JPS63160474A - Sound signal processing circuit - Google Patents

Abstract

PURPOSE:To eliminate the need of an AGC system circuit and to require no adjusting point by separating and selecting a picture intermediate frequency signal (PIF) and a sound intermediate frequency signal (SIF) in an input step, passing the respective signals through a limiter amplifier and detecting a specific sound component in a multiplication circuit. CONSTITUTION:After a signal in the vicinity of the carrier frequency of the PIF of 58.75 MHz is selected in a filter 2, and only the SIF of 54.25 MHz is selected in a filter 3, they are respectively fed to the limiter amplifiers 4, 5. In the input of the limiter amplifier 4, the carrier and the AM modulated part thereof are included as a sideband wave, however, limitter amplified, so that the AM part of the output is attenuated to have only the carrier component. In the input of the amplifier 5, only the SIF of an FM signal is present, the FM part is amplified as it is and outputted, when the two inputs are multiplied in the multiplication circuit 6, the sound FM signal of 4.5 MHz is obtained in the output. Namely, the circuits 4, 5 are limiter amplifier, so that a required sensitivity can be easily maintained and it is not required to apply the AGC.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an audio signal processing circuit, and particularly to a circuit for processing an audio intermediate frequency signal of a television broadcast receiver.

[Conventional technology]

FIG. 2 shows a conventional example of an audio intermediate frequency signal processing circuit for a TV broadcast receiver. In the figure, 1 is a terminal into which a TV intermediate frequency signal obtained from the tuner output is input;
is a bandpass filter that selects only a 58.75 MHz video intermediate frequency signal (hereinafter abbreviated as PIF) and a 54.25 MHz audio intermediate frequency signal (hereinafter abbreviated as SIF). Further, 12 is an AGC type amplifier circuit, 13 is a carrier amplifier circuit that selectively amplifies only PIF from the output of the AGC type amplifier circuit 12, 14 is a multiplier circuit, 7 is a detection signal output terminal, and 16 is an AGC detection circuit.

The operation will be explained below. The intermediate frequency signal output from the tuner has unnecessary components attenuated by a filter 11, and is amplified by an AGC type amplifier circuit 12. One of the amplified signals is sent as is to the multiplication circuit 14, and the other is sent as a switching signal to the shift multiplier circuit 14, which has been amplified by the carrier of the PIF in the carrier amplification circuit 13.

4.5MHz of the signals synchronously detected by the multiplier circuit 14
Although the component is an audio signal, a video signal is obtained as a low frequency component, and the amplitude value of this video signal component is detected by the AGC detection circuit 1.
6, and the gain of the AGC type amplifier circuit 12 is adjusted so that the amplitude value becomes constant. Also, of the detected output obtained from the terminal 7, only the 4.5 MHz audio component is extracted by a subsequent filter, and then subjected to FM detection.

AGC is applied in the amplifier circuit 12 because the amplitude value of the input from terminal 1 fluctuates depending on the input electric field of the antenna, and SIF is an FM signal, but if the amplifier circuit 12 is made into a limiter amplifier, the SIF Intermodulation occurs between the FM signal and the PIF, which not only prevents normal detection, but also causes the problem that the resulting 4-5 MHz audio FM signal is phase-modulated by the video signal, resulting in audio buzz. Therefore, the amplifier circuit 12 has good linearity,
In addition, it is necessary to amplify the signal while ensuring the necessary reception sensitivity, and for this reason, AGC is applied. In addition, the carrier amplifier 13 is necessary for the 31.71 detection, and if the output signals of the amplifier circuit 12 are directly input to the multiplier circuit without providing this carrier amplifier 13, the PIF and SIF will still be intermodulated. , and a similar problem occurs. In other words, the carrier amplification circuit 13 must accurately select only the PIF carrier signal component.

[Problem that the invention seeks to solve]

Since the conventional audio signal processing circuit has the above-mentioned configuration, it has the following two problems.

Firstly, a complicated circuit configuration consisting of an AGC detector and an AGC type amplifier is required, and secondly, the carrier amplification circuit 13 requires a 58°75
A coil is used for the MHz bandpass filter, but the coil needs to be adjusted.

