value cannot be made steep, and the gain of the signal in the lower and upper limits of the pass band of the BPF decreases. Further, when a certain method (for example, the B method) is detected, the pass band of the BPF is wide, so that signals in unnecessary bands other than 5.5 MHz are also passed. (Noise, interference signal, etc.) are input to the FM detector, and are susceptible to unnecessary signals, and the S / N ratio deteriorates.

The present invention has been made in view of the above-mentioned drawbacks, and has as its object to provide an FM signal demodulation circuit which has a simple configuration and does not deteriorate the S / N ratio.

[0008]

According to the present invention, there is provided an FM demodulation circuit for demodulating an audio intermediate frequency signal (hereinafter, abbreviated as an SIF signal) in a television signal. Abbreviated as BPF).
A first F for detecting the SIF signal passing through the BPF
M detector and a second FM detector, wherein the detectable frequency range of the first FM detector is set narrower than the detectable frequency range of the second FM detector, and the second FM detector
An FM demodulation circuit characterized in that a detectable frequency range of the first FM detector is varied by a detection output of a detector.

Further, the present invention provides an FM for demodulating an audio intermediate frequency signal (hereinafter abbreviated as SIF signal) in a television signal.
In the demodulation circuit, a band-pass filter (hereinafter abbreviated as BPF) for passing the SIF signal, a phase detection circuit and a low-pass filter (hereinafter abbreviated as LPF), and a voltage-controlled oscillator (hereinafter abbreviated as VCO). ) And a first FM detector and a second FM detector for detecting the SIF signal passing through the BPF, and the first FM detector has a detectable frequency range of the second FM detector. Wherein the free-run frequency of a VCO constituting the first FM detector is varied by a detection output from an LPF constituting the second FM detector. Is provided.

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
The signal demodulation circuit will be described.

FIG. 1 shows an embodiment of the present invention. In FIG. 1, an SIF signal is input to a BPF 1, which passes an audio subcarrier and suppresses a signal component outside the SIF band. The SIF signal that has passed through the BPF 1 is input to the first FM detector 2 and the second FM detector 4.

Here, the first FM detector 2 has the input / output characteristics shown in FIG. In other words, the frequency band in which the first FM detector 2 can detect the signal has a certain margin with respect to the frequency deviation defined in the broadcasting system, and has a narrower frequency range than the conventional FM detector. It is designed so that only the detection can be performed.

Next, the second FM detection circuit 4 will be described in detail. The second FM detection circuit 4 is a PLL system, and includes a VCO 42, a phase detector 41, and an LPF 5 as shown in FIG.

In FIG. 5, the input SIF signal is
The phase detection with the reference signal from the VCO 42 is performed, and the output signal from the phase detector 41 is integrated by the LPF 5. LPF
The DC voltage integrated at 5 is supplied to the VCO 42,
The oscillation frequency of the VCO 42 is controlled so that the oscillation frequency of the VCO 42 is synchronized with the phase and frequency of the SIF signal. Note that the relationship between the oscillation frequency and the oscillation frequency control voltage in the VCO 42 is as shown in FIG. Further, by changing the free-run frequency of the VCO 42, the detectable frequency range is shifted right and left as shown in FIG. Here, the free-run frequency means a center frequency Fc of a detectable frequency of the FM detection circuit.

As a result, the DC voltage from LPF 5 becomes FM
The detection output is input to the first FM detection circuit 2, and the free-run frequency of the VCO constituting the first FM detection circuit 2 is varied based on the detection output, and the detectable frequency range is shifted right and left. The detection output from the first FM detection circuit 2 is band-limited by the LPF 3, and an audio signal is output.

Here, L constituting the first FM detection circuit 2
The PF5 has the same characteristics as the LPF 5 shown in FIG.

The FM detection characteristic of the second FM detection circuit 4 is the same or similar to the FM detection characteristic shown in FIG. 2, and it is necessary to perform FM detection in a wider frequency range than the first FM detection circuit 2. Can be.

That is, in the second FM detection circuit 4, the FM modulation band in each system is smaller than the detectable frequency range, and the LPF 5 whose cut-off frequency is low enough to sufficiently remove the FM detection output signal is used. ,
A substantially DC voltage can be obtained as a detection output. This D
The value of the C voltage is Vm, Vb, Vi, and Vd in FIG. 6 depending on the audio subcarrier to be received. Note that, as shown in FIG. 7, the LPF 5 in FIG. 1 has the same characteristics as the LPF 5 in FIG. 5, and the cutoff frequency is set sufficiently lower than that of the LPF 3.

The DC voltage from the LPF 5 changes the free-run frequency of the VCO as shown in FIG. 6, and the DC voltage from the LPF 5 changes the variable frequency range of the first FM detection circuit to an appropriate range for each system. Can be designed.

As described above, even if the frequency range that can be detected by the first FM detection circuit 2 can be detected only in a range necessary for one system, the frequency range that can be detected is frequency offset according to each system. Thus, the audio signals of the M, B, I, and D systems can be demodulated.

[0021]

According to the present invention, an FM signal demodulation circuit capable of coping with various audio systems while maintaining a high S / N ratio with a simple configuration can be obtained by employing the above configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an FM signal demodulation circuit according to the present invention.

FIG. 2 is a block diagram showing a conventional FM signal demodulation circuit.

FIG. 3 is a diagram showing input / output characteristics of an FM detection circuit constituting a conventional FM signal demodulation circuit.

FIG. 4 is a first FM constituting the FM signal demodulation circuit of the present invention;
FIG. 4 is a diagram illustrating input / output characteristics of a detection circuit.

FIG. 5 shows a second FM which constitutes the FM signal demodulation circuit of the present invention.
FIG. 3 is a block diagram of a detection circuit.

FIG. 6 shows a first FM constituting the FM signal demodulation circuit of the present invention.
FIG. 4 is a diagram illustrating input / output characteristics of a detection circuit and a second FM detection circuit.

FIG. 7 shows an LPF 1 constituting the FM signal demodulation circuit of the present invention.
FIG. 4 is a diagram illustrating input / output characteristics of the LPF 2 and the LPF 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 BPF 2 1st FM detection circuit 3 LPF1 4 2nd FM detection circuit 41 Phase detector 42 VCO 5 LPF2

Claims (2)

[Claims] 1. An FM demodulation circuit for demodulating an audio intermediate frequency signal (hereinafter, abbreviated as SIF signal) in a television signal, a band-pass filter (hereinafter, abbreviated as BPF) for passing the SIF signal, and A first FM detector and a second FM detector for detecting the SIF signal passing through the BPF, wherein a detectable frequency range of the first FM detector is set narrower than a detectable frequency range of the second FM detector. An FM demodulation circuit, wherein a detectable frequency range of the first FM detector is varied according to a detection output of the second FM detector. 2. An FM demodulation circuit for demodulating an audio intermediate frequency signal (hereinafter abbreviated as an SIF signal) in a television signal, a band-pass filter (hereinafter abbreviated as a BPF) for passing the SIF signal, and a phase. The BPF includes a detection circuit, a low-pass filter (hereinafter abbreviated as LPF), and a voltage-controlled oscillator (hereinafter abbreviated as VCO).
A first FM detector and a second FM detector for detecting the SIF signal passing through the first FM detector, and a detectable frequency range of the first FM detector is set narrower than a detectable frequency range of the second FM detector. An FM demodulation circuit, wherein a free-run frequency of a VCO constituting the first FM detector is varied by a detection output from an LPF constituting the second FM detector.