JPS6262107B2 - - 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, 4 is an audio IF detector, 5 is an audio IF amplifier, and 6 is an audio FM detector.

In this system, first, a television signal input from an antenna (not shown) is input to the 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 the video IF
The signal is amplified by an amplifier 2 and AM detected by a video detector 3. The detection output includes audio as well as video signals.
Beat frequency of IF frequency and video IF frequency 4.5MHz
is output, 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. The AM detected signal is amplified by an audio IF amplifier 5 and then FM detected by an audio FM detector 6. In this way, the audio signal is extracted.

This intercarrier system has the advantage that the circuit is simple, and since the audio IF beat frequency of 4.5 MHz is determined by the video carrier wave and the audio carrier wave, 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. Amplified audio IF is audio
The signal is input to the FM detector 6 and the audio signal is extracted.

Since the separate carrier method separates the audio IF frequency from the tuner output, 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 when the local oscillation frequency fluctuates, this fluctuation is detected by the audio FM detector, and hum noise etc. are added to the audio signal. There was a drawback that noise was mixed in.

Furthermore, the audio FM detector 6 requires a 4.5 MHz filter for extracting the audio signal, resulting in a large size and poor cost performance.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and provide an audio IF circuit for a television receiver that has good sound quality, does not generate hum noise, and has improved cost performance without the need for a 4.5MHz filter. There is something to do.

In order to achieve the above-mentioned object, the present invention
The output signal of a so-called phase locked loop (hereinafter abbreviated as PLL circuit) which controls the voltage controlled oscillator by phase detecting the IF signal (58.75MHz in Japan) and the output signal of the first voltage controlled oscillator, and the output signal of the first voltage controlled oscillator. The audio signal output from the voltage controlled oscillator No. 2 (in Japan,
A signal having a frequency of 4.5 MHz) is extracted, a predetermined beat component is extracted from the multiplication output, and a signal corresponding to the phase difference between the predetermined beat component and the audio IF signal, that is, the demodulated audio The feature is that the signal is obtained.

An embodiment of the audio IF circuit of a television receiver according to the present invention is shown in FIG. Functional blocks in FIG. 3 denoted by the same numbers as in FIG. 2 represent the same functional blocks as in FIG. In the figure, 11 is a 58.75MHz filter for extracting a video transport carrier, 12 and 13 are a phase detection circuit and a voltage controlled oscillator that constitute a PLL circuit to be described later, 14 is a multiplication circuit,
15 is a 54.25MHz filter, 21 is a phase detection circuit, and 22 is a voltage controlled oscillator.

In the figure, the audio signal is separated from the video 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 phase detection circuit 21.

On the other hand, the phase of the video carrier component extracted from the video IF amplifier 2 by the 58.75MHz filter 11 is compared with the output signal of the voltage controlled oscillator 13 in the phase detection circuit 12, and the output signal of the voltage controlled oscillator 13 is converted into the video carrier component. Synchronize the phase with. Since the phase detection circuit 12 and the voltage controlled oscillator 13 are well-known PLL circuits and equivalently constitute a narrow band filter circuit, the output signal a of the voltage controlled oscillator 13 is 58.75M.
Signals other than the video carrier component contained in the output signal of the Hz filter 11, for example, the horizontal synchronization signal component, are sufficiently suppressed, resulting in only the video carrier component. Video carrier component a is supplied to a multiplication circuit 14.

The audio IF signal c is supplied to a phase detection circuit 21, and its phase is compared with the output signal f of the 54.25MHz filter 15. The voltage controlled oscillator 22 is controlled by the output signal g. Here, the phase detection circuit 21 and the voltage controlled oscillator 22 include the multiplication circuit 14,
It will be easily understood that, except for the inclusion of the 54.25MHz filter 15, this constitutes a well-known PLL FM detection circuit.

Below, multiplier circuit 14 and 54.25MHz filter 1
The function of No. 5 will be explained. First, one terminal of the multiplier circuit 14 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 _s . Therefore, the output signal e of the multiplier circuit 14
The frequency of is _p + _s or _p − _s .

Now, if the voltage controlled oscillator 22 is configured so that _s = 4.5MHz, the difference signal output of the multiplier circuit 14 will be
_p −〓 _s becomes _p −〓 _s = 58.75MHz − 4.5MHz = 54.25MHz. Therefore, the difference signal output _p − _〓s matches the frequency _s of the audio IF signal c, and basically has the same function as the well-known PLL FM detection circuit. In this way, an audio signal is obtained as the output signal g of the phase detection circuit 21.

In this embodiment, the center frequency of the filter circuit 15 that extracts the audio carrier signal is 54.25 MHz, but in recent years, a so-called acoustic surface wave filter (SAW) that uses acoustic surface waves has been developed as a technology to eliminate adjustment of such a filter circuit. filters) are well known. This acoustic surface wave filter has good cost performance when used in the video IF frequency band (58.75MHz) because the substrate that constitutes it becomes large when the center frequency is low. Therefore, by combining such solid-state filter technology and monolithic IC technology, audio
By creating an IF circuit, this embodiment has the advantage that it can be formed with better cost performance than the conventional example.

As described above, according to the present invention, it is possible to stably receive audio signals even when the tuner changes locally, and it is possible to obtain good sound quality without hum noise.

Furthermore, since the 4.5MHz filter is not required, the configuration can be simplified and downsized, which has the effect of improving cost performance.

[Brief explanation of the drawing]

1 and 2 are block diagrams of a conventional audio IF circuit, and FIG. 3 is a block diagram showing an 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... Voltage controlled oscillator, 14... Multiplier, 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, a means for extracting the carrier wave of the video IF signal and outputting the first carrier wave, a first voltage controlled oscillator that outputs the second carrier wave, means for controlling the frequency of the first voltage controlled oscillator by a phase difference between a first carrier wave and a second carrier wave, approximately 4.5M;
a second voltage-controlled oscillator that outputs a signal with a frequency of Hz; a multiplication circuit that receives the output signals of the first and second voltage-controlled oscillators; a 54.25MHz filter that receives the output signal of the multiplication circuit; A phase detection circuit receiving the output signals of the audio IF amplifier and the 54.25MHz filter is provided, and the output of the phase detection circuit controls the second voltage controlled oscillator and obtains a demodulated audio signal. An audio IF circuit for a television receiver characterized by: