Audioclinic (Q and A) (Feb. 1974)

by Joseph Giovanelli

Measuring an Unknown Frequency

Q. I should like to find out the frequency of an unknown audio signal.

How can I hook up the unknown signal and a calibrated signal generator, sweep until I hear a beat or null, and then take my reading from the calibrated generator?

-F. Alexander, Maspeth, New York.

A. The simplest way by which one can determine a frequency generated by an uncalibrated source is to obtain a frequency counter. Certainly if you plan to do much along these lines the investment in a counter is justified. The uncalibrated source is fed directly into the input of the counter. If its output level is too low for the counter to handle, an appropriate preamplifier must be used. The frequency of the unknown signal is read directly on the counter's display.

Here is another method for determining the frequency of the unknown signal source. This system is more like the one mentioned in your question.

In this arrangement the unknown signal is compared to a signal of known frequency. The two signals are adjusted to produce equal audio outputs. Each signal is fed to the input of a two position mixer, whose output is connected to an amplifier and loudspeaker system.

Perhaps the best procedure is to listen to the unknown and then attempt to match that sound fairly closely to that of the known generator, by alternating listening to one or the other of the signals, while you adjust the known to equal the frequency of the unknown source.

When you think they are reasonably close in frequency, listen to the two signals together. You probably will hear them, plus a third signal set up by interaction between the two signals.

Adjust the known signal till this third sound disappears. What you will then hear is a variation of the amplitude of what appears to be one signal. Carefully adjust the frequency of the calibrated source until even this effect vanishes. You will then have the two signal sources running at the same frequency. Therefore, the frequency of the unknown source can be read as though it was the known source. The accuracy of your results will depend on the accuracy of the known signal generator's calibration.

If the frequency of the unknown signal is above the range of audibility, the same basic procedure can be used, although it would be subject to error.

In this instance you would sweep the calibrated source until you hear a signal. This signal represents the beat tone created by the known and unknown signals which are close enough in frequency to produce an audible beat. Unfortunately, rather than obtaining a true beat between the known and un calibrated signals, it is possible that the harmonics of the signal of known frequency can beat with the fundamental frequency of the unknown or vice versa. Thus, when you adjust for the absence of such a beat, your reading of the known signal frequency may reflect a reading created by harmonics rather than the fundamental frequencies involved. Therefore, be sure to sweep the known signal's frequency over a sufficient range to pick up more than one beat. The beat which is the strongest will represent the beating of the two fundamental frequencies.

Where supersonic frequencies are not involved, a third procedure is possible. Instead of combining the signals, feed each of them into a stereo amplifier channel. Make the same tests while listening to the output of that amplifier with headphones.

Compatibility Between Dolby Devices

Q. Are the various makes of Dolby decoders compatible? Can I play tapes recorded on a KLH 40 tape-deck and Dolbyized on the unit's built-in Dolby system, though a TEAC deck with the TEAC Dolby unit, and get no degradation of the sound?

-Michael Deutsch, Travis AFB, California

A. Assuming that the various pieces of Dolby equipment are properly calibrated and adjusted, there is no reason why you cannot play a tape made on one machine, on a completely different make of machine and Dolby decoder. The performance of the equipment should be perfect.

The parameters for proper operation of the Dolby System are spelled out carefully by Dolby Laboratories with this idea of compatibility in mind.

(Audio magazine, Feb. 1974)

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