[net.audio] Biasing of tape

swj@fluke.UUCP (Stan Jones) (08/16/83)

In general, increasing the bias reduces the amplitude of the higher frequencies
and reducing decreases.....At the same time, an increased bias level will also
increase the maximum output level (for a given distortion level) for the lower
frequencies, so significant changes in bias can lead to problems with distortion
or SNR depending on the calibration of your record level meters.  Fortunatly,
the response at 15khz changes much faster than 1khz so small bias adjustments
shouldn't cause too much of a problem.

The typical method of adjusting the bias is to record some 1khz tone at a -20Db
level and note the output level on playback (on some kind of VOM ((Preferably
a Fluke DMM))that has reasonable response to 15khz) and then repeat the process
at 15khz.  
Bias is correctly adjusted when the two levels are the same.  Dolby must be off
and you must use -20Db level for recording to avoid compression at the high end.

Bias is used to get a uniform (or almost) orientation to the magnetic particles
in the tape which is then modulated by the recorded audio to vary the average
orientation in one direction or another depenc
depending on the polarity of the signal.  It works well except that there is a
tendancy for the bias signal (70 to 120khz typ.) applied to the record head
to partially erase the recorded signal.  This occures most at the highest 
frequencies, i.e. more bias = less highs.

If you do adjust your bias for your prefrence of highs don't exceed about 
3Db of boost at 15khz to avoid other problems.  Also, this adjustment will
only be 3Db with one brand of tape, although the response will still be 
up about 3Db from normal with any tape.

The material the tape is made from (Fe to Chrome to Metal) doesn't change 
the relative effects, only the levels of bias and record current are 
higher for ch and metal tapes.

Hope this helps, It was good to see a real question on the net, not all this
Horse ...... about speaker/cd phasing!!!!

Stan Jones
Service Products, Fluke

Subject: Re: Questions on Biasing - (nf)
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