Split Sync Scrambling
Split Sync Scrambling – Video scrambling technique, used with horizontal blanking inversion, active video inversion, or both. In split sync, the horizontal sync pulse is “split”, with the second half of the pulse at +100 IRE instead of the standard -40 IRE. Depending on the scrambling mode, either the entire horizontal blanking interval is inverted about the +30 IRE axis, the active video (after color burst and until the beginning of front porch blanking) is inverted about the +30 IRE axis, both are inverted, or neither is inverted. By splitting the horizontal sync pulse, a reference of both -40 IRE and +100 IRE is available to the descrambler. Since a portion of the horizontal sync is still at -40 IRE, some sync separators may still lock on the shortened horizontal sync pulses. However, the timing circuits that look for color burst a fixed interval after the beginning of horizontal sync may be confused. In addition, if the active video is inverted, some video information may fall below 0 IRE, possibly confusing sync detector circuits. The burst is always present at the correct frequency and timing, however, the phase is shifted 180 degrees when the horizontal blanking interval is inverted.
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