7412533660

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By FRANCIS A. GICCA

Raytheon Co.

A serious loss of łhe stereo effecł occurs with łracking error. Here is how łhis may be minimized by proper mounting of straight or offset tonę arms.

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Fig. 1. (A) Groove coordinates. (B) Vec-tor represenlation of leit-channel output.

Kditor’s Notę : This afttclc tlenia irlth thc horizontnlly mensured tracking error. Equally important is thc cffcct produced by vertical misallf/nment of ihe Stylus. For Information on this topie we refer our rearlcrs to the article " Pickup Cnrtridges for Stereo” by Norman Crowhurst in our Octobcr, 195.S issue.

FIS generally well known that un-lcss a phonograph cartridge is at all times tangent to thc groove of a monophonic recording, distortion is produccd. With any pivoted pickup arm it is obvious that thc anglc be-twecn thc axis of thc cartridge and thc tangent to thc groove must changc as thc cartridge moves across thc record. This angular error between thc axis of the cartridge and thc tangent to the groove is called "tracking error" and should always be minimized in order to reduce distortion. (For a discussion of tracking distortion see B. B. Bauer’s article, "Tracking Angle in Phonograph Pickups,” Electronics, March, 1955.)

However, it is not as generally known that tracking error also pro-duces a serious loss of stercophonic effect when reproducing stereo dises. For examplc, if a 9-inch-long straight pickup arm is oriented so that the cartridge’s Stylus passes over the center of thc disc, a loss in stereophonic cfTect of 31 db will occur in the outer grooves.

In order to investigate the cause of this degradation, and how it may be minimized, it is first necessary to es-tablish a set of three-dimensional ref-erence axes cen te red about the bot-tom of a stereophonic groove. Fig. 1A shows the coordinate system chosen. The X axis runs from thc inside to the

Fig. 2. Translation of groove X output into the cartridge coordinates. See text.

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Fig. 3. Comparison between the total

motion vector and the original L vector.

outsidc of the groove, the Y axis runs up and down the groove, and the Z axis runs front to back. These axes are mutually perpendicular.

Assume that the disc is modulated with “left" output only, that is, the inner wali of the groove moves back and forth at a 45-degree angle to the horizontal as shown in Fig. IB. This motion of the inner wali may be repre-sented by a motion vector at a 45-degree angle in the in-out, up-down (X-Y) piane. Such a vector of length L has been drawn in the X-Y piane of Fig. IB.

Since the groove wali is moving at a 45-degree angle, this is equivalent to equal lateral (in-out) and vertical (up-down) motions of the reproducing Stylus. In other words, the L vector may be represented by two other vec-tors of equal length. one up-and-dow-n and the other in-and-out. By trigo-nometry it is apparent that each of

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