That is, a surface acoustic wave filter is used as the bandpass filter 11, and the SIF signal and the signal near the carrier frequency of the PIF signal are simultaneously selected, but no adjustment is necessary here. On the other hand, the carrier amplification circuit 13 has a coil, that is, an adjustment point, as described above, and if a surface acoustic wave filter is used instead of the coil to eliminate this, the delay time will cause the switching signal to The phase of the carrier is delayed compared to the original signal, and the multiplication in the circuit 14 prevents normal synchronous detection as AM detection, and the low-frequency video detection output signal obtained from the output also becomes abnormal, making accurate AGC impossible. I won't be able to do it. In the end, it was not possible to eliminate adjustment points with the conventional circuit configuration.

The present invention has been made in view of the problems of the conventional ones as described above, and it is an object of the present invention to provide an audio signal processing circuit that can eliminate the need for an AGC circuit and adjustment points.

[Means for solving problems]

The audio signal processing circuit according to the present invention has PIF and S at the input stage.
The IF is completely separated and selected, and after each is passed through a limiter amplifier, the 4.5 MHz audio component is detected by a multiplier circuit.

[Effect]

In the present invention, the PIF and SIF are completely separated and selected at the input stage, and after each is passed through a limiter amplifier, the 4.5 MHz audio component is detected by the multiplier circuit. is no longer necessary, and no adjustment points are required either.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an audio signal processing circuit according to an embodiment of the present invention, in which 1.7 is the same terminal as the conventional example, and 2 is a PI
Filter that selects only signals near the carrier frequency of F, 3
4 and 5 are limiter amplifiers, 20 is a first amplifier circuit consisting of filter 2 and limiter amplifier 4, and 30 is filter 3 and limiter amplifier 5.
6 is a multiplication circuit.

Next, the operation will be explained.

Tuner outputs are input from terminal l and sent to filters 2.3, respectively. Since surface acoustic wave filters can be used as filters 2 and 3, sharp band characteristics can be obtained without adjustment. Filter 2 has a 58.75 MHz PI
The signal near the carrier frequency of F is filtered by filter 3 at 54
．． After selecting only the 25 MHz SIF, each is sent to a limiter amplifier 4.5. The input of the limiter amplifier 4 contains the carrier and its AM modulated component as sideband waves, but since the limiter amplifies the carrier, the AM component is attenuated and the output is mostly only the carrier component. Further, the input to the amplifier 5 is only the FM signal SIF, and its output is amplified and output with the FM component unchanged. If these two human powers are multiplied by the multiplier circuit 6, the output will be 4.5M.
A Hz audio FM signal is obtained. Further, since the limiter amplifier 4 or 5 does not have signal components other than PIF and SIF, intermodulation does not occur, and the same applies to the multiplier circuit 6. Also, since circuits 4 and 5 are limiter amplifiers, it is easy to secure the necessary power, and the AGC
There's no need to apply.

Note that the surface acoustic wave filters of filters 2 and 3 have different delay times, but the multiplication of the multiplication circuit 5 of the present invention does not require synchronous detection, and it is sufficient to obtain a simple beat signal of 4 or 5 MHz, so there is no problem. do not have.

〔Effect of the invention〕

As described above, the audio signal processing circuit according to the present invention eliminates the need for an AGC circuit, simplifies the circuit, and eliminates the need for adjustment points.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an audio signal processing circuit according to an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional audio signal processing circuit. In the figure, 2.3 is a filter, 4 and 5 are limiter amplifiers, 6 is a multiplier circuit, and 20.30 is a first degree second amplifier circuit.

Claims (2)

[Claims] (1) An audio signal processing circuit for a television receiver, comprising: a first amplifier circuit that selects only a signal near the carrier frequency of a video intermediate frequency signal from input intermediate frequency signals and amplifies it with a limiter; A second amplifier circuit that selects only the audio intermediate frequency signal from the intermediate frequency signals and amplifies it with a limiter, and a multiplier circuit that multiplies the outputs of the first and second amplifier circuits to obtain an audio FM signal. An audio signal processing circuit characterized by: (2) The audio signal processing circuit according to claim 1, wherein the first and second amplifier circuits each include a filter and a limiter amplifier.