Tape Guide (Aug. 1979)

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Copying Dolby-Encoded Tapes

Q. What is the procedure when copying a Dolby-encoded tape to produce a Dolby-encoded copy?

-David Rowland, Parlin, N.J.

A. There are two schools of thought on this. The first, and apparently preferred one, is that to obtain proper tracking (correct application of treble cut on low-level signals), you should play back through a Dolby decoder to restore a flat signal and then record through a Dolby encoder to make the dubbing. The second school of thought is that the less you tamper with an audio signal, the cleaner the final result will be. You would thus play the original tape without decoding, and copy this by recording without encoding. Accordingly, the copy will retain the original encoding.

I have tried both methods, and with my equipment and to my ears there is no obvious or consistent difference in results. With your equipment and your ears, however, there might be a difference.

External Dolby Hook-Up

Q. I am planning to buy an external Dolby unit. I'm wondering how to hook up this unit to decode FM Dolby broadcasts. Can the unit be put between my receiver and my open-reel tape deck, or would it be necessary to connect it between the preamp and amplifier sections of the receiver?

-Scott MacGregor, Atlanta, Ga.

A. You apparently have two objectives: (1) To produce Dolbyized tape recordings for any signal source (FM, phono, etc.) and (2) to decode Dolbyized FM broadcasts. To meet the first objective, the Dolby unit must be placed between the receiver and the deck, utilizing the "tape-out" and "tape-in" jacks of the receiver. To meet the second objective, you need a switching arrangement that permits the decoding (playback) section of the Dolby unit to be connected to the FM signal. Quite possibly your deck provides such an arrangement, particularly if it is of the three-head type that permits simultaneous record and playback, as it will have a "tape-source" switch for monitoring. The switch can be set to the source position to accept the FM signal, and this can be processed through the decoding section of the Dolby unit. Altogether, the Dolbyized FM signal would be routed through the Dolby unit's recording section but with Dolby encoding off, through the tape deck (without otherwise operating the deck), through the Dolby unit's playback section with Dolby decoding on, and into your receiver via the "tape-in" jacks. In the case of non-Dolbyized FM broadcasts, you would play the FM signal straight through the receiver, without going through the above sequence.

Storing Tape Recordings

Q. I have been led to believe that tape recordings should be stored carefully away from electric motors and electronic equipment lest the recordings be damaged by stray magnetic fields. But I have also read an article with a different view. It summarizes a 3M Company report to the effect that, apart from a magnet placed directly on a reel of tape, no electrical or magnetic device in the home is likely to damage the magnetic impression on a tape.

Has anyone confirmed 3M's research?

- Everett Young, APO New York

A. The 3M Company is one of the primary authoritative sources on magnetic tape recording, and I am inclined to trust their findings. Studies by the National Bureau of Standards have confirmed the point that only an extremely powerful magnetic field very close to the tape (a matter of a very few inches) will endanger the tape.

That danger is erasure.

My own experience is that a very powerful electromagnet (ordinarily used as a bulk eraser) brought as close as about four inches from a reel of tape has no audible effect on the recorded sound.

It seems, altogether, that a safe rule would be to keep tapes at least six inches away from possible sources of magnetic fields, such as those emanating from motors, speakers, etc. If you want to be extra, extra safe, change the rule to one foot.

If you have a problem or question on tape recording, write to Mr. Herman Burstein at AUDIO, 401 N. Broad Street, Philadelphia, PA 19108. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Source: Audio magazine, Aug. 1979; Herman Burstein )

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