TAPE GUIDE (Jul. 1991)

Why Adjust Bias at -20 dB?

Q. I adjust my deck's fine bias knob, using white noise from a test CD, to match the source with the tape playback at a level of -20 dB. But when I increase the record level to 0 dB, the source and tape don't match anymore; the tape is duller. Which procedure is correct, bias adjustment at 20 or at 0 dB, and why?

-Anthony Hudaverdi, Santa Monica, Cal.

A. You should adjust at -20 dB, for the following reasons: Assuming all frequencies have the same amplitude, as is essentially true of white noise, in a cassette system the tape saturates more easily at high frequencies than at others. This is largely due to the great amount of treble boost employed in recording. Such boost is needed to compensate for magnetic and electrical losses encountered in recording, and it may reach something like 15 dB at 15 kHz. Therefore, using white noise, high frequencies are presented to the tape (and tape head) at much greater amplitude than are the middle and low frequencies.

To avoid tape saturation and consequent loss of highs when adjusting bias with a white-noise test signal, it is necessary to keep the signal level distinctly below DIN or Dolby level. (One or the other of these two levels, which are about 2 dB apart, is usually the 0 dB reference.) The usually recommended level is -20 dB.

Fortunately, in most program material the amplitude of the highs is substantially lower than that of the rest of the audio range. This natural fall-off of highs tends to compensate largely or completely for the treble boost employed in recording. Accordingly, if we adjust bias at the -20 dB level when using white noise, we avoid tape saturation at high frequencies when recording so that peaks reach 0 dB or even about 3 to 5 dB higher.

Some program material does contain unusually strong highs, and the recordist may then have to reduce recording level by several dB to avoid saturation. One can't simply "set and forget" when it comes to recording level; judgment must be employed.

Type and brand of tape also enter the picture. Type IV (metal-particle) tape is more immune to saturation than the other types. Within a given type, susceptibility to saturation tends to vary somewhat from one brand to another. In the case of Type IV tape, one could adjust bias at around -10 dB, if one wished, when using white noise.

Finally, there is the matter of what type of noise reduction is employed.

Because of special equalization in the high treble range (spectral skewing and anti-saturation curves) employed by Dolby C and Dolby S, these noise reduction systems help guard against tape saturation more effectively than Dolby B NR. The dbx system guards still more effectively. Accordingly, the need for going below 0 dB when adjusting bias with white noise will vary.

Tape recordists frequently employ FM interstation noise to adjust bias.

Above about 2 kHz, this noise drops steadily in level, owing to the tuner's standard FM de-emphasis of 75 µS; the drop eventually reaches a rate of 6 dB per octave. To illustrate, the inter station noise at 10 kHz is nearly 13 dB lower than at 1 kHz. Accordingly, the need for going below 0 dB when setting bias is reduced.

Safe Head-Cleaning

Q. I have just purchased a dual-well cassette deck and would like to know what the best products are to clean it with and how often to demagnetize it. I am a little wary of using anything right now because I have heard that alcohol-based products can damage the heads and that abrasive tape cleaners do the same.

-Donna Farley, San Marcos, Tex.

A. You should first consult the owner's manual of the deck to discover what, if anything, the manufacturer recommends for cleaning the heads and other parts (capstan, pressure roller, guides) contacted by the tape. If nothing is suggested, it is generally safe to use 91% isopropyl alcohol, which many drug stores carry. Audio stores usually carry special cleaning fluids intended for tape decks; often they contain trichlorofluoroethane. Apply the alcohol or other substance with a cotton swab to all deck components contacted by the tape. Let things dry at least five minutes before using the deck.

Avoid 70% isopropyl alcohol, rubbing alcohol, and similar alcohol compounds because they usually contain contaminants that may leave undesirable residues, such as gums and oils.

It is generally considered that direct cleaning, as described here, is preferable to use of a cleaning cassette, which may be abrasive.

Demagnetization is preferably performed with a wand-type demagnetizer rather than a demagnetizing cassette; wand demagnetizers are apt to be more effective. When you use any type of demagnetizer, be sure that the rest of your audio system is off. If your deck is feeding into your audio system, and if the system is accidentally left on when you are demagnetizing, the resulting loud hum can damage your speakers.

Cleaning and demagnetization should take place after about every eight hours of use, to be on the safe side. Some advocate less frequent demagnetization--say, every 16 hours. If you notice substantial accumulations of tape oxide on the heads and other components, cleaning more frequently than every eight hours may be advisable. Excessive deposits on the heads, etc., may indicate the need to change your brand and/or grade of tape.

"Improved" Type II Tapes

Q. I have recently come upon some tape brands that combine metal particles with ferric oxide and take Type II bias. It is claimed that these tapes approach the extended frequency response of Type IV tapes for less cost. Are these tapes really better than ordinary Type II?

-Rhet Diaz, Playa Del Rey, Cal.

A. The very extensive review of cassette tapes by Howard A. Roberson in the March 1990 issue does show that a few Type II tapes are nearly as good as the best Type IV tapes in this respect. A 10-kHz tone recorded at Dolby level will typically drop about 4 or 5 dB in level, and occasionally much more, when recorded on Type II tapes.

On Type IV tapes, this tone would typically be about 1 dB down. Roberson's article enables you to identify which Type II tapes have the best treble response at high recording level.

(Source: Audio magazine, Jul. 1991, HERMAN BURSTEIN)

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