A New Angle in Record Playing
Stylus rake angle joins the pantheon of phonographic parameters.
by the Editors of HIGH FIDELITY
Diagrams by Discwasher, Inc.
We were delighted when CBS Technology Center informed us, early in 1979, that one of its ongoing research projects had resulted in a formerly unattainable accuracy in the assessment of vertical tracking angles for playback styli. (See "Tracking-Angle Error: A New Slant" by James V. White, HF June 1979.) Though much had been written about the distortion introduced into disc playback by incorrect VTA adjustments, the means of measuring--and therefore standardizing--the angle and of correcting any misadjustment had remained elusive. Now, we thought, a new era of perfection in phonographic reproduction is upon us.
It wasn't that simple. Few pickup manufacturers seemed aware of the work done at CBS and elsewhere. Most continued to use a measurement method that the research had discredited. A few were so frank as to admit that, in essence, they "eyeball it" and then go for best sound; if their subjective findings disagree with a lab test, the latter is discreetly ignored. And we consistently found that, though the VTAs of many quality pickups measured extremely high with the improved CBS technique (a frequency-modulation method, as op posed to the amplitude-intermodulation method that had been used for years), they still sounded dandy. How could they, when theory dictated that we should experience audible-even gross distortion? If Jon Risch and his colleagues at Discwasher are correct, at least some of the inconsistencies may be traced to the angle between the stylus' contact patch (or "footprint") and the plane of the record. Dubbed stylus rake angle, the concept has shown up in a few audio journals over the years, but Discwasher's study appears to be the first full-scale exploration of it as an important element in record playback.
To understand the ramifications of this study, it is necessary to digress a bit and retrace the research procedure.
Bruce Maier, president and founder of Discwasher, had charged his engineering staff with the task of conducting what he termed "an infinite regression analysis" of disc-playback variables.
The research group's first requirement therefore, was a reference playback system with all of the known variables as tightly controlled as possible.
Insuring correct vertical tracking angle was a prime concern, of course. A specially modified tonearm allowed precise adjustment of rear-pivot height-and hence, in theory, vertical tracking angle-during record play. But first attempts to determine correct VTA proved puzzling. As VTA was changed, no clear-cut correlation with distortion developed. Risch's curiosity was piqued, and he sought out a test record designed specifically for VTA adjustments, settling on a disc cut by Telefunken (and distributed in this country by Gotham Audio Corporation in New York) for making measurements according to the DIN specification No. 45,542.
The record contains two sets of IM bands, with each set divided into seven separate bands cut at VTAs ranging from 6 to 30 degrees. One set of bands contains tones at 370 and 630 Hz, which add to an IM product at 1 kHz. The other set has tones of 1.85 and 3.15 kHz, adding to 5 kHz. With a test pickup mounted according to the manufacturer's recommendation, the 1-kHz and 5-kHz tests gave very different results. In theory, both tests should have produced a null in intermodulation when the test cut's VTA matched that of the playback stylus. The 1-I Hz test showed such a null at 26 degrees, though the cartridge was specified for a lower VTA; the minimum in the 5-kHz test was much less pronounced and indicated a VTA of 21 degrees.
---- Diagram explains the relationship between the vertical tracking
angle and the stylus rake angle. Both are shown here in relation to the
record surface.
------- "The shape of the "footprint," or bearing surface
(shown in black) in which the stylus tip actually contacts the groove
wall, is determined by tip geometry. Spherical (far left, representing
a 0.7-mil tip) rests on a round area of groove wall; at the other
extreme is the Shibata tip (far right, with a 3.0-mil bearing radius), which
rests on a tall, narrow area. In between are a typical elliptical (center
left, with a bearing radius of 1.7 mil) and what HF calls a multiradial
(center right, shown with a bearing radius of about 2.0 mils), though most
manufacturers have their own brand designations.
With such equivocal data, Risch hypothesized that some additional form of high-frequency mis-tracing was occurring, caused by a less-than-ideal orientation of the stylus' footprint in the groove. Studying typical stylus geometries, he realized that a spherical stylus would continue to contact the groove area in the same way over a wide range of vertical angles and therefore be largely immune to vertical alignment problems. Not so with the more expensive elliptical and multiradial styli. Their narrow profiles can present larger contact areas to the groove wall, but these shapes will fit the modulation correctly only if vertical tolerances are strictly maintained.
To test this hypothesis, he compared the distortion generated by essentially identical pickups, one with a spherical and the other a Shibata stylus.
The tests confirmed that distortion with the spherical stylus remained relatively consistent between the two sets of test cuts, as it should where VTA alone is involved; inconsistencies in the data with the Shibata tip suggested that rake angle was the prime factor in maintaining lowest possible distortion. For one thing, the data showed distortion at many frequencies-not just the second-order products of VTA misalignment.
The investigation was aided materially when the research group acquired a stylus assembly in which the Shibata tip was misaligned with respect to the cantilever, putting correct stylus rake angle (SRA) and correct VTA about 4 degrees apart. Again, as the system was optimized for SRA-necessarily misadjusting VTA since one can't be changed without altering the other-distortion was minimized, while optimizing the VTA increased the miscellaneous distortion without banishing the second-order products that, in theory, should have been minimized.
Risch theorizes that the increased distortion arising from incorrect SRA comes as a result of groove modulations "grabbing" at the edges of the stylus-torquing and twisting it, thereby exciting vibrational modes in the cantilever. The net result, he says, is a perceivable increase in primarily high-frequency distortion and brightness, all the things that sound extremely nasty on a high-quality system.
What is needed, he says, is an accurate reference for both pickup manufacturers and record-mastering engineers, many of whom take pride in shimming their cutterheads to change VTA for "that personal touch." But if the cure remains elusive, the present era of audiophile discs seems uniquely prepared to see the quest to a fruitful conclusion. HF
------- A plot of the VTA measurements made with the intermodulation
bands on the DIN 45,542 test record. The solid line connects points measured
with the bands that sum to I kHz; the dotted line, those that sum to
5 kHz. In theory--and assuming that VTA alone were involved--the two curves
should be very similar.
-30 SECOND-ORDER IM DISTORTION PLOT VTA bands of DIN 45,542 test record
(High Fidelity, May 1981)
Also see:
Technics by Panasonic -- SL series turntables (ad, Nov. 1977)