The C-trilogy (Part 1) - Center, who are you?

According to Wikipedia "It is the channel that is mostly, or fully, dedicated to the reproduction of the dialogue of an audiovisual program. [...] In material without accompanying visuals (eg music), the center channel simply reproduces sound intended to come from immediately in front of the listener."

The center channel serves to anchor the phantom image which widens the sweet spot for listeners. On the other hand leaving a discrete--and required--signal out of the L+R channels will cause a problem if the C channel is not configured correctly. Insofar many engineers also feed this material to L+R. See the discussion @ mixonline.

In the C-trilogy I'll do some thinking aloud about (1) which microphone to use, (2) where to place it and (3) how to add it to the mix.

Which microphone to use to supply a discrete signal for a dedicated center channel?

Sidetrack: I say "discrete signal" because with coincident main microphone setups (XY & Blumlein; M/S provides you with a dedicated C anyway) you can easily extract the sum of L+R and place that onto the C. This works slightly less good with near coincident arrangements (ORTF) and not at all with time of arrival based / AB setups.

What you want to do is find a spot where you'd place the one microphone required for a monophonic recording. True, this skill has been steadily devalued, but attempting it lets you appreciate stereo and surround all the more--and gives you something fitting the C.

• Cardioid: A microphone of cardioid characteristic attenuates sources arriving off angle. Using one will work only if the ensemble does not expand far in width and depth. On the other hand you will attenuate sound sources from behind the microphone, e.g. favorably reducing audience noise.
  
• Omni: I generally consider an omni, with a uniform frequency response in all directions, to be a good choice to track a dedicated center, especially if it can be placed close to or centrally within the ensemble.
  
• Bidirectional: A fig-8 can also be used facing sideways in a close frontal location, but not centrally within a heterogenous soundfield, e.g. an instrumental ensemble. It works well for a choir, attenuating vocalists singing towards the mic as well as sound / noise coming from the direction of the audience (and conductor).

When recording repertoire with one soloist a close spot can also be fed to the C, suitably delayed (in relation to the main microphone) if you don't want to pull the soloist to the middle--which can be an option on occasion. You'll probably want use a directional mic in this case to have greater control over the pickup range, but if you are close enough an omni will work as well.

[Continued in Part 2]