JP2002165154A - Television signal receiving apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove influence due to interfering waves (26 to 30 MHz) for immunity test, which are especially influential to VHF low band and causes noises in a demodulated audio signal an, where a down conversion scheme is employed for an audio demodulation circuit of L system. SOLUTION: An audio processing circuit 20 includes a local oscillation circuit 8 and a mixer 7 for down-converting an audio intermediate frequency signal from a tuner to a second audio intermediate frequency signal of low frequency for audio demodulation. The local oscillation circuit 8 outputs a local oscillation signal of a frequency higher than the audio intermediate frequency by 6.5 MHz, upon the receipt of a VHF low-band channel of L system and outputs a local oscillation signal in phase-synchronization with a video intermediate frequency signal upon the receipt of other band channels.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a television signal receiver, and more particularly to a receiver suitable for demodulating an L-system television audio signal.

[0002]

2. Description of the Related Art There are various television systems at present, such as the L system (SECAM system) used in France and the B / G system (PAL system) used in the United Kingdom and Germany. , And even Japan,
There is an M system (NTSC system) adopted in the United States and the like.

In the SECAM system of the L system, the relationship between the video carrier frequency fP and the audio carrier frequency fS is VH.
The F low band differs from the VHF high band and UHF. That is, in VHF high band and UHF,
The audio carrier frequency fS is located on a higher frequency side than the video carrier frequency fP, but in the VHF low band, the audio carrier frequency fS is lower than the video carrier frequency fP.
It is located on the lower frequency side.

Therefore, when a television signal of the SECAM system is received by a tuner and an intermediate frequency signal (IF signal) is obtained, the relationship between the IF signals is as shown in FIG. 3A shows the relationship between the audio intermediate frequency (SIF) and the video intermediate frequency (VIF) in the VHF high band and UHF, and FIG. 3B shows the audio intermediate frequency (SIF) and the video intermediate frequency in the VHF low band. The relationship of frequency (VIF) is shown.
That is, for VHF high band and UHF, VIF is 3
8.9MHz, SIF is 32.4MHz and VIF
Is higher than SIF, and in VHF low band, SI
F is 40.4 MHz and VIF is 33.9 MHz,
The SIF is higher than the VIF.

[0005] As a method for demodulating such an L-system audio signal, a down-conversion demodulation circuit is known. This down-conversion method is a method in which an audio IF signal is down-converted to a low frequency and then AM demodulated. The audio IF signal and a predetermined local oscillation signal are mixed by a mixer and converted to a constant low frequency. The signal is output and subjected to AM demodulation by a SIF detection circuit via a predetermined band-pass filter.

FIG. 4 shows an audio demodulation circuit of this down-conversion system. An SIF signal from a tuner (not shown) is supplied to a mixer 32 via an SIF amplifier 31. The local oscillation signal from the local oscillation circuit 33 is supplied to the mixer 32, and a second SIF signal (2nd-SIF) which is a difference between the audio intermediate frequency and the local oscillation signal is generated. This 2nd-SIF signal has its noise component removed by a band-pass filter 34, and furthermore, an amplifier 35
Are supplied to the SIF detection circuit 36 to perform AM demodulation.

The local oscillation circuit 33 includes a PLL circuit synchronized with the video IF frequency, outputs a local oscillation signal equal to the video IF frequency, and outputs a SIF and V
6.5 nd 2nd-S corresponding to the frequency of the IF difference
An IF signal is obtained.

As described above, in the L system, the voice carrier frequency fS and the video carrier frequency fP are reversed in the VHF low band and the other bands.
As shown in FIG. 3B, the relationship between the VIF and the SIF is that the SIF is on the higher frequency side than the VIF, so the frequency from the local oscillation circuit 33 is changed to the VIF frequency (33.9 MHz).
In this case, a 2nd-SIF signal of 6.5 MHz, which is a difference from SIF (40.4 MHz), can be obtained, but the following problem occurs.

