TAPE GUIDE (July 1989)

Dolby NR and Bias Adjustment

Q. I have read that bias setting should be determined with Dolby noise reduction off, and this is the way I have normally established bias settings. A friend who is generally more knowledgeable in such matters than I am has told me that I should set the bias using the NR system that I plan to record with. I have tried his method but have found that recordings often sound somewhat shrill, especially with Dolby C NR, indicating insufficient bias. Which of these is the proper method?

-Larry Craven; Raleigh, N.C.

A. There are two creditable schools of thought as to whether bias should be adjusted with Dolby NR on or off. In theory, bias should be adjusted for flattest possible response with Dolby NR off; the use of Dolby NR would then have the least effect on treble response. But as a practical matter, it is often satisfactory to adjust bias with Dolby NR on. I have discussed the matter with an extremely knowledgeable audio engineer, and he said that, at times, he has achieved better results by adjusting bias with Dolby NR on. Apparently, the way to go varies with the deck. On my Nakamichi deck, I find it satisfactory to adjust bias with Dolby on. Comments are invited.

HX Pro as Noise Reduction

Q. I believe I understand how HX Pro works. Isn't it a secondary form of noise reduction? If you can record high frequencies at a higher level by means of HX Pro, don't the lower frequencies of the program material also get recorded at a correspondingly higher level, with a concomitant increase in signal-to-noise ratio?

-R. Haeffele; Bloomington, Cal.

A. With HX Pro, somewhat less treble boost is required in recording than would otherwise be the case, thus lessening the risk of tape saturation.

This is because, in the presence of high-frequency energy, the bias is somewhat reduced by HX Pro, yielding better treble response. Therefore, either the recording level can be pushed a bit higher or the same recording level can be used as before but with less danger of tape saturation. Only the first alternative would be equivalent to noise reduction.

Curious About Type II

Q. Please clarify one of life's mysteries for me. My cassette deck has automatic selection of bias and equalization, based on the cassette shell. I find that 1 can use Dolby NR only with Type I tapes. If I use Dolby NR with Type II, much of the crispness is lost in playback. On the other hand, without Dolby NR, I find no objectionable hiss on most recordings made with Type II tape. What do you suggest?

-Larry R. Murray; West Palm Beach, Fla.

A. It seems that you have one or both of the following problems when your cassette deck goes into Type II mode: Either excessive bias is supplied in recording, which reduces high frequencies somewhat (but not to an intolerable extent), or Dolby NR tracking is inaccurate, resulting in treble loss. (Tracking refers to the fact that the level of the encoded signal, in recording, must match the level of the playback signal that is decoded.) Noise-reduction circuits tend to accentuate departures from flat response.

Thus, any high-frequency loss due to excessive bias is accentuated by the NR system; this is in addition to losses due to mistracking.

Inasmuch as your deck works properly with Type I tape, including use of Dolby NR, I suggest that you stay with Type I. If you strongly desire use of Type II, try various brands. It may well be that some brands work better than others with the bias and sensitivity settings provided by your deck. At some future time, your deck may need servicing for reasons other than the present problem; have the service shop check its bias and Dolby NR tracking as well.

Improving 78s

Q. I want to improve the sound of my 78-rpm phonograph discs by recording them on tape. Will a DAT system significantly improve their fidelity? If not, what should I use?

-Kenneth A. Stone; San Fernando, Cal.

A. A DAT system simply makes a near-perfect copy of whatever it is fed. If low fidelity is put into a DAT unit, low fidelity will come out. DAT recorders cannot improve on the source.

One of the improvements you can make in playing and/or taping 78s is noise reduction. There are a few dynamic noise filters on the market. Consult your local audio dealers or refer to back copies of Audio's Annual Equipment Directory (which appears in every October issue).

You will probably want to play around with the frequency response, too. For this, a graphic equalizer or a parametric equalizer can be useful.

Again, consult your local dealers and the Annual Equipment Directory or the annual Buyers' Guide published by Stereo Review. The graphic or parametric equalizer can also serve for noise reduction, if judiciously used, but is probably not as effective as a dynamic noise-reduction unit, which operates on the principle of cutting treble only when the program material has little treble content.

Hi-Fi with a VCR

Q. Would a Hi-Fi VCR be capable of faithfully recording the material from a good Compact Disc or LP? Would a dbx noise-reduction unit make a significant contribution to sound quality?

