Analog TAPE GUIDE: Q & A about the still-highest-fidelity audio- recording and reproduction format! (part 2)

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Pick the Right Alcohol

Someone asked why the use of 91% isopropyl alcohol made his tape deck’s rubber rollers dry out and crack. I stated that cleaning my own decks with this solution had never caused problems, but another reader has provided more information:

As a chemist and an enthusiastic tape recordist for many years, I would like to throw a little light on the matter.

Actually, both you and your correspondent are correct. The reason is that the isopropyl alcohol purchased from your local drug store is not just isopropyl alcohol and water; it is denatured isopropyl alcohol. Denaturants are added so that the alcohol will not be very tasty if you try to drink it. That is done by law. There are, however, a number of formulas that can be used to denature the alcohol, and it is impossible to tell by looking at the label or sniffing the alcohol just which has been used. Some of the denaturants are pretty nasty materials. I suspect that your reader bought some alcohol containing denaturants that didn’t agree with the rubber in his pinch rollers and caused them to decay. On the other hand, the drug store where you bought your alcohol may have carried a variety with “tame” denaturants that had no detrimental effect on your recorders at all. I suspect the problem is in the denaturants, not the alcohol itself.

There is a readily available cleaning agent that, for some reason, is ignored: Vodka. Vodka is simply grain neutral spirits (ethyl alcohol) diluted to proof with distilled water. It does not contain sugar or flavoring agents, and should leave no residue of any kind. Ethyl alcohol (ethanol) is also less likely to penetrate rubber or cause any trouble, but it is a very good solvent. In some cases, the normal 86-proof vodka (43% alcohol) will not be strong enough to do a good cleaning job. If that is the case, it is possible to buy a bottle of grain neutral spirits, which is usually 190 proof (95% alcohol). This can be used directly, or you can slightly dilute it with distilled water.

There are a couple of advantages of using vodka or grain neutral spirits as a cleaning agent. These liquids are relatively mild and unlikely to damage most rubber or plastic, certainly much less so than isopropyl alcohol. Second, if it turns out you do need a stronger solvent, you can always consume the remainder; you haven’t wasted any money. Most important of all, however, there is no denaturant present and no chance of these nasty and unidentified materials causing any problems.

MPX Filters and Response Limits

Q. My cassette deck does not have a switch for a multiplex (MPX) filter. Al though this deck has a rated frequency response of 20 Hz to 18 kHz, I have read that if a deck has a nondefeatable MPX filter, its response is limited to 15 kHz. Is this really true?

A. Your deck may have a non-switchable MPX filter, or its recording circuitry may be designed to render a filter unnecessary by rolling off sharply above 18 kHz; check with your dealer or look in your owner’s manual. Without an MPX filter or with the filter disengaged, a cassette deck’s response can extend to 20 kHz or a bit higher. With a filter, response can extend at most to 18 kHz or so.

The filter’s purpose is to cut out any 19- kHz pilot tone and 38-kHz subcarrier in the output signal from an FM stereo tuner. Otherwise, these frequencies may beat with the cassette deck’s bias frequency, producing audible noises, and may interfere with proper operation of the Dolby noise-reduction system. Many tuners filter these components out, but most cassette decks include multiplex filters in case a tuner doesn’t. Usually, this filter is defeatable.

How Many Tracks for Mono?

Q. I record the sermons in my church on a stereo cassette tape. When recording a mono signal, is it better to record that signal onto both tracks A and B or to record it on either track A or B?

A. You probably can’t record tracks A and B at separate times, because a recorder’s erase head operates when in record mode, and most cassette recorders and decks erase both tracks at once. So if you recorded the second half of the sermon on track B, you’d erase the material you’d previously recorded on track A.

Recording each track individually would save tape. But even if you could record tracks individually, it would probably be best to record the mono signal on both tracks and combine them in playback. This ensures that if one track suffers from drop outs or loses treble because of a grain of dirt or magnetic coating that affects just one gap, the other track will serve as a backup.

If you wish to record on only one track, it is best to record only on track B, the right-hand track. This is because contact between the tape and the heads is some times poorer toward the edge of the tape than in the middle, and track A is nearer the edge.

