Digital Sound for Film and Video

By Schilling, Ed | American Cinematographer, December 1984 | Go to article overview

Digital Sound for Film and Video


Schilling, Ed, American Cinematographer


British science-fiction writer Arthur C. Clarke once commented that any sufficiently advanced technology becomes indistinguishable from magic. This statement can be related to any type of high-tech silicon-chip machinery, but it is becoming especially applicable to sound recording and mixing for motion pictures.

The recent introduction of digital recording is making possible feats that sound mixers of the 1950s never dreamed of. Perhaps the most wonder-filled faces belong to the audience members, who are hearing things they never heard before. Spoonfuls of sugar being poured into a cup of coffee, for instance. So what's so great about that? Simply the fact that digital technology allows you to hear it over the rumble of an orchestra tuning! Audiences are also not hearing things they used to - namely the grinding and clicking of a theater's optical soundtrack at operating volume. Digital technology removes the distracting noises that prevented film viewers from that vitally important task - suspended disbelief.

But even if the crystal-clear sounds and effects are awe-inspiring, there is a technology behind the magic. And as time goes by, digital technigues are actually helping editors and mixers by eliminating some tedious tasks, while simultaneously opening up new audio frontiers. Accordingly, anyone associated with film production will have to become familiar with this rapidly expanding science.

What is digital sound? Essentially, it's a method of encoding audio signals in the form of numerical "words." Digital microprocessors sample incoming signals thousands of times a second; each sample is instantly converted to a binary code, consisting of combinations of two numerals, 0 and 1. When a digital tape recorder operates in the playback mode, the microprocessor strmgs together each numerical code word to form a sigoal that can be amplified.

A rule of thumb dictated by the technology holds that the sampling rate must be approximately twice as high as the highest frequency you wish to record. For example, if you want to insure proper recording of a sound at 20kHz, the sampling rate must be at least 40kHz, or 40,000 times per second. Current industry standards have settled on 44 kHz, 44.056 kHz, and 50 Khz.

Exactly what advantage does this technology offer film producers? In the analog medium, each successive dubbing step drastically raises the noise floor level. The last 15 years have brought the film industry such noise reduction processes as Dolby and dbx; still, sound mixers are limited in the number of pre-dubs they can make before the mushrooming noise factor makes any further re-recording out of the question.

Because a digitally recorded tape contains only numbers, the microprocessors can easily distinguish between the recorded signal, and the inherent tape noise. Consequently, digital sound tracks can theoretically be bounced through an infinite number of generations, with the last just as clean as the original. This fact alone makes digital recording extremely important to motion picture sound editors and mixers, who routinely pre-dub hundreds of effects, music, and dialogue tracks to create a submaster for the final mix. Another advantage is that digital tracks recorded "hot" are not likely to cause print-through.

Before proceeding any further, it should be explained that much, if not all of the digital sound used in motion pictures and video production, as well as consumer records and tapes, is not purely digital. It is true that tape can store pristine digital signals. However, normal microphones translate sound waves into analog signals, which pass through mixing consoles, effects devices, and other equipment before and after they are recorded with a microprocessor. Therefore, a minimal loss in signal clarity is inescapable. Only when a signal path is paved with alldigital equipment will a sound be truly pure.

Several forms of this technology have already penetrated the consumer audio market.

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