-Victor S. Zupancic; Kirkland Lake, Ont., Canada

A. With good-quality videocassettes, Hi-Fi VCRs are capable of excellent audio recording in terms of frequency response, distortion, S/N ratio, and tape motion. Therefore, they are able to reproduce CDs and other high-quality sources without noticeable change.

Considering that a Hi-Fi VCR achieves an S/N of well over 80 dB, and even as much as 90 dB, it seems that you would gain very little by using dbx NR along with a Hi-Fi VCR.

Frequency Response Requirements

Q. What is considered good high frequency response? Would frequency response extending to 28 kHz be better than response to 26 kHz? Would a ferrite head or a permalloy head provide better response?

-James Bailey; Brenton, W. Va.

A. Flat response, within 1 or 2 dB, between 50 Hz and 15 kHz is ordinarily considered to be of hi-fi caliber for tape decks. Relatively flat response between 30 Hz and 20 kHz is usually all that one might need and hear in 99.9% of circumstances involving music and humans. Response above 20 kHz tends to be gilding the lily. So far as playback is concerned, the head's material is not a key factor-although it can be in recording. What counts principally is how straight and how narrow the head gap is. The narrower and straighter the gap, the higher the possible treble response.

Bias Variations

Q. My friend and I both own the same brand and model of tape deck, purchased about eight months apart.

Comparing the owner's manuals, we find that the bias settings for various tapes differ greatly. Which manual should I follow?

- Cary Charles, North Charleroi, Pa.

A. During the course of producing a particular model of tape deck, a manufacturer usually makes production changes in order to improve performance, to maintain performance while reducing cost, or to correct mistakes.

Thus, the proper setting of bias may vary from one production run to another. I believe that you and your friend should each follow your own manual.

If the changes in recommended setting affect only a few tape formulations, it may be that those formulations changed between the manufacture of the two decks. If so, the recommendations in the newer manual are more likely to be correct. When in doubt, adjust bias for the most faithful frequency response when recording FM interstation noise at a moderate level.

On the Level

Q. Whenever I record music with my cassette deck, I have to record the left channel higher than the right in order to obtain equal playback levels. 1 have tried using different interconnects, different sources (various CD players, a tuner in both stereo and mono, and a Hi-Fi VCR), and bypassing my equalizer-all to no avail. Also, when I switch from forward to reverse in playback, or vice versa, there is a distinct change in the sound level. Please advise.

-Overton Isaacs; Bronx, N.Y.

A. It may be that the record level indicators of your deck's left and right channels are mis-calibrated relative to each other. This is usually a simple internal adjustment for a competent audio technician. It could also be that the left-channel record electronics, following the point where the signal is taken for the record level indicator, have less gain than the right channel. If the difference in level is not great--say, not more than 3 dB or so--I suggest you live with the problem. Either raise the level of the left channel, as you are now doing, or drop the level of the right channel. Use whatever procedure best enables you to maintain a high recording level (for high S/N ratio) without overloading the tape, which causes distortion and treble loss.

If the playback head of your reversing deck is stationary, rather than of the rotating type, and has two sets of gaps, one for each direction, it could be that one set of gaps has less output than the other. Another possibility is that the playback head, whether stationary or rotating, is in poorer azimuth alignment in one direction than in the other. Azimuth misalignment in playback, with respect to azimuth alignment in recording, causes treble loss.

Substantial treble loss can produce the effect of a change in overall sound level.

Single vs. Dual Capstans

Q. My cassette deck has only a single capstan, yet it has given excellent results. Will my tapes display increased wow and flutter over time because my deck doesn't have dual capstans? Is a dual-capstan drive preferred for optimum stability and shelf life of cassette recordings?

-J. Douglas Schumer; Derby, N.Y.

A. As has been noted here and elsewhere in the past, there is usually more than one good path to a given engineering objective. So far as I know, this applies to the question of dual capstans versus a single capstan to drive the tape. (To take a similar situation, a one-motor deck, if well-engineered, can perform as well as other decks with two or more motors.) Stated directly, a well-engineered deck with a single capstan can provide very good performance in terms of wow and flutter, and can continue to do so for a long period. I see no connection between the number of capstans and the shelf life of recording.

(Source: Audio magazine, July 1989, HERMAN BURSTEIN)

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