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Trigger Tone

From Chace Audio

A trigger tone is a 12kHz sine wave that was used to "trigger," or turn on, the surround channel of a discrete LCRS (Left channel, Center channel, Right channel, Surround channel) Cinemascope™ print.

Early LCRS surround mixes for Cinemascope films contained very little surround content. Instead of having constant amplified film noise and hum coming out of the surround speakers at theaters, a noise reduction scheme was introduced in the form of a noise “gate” to turn off the surround channel when no program material was present.

The surround amps remained shut off until they heard audio, but a high signal threshold was needed to properly control the gate; program material could not be relied upon to open and close it. To solve this problem, a 12kHz sine wave was recorded into the surround track at a fairly hot level to trigger the amplifiers. Because the mechanism controlling the surround amps wasn't very fast, this trigger tone had to precede the program material by a second or two, and had to remain present for the duration of the surround channel playback in order for the amps to stay on. When the section of surround material ended, the trigger tone would stop, the gate would close, and the amplifiers would turn off.

The trigger tone frequency was either too high to be reproduced in the theaters of the day or it was filtered out in their audio monitoring chain. However, it is very apparent on modern playback systems. Since the 12kHz trigger tone was introduced in the final print master elements, and not just the release prints, it is present on any surviving 35mm full coat LCRS Cinemascope print master asset. Luckily, it is a fairly simple job to remove the trigger tone from the track without affecting the audio in the surround channel.

In this clip, the piercing whine of the Trigger Tone is heard as the sound of the horse drawn wagon is directed to the surround speakers.