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Picture makes your screen be a SW emitter








Display following animated picture on your screen. Make it take as much surface as possible. Switch on a standard home radio receiver, tune it on Short Waves. At the right frequency you will hear a buzzing sound that changes pitch together with the picture pattern.

The SW frequency the monitor will emit is one fourth the pixel rate frequency your video card sends to the monitor.
A standard 800x600 video mode at 56 Hz frame rate and 36 MHz pixel rate will emit at a SW frequency of 36/4 = 9 MHz.
A standard 640x400 video mode at 60 Hz frame rate and 25.175 MHz pixel rate will emit at a SW frequency of 25.175/4 = 6.294 MHz.





How comes ? By asking the monitor to display alternatively the white and black pixels of the horizontal picture lines, the video card produces a steady oscillating signal. (When the video card wants a white pixel it sends a given amount of current to the monitor. When it wants a black pixel it sends no current.) This signal has the shape of a SW radio signal. Should your video card and monitor (and the wire that links the two) be perfect this signal should not come to the outside world. But nothing's perfect, and a good radio receiver is very sensitive.

By switching the SW signal on (horizontal lines of black and white pixels) and off (horizontal lines of steady color), the monitor modulates a simple square signal at a hearable frequency.

(When electrons somewhere are moved back and forth, other electrons a while further will be forced to move back and forth too. A radio emitter is an electronic device that makes the electrons inside its vertical antenna move upwards and downwards very strongly. The electrons inside the vertical antennas of every radio receiver nearby will therefor be moved very weakly upwards and downwards. The electronics inside the receiver records and amplifies this movement . Thus the receiver knows at which force the electrons are moved inside the emitter antenna. Thus the emitter can send some information towards the receiver. For example the movement that the loudspeaker of the receiver should make. In the case of our example, while the dotted lines are being displayed the loudspeaker is pushed outwards and while the steady lines are being displayed the loudspeaker is pulled inwards.)

Changing the video mode changes the SW emit frequency. The SW emit frequency can be tuned precisely by scaling the picture horizontally. Multiply the picture width by 1.1 divides the SW emit frequency by 1.1.

The video mode also influences the received sound frequency. This frequency can be tuned precisely by scaling the picture vertically. Multiply the picture width by 1.1 divides the sound heard by 1.1.




Eric Brasseur  -  22 April 2000
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