JPS5811096Y2 - SIF circuit - Google Patents

Description

[Detailed Description of the Invention] This invention relates to an SIF circuit for a television receiver.

FIG. 1 shows a block diagram of a video detection and SIF circuit of a conventional television receiver.

In the figure, 1 is a video detector, 2 is an SIF circuit, 3 is an input filter, 4 is a limiter amplifier, and 5 is an FM detector.

As is well known, the video detector 1 uses a video intermediate frequency carrier wave (
A type of heterodyne detection, that is, ink carrier detection operation is performed using the audio intermediate frequency carrier (hereinafter referred to as the SIF signal) as the local signal and the audio intermediate frequency carrier (hereinafter referred to as the SIF signal) as the local signal. ) is being taken out.

Here, if the frequency of the P-carrier is fp and the frequency of the S-carrier is f5, the output of the video detector 1 is 0UT1.
A signal having the relationship OUT 1=nfp±mfs (n, m are integers) is generated.

This is necessarily derived from the fact that the video detector 1 performs a heterodyne detection operation as described above.

Here n=1. When m=1, OUT 1=f p f s Then, the required SIF signal is extracted.

In this case, n=1. All signals having frequency relationships other than m=1 are unnecessary components.

However, along with this unnecessary component, the output of video detector 1 is 0.
Output 0UT1 that occurs in UT1 and contains these unnecessary components
is led to the SIF circuit 2, which is one of the causes of a harsh noise called a buzz sound being mixed into the audio signal.

Now, if we consider a European television receiver as an example, the above-mentioned fp and fs have the following frequency relationship in the European system.

fP = 38.9M, f 5 = 33.4M East P carrier and S carrier having such a frequency relationship are detected by the video detector 1 as ink carriers, and have a spectrum as shown in Figure 2. U signal occurs at output 0UT1
Only frequency components below S are listed).

Required regular signal SIF signal frequency f SIF
is f 5IF= f p f 5=38.9 33.4
= 5.5 (M City), and the other spectrum is an unnecessary component.

Here, the unnecessary components have the following frequency relationship. kf5-(
k-1) fp=fp-k(fp-fs)=fpkfsI
F and lfs+F (where l is both an integer) Now, looking at the vicinity of f s+F-5,5Mton, which is a regular SIF signal, in addition to the regular SIF signal, we set it to = 6 using the above relational expression. Unnecessary component p 6fs.

F=38.9 33=5.9 (M City) occurs.

On the other hand, the SIF circuit 2 includes an FM detector 5 that provides demodulated output for frequency modulated waves;
It is well known that the characteristic is an S curve as shown in FIG.

Therefore, the signal 0UT containing the above-mentioned unnecessary components 5.9M...
Consider the case where 1 is applied to the SIF circuit 2.

As shown in FIG. 1, in the SIF circuit 2, 5.5 MH
Since there is a human-powered filter that resonates at Produces almost no output.

However, since the unnecessary component of 5.9 MHz is only 7% higher than the resonance point of input filter 3 at 5.5 MHz, it is not sufficiently attenuated by input filter 3, and passes through limiter amplifier 4 to FM detector 5.
, and an output will be generated at the output 0UT2.

The output to the output 0UT2 due to this unnecessary component of 5.9 MHz is a cause of buzz noise prevention.

Buzz noise caused by the above-mentioned generation mechanism cannot be prevented by the conventional method of improving the limiter characteristics of the limiter amplifier 4.

In addition, in the method of improving the skirt characteristic of the input filter 3, the unnecessary component of 5.9 MHz is
It is difficult because it is too close to MHz.

In other words, improving the skirt characteristic of the input filter 3 also narrows the passband width, giving distortion to the output of the FM detector 5.

Furthermore, it is extremely difficult to improve only the skirt characteristics without changing the passband width.

Therefore, an object of this invention is to provide an SIF circuit that can remove buzz caused by unnecessary frequency components caused by ink carrier detection.

In summary, this invention focuses on the fact that the above-mentioned unnecessary components are one of the causes of buzz noise interference, and provides a trap for the unnecessary component frequencies to prevent these unnecessary components from reaching the FM detector.

An embodiment of this invention is shown in FIG.

That is, in the SIF circuit 2', the output 0UT1 of the video detector 1 is applied to the input filter 3 through the trap circuit 6.

This trap circuit 6 constitutes a bridged T-type trap circuit, and its characteristics are as shown in FIG.

At this time, the trap frequency f1 roughly has the following relationship.

If the inductance L of the trap circuit 6 is adjusted so that f = 5.9 MHz, the output 0UT2 of the SIF circuit 2' will be the overall characteristic including the trap circuit 6. As shown in Figure 6, unnecessary component = 5.9M
Hz does not occur at the output 0UT2, and it is possible to eliminate the cause of buzz noise interference that has conventionally occurred, and it is possible to receive good audio.

Although the above explanation has been made regarding a European system television receiver, the same applies to television receivers according to other broadcasting systems.

For example, in the Japanese system, the above-mentioned unnecessary components are as follows.

f p = 58.75 MHz f5 = 54.25 M Regular SIF signal = 4.5 MHz Unnecessary components near 4.5 MH2 = 4.75 M± And it is possible to remove these buzz noise interferences in the same way. it is obvious.

As described above, this invention eliminates the unnecessary frequency components generated by ink carrier detection, that is, the buzz caused by the beat frequency components generated between the harmonics of the video intermediate frequency carrier wave and the audio intermediate frequency carrier wave, using an extremely simple configuration. It has the effect of being able to be removed.

[Brief explanation of the drawing]

Figure 1 shows the video detection and S of a conventional television receiver.
The block diagram of the IF circuit, Figure 2 is the spectrum of the output signal of the video detector in a European television receiver, Figure 3 is the characteristic diagram of the FM detector of the same television receiver, and Figure 4 is the spectrum of the output signal of the video detector in the European television receiver. A block diagram of an embodiment of the invention,
FIG. 5 is a characteristic diagram of the trap circuit, and FIG. 6 is a characteristic diagram of the SIF circuit of FIG. 4. 1... Video detector, 2'... SIF circuit, 6... Trap circuit.

Claims (1)

[Scope of utility model registration request] An SIF circuit comprising a trap circuit connected to an output end of a video detector that performs an ink carrier detection operation to remove a beat frequency component generated between harmonics of a video intermediate frequency carrier wave and an audio intermediate frequency carrier wave.