Features

Back in the spring of 1990, Stereophile introduced its first Test">http://www.stereophile.com//reference/176/">Test CD, featuring a mixture of test signals and musical tracks recorded by the magazine's editors and writers. Even as we were working on that first disc, however, we had plans to produce a second disc which would expand on the usefulness of the first and feature a more varied selection of music. The result was our Test">http://www.stereophile.com//features/338/">Test CD 2, released in May 1992.

"What's that noise?" Bob Harley and I looked at each other in puzzlement. We thought we'd debugged the heck out of the recording setup, but there, audible in the headphones above the sound of Robert Silverman softly stroking the piano keys in the second Scherzo of Schumann's "Concerto Without Orchestra" sonata, was an intermittent crackling sound. It was almost as if the God of Vinyl was making sure there would be sufficient surface noise on our live recording to endow it with the Official Seal of Audiophile Approval. Bob tiptoed out of the vestry where we'd set up our temporary control room and peeked through a window into the church, where a rapt audience was sitting as appropriately quiet as church mice.

If there's one buzzword in high-end audio for the 1990s, it's undoubtedly jitter. "Jitter" describes timing variations in the clock controlling the ones and zeros that represent the analog audio signal. If that clock isn't stable to an extraordinarily precise degree, the sound quality of the digital processor will be degraded.

A CD transport/digital processor combination introduces jitter in three ways: 1) the transport puts out a jittered signal; 2) the S/PDIF or AES/EBU interface between the transport and processor creates jitter; and 3) the digital processor adds its own jitter to the clock. These additive factors are largely responsible for the great range in sound quality we hear from different transports and interfaces.