<<< The problem we are having is getting the black levels to be 7.5 IRE and not
0.0 IRE. …it always renders it at a 0.0 IRE black level whether we turn on
Superblack or not. We are looking at the signal through a waveform monitor to
be sure. >>>
Since you are using an AT Vista, which requires an outboard encoder for NTSC
video, check your video encoder settings, it may have a switch or pot labeled
SETUP or PEDESTAL or BLACK LEVEL. You can adjust this control so that 0 RGB =
7.5 IRE (or it may be just an ON/OFF switch). If your encoder does not have
this control then I'd use Gary's idea of making your black value brighter.
This way if you need 0 black for keying you don't have to mess with your video
hardware.
Paul