A Measurement that Says Something

When we first heard rumors that Shure Brothers was about to unleash something called "trackability" on the audio world, our reaction was mainly one of indifference. We already had loudspeakers with listenability, tape recorders with portability, and amplifiers with stability and dependability. Trackability, we figured, was just another clever sales gimmick; a catchy word that the advertising department had thought up to describe what everyone wanted in a pickup.

How wrong we were! Trackability is a catchy word, but it also describes the most meaningful pickup measurement that's come along in years. (See "The Disc Sound" by Brown & Koval, in Stereophile Vol.1 No.7, and "Trackability" by James Kogen, in the November and December 1966 issues of Audio.)

Briefly, the trackability test determines how much recorded level (modulation velocity) a pickup will track cleanly at a given force over the entire audio spectrum. The results are plotted on a graph of frequency versus velocity, and then compared with another curve that shows the maximum velocities attained at those frequencies on a "typical" disc.

If the pickup's trackability curve lies above the velocity limits of the typical disc, the pickup should be able to track the disc cleanly. If the pickup curve falls below the other at any frequency, this suggests that a typical disc may produce some mistracking at that frequency. Thus, we can not only tell at a glance how clean a pickup is likely to sound on most recordings, we can also make some educated guesses about the type of recorded material—voice, piano, violins—that will bother the pickup, by comparing its trackability curve with the energy distribution curves of the various sound sources.

Unlike some other pickup tests, like those for compliance and mass, the trackability test has the supreme virtue of actually measuring the pickup's performance in the groove. Compliance and mass measurements can only suggest how a pickup will track, for they must be integrated with other factors like stylus damping and tracking force, and every one of these elements tends to affect all the others. The trackability test automatically integrates all these factors to yield a simple Yes or No answer to the question "Will this pickup track the loudest 3kHz signal we're likely to encounter on a typical disc, at the recommended tracking force?"

As far as we're concerned, the old standbys—the compliance and mass specs, which were never very informative anyway—can now go by the board, to be replaced universally by trackability ratings.

Obviously, though, if other manufacturers are going to adopt the trackability tests, we're going to have to have uniform test standards. (Institute of High Fidelity, please take note.) The reference curve for the "typical" disc will have to be standardized, as will the maximum permissible amount of tracking distortion and the playback equalization, if any, that is used in the tests. (High-end rolloff, for instance, will have a profound effect on the amount of measured distortion produced by mistracking.) Once these things are established, it will be possible to state trackability specs as numerical values by merely listing the differences, at key frequencies, between the reference curve and the pickup's trackability measurements. These could then be assigned plus or minus values, depending on whether they he above or below the reference curve.

Now that we have a meaningful index of tracking ability, perhaps we can hope for some other innovations in specs that will have some meaning for the prospective buyer. For example, if pickup manufacturers were to list the weight of their pickups (total weight, not tracking force) and provide a simple compliance curve for low frequencies (3–30Hz, for instance), and tonearm manufacturers were to provide an effective–moving-mass figure, it would be possible to predict the resulting bass resonance frequency instead of just hoping it will not fall right on top of the turntable's primary rumble frequency.

Also: why not publish amplifier distortion ratings at 1 watt output (where we do most of our listening) instead of at full output only? We can think of some other helpful specs, too, but we'll get into them some other time. Meanwhile, our congratulations and heartfelt thanks to Shure Brothers for "trackability." May it prosper and, ultimately, become an industry-wide standard.—J. Gordon Holt