JPH0237895A - Receiver - Google Patents

Abstract

PURPOSE:To demodulate without any trouble even in a frequency band of a mixed AM signal and FM signal by providing a control circuit to detect whether a receiving signal is an AM signal or an FM signal and switch automatically a demodulating circuit. CONSTITUTION:The title receiver provides a control circuit 7 to discriminate whether a receiving signal is an AM signal or an FM signal, from the output of an FM detector 6 in an automatic frequency control circuit(AFC circuit) 12 of an intermediate frequency signal converting part 11 of an ordinary receiver and to switch automatically an AM demodulator 8 and an FM demodulator 9. The difference when the receiving television signal is the AM signal and when it is the FM signal from the output FM-demodulated by the FM detector 6 in the AFC circuit 12 of the intermediate frequency signal converting part 11 of the receiver is detected and modulators 8 and 9 are switched. Thus, even in the frequency band of the mixed AM signal and FM signal, the corresponding procedure and receiving can be automatically performed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a receiving device that has an automatic identification circuit for AM and FM signals and demodulates both signals.

Conventional technology Conventionally, AM broadcasting and FM broadcasting are carried out in different frequency bands, and since there is no frequency band in which both broadcasting systems are mixed, technology for automatic identification of AM and FM signals and automatic switching of demodulators has been established. It has not been. Therefore A.M.
Switching between the demodulator and the FM demodulator 1j13 is performed by a manual switch in conjunction with a reception frequency band changeover switch.

An example of the above-mentioned conventional receiving device will be described below with reference to the drawings.

Figure 4 shows a conventional AM and FM radio broadcast receiving device.
/represents a diagram. In FIG. 4, ■ is a high frequency amplifier, 11 is an intermediate frequency signal converter, 8 is an AM demodulator,
9 is an FM demodulator, 13 is a demodulator output selector switch, C
is an AM high frequency signal input terminal, d is an FM high frequency signal input terminal, and b is a demodulated signal output terminal.

The operation of the receiving device configured as described above will be described below.

A signal input from the AM high frequency signal input terminal C is amplified by the high frequency amplifier l, converted into a fixed intermediate frequency signal by the intermediate frequency signal converter, and then demodulated by the AM demodulator. On the other hand, similarly, a signal input from the FM high frequency signal input terminal d is amplified by the high frequency amplifier 1, converted to a fixed intermediate frequency signal by the intermediate frequency signal converter, and then demodulated by the FM demodulator.

The demodulated output obtained as described above is selected by the demodulator selection switch 13 which is linked to the reception frequency band selection switch and is output to the demodulated signal output terminal. Note that the high frequency amplifier 1 and the intermediate frequency signal converter 10 are used for AM signals and for F signals.
Since the frequency of the carrier wave for the M signal is significantly different, the same product cannot be used in common.

Problems to be Solved by the Invention However, with the above configuration, the retransmission by the ground station of satellite television broadcasts, which is expected to be implemented in the future, will be difficult due to UH.
When FM is transmitted using the sky area in the F band, UHF
Since the existing AM transmission of the band and the two types of transmission methods will coexist within a series of bands, it will be necessary to switch the demodulation circuit as appropriate, and unlike conventional radio broadcasting, the transmission method is not determined depending on the broadcast frequency band. Therefore, switching the demodulator using a conventional manual switch is extremely inconvenient.

In view of the above-mentioned problems, the present invention provides a receiving device that can receive television signals without any trouble regardless of the broadcasting method by automatically distinguishing between AM broadcasting and FM broadcasting and automatically switching the demodulator. .

Means for Solving the Problems In order to solve the above problems, the receiving device of the present invention usually includes an automatic frequency control circuit (
The device is equipped with a control circuit that determines whether a received signal is an AM signal or an FM signal from the output of an FM detector in the AFC circuit (hereinafter abbreviated as AFC circuit) and automatically switches between an AM demodulator and an FM demodulator. It is something.

Operation The present invention has the above-described configuration, and when the received television signal is an AM signal and the output from the FM demodulation by the FM detector in the AFC circuit of the intermediate frequency signal converter of the receiving device.
By detecting the difference in the case of an M signal and switching the demodulation circuit based on that signal, it becomes possible to automatically correspond and receive signals even in a frequency band where AM signals and FM signals coexist.

Embodiment Hereinafter, a receiving apparatus according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a block diagram of a receiving apparatus in a first embodiment of the present invention. In FIG. 1, 1 is a high frequency amplifier, 2 is a mixer for intermediate frequency conversion, 3 is a voltage controlled local oscillator, 4 is a low pass filter (hereinafter abbreviated as LPF), 5 is an amplitude limiter, 6 is an FM detector,
7 is a control n circuit, 8 is an AM demodulator, 9 is an FM demodulator, 1
0 is an audio subcarrier separation circuit, 11 is an intermediate frequency signal converter, 12 is an automatic frequency control circuit, a is a high frequency signal input terminal, and b is a demodulated signal output terminal.

