FOCUS Enhancements 3.2 - Manual (Page 142)

External Control Commands
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mpgBlack
This command covers MPEG video on a channel, causing the
corresponding video displays to go “black.” Note: This black may
not be recognized by all displays as a true video signal.
To cover the video again, use the mpgUnblack command.
Syntax
mpgBlack 0 channel
Parameters
channel
The channel on the Firefly (0)
Return Values
This command returns ACK if mpgBlack was started successfully,
or NAK if mpgBlack could not be started.
Remarks
Video playback is completely independent of whether the video is
muted with mpgBlack. Videos will play “underneath” the black
normally, audio will be unaltered and callbacks will be sent
normally.
If a playlist is played with one of the commands, such as
playlistPlay, unused channels (channels with no video files) will be
set to the blackened state. Therefore, you must call mpgUnblack on
those channels if you intend to play video on them with commands
such as mpgLoad/mgpPlay.
See mpgUnblack for more information on “unblacking” video.