Digital Audio for Film and Video

By Plushner, Rick | American Cinematographer, November 1983 | Go to article overview

Digital Audio for Film and Video


Plushner, Rick, American Cinematographer


In the relatively short time since its introduction, digital audio has significantly affected the recording and film industries. Recent major motion pictures (E. T.. Poltergeist Star Trek II, Annie, et. al.) have employed digital audio recording, and television producers are also implementing digital audio techniques to deliver superior audio for home video production.

Digital Audio: The Fundamentals

Digital audio is truly a quantum leap forward in the science of sound storage and reproduction. Previous to the development of digital techniques all audio recording was accomplished by analog methods, from the first wax cylinders to today's sophisticated multitrack tape machines.

Analog recordings suffer inherent limitations resulting from the fact that they are attempts to store the musical waveform directly. Digital audio, however, records sound in the form of numbers, exactly as a computer stores information. Digital recording eliminates such problems as noise, limited dynamic range, tape print-through, cross-talk, mechanical wow and flutter and generation loss suffered in the copying process. In addition, digital audio provides a wide range of production flexibility.

Analog to digital conversion, the heart of the new process, involves taking an analog or continuously variable waveform and expressing it as a discretely stepped or digital signal. This digital signal alternates between two fixed levels which are either on or off, and are expressed as a series of Ts and O's. The advantage of a digital signal is that the audio information is much less susceptible to degradation in processing or reproduction. Once all the subtle variations in level in the original analog signal have been digitized, any noise in the digital signal itself has no effect on the audio information which will be retrieved when the signal is converted back into an analog one for playback.

The fidelity of a digital signal derived from an analog signal is a function of two things: the sampling rate and the "word" size. An analog signal is digitized by reading its amplitude at regular intervals, and the sampling rate is the frequency of those "readings." If the sampling rate is at least twice the highest frequency in the analog signal, we can recover the complete original signal from the discrete data of the digital signal. Since the highest end of the audible frequency range is 20,000 Hz (cycles per second), the sampling rate for digital audio needs to be at least 40,000 times a second.

The word size is the number of distinct levels which the system can recognize each time it reads the amplitude. The word size for professional digital audio has been standardized at 16 bits. This means that the amplitude of the analog signal can bjj specified at any one of 65,536 (i.e. 216) discrete levels. It is the word size which sets the limit for the signal-to-noise achievable in a system and a 16 bit word permits a signal-to-noise ratio in excess of 90 rJB.

The Sony Digital Audio System

The Sony digital audio system involves a processor, two types of recorder capable of recording digital audio signals, and a digital audio editor. The processor converts the analog signal to a digital signal and processes the signal for error correction. The error correction process protects against drop-outs, analyzes and replaces missing information and eliminates pulse noise or "crossword error".

Reconstruction of the analog waveform takes place in a separate section of the processor and results in the following sonic specifications: greater than 90 dB signal-tonoise, greater than 90 dB channel separation, and greater than 90 dB dynamic range. Because the energy of the tape itself is not read as in analog recording, there is no surface noise, tape hiss, print-through or degradation of the signal through the tape's inevitable loss of magnetic properties.

Recording a digital audio signal requires much greater frequency response or bandwidth in a tape recorder than recording an analog audio signal.

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