DE2645567A1 - CIRCUIT ARRANGEMENT FOR THE AUTOMATIC TUNING CONTROL OF A TELEVISION RECEIVER - Google Patents

Description

INDESIT INDUSTRIA ELETTRODOMESTICI ITALIANA S.p.A. Rivaita (Turin), Italy

"Circuit arrangement for automatic tuning control of a television receiver "

Priority: October 13, 1975, Italy, No. 69543-A / 75

The invention relates to a circuit arrangement for automatic tuning control of a television receiver for a television system, in which a video carrier is amplitude modulated and an audio carrier is frequency modulated, the television receiver at least one common amplifier for the two modulated carriers, a video detector to obtain the demodulated video signal and an audio frequency discriminator for obtaining the demodulated audio signal.

Television receivers of this type are in common use in most countries (excluding France and a small number of other countries that use amplitude modulation for the audio signal). With such recipients the audio signal is normally obtained downstream from the video detector by a detection process

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for picking out slips between the two beams. This so-called intermediate carrier system has the advantage that the vote is not very critical and a. more decisive Factor is only for the video signal, which allows for a much larger mismatch than the audio signal. A mismatch of 50 KHz, for example, destroys the audio information while it is completely acceptable for the video information.

However, the subcarrier system suffers from the disadvantage that there is always a certain intermodulation between the audio and the video signals is generated, which is kept within acceptable limits, but is still present and an obstacle represents for television receivers to produce a sound reproduction with high acoustic fidelity, moreover, this Intermodulation emphasizes video signals that correspond to high visual contrast or contain certain frequencies.

In many quality television receivers an automatic tuning control circuit is provided which generally consists of an There is a frequency discriminator, which is tuned to the frequency of the converted video frequency and from which a control signal (generally a DC voltage) is obtained for the frequency conversion receiver oscillator of the television receiver. this is the reason why, despite the mentioned non-critical property of the intermediate carrier system with regard to Misalignment (in terms of sound) as desirable

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has been considered television receivers with an automatic

To provide frequency control, on the one hand, to prevent a drift of the (local) /

/ To correct the frequency of the receiver oscillator and it on the other hand to make it easier for the user (who usually cannot improve the tuning). This consideration applies especially for color television receivers, where a mismatch is more damaging.

The object of the present invention is to provide a simple but effective automatic tuning control circuit for a television receiver to make available, which avoids the aforementioned disadvantages of the intermediate carrier system.

This object is achieved with a circuit arrangement for the automatic tuning control of a television receiver for a television system in which a video carrier is amplitude modulated and an audio carrier is frequency modulated, the Television receiver at least one common amplifier for the two modulated carriers, a video detector for receiving of the demodulated video signal and an audio frequency discriminator for obtaining the demodulated audio signal, the circuit arrangement according to the invention being characterized in that the audio signal from the video detector upstream amplifier is removed and that the signal for tuning control is derived from the audio frequency discriminator is.

The circuit arrangement according to the invention has the advantage that

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a good separation of the audio and video modulation can be obtained while automatic control of the frequency of the local oscillator and that therefore correct tuning of the television receiver is maintained.

In the following the invention is based on an embodiment explained in more detail. The accompanying drawing shows a schematic block diagram of part of a television receiver circuit, which contains a circuit arrangement according to the invention for automatic voice control.

In the drawing, the reference number 1 denotes a receiving antenna, which picks up the television signals and passes them on to a high-frequency input amplifier 2.

From the input amplifier 2, the amplified signals pass to a mixer M, which, according to the known superposition method, also receives a receiver oscillator signal that is transmitted by a receiver oscillator VCO has been generated with a controllable frequency. From the output of the mixer M v / ground signals with reduced Frequency (intermediate frequency) given to an intermediate frequency amplifier IF, the selective circuits for implementation a separation of the video and audio components. The intermediate amplifier IF generates two output signals, viz a video signal (which consists of an amplitude-modulated video carrier tuned to an intermediate frequency of, for example 38.9 MHz) and an audio signal (which consists of a

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frequency-modulated audio carrier that is converted into an intermediate frequency of, for example, 33.4 MHz is implemented).

The video signal at the intermediate frequency passes through a further amplifier 3 and from there to a video demodulator 4, from which the demodulated video signal via a line 5 to amplification circuits (not shown) for the demodulated video signal, to sync separation (or sync split) circuits and in the case of a color television receiver is placed on a chrominance section. These circuits are not shown in the drawing as they are used for are not important to an understanding of the present invention.

The audio signal with an intermediate frequency arrives at a further amplifier 6 and from there to a frequency discriminator 7. The discriminator 7 receives a demodulated audio signal on a line 8 and sends it to a (not shown) Given low frequency audio section. The discriminator 7 also generates a control signal (DC voltage) having a value and a polarity which is a function of the misalignment the center frequency of the audio signal with respect to a selected one Frequency are, and which is given over a line 9 to the receiver oscillator VCO in order to control its frequency.

The operation of the circuit arrangement is from the preceding Description and understandable from the drawing. The control voltage obtained from the audio signal discriminator 7 serves to set the receiver oscillator VCO on the correct one

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Maintain frequency and thus always ensure correct coordination. It is because of this control that the main disadvantage is of the non-subcarrier system, i.e. the critical nature of voting, is eliminated. It is therefore possible to use the to benefit from the known advantages of the non-subcarrier systems, i.e. from the better separation of the sound modulation from the video modulation.

It is well known that in the case of subcarrier systems, despite every precaution, a certain modulation with 50 Hz (the vertical frequency) superimposed on the audio signal, especially in the case of scenes with high visual contrast (e.g. white Inscriptions on black background), due to the non-linearity of the signal paths of the two signals in common Circuits.

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Claims (3)

Claims Χ. ¹ circuit arrangement for the automatic tuning control of a television receiver for a television system, in which a video carrier is amplitude-modulated and an audio carrier is frequency-modulated, the television receiver having at least one common amplifier for the two modulated carriers, a video detector for receiving the demodulated video signal and an audio frequency discriminator for receiving the demodulated audio signal characterized in that the audio signal is picked up by the amplifier (IF) connected upstream of the video detector (4) and that the signal (9) for tuning control is derived from the audio frequency discriminator (7). 2. Circuit arrangement according to claim 1 for a television receiver, which is a frequency conversion receiver with a receiver oscillator whose frequency can be controlled , acts, characterized in that the tuning control signal (9) controls the frequency of the local oscillator (VCO). 3. Circuit arrangement according to claim 1 or 2, characterized in that that the control signal is a DC voltage, the value and polarity of which depends on the Mismatch of the audio signal with respect to a selected frequency depends. 709815/0925