JPS622751B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an audio IF circuit for a television receiver.

A television signal consists of a video carrier wave that is AM-modulated with a video signal and an audio carrier wave that is FM-modulated with an audio signal.A television receiver separates and demodulates these waves to obtain a video signal and an audio signal. ing. Audio on conventional television receivers
There are two types of IF circuits.

One is called the intercarrier system, and a block diagram of this system is shown in FIG. In the figure, 1 is a tuner, 2 is a video IF amplifier, 3 is a video detector, and 4 is a video detector.
is audio IF detector, 5 is audio IF amplifier, 6 is audio
It is an FM detector.

In this system, first, a television signal input from an antenna (not shown) is input to a tuner 1 and converted into an IF frequency. In Japan, the video IF frequency is 58.75MHz and the audio IF frequency is 54.25MHz. The signal converted to the IF frequency is amplified by a video IF amplifier 2 and subjected to AM detection by a video detector 3. The detection output includes an audio IF as well as a video signal.
The beat frequency of 4.5MHz is output as the frequency and video IF frequency, but only the video signal is extracted by a low-pass filter and trap.

On the other hand, from the video IF amplifier 2 to the audio IF detector 4
The input signal is subjected to AM detection and the beat frequency 4.5MHz component of the audio IF is extracted. After the AM detected signal is amplified by the audio IF amplifier 5,
FM detection is performed by the audio FM detector 6. In this way, the audio signal is extracted.

This intercarrier system has the advantage that the circuit is simple and that the audio IF beat frequency of 4.5 MHz is determined by the video carrier wave and the audio carrier wave, so it is stable regardless of the local oscillation frequency of the tuner. However, when extracting the beat frequency between the video IF frequency and the audio IF frequency, the video signal interferes, resulting in so-called buzz interference, which has the disadvantage that the sound quality is somewhat poor.

A method with good sound quality is the separate carrier method. This block diagram is shown in FIG. 7
is an audio IF amplifier. The audio IF amplifier 7 differs from the audio IF amplifier 5 used in the intercarrier system in that it extracts and amplifies the audio IF frequency component of 54.25 MHz. The amplified audio IF is input to the audio FM detector 6 and an audio signal is extracted. Since the separate carrier method separates the audio IF frequency from the output of the tuner, it provides good sound quality without buzz interference. However, the sound at this time
The IF frequency of 54.25MHz is obtained by the beat between the audio carrier frequency and the tuner's local oscillation frequency, so if the local oscillation frequency fluctuates periodically, the audio
This fluctuation was detected by FM detection, and the problem was that noise such as hum noise was mixed into the audio signal.

SUMMARY OF THE INVENTION An object of the present invention is to provide an audio IF circuit for a television receiver that eliminates the drawbacks of the prior art described above, has good sound quality, and does not generate hum noise.

In order to achieve the above-mentioned object, the present invention detects the phase difference between the video signal carrier (58.75MHz in Japan) and the output signal of the reference oscillator, and based on the detected phase difference, local Audio separated from intermediate frequency signal by controlling oscillation frequency
Multiply the IF signal (54.25MHz in Japan) by the output signal of the reference oscillator above to extract the frequency of the difference between the two signals, that is, the audio IF signal of 4.5MHz,
The feature is that this is amplified and FM detected.

The present invention will be explained below with reference to Examples. FIG. 3 is a block diagram showing one embodiment of the present invention. In Figure 3, 11 is a 58.75MHz filter, 12 is a phase detection circuit, 13 is a reference oscillator, 1
4 is a multiplication circuit, and 15 is a 4.5MHz filter.
Further, the symbols other than these indicate the same things as those in FIG. 2.

In the figure, the audio IF signal is separated from the video IF signal at the output of the tuner 1, as in FIG. 2, and the audio IF signal c amplified by the audio IF amplifier 7 is supplied to the multiplier circuit 14. The output signal a of the reference oscillator 13 is supplied to the other terminal of the multiplier circuit 14, and the beat component signal b of the audio IF signal c is obtained from the multiplier circuit 14.

On the other hand, 58.75MHz filter 11 is applied from the video IF signal.
The video carrier component extracted by is compared with the output signal a of the reference oscillator 13 in the phase detection circuit 12. The phase detection circuit 12 controls the local oscillation frequency of the tuner 1 so that the video carrier component and the output signal a of the reference oscillator 13 are in the same phase. The oscillation frequency of the reference oscillator 13 is set in advance to the frequency _p of the video carrier component,
Furthermore, if the audio IF signal frequency is _s , two signals are obtained as the output signal b of the multiplier circuit 14: _p - _s =58.75MHz-54.25MHz=4.5MHz _p + _s =58.75MHz+54.25MHz=113MHz. 4.5MHz filter 15 _p −
_s signal and pass this extracted signal to the audio IF
A predetermined audio signal is obtained by processing with the amplifier 5 and the audio FM detector b.

