TAPE GUIDE: Q & A about the still-highest-fidelity audio-recording and reproduction format! (Aug. 1984)

Tape Sensitivity and dbx NR

Q. I plan to buy a cassette deck with Dolby B and C, and also plan to buy a dbx noise-reduction unit. Will there be any problem in adjusting the dbx NR unit to work with the deck? How will I adjust it for tape sensitivity?

A. I foresee no problems in getting the dbx unit to work properly with your deck. In the unlikely event that a problem occurs even if you have carefully followed the dbx instruction manual, this type of question should be ad dressed to your audio dealer or to dbx.

No sensitivity adjustment is required for dbx, although such an adjustment is necessary in the case of Dolby NR if good tracking and, therefore, good treble response are to be obtained.

I trust you do not plan to record with both Dolby (B or C) and dbx simultaneously. With the approximately 30 dB of noise reduction achieved by dbx, further noise reduction is probably superfluous and is apt to lead to problems, including poor Dolby tracking. On the other hand, it may be interesting to experiment with simultaneous operation of both NR systems. Some times things that are not supposed to work do work—and vice versa.

‘Thin’ Sound at Low Volume

Q. When I play my tapes at very low volume, the sound seems to be very thin, not rich in tone. My preamp doesn’t have a loudness control. What can I do to help improve the sound at low volume?

A. When music is reproduced be low its original level, there is an apparent loss of bass to the human ear. This is known as the Fletcher-Munson effect. Therefore, many preamplifiers (separate or as part of a receiver or integrated amplifier) incorporate a loudness switch which automatically boosts the bass relative to the rest of the audio spectrum when volume is reduced. Whether such automatic boost is the right amount—neither too much nor too little—is another question. It appears that most preamps do not answer this question satisfactorily. A few do, by incorporating both a volume control and a continuous loudness control (rather than a switch). The volume control is used to adjust the sound level to that which approximately corresponds to the original level; the loudness control is used to reduce the listening level to a comfortable one and at the same time automatically pro vides bass boost of more nearly the correct amount than does a mere loudness switch.

. Open-Reel NR

Q. Why is it that the vast majority of open-reel tape decks do not incorporate a noise-reduction system of any sort?

A. As a general principle, noise-reduction systems are most audibly effective with noisy tape decks. In as much as the signal-to-noise ratios of fine open-reel decks reach well above 60 dB and even into the 70s—without benefit of NR—their manufacturers have tended to eschew such systems.

In the beginning, virtually the only NR system available for home use was Dolby B, which offered an improvement of about 8 to 10 dB in S/N. The audible improvement with cassette decks, which had S/N in the high 40s or low 50s, was quite marked, but it was less so with the superior open-reel decks. Furthermore, every NA system has some undesirable side effects, chiefly pumping and breathing (volume changes and bursts of hiss), al though by now these side effects have been reduced to insignificance for most listeners. Then, in the case of the Dolby NR systems, matching of input and output levels is required to avoid what is called mistracking, which results in some loss or excess of treble at the very higher Altogether, then, it appears that most manufacturers of open-reel decks have elected not to gain a modest audible improvement in S/N in exchange for higher cost, possible side effects, and special adjustments to avoid mistracking.

Of course, today we have Dolby C with its improvement of approximately 20 dB in S/N and dbx with its improvement of approximately 30 dB—both virtually free from side effects to most listeners. Hence, it appears desirable and profitable for open-reel decks to include an advanced NR system. By doing so, they could achieve S/N well into the 80s and 90s in terms of dB— and this would rival digital performance. Why this isn’t happening may have to do with the manufacturers’ evaluations of the market.

Magnetization Worry

Q. I recently read that one should never turn on a tape deck when it is already in the record mode. That is just what my little girl innocently did, and I am wondering whether my deck is irrevocably ruined. I demagnetized the heads, using a cassette-contained de magnetizing device, and have not noticed any problem with recording or playing tapes since then. However, / am not all that confident about my ear’s ability to detect losses of high frequencies, etc., and wonder whether I am going to have to replace a deck / like or run a serious risk of all the tapes I play.

A. There might be a slight chance of magnetizing the record head if the power were turned on just when a very high signal level was fed to the head, although I am inclined to doubt it. Bias current, which is several times the magnitude of the audio signal, presents more of a magnetization threat from a current surge when one enters or exits the recording mode, but a properly designed deck protects the record head against such surges.

If head magnetization did take place, a good-quality demagnetizer should take care of the problem. If you are still worried, despite the evidence of your ears that the deck works properly, you might have a service shop check it out. If the shop has a magnetometer, do not be alarmed if the head exhibits a slight amount of magnetism. It is very difficult, or even impossible, to reduce head magnetism to zero.

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