Audio with DCP

Channels

Most cinemas have at least 6 different audio channels. Although on your home audio system you might have 2 channels (left and right), with the speakers fairly close together, a cinema will almost always have:

• A centre speaker — located between left and right; usually used for dialogue.
• Left and right surround speakers — behind and to the left and right of the listener; usually used for effects.
• A subwoofer (also called Lfe, for ‘low-frequency effects’ or ‘low-frequency enhancement’) — used for bass in music and some effects.

Also, a cinema's left and right speakers will usually be quite a long way apart.

You can make a DCP with all these channels present, if your sound mixer has made a ‘5.1’ mix. If you only have 2 channels (stereo) you have a few options:

• Leave it as it is — a safe option; your audio will be heard, though any dialogue might sound a little strange due to phase cancellation effects.
• Use a Mid and Side (M/S) processor to create a centre channel — DCP-o-matic offers a processor which can extract the common parts of your left/right stereo signals (i.e. the things that are panned centrally; usually dialogue) and put them into the centre speaker in the cinema. This will almost always have a positive effect, and is recommended. The surround and Lfe channels will still be silent, but your soundtrack should sound good.
• Use an upmixer — programs exist which will claim to create surround mixes from stereo signals. They are never ideal, as they are trying to create something (the surround signals) from nothing. If you use an upmixer it is advisable to listen to the results in a 5.1 listening environment (like a cinema) to check that they sound good.

Levels

The other major thing to consider with audio is the level; i.e. how loud it is.

The most important thing is that the audio must not clip. This happens when the audio is so loud that it hits the limits of the levels that the DCP can describe. If this happens, your audio will sound terrible: extremely loud, and badly distorted.

It is also a good idea to get the audio to approximately the right level. Commercial DCP content is mastered so that if a cinema plays it at their “usual” audio level it will sound about “right”. Often they will assume that your DCP is at a similar level. Cinemas can turn the content up and down, but this requires someone to be checking the levels in the auditorium, and this does not always happen for every screening.