Archive of the former Yahoo!Groups mailing list: Mellotronists

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]

Alan M400s #343