JPS6031339Y2 - Audio intermediate frequency signal identification circuit for television receivers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an audio intermediate frequency signal identification circuit used in a television receiver or the like which is capable of receiving several types of television broadcasts having different ink carrier frequencies.

For example, audio intermediate frequency (SIF) signals with different ink carrier frequencies, such as 4.5 MHz for CCIR M system and 5.5 MHz for BG system, are received and reproduced by a single audio receiving circuit of a television receiver. In order to make this possible, special measures are required for the audio receiving circuit, and the audio receiving circuits shown in FIGS. 1 and 2 have been proposed as audio receiving circuits that take such consideration.

The one shown in Figure 1 is called the converter method,
For example, now, from the SIF' detection circuit 1, the above-mentioned 4.5MHz
Assuming that a 5.5 MHz SIF signal is derived, an IMHz local oscillator 3 provided in the audio reproduction circuit 2 is used, and the bandpass filter 5 and SIF demodulation circuit 6 are configured for 5.5 MHz. By setting the oscillator 3 and operating the oscillator 3 when receiving 4.5 MHz, the mixing circuit 4 converts the 4.5 MHz SnF signal into a 5.5 MH2 signal, thereby causing the SIF' demodulation circuit 6 to always have a 5.5 MHz signal. A 5 MHz signal is introduced.

Moreover, in the one shown in FIG. 1, the bandpass filter 5 and the SIF
'The demodulation circuit 6 is set for 4.5MHz, and the
It can also be configured to convert a 5.5 MHz signal into a 4.5 MHz signal by operating the local oscillator 3 when receiving MHz.

On the other hand, the circuit shown in Fig. 2 is called a demodulation band switching system, and in this circuit, the bandpass filter 7 in the audio reproduction circuit 2 is made into a wideband filter that can cover the 4.5MHz band and the 5.5MHz band. Also, when receiving 4.5MH2 and 5.5
The demodulation band of the SIF' demodulation circuit 8 is changed by switching the band selection circuit 9 depending on when receiving MHz.

Now, in the audio receiving circuit of FIGS. 1 and 2, the SI
The SIF signal derived from the F detection circuit 1 is 4.5MHz
It is necessary to turn on and off the local oscillator 3 or switch the band selection circuit 9 depending on whether the frequency is 5.5 MHz or 5.5 MHz.

For this reason, the frequency of the SIF signal (ink carrier frequency) is identified, and the above-mentioned switching operation is performed based on the identification output. It means a signal for.

Conventionally, an SIF identification circuit for creating such a signal has been configured as shown in FIG. 3, for example.

In FIG. 3, 10 is an SIF identification circuit, and this circuit includes a bandpass filter 11 to which the SIF signal from the SIF detection circuit 1 is input, a demodulation circuit 12 to which the output is input, and a demodulation circuit 12 to smooth the output. It is composed of a DC conversion circuit 13.

At that time, the bandpass filter 11 is the SIF detection circuit 1.
The SIF signal derived from 4.5 MHz and 5.5 MHz
Hz, a wideband filter that allows both to pass through is used, and a narrowband filter that allows only one of the SIF signals to pass is used. The demodulation circuit 12 is also selected according to the frequency band 11.

That is, when a narrowband filter 11 is used, the demodulation circuit 12 can also be configured as an AM detection type.

Note that 2 is an audio reproduction circuit similar to that shown in FIGS. 1 and 2.

By the way, in the identification circuit 1 of FIG. 3, since the SIF' signal derived from the SIF detection circuit 1 is identified over its entire period, the following problem arises.

That is, (a) malfunction due to harmonic components of the video signal having a frequency comparable to the ink carrier frequency included in the SIF signal, (malfunction due to color subcarrier included in the βIF signal signal, and (c) demodulation circuit 12 This is due to amplitude changes in the SIF signal (malfunctions caused by AM modulation intensity components due to video signals) when an AM detection circuit is used.

Therefore, the present invention proposes an SIF identification circuit that eliminates these drawbacks.

That is, in the identification circuit of the present invention, in FIG. 4, which shows the same components as those in FIG.
4 is inserted at the illustrated position in the identification circuit 10 or between the SIF detection circuit 1 and the bandpass filter 11.

FIG. 5 shows a specific embodiment of such an identification circuit 10.