In the figure, when the oscillation frequency of the reference oscillator 13 drifts by + _Δp from the video carrier frequency of 58.75MHz, both the video IF frequency and the audio IF frequency change by _Δp . Therefore, the output signal b of the multiplier circuit 14 is ( _p + _Δp )−( _s + _Δp )=4.5MHz, and a 4.5MHz signal is always obtained.

Furthermore, as is clear from the above explanation, the periodic fluctuation ± _Δp ' of the local oscillation frequency, which is a problem in the prior art, is due to the difference between the reference oscillation circuit 13 and the phase detection circuit 1.
2. Since the PLL loop consisting of tuner 1, video IF amplifier 2, and 58.75MHz filter 11 provides a loop gain of ±Δ _p ′/loop gain, the incorporation of hum noise is significantly improved.

FIG. 4 shows another embodiment of the invention. In this diagram, functional blocks with the same symbols as in Figure 3 are
Represents the same thing as the figure.

In FIG. 4, 14' is a multiplication circuit, and 15' is a multiplication circuit.
54.25MHz filter, 21 is phase detection circuit, 22
is a voltage controlled oscillator.

In the figure, the audio IF signal c is supplied to a phase detection circuit 21, and its phase is compared with the output signal of a 54.25MHz filter 15'. The voltage controlled oscillator 22 is controlled by the output signal g. Here, it is easily understood that the phase detection circuit 21 and the voltage controlled oscillator 22 constitute a well-known PLL FM detection circuit, except that a multiplier circuit 14' and a 54.25MHz filter 15' are included. .

Multiplier circuit 14' and 54.25MHz filter 1 are shown below.
The function of 5' will be explained. First, multiplication circuit 1
4' is supplied with the aforementioned video carrier component a of frequency _p , and the other terminal is supplied with the output signal h of the voltage controlled oscillator 22 with frequency _ls . Therefore, the frequency of the output signal e of the multiplier circuit 14' is _p + _ls or _p - _ls .

Now voltage controlled oscillator 2 so that _ls = 4.5MHz
2, the difference signal output of the multiplier circuit _p − _l
s is _p - _ls = 58.75MHz - 4.5MHz = 54.25MHz. Therefore, the difference signal _p − _ls is the audio IF
It matches the frequency _s of the signal c and is basically the well-known
Functions similar to PLL FM detection circuit. In this way, an audio signal is obtained as the output signal g of the phase detection circuit 21.

Comparing the above two embodiments explained in Fig. 3 and Fig. 4, in the embodiment of Fig. 3, the audio IF signal d
The center frequency of the filter circuit 15 that extracts
The difference between the two is that the center frequency of the filter circuit 15' for extracting the voice control signal f is 54.25 MHz, whereas in the embodiment shown in FIG. 4, the frequency is 4.5 MHz. In recent years, so-called surface acoustic wave filters (SAW filters) that utilize acoustic surface waves have become well known as a technique for eliminating the need for adjustment of such filter circuits.
This acoustic surface wave filter has good cost performance when used in the video IF frequency band (58.75MHz) because the substrate that makes up the filter becomes large when the center frequency is low.

Therefore, if an audio IF circuit is created by applying such solid-state filter technology and monolithic IC technology, the embodiment shown in FIG. 4 has the advantage of improving cost performance compared to the embodiment shown in FIG. 3. be.

As described above, according to the present invention, good sound can be obtained without generating hum noise even with periodic fluctuations in the local oscillation frequency of the tuner.

[Brief explanation of the drawing]

Figures 1 and 2 are audio based on conventional technology, respectively.
FIG. 3 is a block diagram showing one embodiment of the present invention, and FIG. 4 is a block diagram showing another embodiment of the present invention. 5...Audio IF amplifier, 6...Audio FM detector,
7... Audio IF amplifier, 11... 58.75MHz filter, 12... Phase detection circuit, 13... Oscillator, 1
4, 14'... Multiplier circuit, 15... 4.5MHz filter, 15'... 54.25MHz filter, 21... Phase detection circuit, 22... Voltage controlled oscillator.

Claims (1)

[Claims] 1. An audio IF amplifier that separates and amplifies the audio IF signal from the video IF signal, means for extracting the carrier wave of the video IF signal and outputting the first carrier wave, an oscillator for obtaining the second carrier wave, and the first carrier wave. means for obtaining a control signal according to a phase difference between the carrier wave and the second carrier wave;
means for controlling the local oscillation frequency of the tuner using the control signal; means for generating a beat component of the audio IF signal amplified by the audio IF amplifier and the second carrier; and extracting only a predetermined beat component. An audio IF circuit for a television receiver, comprising a filter and a detector for FM detecting the beat component.