The operation of the receiving device configured as described above will be described below.

An AM-modulated or FM-modulated high-frequency television signal input from an input terminal a is amplified by a high-frequency amplifier 1 and input to a mixer 2. Further, the local oscillation signal generated by the voltage controlled local oscillator 3 is also input to the mixer 2, and an intermediate frequency signal which is the difference frequency between the two signals is obtained. After removing the amplitude modulation component from the obtained intermediate frequency signal with the amplitude limiter 5, the frequency spectrum of the output detected by the FM detector 6 is
As shown in FIG. Figure 2 shows that the received TV signal is AM.
This is the case for signals. Intermediate frequency fluctuations due to frequency fluctuations of the voltage controlled oscillator 3 are detected as a low frequency signal 13. In addition, the spectrum of the demodulated output 14 of the FM-modulated audio signal and the amplitude variation 15 that could not be removed by the amplitude limiter 5 of the video signal also appears. Next, FIG. 3 shows a case where the received television signal is an FM signal, and a signal 13 showing fluctuations in the intermediate frequency appears as in FIG. 2. In addition, since the video signal is also demodulated by the FM detector, the luminance signal 16 of the video signal
and a color signal 17 centered on the color subcarrier 3.58 MHz, and an audio signal 18 centered on the audio subcarrier 5.73 MHz.
Each spectrum appears. Note that the current FM child television image signal (satellite broadcasting) is limited to 4.5 MHz. The output of the FM detector 6 is input to the LPF 4 to extract a frequency component near DC that indicates a change in the intermediate frequency based on the frequency fluctuation of the voltage-controlled local oscillator 3, and applies it to the voltage-controlled local oscillator 3 to oscillate the local oscillator. The frequency is automatically controlled and the intermediate frequency is stabilized at a constant value. Here, the voltage controlled local oscillator 3, LPF 4, amplitude limiter 5, and FM detector 6 are used as an AFC circuit 12, and a closed loop obtained by adding the mixer 2 to the AFC circuit 12 is used as the intermediate frequency signal converter 11.

On the other hand, if the output of the FM detector 6 is input to the audio subcarrier separation circuit 10, which is composed of a bandpass filter of 5.73M1 (centered on z and has a passband width IM7), from FIGS. 2 and 3, As is clear, only when receiving an FM sub-TV signal, an output of a frequency component centered at 5.73M1 (z) corresponding to the audio subcarrier is obtained. Therefore, the control circuit 7 selects the subsequent demodulator depending on the presence or absence of this signal. In other words, if there is an output from the audio subcarrier separation circuit, select the FM demodulator 9, and if there is no output, select the AM demodulator 8, and the demodulated output will be the demodulated signal regardless of the modulation method of the input signal. Output to the output terminal.

As described above, according to this embodiment, the presence or absence of an audio subcarrier, which is a frequency component centered around 5.73 MHz, in the FM detector output in the AFC circuit of the intermediate frequency signal conversion section of the television signal receiving device is detected. By automatically switching between the AM demodulator and the FM demodulator, demodulation can be performed regardless of the transmission method of the received signal.

Effects of the Invention As described above, the present invention provides an FM signal in the AFC circuit of the intermediate frequency signal converter when an AM signal and an FM signal are input.
By providing a control circuit that detects whether the received signal is an AM signal or an FM signal based on the difference in the detector output and automatically switches the demodulation circuit, it is possible to demodulate without problems even in a frequency band where AM and FM signals are mixed. Can be done.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a receiving device according to the first embodiment of the present invention, FIG. 2 is an output frequency spectrum diagram of the FM detector 6 shown in FIG. 1 (when an AM signal is input), and FIG. 3 is a diagram similar to that shown in FIG. FM
Output frequency spectrum diagram of detector 6 (when FM signal is input)
, FIG. 4 is a block diagram of a conventional receiving device. 1...High frequency amplifier, 2...Mixer,
3...Voltage controlled local oscillator, 4...L
PF, 5...Amplitude limiter, 6...FM
Detector, 7... Control circuit, 8... AM
Demodulator, 9...FM demodulator, 10...
Audio subcarrier separation circuit, 11...Intermediate frequency signal converter, 12...AFC circuit, a...
High frequency signal input terminal, b...Demodulated signal output terminal. Name of agent: Patent attorney Shigetaka Awano (1 person) Figure 2 Figure 3 Figure j, 5511'lff1 Yami wave sheet

Claims (1)

[Claims] When AM modulated and FM modulated TV signals are input, the FM in the automatic frequency control circuit of the intermediate frequency signal converter
By providing a control circuit that identifies whether a received television signal is AM modulated or FM modulated based on the presence or absence of an audio subcarrier in the demodulated television signal that appears in the detector output, and automatically switches the demodulation circuit, it is possible to distinguish between both signals. A receiving device characterized by being capable of demodulation.