stekas@hou2g.UUCP (J.STEKAS) (05/16/84)
What is sine wave encoding and who uses it? How does the scrambled
picture look compared to one with the synch pulses "clipped" off?
Just curious,
Jimfish@ihu1g.UUCP (Bob Fishell) (05/16/84)
Sine wave encoding is used by ON TV and several cable companies. It
is a very simple encoding scheme which consists of a 15.1 KHz sine
wave algebraically added to the video signal prior to CW modulation.
The scrambled picture usually has an large, unstable wavy line in
the middle of the screen that occasionally moves aside to reveal an
almost-intelligible picture, especially on sets with good AGC sections.
The audio is on a subcarrier, not in the usual audio band.
Pirate decoder kits defeat sine wave encoding by extracting the 15.1 KHz
scrambling signal, inverting it, and feeding it back into the AGC of the
IF section, creating a negative feedback loop which stabilizes the video
signal -- sort of. The set's or VCR's AGC still plays a role in
stabilizing the picture. Picture quality on the pirate decoders that
I've seen ranges from poor to fair, and the sound leaves a lot to
be desired.
--
Bob Fishell
ihnp4!ihu1g!fish