In the figure, a bandpass filter 11 made of a ceramic filter or the like is used for 5.5 MHz, so only the 5.5 MHz signal among the SIF signals is passed, and the 4.5 MHz signal is blocked.

On the other hand, a horizontal pulse is introduced into the gate circuit 14 via the resistor R, and therefore, only during the horizontal pulse period, the first transistor TR□ is forward biased and becomes active.

Therefore, only the 5.5 MHz SIF' signal extracted during the horizontal pulse period appears at the collector of the first transistor 'm□.

The output signal from the collector of the first transistor TR1 is applied to the base of the second transistor TR2 in the demodulation circuit 12 via the coupling capacitor C1.

This second transistor operates as an amplifier, and its output signal is passed through a coupling capacitor C3 to a third transistor T.
Applied to the base of R3.

The third transistor TR3 is set by resistors R0 and R1o so that its base bias voltage is slightly lower than the cutoff point, and operates as a kind of peak detector.

Therefore, this third transistor is cut off when there is no input signal, that is, when receiving 4.5M H2, and therefore the fourth transistor of the DC conversion circuit 13 is also cut off, and the collector voltage of this fourth transistor is The identification signal becomes O■.

On the other hand, the second transistor T is connected to the third transistor T'R3.
When the signal from R2 is applied, that is, when receiving 5.5 MHz, a pulse voltage having a peak at the power supply voltage (10 B) appears periodically at the collector of this third transistor.

Since this pulse voltage is smoothed by a capacitor C1 having a sufficiently large capacity with respect to the horizontal frequency, the base-emitter of the fourth transistor 2 is forward biased.

As a result, this fourth transistor 'rR4 is turned on and a positive voltage appears at the collector, and when this voltage is smoothed by a large capacity capacitor similar to 04, a positive voltage is extracted as an identification signal. .

In addition, in the explanation so far, 4.5M H2 and 5.5M
We took the case of receiving a Hz SIF signal as an example, but
According to other methods of CCIH, there are 6 other SIF signals as well.
．． Since there are 0 MHz and 6.5 MHz, the same applies to the case where two or more of them are to be identified.

As described above, the identification circuit of the present invention gates the SIF signal derived from the SIF detection circuit with a horizontal pulse or horizontal synchronization signal, and identifies the ink carrier frequency with respect to the gated signal. , has the following advantages:

That is, there is no video signal during the horizontal retrace period, and furthermore,
Since the degree of modulation is also very stable during this period, malfunctions caused by harmonics of the video signal or AM modulation components of the SIF signal described above can be eliminated.

In addition, since there is no influence from the AM modulation component, the demodulation circuit in the identification circuit can be replaced with a relatively inexpensive AM modulation circuit as in the embodiment shown in FIG.
This means that it can be configured as an M (peak) detection type.

In addition, if the influence of the burst signal inserted on the back porch during the horizontal retrace period becomes a problem, the SIF
The signal may be gated with a horizontal synchronization signal instead of a horizontal pulse.

Which one to adopt may be appropriately selected depending on the type of bandpass filter and demodulation circuit in the identification circuit.

[Brief explanation of the drawing]

Figures 1 and 2 show SI of different ink carrier frequencies.
A block diagram showing the general configuration of a television audio receiving circuit capable of receiving the F' signal, FIG. 3 is a block diagram showing a conventional example of an identification circuit used therein, and FIG. 4 shows an example of an identification circuit according to the present invention. FIG. 5 is a block diagram schematically showing an example of an implementation circuit.

Claims (2)

[Scope of utility model registration request] (1) In a television audio receiving circuit capable of reproducing various audio intermediate frequency signals having different ink carrier frequencies, the operation of the audio receiving circuit is controlled according to the ink carrier frequency of the received audio intermediate frequency signal. The circuit for identifying the ink carrier frequency for switching, gates the audio intermediate frequency signal with a horizontal pulse or horizontal synchronization signal, and identifies the ink carrier frequency with respect to the gated signal. An audio intermediate frequency identification circuit for a television receiver, characterized in that: (2) The identification of the ink carrier frequency is configured by a combination of a narrow band filter whose pass band is one of the ink carrier frequencies and a detection circuit that detects the peak of the output as described in claim 1. audio intermediate frequency signal identification circuit.