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Audio Alphabet Soup


By Derek Casari
THX, SDDS, SRD, DTS, what’s it all mean? If you look closely at the ads for a movie these days, you will be confronted with a plethora of acronyms. In the past, simply knowing the movie schedule was sufficient. Cineplex renovations in recent years, coupled with advances in audio technologies, provide today’s audiences with a quantum leap forward in quality.
Some explanation is in order to help understand what all these terms mean. Let’s start with the acoustic reproduction chain, in industry parlance, the “B” chain. This is to differentiate it from the “A” chain, which is the electronic path; additionally, not to be confused with the “A” train! This is where the term “THX” comes into play. This is a specification for the noise criteria of the room, also known as the “NC” value, as well as specifications for the speaker elements, loudness level, equalizers, crossovers and power amps. The infinite baffle speaker wall was a key component, as was a specification for reverberation time, as well. Credit is attributed to Tomlinson Holman of former Lucasfilm fame, who did an extraordinary job researching the various deficiencies of modern theatres and ways to improve them. The term “THX” stands for T (omlinson) H (olman’s) X (periment). The name also refers back to George Lucas’ early feature film “THX-1138”, which coincidentally is the model number for the current THX crossover/monitor. A crossover is a device that divides frequencies to the appropriate speakers e.g. low frequencies to bass drivers and high frequencies to tweeters. A noise criterion is a figure that determines how quiet a room is. This is important in order to discern many of the subtleties in today’s motion picture sound mixes.
Next is the subject of film sound formats, which describes what is played out into the THX sound system. First up is Dolby SRD. The actual trademark is Dolby Digital. “SRD” was (and is) industry shorthand to denote a print with both a Dolby SR analog track (The analog track is a failsafe in the event of problem with the digital track) and a Dolby Digital track. This is Dolby’s digital audio system that delivers 5.1 channels of audio. 5.1 you say? How can you have a 0.1 channel? The 0.1 channel is known as the subwoofer channel or LFE (Low Frequency Enhancement or Effects). It is called 0.1 because it is not “full bandwidth”; rather it is limited typically to between 20 hertz to 100 hertz. This is the channel that contains the low frequency “booms” that can really make an action movie come alive. The other 5 channels are left, center, right, left surround and right surround. (The predecessor to this was Dolby Pro Logic, which has a mono surround.) All digital sound formats e.g. SRD and SDDS, with the exception of DTS, reside on the film and each one in a different physical locale. The latest variant in Dolby is Dolby EX, which is 6.1 channels e.g. L, C, R, L side surround, R side surround, mono rear, and subwoofer.
DTS is a 5.1 channel system developed by Digital Theater Systems. The audio resides on a compact disc that runs in sync with the film picture by virtue of a synchronizing signal called timecode. This signal is recorded on the film and the compact disc and the associated electronics maintains sync between the two.
SDDS, which stands for Sony Dynamic Digital Sound has the capacity for 7.1 channels, adding two intermediate screen channels, left center, and right center. Although it is less common, it is still in use.
In a modern projection booth there is a cinema processor that decodes all of these various formats presenting the appropriate channels to the correct speakers. All of this goes on behind the scenes, oblivious to the listening audience, with the ultimate goal of delivering the most vibrant and thrilling movie going experience.

Derek Casari is a systems engineer in the postproduction department of a major motion picture studio and has worked in this industry for over 20 years.

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