Hi-Fi VCRs and Dolby NR

Q. Can I use the Dolby NR system of my Hi-Fi VCR for audio recording? The instruction manual isn’t clear on this. Also, is the S/N ratio obtainable with my VCR comparable to that which I can obtain with Dolby C NR on a first-rate cassette deck when using metal tape?

A. In a Hi-Fi VCR, Dolby noise reduction is used only for the linear (edge) track, which is recorded in much the same way as an audio Compact Cassette. Without Dolby NR, the linear track would have a signal-to-noise ratio of only about 45 to 55 dB. Hi-Fi recording with a VCR employs FM modulation and takes place on the same tracks as the video recording, whether or not video recording takes place.

With metal tape, most of the best audio cassette decks achieve SIN ratios of roughly 75 to 78 dB. Some good Hi-Fi VCRs achieve S/N ratios greater than 80 dB. The difference is not apt to be profound to the human ear, except perhaps at very loud levels. The areas in which Hi-Fi videotape is superior to audio cassette are low wow and flutter, extended and flat frequency response, and long recording time (six hours or more with a VCR).

Vicissitudes of Temperature

Q. While attending school I live in a room that used to be a porch, and at night it can get mighty cold. What effect will such cold have on my tape collection? During the day, though, my room warms to a reasonable temperature. Will this fluctuation cause any harm? Will playing the tapes while they’re cold cause harm? Is there any risk to the tape deck from the cold temperatures?

A. Generally, tapes should not be subject to temperatures lower than 30° F or higher than 130° F. Rapid cycling between very cold and very hot might be injurious to tapes, but a fairly slow change from quite cold to room temperature is tolerable. I suggest not playing the tapes when they are still cold; their ability to withstand physical stress improves as they warm up. I doubt that cold temperatures will harm your deck, but to avoid physical stress it should be brought up to room temperature before operation.

DAT Life

A question was posed: “How long will a digital audio tape (DAT) last?” A substantial answer given by Brian T. Karda, Engineering Manager of Loran Cassettes and Audio Products, follows.

Metal-particle tape (MPT) is employed for DAT, which is similar to the metal tape employed for Compact Cassettes. Karda states that MPT can degrade at temperatures as low as 131° F, due to deterioration of the binder and distortion of the base film. Several brands of metal-particle DATs have been tested by Loran; they were all able to withstand 113°, but at 122°, “One particular brand began to show audible defects and excessive error rate,” defined as more than one bad bit in 100 bits. He concludes that “excessive environmental conditions should not be of concern in the studio or home; however, for portable or automotive applications, care must be exercised in storing and playing DAT cassettes.” For example, the temperature may go well above 130° in a closed, standing automobile on a sunny summer day.

DAT appears to perform “quite well” in long-term storage. “In an ongoing test, we are attempting to determine the shelf-life of DAT.. . one-hour DATS were recorded with a music program and tested immediately and every 12 months for error rate, signal level, and audible defects. After three years, none of the recorded DATs have demonstrated any effects or an excessive error rate.”

Loran has found DAT to display “exceptional performance” with respect to repeated-play capability. A number of 5-minute sections were recorded with a test tone. (Flaws are audibly more noticeable on test tones than on program material.) Each section was played and rewound 2,000 times, and after every 50 plays critical listening tests and a check of the error rate were performed. “After 1,500 plays, some of the tapes exceeded the allowable error rate and demonstrated some slight audible defects. However, more than half the tapes were still within the error-rate spec [ more than one bad bit in 100 bits, well within the error-correction ability of a DAT deck —H.B.J] and audibly unaffected even after 2,000 plays.” Thus, Karda concludes, “Some tapes may be more durable than others, but with proper handling of a quality DAT, a quite acceptable life span can be expected—longer than I have played my favorite cassette or CD.”

While DAT may never extensively re place the Compact Cassette or CD at the consumer level, Karda points out that DAT has a distinct place in several areas: In the recording studio, in the duplication house, in broadcast stations, and in computer applications (“where there is constant battle for more data in a smaller space”). Altogether, “DAT is a very robust format—a very useful tool for those using the format within its niche.”

Also see: TAPE GUIDE part 1

[adapted from HERMAN BURSTEIN ‘Tape Guide’ series, ]

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Updated: Wednesday, 2015-04-29 17:04 PST