TAPE GUIDE (Feb. 1990)

Info on Dolby NR

Q. I have the following questions about the various Dolby noise-reduction systems: How many different systems are there, and what are the advantages of each? Can you record with one system and play back with another? When you purchase a commercial cassette, how do you determine which system it has been recorded with? Can the various Dolby NR systems be used with all types of tapes? My equipment has both Dolby B and C NR; why do I need both?

-Jack Speakman; Shawnee Mission, Kans.

A. For home audio equipment, there are two Dolby noise-reduction systems: Dolby B and Dolby C NR. There are also two Dolby systems for professional use: Dolby A NR and Dolby Spectral Recording; these provide greater noise reduction and other advantages. Dolby B NR was the first noise-reduction system for home equipment introduced by Dolby Labs.

It offers about 8 to 10 dB of noise reduction. Dolby C NR came along more recently and offers about twice as much noise reduction. In addition, it has a special equalization curve in the upper treble range that reduces the chance of tape saturation, and of consequent distortion and treble loss, at high recording levels.

If a commercial prerecorded cassette has the Dolby double-D logo but fails to clearly indicate which Dolby NR system has been used in its recording, it almost certainly has Dolby B NR; commercial recordings using Dolby C NR are, in any case, comparatively rare. If you buy one, you can experimentally determine whether it sounds better in the Dolby B or C mode. Depending on the nature of the mismatch, the sound will be either shrill or dull if you use the wrong NR system in playback. No harm will befall the tape if you use the wrong one.

The various Dolby systems can be used with all types of tape, namely Type I (normally ferric oxide), Type II (usually chrome or ferricobalt), and Type IV (almost always metal particle). Although Dolby C NR is superior, most decks that include it also have Dolby B NR, in order to be compatible with decks that provide only Dolby B NR and to properly play commercially prerecorded cassettes that use it.

Desirability of Level Controls

Q. I plan to buy my first cassette deck, and I wonder what difference it will make if I buy a deck without record and playback levels controls.

-Jesse Morales; San Antonio, Tex.

A. If a deck has no level control for recording, the deck employs automatic level control (ALC). While some ALC circuits do a better job than others, the consensus is that for high-fidelity results, the record level should be set manually by the user-not automatically-to avoid the risk of compression and/or distortion. If you plan to do only "utility" recording of such things as background music, conversation, etc., you should find ALC satisfactory.

For playback, a level control is less important but is nevertheless desirable. A level control enables you to make the level of tape playback comparable with that of other sources--such as tuner, CD, etc.--and lets you compare the signal going to the tape deck with the signal coming from it.

Overheated Equipment

Q. I recently bought a cassette deck which works well, considering its low price. However, I'm concerned about the heat it develops after some three hours of continuous use. Not only is the deck hot, but so are the cassette tape and shell. Will this weaken, destroy, or otherwise affect my tapes? The unit is a double cassette deck, and I'm wondering if I should buy a more expensive one.

- Fred McCullers, New York, N.Y.

A. Tape decks are designed to withstand considerable heat, and so are cassettes. If the deck stabilizes--in the sense that it and the cassette do not become very hot--there is probably nothing to fear. But if the deck becomes hot enough to burn your finger if you touch it more than momentarily, and/or if the cassette shell becomes so hot that it warps, either you have to confine yourself to shorter periods of use or buy a new deck. Repair is probably inadvisable for a low-priced deck because of the high cost of the repairs relative to the deck's value.

Precautionary Fast-Winding

Q. I have read that when recording a brand-new cassette tape for the first time, it is beneficial to fast-wind the tape from end to end before recording.

What is the reason for this? Also, should this be done with open-reel tapes?

-Darren Hovsepian; West Bloomfield, Mich.

A. The advice you cited is usually given for tapes that have been recorded on a home deck and stored for a period. Fast-winding, if not perfectly even (it is more likely to be even with a professional-grade deck than a home deck), tends to stress the tape, and the tape may acquire a set during long storage. Also, print-through tends to develop during storage of a recorded tape. Fast-winding the tape before use tends to relieve the stresses and to diminish print-through.

I haven't previously encountered this advice in connection with virgin tape, but there's no harm in winding such tape just before initial use, in case the tape has been exposed to stresses.

This also applies to open-reel tape.

Type I vs. Type II Tape Q. I have read that a good-quality Type I tape (ferric oxide) is better than a Type II tape (chromium dioxide or ferricobalt) for sound quality. But is there a trade-off with respect to tape durability or deterioration of the sound?

-Greg Shettler; Winnipeg, Man., Canada

A. Some Type I tapes are better overall than some Type II tapes. Typically, Type I tapes have less distortion than Type II as far as measurements go, but it is open to question whether the difference is audible. Type II tapes usually have somewhat more extended treble response because of the characteristics of their magnetic coatings.

Also, Type II tapes usually have a somewhat higher S/N ratio by virtue of the fact that Type II calls for a playback equalization which reduces the treble more than does Type I equalization; the greater reduction in treble also brings down the noise more. But if you play at a moderate level and use Dolby C or dbx NR, the difference in S/N may be inaudible or audible only during very quiet passages. Summing up, on average Type II tends to produce performance that is a bit better than that of Type I, but there are individual Type I tapes that outperform some of the Type IIs. To my knowledge, there is no important difference with respect to durability and preservation of sound.

Mixing Facility

Q. Several years ago I acquired a cassette deck which had microphone inputs and allowed one to mix these with the line inputs. This was very useful; for example, it allowed me to record short stories with background music. I have recently upgraded, but at the expense of losing the mixing capability. My current deck's line inputs are cut off when one inserts mike plugs into the mike jacks. Are you aware of any cassette decks that permit mike and line mixing? I suspect deck manufacturers are more interested in providing dubbing facilities than in supporting creative home recording.

-John Robbins; Annapolis, Md.

A. Cassette decks with mike and line mixing facilities are a rarity. Refer to the Annual Equipment Directory in the October issue of Audio to learn which decks have mike inputs, and then contact their manufacturers to ask if any of these models provide mixing facilities.

Your best bet, probably, is to buy a mixer. Check with your local audio dealers on what they have. Several readers report that they have had satisfactory results with Radio Shack mixers.

Options for Boosting Treble

Q. If I wish to accentuate the high frequencies moderately, perhaps 3 to 5 dB, which of the following methods would give the least distortion and which would give the most: Reducing bias current by means of the bias control on my deck; using an equalizer in recording; using an equalizer in playback only, and increasing my speaker's tweeter output by means of the tweeter control on the back?

-Anthony Hudaverdi; Santa Monica, Calif.

A. In theory, reducing bias is least desirable, because this causes an increase in distortion. However, if you are aiming at only a moderate elevation of the treble, no more than about 3 dB, the increase in distortion might be inaudible, depending on the individual listener.

If you are going to use an equalizer, it is far more preferable to do so in playback than in recording. If you accentuate treble in recording, you may overload the tape at high frequencies, with consequent distortion and treble loss, particularly at high recording levels. Boosting treble in playback will increase noise but will avoid distortion.

I am dubious about increasing tweeter output because you may be elevating a broader range of treble frequencies than you really wish to. If you play at very high levels, you may put your tweeter in physical jeopardy by elevating its output as much as 5 dB. The best way to go, in my opinion, is to use an equalizer in playback.

(Source: Audio magazine, Feb. 1990, HERMAN BURSTEIN)

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