The CD revolution of 1984 unleashed a massive campaign of rerelease of vintage analog recordings. To their collective horror, record labels found that many of the original master recordings had deteriorated. Often the only existing copy of a recording was a vinyl pressing. This rush to release beloved music in the new digital form spawned intense interest in digital methods of restoring these old recordings. A number of DSP techniques have been developed to give new life to these recordings. This paper is a survey of some of those methods.

In professional audio, the transition from analog to digital technology is nearly complete. The more recent shift towards systems based on general-purpose computers has been gradual, because such computers lack the processing and I/O performance needed for professional audio applications. The SonicSystem, a professional digital audio system, solves these problems using DSPs and a specialized file system. We describe the uses and implementation of the SonicSystem.

There is a fundamental conflict between algorithms which must operate in real time and algorithms which may operate out of real time. For real time operation, the compute time must be approximately constant for each sample, and the overall delay introduced by the computation must be small. We present an implementation of linear prediction coefficient calculation and fast Fourier transform computation which are "unrolled" so that a small and constant portion of the calculation is done on each sample.

The requirements of audio processing for motion pictures present several special problems that both make digital processing of audio very desirable and relatively difficult. The difficulties can be summarized as follows: (1) Large amounts of numerical computation are required, on the order of 2 million integer multiply-adds per second per channel of audio, for some number of channels. (2) The exact processing involved changes in real time but must not interrupt the flow of audio data. (3) Large amounts of input/output capacity is necessary, simultaneous with numerical calculation and changes to the running program, on the order of 1.6 million bits per second per channel of audio. To this end, the digital audio group at Lucasfilm is building a number of audio signal processors the architecture of which reflects the special problems of audio.