JPS6228911B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improving the characteristics of an audio detection circuit for a television receiver.

In the audio circuit of a television receiver, the video intermediate frequency _P = 58.75MHz and the first audio intermediate frequency _s1 =
54.25MHz to second audio intermediate frequency _s2 = 4.5MHz
The circuit system for generating the video intermediate frequency _p
The second audio intermediate frequency _s2 is obtained by multiplying the mixed signal of the first audio intermediate frequency s1 and _the video intermediate frequency _p , or the method of multiplying the mixed signal of the video intermediate frequency _p and the first audio intermediate frequency _s1 . There are methods such as transistor detection to obtain the second audio intermediate frequency _s2 . Moreover, most of these are integrated circuits (ICs). However, the second audio intermediate frequency _s2 obtained by these methods includes a considerable amount of the video intermediate frequency _p and the first audio intermediate frequency
There is carrier wave leakage of _s1 , and it is output as is to the IC terminal. This second audio intermediate frequency _s2 with a lot of carrier wave leakage passes through a band pass filter and is led to the next stage as almost only the second audio intermediate frequency _s2 .

Conventionally, as described above, unnecessary carrier waves leak at the terminals of ICs, and when ICs are integrated on a larger scale, this carrier wave leakage becomes impossible to ignore.
For example, the capacitance between terminals can reach a maximum of 1.5 pF, so when a high frequency carrier is output to an IC terminal, it has a large effect on other terminals, and therefore on other circuits.

This invention was developed in consideration of the above points.Since carrier wave leakage has a large effect on other circuits, an active low-pass filter is added inside the IC to suppress the carrier wave. lead)
This reduces mutual interference between the two. This invention will be explained below.

FIG. 1 is a block diagram showing one embodiment of the present invention. In this figure, 1 is a detector, 2 is an active low-pass filter, 3 is an input terminal, and 4 is an output terminal. To explain its operation, the video intermediate frequency _p
A mixed signal of the first audio intermediate frequency _s1 and the first audio intermediate frequency s1 is applied to the input terminal 3, and is detected by the detector 1 to obtain the second audio intermediate frequency _s2 at the output terminal 4. If there is no bandpass filter 2, the video intermediate frequency _p and the first audio intermediate frequency _s
The leakage of ₁ is large. Note that since the active low-pass filter 2 itself is well known, its details will be omitted.

FIG. 2 is a graph showing an example of the output level. In this figure, the shaded portions show the one according to the present invention, and the non-shaded portions show the conventional example. 2 _s2 to 4 _s2 represent harmonics of the second audio intermediate frequency _s2 . As can be seen from FIG. 2, there is leakage of the video intermediate frequency _p , which is equal to or higher than the second audio intermediate frequency _s2 , and the first audio intermediate frequency _s1 . If this is output as is to the IC pin, it will have a large effect on other pins. Therefore, an active low-pass filter 2 according to the present invention is installed after the detector 1. The effect is as indicated by the diagonal lines shown in FIG. 2, and the unnecessary components can be attenuated by more than 20 dB compared to the second audio intermediate frequency. Therefore, unnecessary carrier waves often leak when passing through a detection circuit, so a second audio intermediate frequency as shown in Figure 2 is passed through such a portion to eliminate leakage components of the video intermediate frequency and the first audio intermediate frequency. A similar effect can be expected by adding an active low-pass filter 2 for attenuation.

As explained in detail above, the present invention provides an active low-pass filter on the output side of the audio detection circuit that passes the second audio intermediate frequency and attenuates the leakage components of the video intermediate frequency and the first audio intermediate frequency. Therefore, unnecessary components are not output to the terminals of the IC, which is particularly effective in reducing mutual interference of signals in the case of large-scale integrated ICs.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a graph showing the output level of each frequency component of a detector output or an active low-pass filter output. In the figure, 1 is a detector, 2 is an active low-pass filter, 3 is an input terminal, and 4 is an output terminal.

Claims (1)

[Claims] 1. In a circuit for a television receiver that generates a second audio intermediate frequency from a video intermediate frequency and a first audio intermediate frequency, the second audio intermediate frequency is connected to the output side of the circuit that generates the second audio intermediate frequency. An audio detection circuit characterized in that it is provided with an active low-pass filter that allows leakage components of the video intermediate frequency and the first audio intermediate frequency to pass through and attenuates them.