Tape Guide (Oct. 1979)

EQ of Dolby

Q. When an add-on Dolby unit is used, should the tape deck's internal equalization be disabled? When a Dolby unit is used, is the equalization the same for different tape speeds?

-Chacko Neroth, El Cerrito, Cal.

A. Dolby N/R does not affect equalization, that is, using an external Dolby unit does not require equalization changes in the tape deck. By the same token, different tape deck equalization continues to be required at different tape speeds.

Pink Noise

Q. What is "pink noise"? How is 1/3-octave pink noise used in testing equipment?

-Paul C. Lutz, Louisville, Kentucky

A. If one built a noise generator, chances are that the noise would pro duce signal over the entire audio spectrum. If one devised a filtering system, however, this noise could be produced over a portion of the spectrum only. It is this reduced noise bandwidth which we refer to as "pink noise." The 1/3-octave pink noise is a special case, where the spectrum is divided into very narrow segments, each of which is 1/2-octave wide.

Such pink noise segments are often used to evaluate the performance of equipment because its waveform is of ten difficult for equipment to reproduce, especially with transducers, such as loudspeakers. The waveform produced by the speaker or other device is compared to the waveform of the pink noise which feeds into the device under test. The closer the input and output waveforms appear to be alike, the better is the equipment under test.

Listening rooms contain very sharp peaks and dips in their frequency response. They are often narrower than a 1/2-octave portion of the audio spectrum. However, these 1/3-octave segments represent a good compromise between performance and the complexity of the controls required on equalizers designed to correct for these peaks and dips of listening rooms. What happens is that signals are transmitted by means of calibrated transducers, and picked up at some other point in the room to be adjusted by suitable, calibrated microphones.

The signal consists of pink noise. A room equalizer, consisting of 1/2-octave boost and cut modules is used, and the controls are adjusted for best over all frequency response at some given listening point in the room. Note that the equalizer modules are made to match the segments of the pink noise used for calibration.

Noise Reduction Improvement

Q. My tape deck claims a 59-dB signal-to-noise ratio. I am considering the purchase of a Dolby noise reduction unit, and am wondering how much improvement I might expect to get.

-Tom Collinson, Mather AFB, Cal.

A. The higher the S/N of a basic tape system, the less important a further improvement in S/N becomes. Dolby units are capable of improving audible S/N about 6 to 10 dB. Such improvement would make a quite noticeable difference in the case of a tape deck with, say, 45 dB S/N. But it would make an unnoticeable difference in the case of a deck with 100 dB S/N (not that there are any such home tape decks as yet, but I use this extreme to illustrate my point). A deck with 100 dB S/N would be dead quiet, and you can't improve significantly on dead quiet.

As for your deck, the S/N of 59 dB is definitely in the category of high fidelity but is still subject to audible improvement. The finest professional decks reach out to S/N of about 80 dB, and digital machines get something like 90 dB. Thus, Dolby may audibly improve the S/N of your tape system, particularly if you play material with a wide dynamic range, and if you play it at high volume. However, if you operate your system at subdued levels, and especially if you play material with little dynamic range (such as "back ground music" or hard rock), you may wonder why you ever laid out the sum you did for Dolby, particularly for 7 1/2 ips operation.

(Adapted from: Audio magazine, Oct. 1979; Herman Burstein)

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