Audioclinic (Feb. 1982)

Disc Fulfillment

Q. Why will one record, say with 3.5 inches of groove radius, have perhaps a 13-minute playing time, whereas another record with the same amount of groove radius will have 26 minutes per side? Certainly spacing between bands is a factor, but there must be something else.

-Rodolf Schmid; Berkeley, Cal.

A. When a record company produces a disc, the engineer knows that the program material will run X minutes per side. He also knows that, at least for cosmetic purposes, he must fill the disc completely. Thus, whether a side is 13 or 26 minutes long, the last modulated groove should be at approximately the same diameter for both discs. (All measurements are made in terms of diameter rather than in terms of radius.) In order to make it possible to finish at a chosen diameter, the cutter is programmed for the number of grooves per inch which will bring about that result.

This is not difficult when the engineer knows the distance between finishing diameter and the starting diameter, the time needed to traverse this distance and the speed of turntable rotation. However, there are complicating factors. The number of grooves or "lines" per inch also varies in accordance with the dynamics of the program. This allows the recording engineer to put many lines very close together when the music is soft, because there is little groove displacement during soft musical passages.

This in turn, means that there is little chance of groove overlap. During highly modulated passages, on the other hand, groove spacing is widened and grooves are deepened to allow for both greater lateral and vertical groove swings. If this variable grooves-per-inch system were not used, the loudest volumes possible for long sides would be considerably less.

Detects Deflect

Q. Lately I have noticed that when the selectivity switch for the FM section of my receiver is in its "Narrow" position, the signal strength meter deflects somewhat, in proportion to the audio of the program. Why should this be?

-Larry Cook; Albany, Ga.

A. When an FM station is modulated, the effect is much like that produced when tuning slowly past it. Here, however, the signal frequency itself is shifting, and not that of the local oscillator within the tuner. The tuner frequency is moving back and forth, above and below its center frequency. It is this movement of the station's carrier frequency which produces the desired program.

Just as a signal can disappear if one tunes off to one side or the other, so it can tend to disappear if the signal moves from one side of center to the other. The narrower the bandwidth of the i.f. strip in the tuner, the more this effect will be observed. Because of this decrease in signal strength as the signal frequency shifts away from its assigned center frequency, there is some loss in fidelity because the i.f. bandwidth is not linear.

In the wide-band position the selectivity of the tuner is such that the meter will not show changes in signal strength, even though the signal itself is still shifting above and below its assigned center frequency. Notice that when you tune off a signal you must move the dial a greater distance (when in the wide-band position) before the signal becomes noticeably distorted than when the tuner is in its narrow-band position. Therefore, the narrow-band position of your tuner should be used only in those instances where there is alternate channel interference to the desired signal or where multipath distortion takes place.

Lowest Frequency

Q. How low can a pipe organ play?

-James D. Gibbs, Cedar Rapids, Iowa

A. The lowest frequency produced by most pipe organs is 32 Hz-corresponding to what are known as "1 6 foot" pipes. Some pipe organs have what are known as "32-foot" pipes; their lowest note is 16 Hz.

Straight and S-Shaped Tonearms

Q. Does the use of a straight tonearm on a turntable have any advantages over the use of an S-shaped one?

-Michael Lemieux, Rouses Point, N.Y.

A. The straight arm does not have any intrinsic advantages over an S-shaped tonearm.

(Source: Audio magazine, Feb. 1982; Joseph Giovanelli )

= = = =