4.2 - What is bias? What is overbias?
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This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
4.2 - What is bias? What is overbias?
With just the audio signal applied to a tape, the frequency response
is very poor. High frequency response is much better than low
frequency, and the low frequency distortion is very high. In 1906,
the Poulson Telegraphone managed to record an intelligible voice on
a magnetic medium, but it was not until the 1930s when this problem
was solved by German engineers.
To compensate for the tape characteristic, a very high frequency
signal is applied to the tape in addition to the audio. This is
typically in the 100 KHz range, far above the audio range. With the
bias adjusted properly, the frequency response should be flat across
the audible range. With too low bias, bass distortion will be the
first audible sign, but with too much bias, the high frequency
response will drop off.
Incidentally, digital recording equipment takes advantage of the
very nonlinearity that is a problem with analogue methods. It
records a square wave on the tape, driving the tape into saturation
at all times, and extracts the signal from the waveform edges. As a
result, no bias is required. (For a good example of the various
digital recording methods, check out NASA SP 5038, _Magnetic Tape
Recording_.) [Scott]
[Ed. note: For those looking for an understanding of why we need
bias in the first place, here is one way to think about it. Tape
consists of lots of small magnetic particles called domains. These
domains are exposed to a magnetic field from the record head and
oscillate in polarity as the AC signal voltage changes. Domains,
being physical objects, have inertia. Every time the analog signal
crosses from positive to negative and back again, the voltage passes
the zero point for an instant. At this moment, the domain is at
rest, and like any other physical object, there is a short period of
inertia before it gets moving again. The result is the bizarre
high-frequency performance characteristic that Scott described. The
high frequency of a bias signal simply ensures that the domains are
always kept in motion, negating the effect of inertia at audio
frequencies. -Gabe]
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