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Old 03-24-2015, 04:05 PM
Gleb's Avatar
Gleb Gleb is offline
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Join Date: Mar 2015
Location: Russia
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Color subcarrier visible in retrace on early TVs

Hello colleagues!

When I tried to connect a DVD player to one of my antique TVs, I saw a strange interference on the screen:



I attached that player to a scope and found the source of interference. In the back porch of the horizontal blanking interval there is some oscillation burst, next to the sync pulse:






According to the frequency (4,43MHz), that's the "colorburst" signal used to synchronize color decoders. But it's amplitude is enormously big, exceeding the black level and causing it's visibility on the retrace. Of course, later TVs have built-in retrace blanking circuits. Depending on it, modern video signals often use "unnecessary" blanking intervals as a free space to carry any additional information. First ones were color signals, then they added teletext, copy protections etc. But very early TVs lack their own retrace blanking, depending only on blanking pulses of standardized video signal. So, any shit stuffed to blanking intervals will be shown by retrace lines.

Anybody knows the solution?
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Last edited by Gleb; 03-24-2015 at 05:54 PM. Reason: addition
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