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Magnetic Query Q. The operating manual for my cassette deck says not to place magnets, magnetic screwdrivers, etc. near the heads. Is it safe to demagnetize the heads? -Thomas Burt, Las Vegas, Nev. A. the reason for keeping magnets, screwdrivers, etc, away from the tape heads is that these may magnetize them, causing noise and possible era sure of the high frequencies of the cassette. However, it is safe to demagnetize the heads by means of the demagnetizer made for this purpose. The de magnetizer is not a permanent magnet, but has an alternating magnetic field, which means that it will demagnetize the heads as it is gradually withdrawn from them. Cassette Tape Decks Q. What factors contribute to preventing 2- or 3-speed cassette tape decks from appearing? - Rajiv Mehta, New York, N.Y. A. At speeds faster than 1 7/8 ips, the running time for a cassette would be too short. At speeds lower than 1 7/8 ips, it would be very difficult to maintain good treble response along with low distortion and low noise; also, wow and flutter would become more troublesome. On the other hand, development of revolutionary new tapes, such as those with a coating of fine metallic particles, may lead to introduction of 15/16 ips as a second speed. Performance Parameters Q. In comparing the high frequency performance of my open-reel deck at 7 1/2 and 3 3/4 ips, using FM broadcasts as the program source, I notice no difference in treble performance. Is this be cause the treble content of FM broad casts is limited to the capabilities of the 3 3/4 ips speed? -Jeffrey Pratter, Brooklyn, N.Y. A. Today's open-reel decks of good quality can maintain flat treble response well beyond 15 kHz--often beyond 20 kHz--at 3 3/4 ips. Hence there is, often, no apparent difference between this speed and the higher speeds in terms of treble response. However, the higher speed of 7 1/2 ips offers greater "headroom"--namely, less chance of high amplitude signals in the treble range causing tape saturation. If you were recording live pro gram material with very strong transients, as caused by a guitar, you would probably need the greater headroom provided by 7 1/2 ips. But in recording from FM, you deal with a signal which has been limited and/or compressed, so that the danger of tape saturation is much less. S/N Ratio Differences Q. Why is the playback signal-to noise ratio on an open-reel deck al ways greater than, or at least equal to, the record/playback signal-to-noise ratio? -Jeffrey Pratter, Brooklyn, N.Y. A. Playback signal-to-noise ratio tends to be greater than the record/playback signal-to-noise ratio because the former excludes noise in recording. Recording noise is due to noise in the record electronics and to bias employed in recording. Unless the bias waveform is very, very pure--containing a minimum of distortion--it can add appreciable noise to a tape recording. Tape Backing Q. Some of the tapes have a rough backing to provide better traction for the drive mechanism. Will these tapes cause excessive wear of the cross field head of my tape deck? -Morris Schoenberg, Madison, Wis. A. First let me explain, for the benefit of the other readers, that the crossfield head, which supplies bias in recording, is located opposite the re cording head so that the back of the tape goes past the crossfield head. There is no problem in using a cross field head with back-coated tape. The tape is kept a very small distance, something like one-thousandth of an inch, away from the tape. (Audio magazine, Jul. 1979; Herman Burstein ) Also see: = = = = |
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