Distortion In Phono Cartridges

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(source: Electronics World, Aug. 1964)

By JAMES H. KOGEN / Chief Engineer, R&D, Shure Brothers, Inc.

By matching the playback stylus vertical angle with that of the cutting stylus, distortion is reduced.

THERE are many factors which can contribute to distortion in a phono cartridge. Assuming that any good cartridge design will eliminate the flagrant sources of distortion, we can still cite at least five significant contributors.

1. Lateral tracking error. Lateral tracking distortion occurs when the center of rotation of the playback stylus is not on a line tangent to the record groove at the point where the stylus makes contact. Distortion from this source is minimized by proper geometrical configuration of the tonearm.

Lateral tracking error in all good tonearms, properly mounted, is less than 5°--and the resultant distortion is quite small.

2. Vertical tracking error. Vertical tracking distortion is caused when the playback stylus moves in a different vertical arc than that which is cut in the record.

3. Tracing error. Tracing distortion results from the fact that the round playback stylus differs in shape from the wedge-shaped cutting stylus.

4. Dynamic distortion. Dynamic distortion results from the fact that the playing surface of the record is not infinitely hard and is, therefore, indented to some extent by the playback stylus.

5. Rattling in the groove. This distortion occurs when the needle loses contact with the groove wall. Such a situation can happen during very highly modulated passages.

Fig. 1. (A) Motion of the cutting stylus on the master record. (B) The motion of the playback stylus with respect to record.

Vertical Tracking Error

The cutting stylus used in making the master record follows a motion as depicted in Fig. 1A. It has been known for years that distortion will result if the playback stylus does not have the same effective center of rotation as the cutting stylus. Formulas have been developed from which the expected distortion can be calculated. The significant variable in the formula is the vertical tracking angle error, defined as the difference between the effective cutting angle A (Fig. 1A) and the effective playback angle B (Fig. 1B) . Although this theoretical information had been used for some time, something seemed to be amiss. Measurements consistently showed distortion to be considerably higher than we would expect from the theory. Late in 1962, engineers at both the CBS and RCA Laboratories discovered at least one major cause for the discrepancy. It had previously been assumed that the effective cutting angle of the record could be determined by measuring the free motion of the cutting stylus. Precise measurement proved, however, that the effective angle at which the record is cut differs from this free motion by as much as 21°, a startling difference.

The reasons for this difference have not been thoroughly proved as yet, but at least two major contributors are known.

One factor is the bending of the cutting stylus as it slices through the surface of the master disc. A second factor is the so-called "spring-back" of the record material as it is being cut. Although this material is literally being cut, there is a certain amount of springiness in the record material which results in a slight alteration of the groove modulation after the cutting stylus passes.

Clearly the way to eliminate this source of distortion is to eliminate the tracking-angle error. To do this, it is necessary that the cartridge manufacturer have a single target to shoot at. This means that the record companies must standardize on a vertical cutting (recorded) angle. Such a standard has been proposed as 15° by several standardizing organizations both in the United States and abroad. Many of the major recording companies are now cutting records at this angle. The way is now clear and cartridge manufacturers are designing pickups to match this standard.

The dramatic decrease in harmonic distortion that can be obtained by matching the playback stylus tracking angle to that on the record is shown in Fig. 2. This curve was drawn from measurements made with the CBS Labs' test record STR-160 and the Shure M44-7 cartridge, designed for a 15° vertical tracking angle. This curve shows that a significant reduction in distortion can be obtained when the angles are matched. A considerable increase in distortion can be noted when the tracking angle error is 20 °, which was the best match possible for most of the older style high-fidelity cartridges and records.

To guarantee a continued reduction in distortion, therefore, it is imperative that:

1. The recording industry agree on standard measuring methods for determining vertical tracking angle.

2. Recording companies standardize on a single vertical tracking angle, preferably 15 °.

3. Playback cartridges be designed to match the recording standard.

Fig. 2. Distortion characteristics of Shure M44-7 cartridge at 2 grams tracking force on CBS Labs STR-160 test record.

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