That is, in the European market, an interference wave test (immunity test) of a receiver is performed based on the EMC command, and a test is performed by generating an interference wave of 26 to 30 MHz. Therefore, when the local oscillation circuit outputs the same local oscillation signal as the VIF frequency from the local oscillation circuit 33 during the VHF low band reception of the L system, this local oscillation signal (33.9 MHz)
z) and the frequency of the difference between the interfering wave (26 to 30 MHz) are in the same band as the 2nd-SIF signal (6.5 MHz), causing interference and causing noise in the audio signal. For this reason, the inconvenience of not being able to conform to the immunity standard is caused.

An example of a down-converting audio demodulation circuit is disclosed in Japanese Patent Laid-Open No. Hei 10-136287. In this publication, a local oscillation circuit of a PLL system is used, and an audio intermediate frequency and a local oscillation signal are mixed to obtain 1.5
Although an example is described in which a 2nd-SIF signal of a low band frequency equal to or lower than MHz is obtained, no countermeasure against interference in the immunity test is described.

[0011]

As described above, in the conventional down-converting audio demodulation circuit, when an immunity test is performed, particularly when a VHF low-band reception of the L system is performed, a local oscillation signal and an interference wave for the immunity test are generated. The frequency difference is in the same band as the 2nd-SIF signal, causing interference and causing noise in the audio signal.

An object of the present invention is to provide a television signal receiving apparatus which eliminates such disadvantages and suppresses noise generation without interference.

[0013]

According to the present invention, there is provided a tuner for receiving a television signal of a predetermined channel and obtaining a video intermediate frequency signal and an audio intermediate frequency signal, and a second audio having a low frequency band. In a television signal receiving apparatus having an audio processing circuit for down-converting to an intermediate frequency signal and performing audio demodulation, the audio processing circuit includes:
Local oscillation means for generating a local oscillation signal; frequency conversion for mixing the audio intermediate frequency signal and the local oscillation signal from the tuner to obtain a down-converted second audio intermediate frequency signal corresponding to the difference between the two signals; Means, and demodulation means for demodulating the second audio intermediate frequency signal and outputting an audio signal, wherein the local oscillation means comprises a VHF of an L system.
At the time of receiving a channel other than the low band, a first local oscillation signal having the same frequency as the video intermediate frequency is output,
2. The television signal receiving apparatus according to claim 1, wherein a second local oscillation signal higher than the audio intermediate frequency by a frequency of a difference between the video intermediate frequency and the audio intermediate frequency is output when receiving a VHF low band channel. apparatus.

According to the present invention, there is provided a tuner capable of receiving a television signal of the L system and capable of outputting a video intermediate frequency signal and an audio intermediate frequency signal of a predetermined channel, and control means for selecting and controlling a channel to be received by the tuner. In order to down-convert the audio intermediate frequency signal into a second audio intermediate frequency signal of a low frequency, the audio intermediate frequency signal from the tuner and the local oscillation signal from the local oscillation circuit are mixed and both signals are mixed. A mixer that obtains a second audio intermediate frequency signal of a low frequency corresponding to the difference; and a demodulation circuit that AM-demodulates the second audio intermediate frequency signal and outputs an audio signal. Under the control of the control means, when receiving a channel other than the VHF low band of the L system, the first local oscillation having the same frequency as the video intermediate frequency is performed. A second local oscillation signal which is higher than the audio intermediate frequency by a difference frequency between the video intermediate frequency and the audio intermediate frequency when receiving a VHF low band channel. Device.

According to the present invention, the local oscillation signal for down-conversion is switched between the reception of the VHF low band and the reception of the other bands, and the reception of the VHF low band is not affected by the interference wave for the immunity test. 2 which is down-converted by outputting the local oscillation signal
Interference and obstruction to the nd-SIF signal can be prevented, and generation of noise can be suppressed.

[0016]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a circuit diagram showing a television signal receiving apparatus according to one embodiment of the present invention.

In FIG. 1, a broadcast signal of a predetermined channel is selected by a tuner 2 and a tuning circuit 3 from a television signal received by an antenna 1, and a video and audio IF signal is output from the tuner 2. This IF signal is input to the SAW filter 4 to extract a signal component in a desired IF band, and is supplied to the VIF detection circuit 5 and the audio processing circuit 20.

The VIF detection circuit 5 reproduces the video IF carrier, detects the composite video signal, and outputs a video signal from the video output terminal 21. On the other hand, the audio processing circuit 20
F amplifier circuit 6, mixer 7, local oscillation circuit 8, 2nd-
SIF filter circuit 9, 2nd-SIF amplifier circuit 10,
And an SIF detection circuit 11.

