Accom ASWR8150 Technical Manual page 119

Signal Chassis
The video for each channel is read out of the transform framestores, with
addresses generated by that channel's transform controller, and sent to
the output interpolators. The transformed video and generated key
information from each channel passes to the combiner, which
composites the two channels together.
The combining process uses either a fixed key priority (A over B or B
over A) or a Z (depth) based key priority. In Z key mode, the Z position
of each channel in 3D space determines its priority relative to the other
channel.
The graphics output consists of axis grids and channel identifiers for
each channel.
Video/Key Mode
The Video/Key mode differs from the Video/Video mode only in that
the board processes the B channel as the key signal for the A channel,
with additional key clip, gain, and horizontal phase adjustments. Note
that the chrominance processing of the B channel is not used in this mode
since key signals have luminance information only. This makes the A
channel 4:2:2 and the B channel 4:0:0.
The A/B Combiner's key output is effectively that of the B channel, and
not the raster-based signal seen in the Video/Video mode. Since the key
(B) transform can be independent of the video (A) transform, it is
possible to position the transformed key so that the transformed video
is not visible, or only partly visible, in the final composited output.
4-10
System Overview
Figure 4-5 The DVE Board in Video/Key Mode
8150 Technical Guide
9100-0212-04 - May 2000