What is the usefulness of the 4 talkback inputs ?
Actually they are all mixed and routed simultaneously to the 4 cues.
This mean that the 4 talkback buttons are simply selecting witch talkback mic is sent to the 4 cues at the same time.
I think that it would more useful to use the 4 talkback buttons as a selector to choose witch performer cue circuit need to receive the control room talkback.
So we should have 5 buttons :
a talkback to all button
a talkback button for each cue circuit
So that we can select to witch performers cue circuit the talkback is sent.
Generally there is a listen talkback button too on studio recording consoles. This is a talkback in the opposite direction. This is a mic placed in the studio. This button route the studio listen mic to the mini loudspeakers and dim the main loudspeakers at the same time.