Abstract: To improve pictures to be displayed on a display screen of a display device, it is known to use scan velocity modulation. In scan velocity modulation the (horizontal) deflection rate of the electron beam(s) is modulated with the luminance component of the video signal. As a result of scan velocity modulation, the information of the video signal will no longer be displayed at the correct position on the display screen. By using the modulation signal applied to the scan velocity modulator also for modulating the (read) clock rate of the video signal from the memory, it can be ensured that the video signal and the (modulated) deflection signal are always in synchronism with each other.

Abstract: A ventilation panel comprising a rectangular grating for electromagnetic shielding which is arranged in a rectangular frame and has a cell structure. The grating is resiliently enclosed in the frame by means of a comb-shaped mounting spring which also holds the complete ventilation panel resiliently clamped between two walls of a box with electrical components. The mounting spring further provides an electromagnetic shielding at the area of the separation between the ventilation panel and wall portions of the box. The ventilation panel is used in racks for data-processing apparatus, such as public telephone exchanges.