1. Field of the Invention
The invention relates to a display device comprising a display tube for displaying a video signal, the display tube comprising focusing electrode means, a video signal processing circuit for receiving an input video signal, processing this video signal and supplying this processed video signal to a video output stage which is coupled to at least one control electrode of the display tube for supplying an output video signal, said video signal processing circuit including a black level setting circuit for setting the black level of the video signal, an output of the video output stage being coupled to a measuring input of the black level setting circuit for supplying a measuring signal, and a focusing voltage generating circuit for driving the focusing electrode means, the focusing voltage generating circuit having an input for receiving a line-frequency signal.
2. Description of the Related Art
A display device of this type is described in the non-prepublished European Patent Application no. 92203393.1 corresponding to U.S. Pat. No. 5,278,476 (PHN 13.885). The display device described in this Patent includes a black level setting circuit for setting the black level. During the field retrace interval, for example, two different control signals are consecutively applied from the video signal processing circuit to the video output stage. The video output stage applies the output video signal to a cathode of the display tube. The video output stage also has an output for supplying a measuring signal which is applied as a feedback signal to the video signal processing circuit. This measuring signal is compared in the black level setting circuit of the video signal processing circuit, with a reference value and, if necessary, the black level is subsequently adapted.
A drawback of this known display device is that the measuring signal comprises line-frequency interference components so that the black level cannot be accurately adjusted, resulting in the occurrence of color errors.