NotTaR of Television Sets : What is Scan Velocity Modulation?       
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What is Scan Velocity Modulation?

(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

Scan velocity modulation occurs around the transients in the luminance signal. The beam is sped up just before and just after the edge and it is slowed down during the edge. This makes for a sharper edge. On an alternating B/W pattern (stripes, checkerboard) you will see that the white parts get smaller and the black parts get whiter. This geometry error is a side-effect. Some say that this is the main intended effect of SVM.

SVM is SUPPOSED to be used to compensate for the spot blowup at high beam current. Peaking does not help to improve sharpness because the higher peak beam current also gives a fatter spot. SVM CAN work in that case. Unfortunately it is often misapplied, too much SVM will give a very unnatural picture, with obvious horizontal geometry errors.

If applied properly, SVM can improve the picture. Unfortunately there has been a rat race, led by Japanese, suggesting that more is better.

Some people will simply advise turning the contrast down. At low beam current the spot size will be acceptably small and SVM is not needed. In most, if not all, cases they will disable the SVM circuit, usually by pulling the supply connector to the SVM panel. That panel is often fixed to the neck of the picture tube, behind the video amplifier panel.