Reference

As Hans Christian Oersted, the Danish physicist and founder of electrodynamics, discovered in 1819, an electric current passed through a wire generates a magnetic field. Place that wire close to a permanent magnet and the interaction of the two fields will generate a force. That, in two sentences, summarizes the operating principle of the motor that energizes every moving-coil drive-unit in millions of loudspeakers worldwide. It sounds simple, but—like everything in audio—it isn't.

I want to talk about the acoustics of live music and recordings. As I write this I'm back in Boston for a week, re-calibrating my ears with (excuse the expression) the "absolute sound" of live music in various concert halls. On Friday the Boston Symphony played symphonies by Mozart and Shostakovich, producing (as always) magnificent sound with the aid of Symphony Hall's near-legendary acoustics.

When sociologists tell us America is a highly mobile society, they don't just mean we do lot of driving. What they mean is, we do a lot of moving. The good old three-generation family homestead, immortalized in nostalgia TV and literature, is a thing of the past. According to census information, almost 20% of America's population changes its address every year. Of course, it's usually a different 20% every year, but pulling up roots and moving---to a bigger house, a better neighborhood or a nicer city, not to mention a place where your employer decides to transfer you---is almost as commonplace across the US of A as marriage, divorce, and unbridled greed.

Author's Note: Although I started accompanying Stereophile's loudspeaker reviews with measurements soon after I joined the magazine in 1986, it wasn't until 1989, when we acquired an Audio Precision System One electronics analyzer and the then-new MLSSA speaker measurement system from DRA Labs, that I developed the standardized data presentation that is still featured in our reviews more than three decades later. In this article from October 1991, I summarize the results from the first two years of using MLSSA to test 69 loudspeakers.—John Atkinson

Until recently, all problems in digital audio systems were blamed on either the analog/digital converters (ADCs) used in mastering or the digital/analog converters (DACs) needed for playback (footnote 1). As the performance of both ADCs and DACs improved, however, a previously unrecognized mechanism for distortion was unmasked: jitter. As we shall see, jitter—or, more correctly, word-clock jitter—can be a significant limitation in the technical and sonic performance of digital audio systems (footnote 2).

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.

Why cable again?

Well, the obvious reason is that it has been a while since my last foray into Cableland (July 1988). Many new products have been introduced in the interim, so it appeared appropriate to once again open Pandora's Box. Those of you who still remember my speaker cable article of 2½ years ago will recollect the considerable controversy that evolved from that project.

Some of the response was quite predictable, though the venom with which it was laced was not. The manufacturers of those outrageously priced "garden-hose"–type cables that I failed to rave about were more than just perturbed.