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.
