DE3733006A1 - Circuit arrangement for fixing the black level of colour signals in a colour television set - Google Patents

Abstract

In colour television sets which are suitable, for example, for teletext and satellite reception, the black levels of the individual colour signals have to be fixed to a constant value in order to avoid disturbing colour tinges on the screen. The black levels of a number of colour signals (1, 2, 3, 4) of the same chromaticity are fixed to a constant value according to a predeterminable sequence during line flyback. In this process, one colour signal (1, 2, 3, 4) at a time during a line flyback is fed via a switching network (U1, U2, U3), which is driven by a logic circuit (L), to the input of a comparator (V), at the second input of which there is a reference voltage (UR). The output of the comparator (V) is connected to the input of a multiplexer (M1). One output of the multiplexer (M1) is in each case connected to one input of the switching network (U1, U2, U3). <IMAGE>

Description

The invention relates to a circuit arrangement for clamping the Black level of color signals in a color television set.

A precise clamping of the black level is necessary, otherwise it will Image on screen disturbed by a red, blue or green tinge can be. Circuits for clamping the black level should therefore not ne drift due to temperature fluctuations.

Color television sets, e.g. Can play video and teletext are suitable for the reception of satellite signals and that with a SCART input, the color signals of several color si can process signal sources. With the color distant mentioned as an example The red, green and blue signals of the video are therefore visual devices text decoder, teletext decoder, satellite decoder and SCART input to process and to a constant black level jam.

It is therefore an object of the invention to provide a circuit arrangement for the terminal the black level of several color signals of the same color type give.

The invention solves this problem in that the black levels of several Select color signals of the same color type in a predefined order be clamped to a constant value during the line return.

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Fig. 1 shows a first embodiment of the invention

Fig. 2 shows a second embodiment of the invention.

Based on the embodiment shown in FIG. 1, the invention will be described and explained.

In the first exemplary embodiment, four color signals 1 , 2 , 3 and 4 of the same color type, for example red, are clamped to a constant black level during the line return.

The color signal 1 is fed to the first input of a controllable changeover switch U 1 , at the second input of which the color signal 2 is present. The color signal 3 is fed to the first input of a controllable changeover switch U 2 , at the second input of which the color signal 4 is present. The output of the controllable switch U 1 is connected to the first input of a controllable switch U 3 . The output of the controllable switch U 2 is connected to the second input of the controllable switch U 3 , the output of which is connected to the input of a comparator V. At the other input of the comparator V there is a reference voltage UR . The output of the comparator V is connected to the input of a multiplexer M 1 , the outputs of which are connected to an input of the controllable changeover switches U 1 and U 2 . One output each of a logic L is connected to the control input of the controllable changeover switch U 1 , the controllable changeover switch U 2 , the controllable changeover switch U 3 and the multiplexer M 1 .

In Fig. 1, the color signal 1 passes through the controllable changeover switch U 1 and U 3 to the comparator V , where it will be compared with the reference voltage UR . Because the output of the comparator V is connected via the multiplexer M 1 to the first input of the controllable switch U 2 , the black level of the color signal 1 is now clamped to the constant value.

During the next line return, for example, the black level of the color signal 2 is clamped. For this purpose, the logic L switches the controllable switch U 1 and simultaneously actuates the multiplexer M 1 , so that the output of the comparator V is connected to the second input of the controllable switch U 1 . Now the black level of the color signal 2 is clamped.

At the next line return, for example, the black level of color signal 3 may be clamped. For this purpose, the logic L switches the controllable changeover switch U 3 and at the same time brings the controllable changeover switch U 2 into the position which connects its first input to its output. At the same time, the multiplexer M 1 is advanced by the logic L by one step, so that the output of the comparator V is connected via the multiplexer M 1 to the first input of the controllable changeover switch U 2 .

To clamp the black level of the color signal 4 , the controllable changeover switch U 2 is switched over by the logic L and the multiplexer M 1 is simultaneously switched on by one step.

The order of the clamping is arbitrary. Between the individual clamping processes and thus between the individual line returns, the position of the controllable changeover switch U 1- U 3 depends on which color signal, for example that of the teletext decoder or that of the SCART input, is shown on the screen.

For each color type - red, green and blue - such a circuit for clamping the black level is provided in the color television.

However, the invention is by no means limited to the color signals from four color signal sources. Any number of color signals can be processed and their black levels clamped, because all that is required is to expand the switching network built up by the controllable changeover switches U 1- U 3 and to provide the multiplexer M 1 with as many outputs as there are color signals.

In the embodiment shown in FIG. 2, the switching network is constructed as a multiplexer M 2 , with one color signal being fed to an input of the multiplexer M 2 .

Claims (3)

1. Circuit arrangement for clamping the black level of Farbsi signals in a color television, characterized in that the black level of several color signals ( 1 , 2, 3, 4 ) of the same color type are clamped to a constant value according to a predeterminable sequence during the line return. 2. Circuit arrangement according to claim 1, characterized in that each a color signal ( 1 , 2 , 3 , 4 ) via a switching network ( U 1 , U 2 , U 3 , M 2 ), which is controlled by a logic ( L ) is, while a line return is fed to the first input of a comparator ( V ), at the second input of which there is a reference voltage ( UR) , that the output of the comparator ( V ) is connected to the input of a multiplexer ( M 1 ) and that each an output of the multiplexer ( M 1 ) is connected to an input of the switching network ( U 1 , U 2 , U 3 , M 2 ). 3. Circuit arrangement according to claim 1 or 2, characterized in that the switching network as a multiplexer ( M 2 ) with one output and as many inputs as Farbsi signals of the same color type are to be processed.