The output of the SIF amplifying circuit 6 is supplied to a mixer 7. The mixer 7 is also supplied with a local oscillation signal from a local oscillation circuit 8. The mixer 7 mixes the SIF signal and the local oscillation signal and performs low-frequency conversion. A 2SIF signal (2nd-SIF signal) is generated and supplied to the 2nd-SIF filter circuit 9 at the next stage. The filter circuit 9 is a band pass filter, extracts a signal of the second IF band,
The signal is supplied to the SIF detection circuit 11 via the SIF amplification circuit 10, where the signal is subjected to AM demodulation, and an audio signal is output from the audio output terminal 22.

Next, the local oscillation circuit 8 will be described.
The local oscillation circuit 8 includes a PLL circuit 81 and a signal generation circuit 82.
And an oscillation signal from these PLL circuits and a signal generation circuit 8
And a switch circuit 83 for switching and outputting the oscillation signal from the microcomputer 2. The switch circuit 83 is switched under the control of the microcomputer 12 (hereinafter, referred to as a microcomputer). The microcomputer 12 controls the channel selection circuit 3 to select a predetermined channel, and performs various controls of the receiver in response to an instruction from a user.

The PLL circuit 81 includes an input terminal 84 to which a VIF signal is input, a phase comparison circuit 85, a low-pass filter 86, and a voltage controlled oscillator (VCO) 87.
The phase of the VIF signal supplied to the input terminal 84 and the phase of the oscillation signal from the VCO 87 are compared by a phase comparison circuit 85,
The comparison result is converted to a DC voltage by a low-pass filter 86 to control the VCO 87, and the VCO 87
And outputs a local oscillation signal that is phase-synchronized.

The signal generating circuit 82 is composed of a crystal oscillator or the like and generates a fixed frequency signal.
In this embodiment, a local oscillation signal having a frequency 6.5 MHz higher than the frequency of the SIF signal from the tuner 2 is output.

The microcomputer 12 supplies the local oscillation signal from the PLL circuit 81 to the mixer 7 when receiving the VHF high band and the UHF band, and mixes the local oscillation signal from the signal generation circuit 82 when receiving the VHF low band. 7, the switch circuit 83 is controlled. Under the control of the microcomputer 12, the signal generating circuit 8 is used when receiving the VHF high band and the UHF band.
2 is stopped, and when receiving VHF low band, PL
If the operation of the L circuit 81 is stopped, unnecessary radiation can be suppressed.

Next, the operation of the present invention will be described with reference to FIG. First, the case of receiving the VHF high band and the UHF band will be described with reference to FIG. When receiving the VHF high band and the UHF band, the microcomputer 12
Switches the switch circuit 83 to the PLL circuit 81 side. As a result, a local oscillation signal (Ref. 1) synchronized with the VIF signal output from the PLL circuit 81 is supplied to the mixer 7.

As a result, the mixer 7 outputs a second SIF signal (6.5 MHz) having a difference between the SIF signal (32.4 MHz) and the local oscillation signal (38.9 MHz) that matches the frequency of the VIF signal. Is demodulated by the SIF detection circuit 11, and an audio signal is obtained from the output terminal 22.

On the other hand, when receiving the VHF low band, the microcomputer 12 switches the switch circuit 83 to the signal generation circuit 82 side. As a result, as shown in FIG. 2B, SIF + 6.5 MHz (= 46.9 MH)
The local oscillation signal (Ref. 2) of z) is output and supplied to the mixer 7. As a result, the SIF signal (40.4 MHz) and the local oscillation signal (46.9 MHz) are output from the mixer 7.
A second SIF signal (6.5 MHz) having a difference of
AM demodulation is performed by the SIF detection circuit 11 at the subsequent stage, and the output terminal 2
2, an audio signal is obtained.

Therefore, even if a signal for immunity test (26 to 30 MHz) is present at the time of VHF low-band reception, the interference wave signal having a difference from the local oscillation signal (46.9 MHz) is 16.9 MHz. It does not enter the signal (6.5 MHz) band and does not generate noise in the audio signal.

