Maybe this will help...
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]
Alan M400s #343
-------Original Message-------
Date: Thursday, June
06, 2002 03:41:35
Subject: Re:
[Mellotronists] Obscure bits
<< I think it's a tape recording of a single
sustained note slowed down
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.
To unsubscribe from this group, send an email
to:
Mellotronists-unsubscribe@yahoogroups.comYour
use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/
.
____________________________________________________
IncrediMail - Email has finally evolved -
Click
Here