Maybe this will help...
This taken from http://www.faqs.org/faqs/AudioFAQ/pro-audio-faq/
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]
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]
Alan M400s #343
-------Original Message-------
From: ferrograph@...
Date: Thursday, June
06, 2002 03:41:35
Subject: Re:
[Mellotronists] Obscure bits
so much that the ultrasonic bias oscillator frequency is audible as
the high pitch. The quivering effect on the high pitch would be the
wow and flutter of the tape. >>
that seems reasonable- bias frequencies were generally lower in those days,
even on professional machines. has anybody seen a really good explanation of
how bias works? I'll have to dig this out and have another listen, but
there's a similar effect on "are you experienced?".
d./400nr1098 (not biased at all) and a small army of 1/4" machines.
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