TAPE GUIDE (Dec. 1992)

Attention, Norm Strong!

I believe I have a solution for Norm Strong's dbx woes ("Whatever Happened to Mike Inputs and dbx NR?" in "Tape Guide," January 1992). I have a dbx Model NX-40 encoder/decoder that I would be willing to part with for a very low fee (plus shipping). I don't use the unit, and it is in virtually new condition. It will also decode dbx LPs-rare products, but I will give Mr. Strong the few that I have, gratis.

-John D. Cheatham, P.O. Box 1224, Redondo Beach, Cal. 90278

Mike Specs

Q. I am looking for a pair of micro phones that are unidirectional, dynamic, and have a range of about 30 Hz to 16 kHz; these are to feed my portable tape deck, whose specs for the mike inputs give only the following: "0.25 mV (-70 dB), for low impedance." I'm not sure of the relationship of mike impedance, in ohms, to the deck's specs. I need mikes with high output. Would this mean mikes with 600-ohm impedance? Is a mike spec for open-circuit sensitivity important in connection with output?

-Richard S. Hartley; Dover Foxcroft, Maine

A. Low-impedance microphones generally have output impedances in the range of 50 to 600 ohms, although a few go as high as 1,000 or even 2,000 ohms. High-impedance mikes are those between about 10 and 50 kilohms. While there is a basic relation ship between impedance and output, the relationship is not very linear. The higher the output impedance, the higher tends to be the output. High-impedance mikes have the obvious advantage of producing greater signal volt age, which helps make for a high signal-to-noise ratio, but they have the disadvantage of permitting only a few feet of connecting cable--about 10 to 15 feet-before cable capacitance causes serious treble loss.

It is quite possible that a 200-ohm mike will produce more output than a 600-ohm one. In other words, for a given sound input, the 200-ohm mike can be more sensitive than a 600-ohm mike. The sensitivity rating of a mike is ordinarily based on output voltage for a sound pressure of 1 microbar (u-bar) applied to the mike. The output voltage is stated as so many dB below 1 V. For example, a given mike might have a spec of -65 dB/u-bar, signifying that its output is 0.00056 V, or 65 dB below 1 V, for a sound pressure of 1 µbar.

The specification of your tape deck states that a signal 70 dB below 1V (which is 0.25 mV, or 0.00025 V) can drive your deck to full recording level.

Accordingly, you should look for a mike rated at no less than -70 dB; for example, -65 dB would be okay. As for output impedance, unless you plan to use extraordinarily long connecting cable, perhaps beyond 100 feet, it probably is not important whether the mike's impedance is 200 or 600 ohms.

To find the mike best suited to your needs, it is advisable to deal with a quality audio store, even though you may pay somewhat more than in a store that discounts heavily. Some times a store will try to push items it has heavily overstocked or on which it makes maximum profit, rather than try to truly serve the customer's needs. A high-quality dealer may provide a better opportunity to buy a mike on a money-back or trial basis.

Recording Level and Tape Type

Q. Why do most deck manuals ad vise using +2 or +3 dB recording level for peaks with Type I tape, and +5 or +6 dB with Type II tape? With metal tape (Type IV), they say one can go as high as +7 dB.

-Anthony Hudaverdi, Santa Monica, Cal.

A. Type II tapes generally have higher coercivity than Type I tapes. This refers to the force required to magnetize a tape. Thus, a stronger signal must be applied to Type II to achieve a given recorded level. Furthermore, Type II can accept a stronger signal in the treble region before it goes into saturation. Accordingly, the optimum signal level employed in recording that which maximizes S/N without incurring excessive distortion and treble loss-tends to be roughly 3 dB higher for Type II than for Type I. Type IV tape has still higher coercivity and so can accept a still higher recording level.

Differences in permissible recording level vary not only according to tape type but also according to brand.

Whatever type and brand you decide to use, experiment to find the optimum recording level. Also, the optimum re cording level may vary somewhat with the nature of the program material. All in all, there is some art to getting the best out of a recording.

Dolby S Progress

Q. I awn two cassette decks-both with Dolby B, Dolby C, and Dolby HX Pro-that do a very good job of recording. Recently I read about a new form of Dolby noise reduction, Dolby S. As a typical audiophile, I am always interested in upgrading the performance of components, even those with performance that is very good. However, I don't wish to part with either deck, so I wonder if Dolby S encoding/decoding is available in an outboard unit, similar to the way dbx was once offered: If not, are there plans to do this in the near future?

-Frank Ciccone; Wallingford, Conn.

A. As yet, the cassette deck industry has not been in a mad rush to incorporate Dolby S NR, evidenced by the fact that, according to the Equipment Directory in the October 1992 issue of Audio, only seven decks offer this feature. One reason is price: Dolby S NR itself adds to cost, and a deck must meet high standards set by Dolby Laboratories before it can include this system. Another reason is that the difference between Dolby C and Dolby S NR is not profound, particularly if you listen at fairly moderate levels, where noise is usually unobtrusive. This is not to say that Dolby S NR has no advantages; it does extend noise reduction to the bass range, and it is less sensitive than Dolby C NR to mistracking (mismatching of the record and play back levels).

I have no information as to whether or when Dolby S noise reduction will be available in a stand-alone unit.

Setting Recording Levels

Q. I am searching for a good way to set tape recording levels. My present method entails skipping through the entire recording, comparing the source material with the taped material for distortion, and making appropriate adjustments. While this is not extremely difficult with my three-head deck, it does take some time. Is there a better way to check for saturation levels? I've read that the human ear is most sensitive to frequencies around 2,000 Hz. Would a test tone of this frequency provide a suitable tape saturation check?

-Mark E. Richards; South Bend, Ind.

A. Let's assume that you are taping CDs. (Similar comments would apply to phono discs.) Your present procedure seems more laborious than necessary. Your guides to proper recording level should be the record level indicator of your deck, a reference set ting of the record level control, and your own experience.

Use a CD with substantial dynamics to find the maximum setting of the record level control which permits undistorted recording to your ear. Note the corresponding reading of the record level indicator on peaks. For a safety margin, back down on the level control to reduce recording level about 2 or 3 dB. You now have a reference setting for your level control. However, this doesn't mean that the control should remain in exactly the same position for all recordings. Based on your knowledge of the CD you wish to copy, you might reduce or advance the setting of the control a bit. For example, if you were to tape Debussy's "Prelude to the Afternoon of a Faun," you might in crease the level a little; if you were to record Stravinsky's Firebird Suite, you might reduce the level somewhat.

When recording, take frequent note of the record level indicator. If it goes well beyond the reference level previously noted, and if it does so frequently, the record level might be too high. It may be desirable to redo the recording at a lower level, but this is unlikely to hap pen often.

What do I mean by changing the recording level "a bit" or "somewhat"? This is where your experience comes in to inform you. For most music, maxi mum amplitude tends to occur in the general vicinity of 400 Hz or so. On the other hand, owing to the large amount of treble boost employed in a cassette deck for recording, high signal amplitudes presented to the record head and to the tape may well occur in the range above 10 kHz. Therefore, if you were to check distortion on the basis of single tones, I would suggest 400 Hz and something like 12 kHz (or higher, if your hearing is good). But I don't think that you should use single tones to find the proper recording level. The ear is considerably less sensitive to distortion for mixed tones than for single tones. For example, distortion might become noticeable to a given individual when it reaches 1% on a single tone, but not until it reaches 5% on complex tones. Thus, you should use actual music to determine how high you can set the record level before distortion becomes audible.

(adapted from Audio magazine, Dec. 1992, HERMAN BURSTEIN)

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