TAPE GUIDE (Nov. 1987)

The What and How of Bias

Q. I'm confused about what the bias frequency is supposed to do in recording. I have seen bias described as a signal that (1) promotes flat frequency response during recording; (2) excites the magnetic particles in the tape coating, promoting a stronger recorded signal by making the magnetic particles more receptive; and (3) allows the tape to record loud and soft sounds evenly and proportionately. Which definition is accurate?

-Steve Turner, Pepperell, Mass.

A. The second definition is easily the best, although incomplete because it fails to point out that bias, very importantly, also reduces distortion. The third definition is next best, but nebulous because "evenly and proportionately" doesn't tell us enough. The first definition is outright wrong; bias in fact causes treble loss.

Let's start over. What bias does is to greatly increase the amount of audio signal that can be recorded on tape for a given amount of signal applied to the tape; it also greatly reduces distortion.

How bias does this has been explained in two basic ways.

Perhaps the more common explanation refers to the tape's transfer function, which is the relationship between the magnetizing force (signal applied to the tape) and the remanence (signal recorded on the tape). Ideally this relationship should be linear, so that the recorded signal varies in direct proportion to the applied signal. However, remanence lags with respect to magnetizing force, a phenomenon called hysteresis. The result is a transfer function which is nonlinear when recording the lowest amplitude portions of the applied signal waveform, then linear as the waveform's amplitude increases, and finally nonlinear again as the amplitude becomes great enough to saturate the tape. Adding bias in the correct amount (typically about 10 times the maximum audio signal level) carries the low-amplitude portions of the audio signal into the linear portion of the transfer function. Thus, the recorded signal is largely free of distortion and recorded at high amplitude. So goes the first basic explanation, of which there are several variations. (For this explanation in detail, see "Dynamic Bias Control with HX Professional" by J. Selmer Jensen and S. K. Pramanik, Audio, August 1984.) The second basic explanation is pretty close to your second definition, stating that magnetic domains in the tape's coating are excited by the bias current so that they respond more readily-with less distortion and greater amplitude-to the applied audio signal.

In his book Magnetic Recording Techniques (McGraw-Hill, New York, 1972), Charles E. Lowman writes: "From the studies of magnetism it is known that the domains are tightly locked together. To loosen these magnetic bonds and allow the signal [to be recorded], a large amount of magnetic energy is required. It would appear that until a particular threshold of response is reached, little or no signal recording can take place .... Once unlocked [the domains] move with relative ease."

Another version of this explanation comes from Dale Manquen ("Magnetic Recording and Playback," Handbook for Sound Engineers, Howard W. Sams & Co., New York, 1987): "The magnetization of the tape particles is not easily changed, due to the memory force or hysteresis of the particles. In fact, we could think of the particles as lethargic--sleepy little particles that must be aroused before they will do anything, and then being quite content to immediately fall asleep as soon as the excitation stops. This lethargy produces a jerky recording characteristic that ignores weak signals and responds only to strong signals. Since the resulting distortion levels are intolerable for audio application, a method of waking up the sleepy particles must be employed. If a rapidly varying signal of sufficient amplitude to just begin magnetizing the particles is added to the audio flux signal, the magnetic particles will more readily conform to changes in the audio waveform. The high-frequency biasing signal produces a hysteresis-free or anhysteretic recording." Manquen also writes: "The high-frequency bias signal provides enough excitation to jolt the magnetic particles into an active state."

Turning to the relationship between bias and frequency response (stated incorrectly in your first definition), the bias signal has an erasing effect akin to that produced by the erase head; this grows more severe as the audio frequency rises because the high frequencies are less deeply imbedded in the tape than are the lower frequencies. This erasing effect increases with bias. In short, treble loss increases with bias. Treble boost in the record electronics helps overcome treble loss due to bias and other factors. But there is a limit to the amount of treble boost which can safely be used, and there fore to the amount of bias. At a high recording level, excessive treble boost can cause tape saturation and a situation called fold-over, where the recorded treble is actually decreased rather than remaining at the saturation level; this is accompanied by excessive distortion.

In cassette recording, bias typically must be at a level below the amount required for minimum distortion. How ever, too little bias produces excessive distortion. Therefore, the deck manufacturer strives for optimum bias, which achieves a satisfactory combi nation of low distortion and extended treble response. Listening tests con ducted by Dolby Laboratories indicate that the trade-off between distortion and treble response should somewhat favor low distortion. Optimum bias also takes into account minimization of noise and the maximum record treble boost that can safely be used.

Deck-to-Deck Compatibility

Q. Many tapes I record on my Nakamichi deck sound muddy when played back on other decks, presumably be cause, as I understand it, Nakamichi decks use their own equalization curve. Does owning a Nakamichi isolate me from all other decks? Does everything recorded on a Nakamichi have to be played back only on a Nakamichi?

-Anthony Hudaverdi, Santa Monica, Cal.

A. First, let me point out that Nakamichi employs standard equalization, conforming with RIAA and NAB Standards. Where Nakamichi units deviate from some decks is in applying some treble boost at the very high end in playback to compensate for gap loss at the playback head. But this is consistent with the Standards.

Your problem may lie in azimuth mis alignment-either of the Nakamichi or of the other decks. I suggest that you have your deck's azimuth checked out by an authorized service shop.

Another possibility is that of mis-tracking, if you are using Dolby B or C noise reduction. Recording and play back levels have to be matched if Dolby NR is to work properly. If there is serious mistracking, this can cause treble loss, leading to what you describe as "muddy" sound. Is your problem less serious when you record without Dolby NR? If so, this suggests the culprit is mistracking, in either the Nakamichi or the other decks.

Damage from Magnetic Fields

Q. I have read several times about the importance of keeping recorded tapes away from magnetic fields. It has been my habit to place recorded tapes on top of one of my speaker cabinets for long periods. Recently it dawned on me that this might be harmful, so I put a compass on top of the cabinet to check things out. I was shocked when the needle, which had been pointing north, spun to the south when I put the compass on the cabinet. The tweeter is only about 2 inches below the cabinet top, the midrange speaker about 6 inches below, and the woofer about 18 inches below. Have I done my tapes much harm?

-Philip H. Leak, Roseville, Cal.

A. I doubt that you have harmed your tapes, although I can't be sure.

Usually a distance of about 3 inches between the tape and a strong magnetic field is sufficient protection. In your case, the distance is less, but I presume it is only a moderate field.

You can check whether your practice is harmful by recording two cassettes from the same source, placing one of them on your speaker, and then playing both. If the one that was on the speaker has duller treble response, harm has been done-namely, some erasure of high frequencies.

For more information on the dangers presented by magnetic fields, see my article "Magnetic Shielding" in the April 1979 issue.

Effects of Wrong Bias

Q. I have an old cassette deck with out switchable bias, and therefore I have always used normal-bias cassettes. If I were to use high-bias cassettes, would I damage or decrease the life of my tape heads?

-Ivo Rokovich, Hialeah, Fla.

A. You will not harm your tape heads or tape deck by using other than normal-bias cassettes. What will happen, if you use Type II (chromium dioxide, ferricobalt, and a few metal) tapes or Type IV (the majority of metal) tapes, is an increase in high-frequency response and in distortion. These effects will be largest with Type IV.

(Source: Audio magazine, Nov. 1987, HERMAN BURSTEIN)

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