The invention relates to a video circuit for generating video signals for displaying pictures on a picture screen, the video circuit comprising a picture signal source, at least one luminance circuit coupled thereto for generating a luminance signal, and a protection circuit for protecting the picture screen from burning-in due to a prolonged stationary picture.
Video signals of this type are used for generating at least a Y-luminance signal and/or R-, G- and B- luminance signals for the pictorial display of information from an information processing device, including Teletext and Viewdata systems as well as video-games.
The luminance signals can be applied to corresponding inputs of a display device or can be modulated on a carrier and applied to an aerial input of a television receiving device.
It frequently happens that a certain picture remains stationary on the picture screen for a long period of time, so that bright portions of the picture may cause an accelerated local burning-in of the picture screen. Particularly, it often happens that users forget to switch off a television game so that, for example, a line pattern of a playing-field remains on the screen for a whole night. The video circuit is provided with a protection circuit which tries to obviate this harmful effect.
A video circuit of the type defined in the preamble is known from Canadian patent application No. 267,742. In this video circuit the protection circuit comprises a temperature-dependent element which has the effect that the picture to be displayed is slowly moved over a short distance across the picture screen due to variations in the ambient temperature. However, it appeared that in rooms having a thermostatically controlled heating system, a temperature is adjusted at the beginning of a night which remains very constant thereafter, which is partly caused by the fact that, for example, doors and windows remain closed for a long period of time so that the picture can still remain stationary hours on end in the same position on the picture screen for the remainder of the night.