[0028]

As described above, according to the present invention, it is possible to avoid a situation in which a signal for an immunity test interferes and noise is generated in a voice signal, and conforms to the immunity standard and eliminates noise. A television signal receiving device can be provided.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a television signal receiving apparatus according to one embodiment of the present invention.

FIG. 2 is an explanatory diagram for explaining the operation of the audio demodulation circuit of the present invention.

FIG. 3 is an explanatory diagram showing a relationship between an audio intermediate frequency and a video intermediate frequency of the L system.

FIG. 4 is a circuit diagram showing a conventional audio demodulation circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 2 ... Tuner 3 ... Tuning circuit 4 ... SAW filter 5 ... VIF detection circuit 6 ... SIF amplification circuit 7 ... Mixer 8 ... Local oscillation circuit 9 ... 2nd-SIF amplification circuit 10 ... 2nd-SIF filter circuit 11 ... SIF detection circuit 12 ... microcomputer (control circuit) 20 ... audio processing circuit 81 ... PLL circuit 82 ... signal generation circuit 83 ... switch circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference)

Claims (5)

[Claims] 1. A tuner for receiving a television signal of a predetermined channel to obtain a video intermediate frequency signal and an audio intermediate frequency signal, and down-converting the audio intermediate frequency signal into a second audio intermediate frequency signal having a low frequency. In a television signal receiving apparatus having an audio processing circuit that performs audio demodulation, the audio processing circuit mixes a local oscillation signal that generates a local oscillation signal with an audio intermediate frequency signal and a local oscillation signal from the tuner. Frequency conversion means for obtaining a down-converted second audio intermediate frequency signal corresponding to the difference between the two signals, and demodulation means for demodulating the second audio intermediate frequency signal and outputting an audio signal, The local oscillation means, when receiving a channel other than the VHF low band of the L system, receives a first local oscillation signal having the same frequency as the video intermediate frequency. 2. The television according to claim 1, further comprising: outputting a second local oscillation signal which is higher than the audio intermediate frequency by a difference frequency between the video intermediate frequency and the audio intermediate frequency when receiving a VHF low band channel. John signal receiving device. 2. The PLL according to claim 1, wherein said local oscillation means generates a first local oscillation signal phase-synchronized with said video intermediate frequency signal.
A signal generating circuit for generating the second local oscillation signal; selecting a local oscillation signal from the PLL circuit when receiving a channel other than the VHF low band of the L system; and selecting the signal when receiving a VHF low band channel. 2. The television signal receiving apparatus according to claim 1, further comprising a switch circuit for selecting a local oscillation signal from a generation circuit and supplying the selected signal to the frequency conversion means. 3. A tuner capable of receiving a television signal of the L system and capable of outputting a video intermediate frequency signal and an audio intermediate frequency signal of a predetermined channel; control means for selecting and controlling a channel to be received by the tuner; In order to down-convert the audio intermediate frequency signal to a second audio intermediate frequency signal of a low frequency, the audio intermediate frequency signal from the tuner and the local oscillation signal from the local oscillation circuit are mixed to correspond to the difference between the two signals. Low frequency second
And a demodulation circuit that AM-demodulates the second audio intermediate frequency signal and outputs an audio signal. The local oscillation circuit controls the L system under the control of the control unit. When receiving a channel other than the VHF low band, a first local oscillation signal having the same frequency as the video intermediate frequency is output. When receiving a VHF low band channel, a difference between the video intermediate frequency and the audio intermediate frequency is higher than the audio intermediate frequency. A second local oscillation signal that is higher by the frequency of the second signal. 4. A PLL for generating a first local oscillation signal phase-synchronized with the video intermediate frequency signal.
A signal generation circuit for generating the second local oscillation signal; and selecting a local oscillation signal from the PLL circuit when receiving a channel other than the VHF low band of the L system under the control of the control means. 4. The television signal receiving apparatus according to claim 3, further comprising: a switch circuit that selects a local oscillation signal from the signal generation circuit when receiving a channel and supplies the signal to the mixer. 5. The television signal receiving apparatus according to claim 3, wherein said second local oscillation signal is a signal having a frequency 6.5 MHz higher than said audio intermediate frequency signal.