Most cable systems have used / or still are using a scrambling method using pulsed sync suppression.
This method involves reducing the amplitude of the picture carrier signal by 6db during the
horizontal blanking interval and possibly during the vertical blanking interval. This video
signal has sync tips located between the black and white levels. Sync seperators in the
television cannot operate properly with this signal, nor can AGC and color circuits, since
the picture cannot sync properly the signal appears to be scrambled.
The decoder compensates by attenuating this signal during the time in which the transmitted
signal was not attenuated, in which the logic controlled gain switch must receive the timing
data for proper sync.