Cable TV - Sync Attentuation or Suppression

This technique is used to reduce the amplitude of the sync pulses of the video signals by attenuating them at the cable 'headend'. Sometimes the required sync-pulse-timing information is then sent as an amplitued-modultaed signal along with the audio signal, (I.E. the encryption data)legitimate descramblers use this timing information to trigger the generation of new sync pulses which are added to the video signals in the descrambler.

Other versions of this technique can vary the amount of sync-pulse attenuation from line-to-line in a pesudo- random manner, with the encryption signal telling the descrambler how to keep up with the changes in the transmitted waveform.

These techniques are only partially successful since many modern receiver designs have synchronising circuits that can keep a picture synchronised even if only a few properly timed synchronizing signals are received from time to time, and in any case most video signals contain enough repetitive data to enable some receivers to synchronise in the absense of perfect sync pulses.

A more effective method of using sync supression to achieve a secure scrambling method is to add a large-amplitude sine wave at line frequency to the baseband video signal. Since the receiver sync-detection circuits will be looking for the most-negative-going portion of the demodulated video signal which is ususally the bottom of the sync pulses, they will lock on to the most negative part of the sine wave, thus making nonsense of the transmitted picture. Once again the encrypted data tells the receiver to compensate for the sine wave in which has been transmitted in the form of low-level amplitude modulation of the audio carrier.

Yet another form of using sync supression is by modification of the sync pulses is to gate-out the sync pulses on transmission by adding a line-frequency gating waveform of the opposite polarity to that of the sync pulses, so that they are effectively cancelled out, or significantly reduced in amplitude. The inverse of the gating waveform is then transmitted as amplitude modulation of the audio carrier or sometimes on a completely seperate carrier, and when this signal is added to the main carrier signal in the descrambler the sync pulses are effectively restored to normal.

One simple way of implemeneting sync supression is to covert the incoming video signal into a composite video signal and ad a DC level to the signal giving the signal a DC offset, if enough of an offset is obtained the waveform is raised high enough so the television can longer synch thus the picture begins to roll and jitter from side to side, appearing to be a scrambled signal when this is just the TV simply not being able to find the sync pulses at the usual location.

To identify a sync suppressed signal simply tune to the channel and look for a wavy bar in the middle of your screen. This bar will contain 4 colors [ WHAT, BLACK, MULTIPLE, WHITE ]:--BLACK = Horizontal Blanking Interval--MULTIPLE = Colorburst, The picture is partially visible in the background but is out of alignment.

SSAVI = Suppressed Sync And Video Inversion

Identifying SSAVI Suppression Signals

To identify these scrambled signals simply tune to the channel look for the same type of signal as a normally sync supressed signal would appear but look for a line near the top portion of the screen for a diced white line, kinda looks like a row of white blocks, this is actually the data needed to descramble this particular channel. The data is being sent during the Vertical Blanking Interval on lines 10 to 13.

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