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Cinema sound - A-chain and B-chain

The sound system in a movie theater is separated into an A-chain and a B-chain. Why is this? And why is the projection screen considered part of the sound system?

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The place where many people enjoy the highest quality audio that is available to them is in the movie theater. An immense amount of effort is put into cinema sound because it can be very impressive, and that is exactly what film should be - a very impressive experience.

The sound system a movie theater (or motion-picture theater if we want to be formal) is divided into two parts - the A-chain and the B-chain.

You can think of this like a great big hi-fi system where the A-chain is the CD-player, radio tuner or vinyl turntable (or even - horrors - a cassette deck). The B-chain is the amplifier and loudspeakers. In hi-fi, we wouldn't bother thinking about these as separate entities, but in cinema things are much larger in scale and there are conceptual benefits.

The A-chain in cinema consists of the sound recording on the film print, which will be available in Dolby analog, and also a selection among Dolby Digital, DTS and SDDS digital formats. Also, the equipment that retrieves the audio from the print and processes it so that is ready for amplification is part of the A-chain. This is not all internal to the projector - audio systems can be retrofitted to older projectors, and the electronics will be in physically separate racks.

The B-chain consists of multi-channel volume control, equalizers, amplifiers, loudspeakers and the acoustics of the theater itself. The B-chain also, perhaps surprisingly, includes the projection screen. The reason for this is that the left, center and right loudspeaker systems are behind the screen, therefore the screen has to be acoustically transparent. Naturally, the degree of transparency will vary with frequency, so the screen has to be included in the overall acoustic design.

The A-chain is something that can readily be changed. An improved projector could be installed. If a manufacturer such as Dolby came up with a new and improved digital sound system, this could be retrofitted to the projector. The B-chain is much more of a fixture - changing the acoustics of the auditorium would be a major task.

There is a high degree of standardization in both the A-chain and B-chain in movie theaters around the world. This is so that a film soundtrack can be mixed with confidence that it will sound pretty much the same wherever the film is shown. Artistic decisions are made in the dubbing theater, the rest of the process all the way through to the ears of cinema goers is purely technical.

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By David Mellor Friday March 4, 2005
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