Tape bias
Tape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analog magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal (generally from 40 to 150 kHz) to the audio signal. Magnetic tape has a nonlinear response at low signal strengths, as measured by its coercivity. Bias increases the signal quality of most audio recordings significantly by pushing the signal into the linear zone of the tape's transfer function.
History
The earliest magnetic recording systems simply applied the unadulterated input signal to the record head, resulting in recordings with poor low-frequency response and high distortion. Within short order, the addition of a suitable direct current to the signal was found to reduce distortion by operating the head substantially within its linear response region. The principle disadvantage of DC bias was that it left the tape with a net magnetisation which because of the grain of the tape particles generated significant noise on replay. Some early DC bias systems used a permanent magnet that was placed near the record head. It had to be swung out of the way for replay. DC bias was re-adopted by some vey low cost cassette recorders.
Although the improvements are marked with such DC bias, even more dramatic improvement results if an alternating current bias is used instead. The first patent for AC bias was filed by W. L. Carlson and Glenn L. Carpenter in 1921, eventually resulting in U.S. patent 1,640,881. The value of AC bias was somewhat masked by the primitive state of other aspects of magnetic recording, however, and Carlson and Carpenter's achievement was largely ignored. The striking improvement in distortion and noise provided by AC bias was rediscovered in 1940 by Walter Weber, while working for Hans Joachim von Braunmühl at the German Broadcast Company (RRG).
The UK company Boosey and Hawkes produced a steel wire recorder under Government contract during the Second World War that was equipped with AC bias . It is unlikely that they were aware of the German developments (otherwise they would probably have copied the tape system in its entirety). Examples still surface from time to time, many having been disposed of as Government surplus stock. Boosey and Hawkes abandoned recorder production at the cessation of hostilities and concentrated on music publishing (and are still in business).
Theory
As the tape leaves the tape head, the bias partially demagnetizes the tape and the remaining net induction is essentially the difference between the positive and negative half-cycles of the previously recorded signal. This differencing operation further cancels some of the nonlinearity.
Note: that this is a very simplistic explanation of AC bias. In practice, the full modus operandi of AC bias is not adequately understood. There are several prevailing theories, though none of them fully explains the operation, and in particular, what happens if too little or too much bias is applied.
Practice
The characteristics of the recording system change quite markedly as the level of the bias current is changed. There is a level at which the system gives the minimum distortion. There is also a level at which the frequency response is a maximum. Unfortunately, these conditions do not occur at the same bias level. Professional recorders are invariably set up for minimum distortion, the necessary frequency response being achieved by choosing an appropriate tape speed. Consumer equipment, and in particular Compact Cassette recorders have the bias set at a compromise level to give good frequency response and acceptably low distortion.
Different levels of bias are needed for different types of tape, so most recorders offer a bias setting switch on the front panel, or, in the case of the compact audio cassette, may switch automatically according to cutouts on the cassette shell. Ferric based tapes require the lowest bias current, with Chrome based tapes (including the pseudo chromes) requiring a higher level. Metal particle requires even more. Metal Evaporated tape requires the highest level of bias, but it fell out of favour for most analogue purposes.
See also
External links
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