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SOUND

On this page i will discuss sound in DCPs.

What about STEREO sound in a DCP?

There are a lot of videos and even DCPs around that have 2 channel sound: Left and Right. This is fine in a living room with both speakers of the TV-set close to each other. But a cinema is not a living room. When playing a stereo mix (just L and R) people on the left side of the auditorium will hardly hear the sound of the right speaker, it will sound like all dialogue comes from left. But not so if you make a Centre track too.

A few pictures will hopefully make this clear.

mono

First of all let’s go back to the old times, let’s say before 1980, when standard cinema sound was mono. Yes: mono, and a good sound designer can make that sound beautiful! There is nothing wrong with good mono sound in cinema. All sound comes from behind the screen and wherever you are in the theatre, the dialogue seems to come from the (center of) the screen.

mono on two speakers

This is what happens if we put the mono sound on two tracks, make a 2.0 DCP and play it. Only in the darkest area the audience will have the impression all sound comes from the center of the screen. For this to understand we need to know a little about human hearing. We listen with both ears and with that we can determine from which side sound is coming. Most important is the timing. If somebody to the right of us drops a pencil, the sound of the pencil hitting the floor will reach our right ear about 0.6 millisecond (ms) before it reaches our left ear. Another difference is that the sound that reaches our right ear will be a fraction louder than the same sound reaching our left ear. Our brain detects these differences and we hear the dropping pencil to the right of us, where the timing will play the major part in the determination of direction. If we play mono sound on two speakers all the people not sitting in the darkest colored area will hear the sound of the speaker that is closest earlier and louder. This makes them feel like the sound is coming from the corner of the screen, which is an awkward experience.

If we put stereo sound on the two speakers we still have the same problem. Sounds that are much stronger on one of the tracks, will be noticed coming from the appropiate speaker but the timing issue will still result in a confusing experience. Only sounds that are absent in one of the tracks will be experienced as coming from the one side as intended.

So how do we solve this? Best is to make a nice 5.1 theatre mix. If that is not an option there is a simple way to get things better than a straight 2.0 DCP. (In fact somebody must have been sleeping when the DCP specifications were put together. There is no valid argument to admit 2.0 sound in theatres).

Starting with 2 mono tracks, Left and Right, we use sound editor software (which may be very simple) to make a new mono track: Centre, which is simply Left and Right put together. The result is in fact a 3.0 mix: LCR. But this is (unfortunately) not DCI compliant, so we make it a 5.1 version. For that we need a silent track with the exact same length as L, R and C. This can be made as well with virtually any sound editor software. This silent track is used for the Ls, the Rs and the LFE tracks of the 5.1 DCP.

We will NOT make a LFE track. LFE stands for Low Frequency Effect. This is different from your subwoofer at home. The main speakers in a proper cinema are capable of reproducing the entire frequency spectrum, including the very low frequencies. The LFE (sub) speaker is only used for effects like rumbling thunder. It is a seperate kind of track and only useful when doing a proper 5.1 theatre mix.

The result is that we have a mono Centre track and the original stereo Left and Right track, which will be played in the theatres like this:

added center channel

The audience sitting in the dark area will have the feeling that dialogue (and other sounds) are coming from the center of the screen. This is far better than the narrow strip when playing a straight 2.0 mix as shown above in the second picture where only two speakers (L and R) are used. In fact we have the same coverage as with good old fashioned mono. (There is another advantage: in the unlikely event of one of the speakers failing, still all the sound will be played in the theatre)

Please download this text as pdf and keep for future reference.

Many thanks to Freek Weijtering for making this clear.

When ready i will publish more on sound in DCPs.