FOCUS Enhancements 3.2 - Manual (Page 140)

External Control Commands
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mpgPlay
This command plays a previously loaded MPEG file on the
specified channel. The file must have already been loaded using
the mpgLoad command.
Syntax
mpgPlay 0 channel
Parameters
channel
The channel on the Firefly (0)
Return Values
This command returns ACK if playback was started successfully,
or NAK if playback could not be started (e.g., no file previously
loaded).
mpgStop
This command stops playback of an MPEG file that is currently
playing on the specified channel.
Syntax
mpgStop 0 channel
Parameters
channel
The channel on the Firefly (0)
Return Values
This command returns ACK if playback was stopped successfully,
or NAK if playback could not be stopped (e.g., no file currently
playing).