Groove And Stylus

The only function of a phonograph reproducer is to convert the mechanical motion of a stylus sliding in a spiral record groove to electrical signal output, which can then be amplified and applied to a loudspeaker. Although this is a very simple task, many details arise which cause great difficulty in the successful realization of perfect record reproduction. It might well be in order to review the manner in which the recorded information is presented to the pickup, and then explore the requirements of a pickup to make use of this information. •

When a record is made, a formed cutting tool is forced into the surface of a relatively soft material in disc form which is being rotated beneath the cutting tool. At the same time the mechanism (or cutting head) which holds the cutting tool is advanced slowly, so that when the disc has made a complete revolution, the tool has moved forward a few thousandths of an inch and the next groove starts alongside the first one. This covers the entire surface of the disc with a single spiral groove which is continuous from beginning to end of the record. On an average twelve inch record this groove is about 800 feet long. On a microgroove record, the groove can be as much as 2000 feet long! The shape of this groove in cross-section is shown in Fig. 1.

If we place a ball-point stylus in this continuous spiral groove, the stylus will be slowly drawn across the disc as the turntable is rotated. If the stylus holder is mounted on a pivoted arm, the arm will swing across the record under the "screw" action of the spiral groove. The stylus may be said to "scan" the entire length of the groove. A portion of the record surface is shown in Fig. 2a. Fig. 2b shows what happens to the groove if the cutting stylus is caused to oscillate back and forth sideways while it is cutting the spiral groove. Now, although the average groove configuration is still a spiral, there are numerous "wiggles" which the playback stylus must follow to remain in the groove. These wiggles represent the modulation or useful output information to be obtained from the record. It is the function of the pickup to convert this modulation of the record grooves into an electrical signal which can then be amplified.

Regardless of the type of electrical generator element used in the pickup, whether it be crystal, ceramic, magnetic or moving coil, all of the mechanical motion originates in the fit of the playback stylus in the record groove. This is purely a problem in geometry, and has nothing to do with the electrical or mechanical properties of the pickup. Fig. 3 shows the tip of the reproducing styii as seen from the side. A section through the tip at the point where it engages the

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