DE2645567C2 - Circuit arrangement for the automatic tuning control of a television receiver - Google Patents

Description

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

Television receivers of the type mentioned are in common use in most countries (Except France and a small number of other countries that use amplitude modulation for the audio signal use). In such receivers, the audio signal is normally downstream from the video detector, through a detector process to pick out beats between the two carriers. This so-called intermediate carrier system has the advantage that the vote is not very critical and is only a determining factor for the video signal is. which allows for a much larger mismatch than the audio signal. A mismatch of 50 kHz, for example, destroys the audio information. while for the Vidcoinformation completely is acceptable.

However, the subcarrier system suffers from that Disadvantage that there is always a certain intermodulation between the audio and video signals which is kept within acceptable limits, but is still present and an obstacle for television receivers represents to produce a sound reproduction with high acoustic fidelity. Besides

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55 these intermodulation is emphasized video signals corresponding to a high visual contrast or contain certain frequencies.

In many quality television receivers (see e.g. DE-AS IO 96 408) is an automatic tuning control circuit provided, which generally consists of a frequency discriminator based on the frequency of the implemented Video frequency is tuned and from rlem a control signal (generally a DC voltage) for the Frequency conversion receiver oscillator of the television receiver is obtained. This is the reason that it despite the mentioned non-critical property of the subcarrier system with regard to mismatching considered desirable (in terms of sound) has been. To provide television receivers with an automatic frequency control to the one hand a To correct the drift of the (local) frequency of the receiver oscillator and, on the other hand, to the user easier to make (which usually does not improve the tuning). This consideration is especially true for color television receivers where mis-tuning is more damaging.

The invention is based on the object of providing a circuit arrangement of the type mentioned at the beginning create a good one while maintaining the automatic control of the frequency of the receiver oscilloscope To enable separation of the audio and video modulation.

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. wherein the television receiver has at least one common amplifier for the two modulated carrier, a video detector for obtaining the modulated video signal and an audio frequency discriminator for obtaining the demodulated audio signal, wherein the circuit arrangement is characterized according to the invention. that the audio signal from the upstream of the video detector Amplifier is removed and that the signal for tuning control from the audio frequency discriminator is derived.

The circuit arrangement according to the invention has the advantage that a good separation of the audio and the Video modulation can be obtained while automatically controlling the frequency of the receiver oscillator and that therefore correct tuning of the television receiver is maintained will.

In the following the invention is illustrated by means of an embodiment explained in more detail. The drawing shows a schematic block diagram of part of a television receiver implementation. a circuit arrangement according to the invention for automatic tuning control contains.

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

From the input amplifier 2, the amplified signals pass to a mixer M. which, in accordance with the known superposition method, also receives a receiver oscillator signal that has been generated by a receiver oscillator VCO with a controllable frequency. From the output of the mixer Λ / signals with reduced frequency (intermediate frequency) are sent to an intermediate frequency amplifier IF , the selective one

Includes circuitry for effecting separation of the video and audio components. The repeater / Fer generates two output signals, namely a video signal (that of an amplitude-modulated There is a video carrier, which is converted to an intermediate frequency of 38.9 MHz, for example) and an audio signal (which consists of a frequency-modulated audio carrier converted to an intermediate frequency of, for example 33.4 MKz is implemented).

The video signal with the intermediate frequency passes through a further amplifier 3 and from there on a video modulator 4, from which the demodulated video signal is fed via a line 5 to amplification circuits (not shown) for the demodulated video signal, on synchronization separation (or synchronization separation) circuits and, in the case of its color television receiver, is given to a chrominance section. These Circuits are not shown in the drawing as they are not necessary for an understanding of the present invention are important.

The audio signal with an intermediate frequency arrives at a further amplifier 6 and from there to a frequency discriminator 7. A demodulated audio signal is received from the discriminator 7 on a line 8 and sent to a low-frequency audio section (not shown). The discriminator 7 also generates a control signal (DC voltage) having a value and a polarity which are a function of the misalignment of the center frequency of the audio signal with respect to a selected frequency, and which is sent via a line 9 to the local oscillator VCO in order to control its frequency .

The mode of operation of the circuit arrangement can be understood from the preceding description and from the drawing. The control voltage received from the audio signal discriminator 7 is used to keep the external oscillator VCO at the correct frequency and thus always ensure correct tuning. Because of this control is the main drawback! of the non-media carrier system, ie the critical nature of voting, is eliminated. It is therefore possible to benefit from the known advantages of the non-intermediate carrier systems, ie from the better separation of the sound modulation from the video modulation.

It is known that in subcarrier systems, despite every precaution, a certain modulation with 5 ^r : Hz (the vertical frequency) is superimposed on the audio signal, especially in the case of scenes with high visual contrast (for example with white inscriptions on a black background), due to the non-linearity of the signal paths of the two signals common circuits.

1 sheet of drawings

Claims (3)

Patent claims: 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 taken from 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! for a television receiver which is a frequency conversion receiver with a receiver oscillator whose frequency can be controlled, characterized in that the tuning control signal (9) controls the frequency of the receiver oscillator (VCO). 3. Circuit arrangement according to claim 1 or 2, characterized in that it is the control signal is a DC voltage whose value and polarity depend on the misalignment of the Audio signal with respect to a selected frequency «> iib'.i depends.