From: st_waldman@fencer.cis.dsto.gov.au
Newsgroups: rec.audio.pro
Subject: Re: Tape deck bias
Date: 27 Jan 93 02:13:56 GMT
Organization: D.S.T.O. Aeronautical Research Laboratory

In article <1993Jan22.185157.22090@city.cs>, lionel@cs.city.ac.uk (Lionel Tun) writes:
> There is a bias knob on my church's analogue tape deck (Sony TCK570)
> but I have always set it in the middle. What can I do with it? Is it
> that with `good' tapes I should increase bias and with el cheapo
> tapes reduce the bias?

The bias control only works during the recording phase. It is used to
obtain the optimum high-frequency (and possibly distortion) performance
of your tape deck with the particular tape type that you are using.

A higher bias level will decrease high-frequency output, but will 
increase mid-band output marginally. A lower bias level will raise
the output at high-frequencies, say above 5 kHz or so, and too low
a bias level leads to a peak in the high frequency response in the
10-15kHz range. Please note that some controls may work in reverse,
i.e. "increasing" the bias will actually decrease the bias current
going to the tape head, leading to decrease in HF response.

It is possible to adjust the bias level by ear using music as the
reference. As to how easy it is to do this, well, it will of course
depend on the high-freqency signal content of the music. A pink noise
signal generator may be preferred.

Overall, I would recommend the following procedure:

1. Use a sine wave generator to apply a 400 Hz input signal.
   If you have a Dolby level calibration, adjust the level so that
   the output level matches the input level at about -10dB on the
   meter. This corrects for any errors in input/output relationship
   caused by different tape sensitivities.

2. If you cannot adjust the overall level to get -10dB at both input
   and output, then proceed as follows. Set the record level to -15dB
   or -20dB (the lower level may be too hard to read on your meter).
   Record the 400Hz tone and play it back. Note down the playback
   level, which now becomes the target reference level for bias
   calibration.

3. Now set your sine wave generator to 10kHz (or slightly higher if
   your deck is up to it), and record the tone at the same level
   as you did the 400Hz tone. Check the playback level of the 10 kHz,
   and note how much higher or lower it is relative to the 400 Hz
   --playback-- level. You may find your deck is +- 5 dB out.
   If your 10 kHz tone is played back at a greater level than the
   the 400 Hz reference playback, you will need to increase the bias
   level. If it is lower, then decrease the bias level. If it is the
   same, then do nothing, as the bias is correctly aligned.

4. Having changed the bias level, go back to step 3. Repeat this 
   until the 10 kHz playback level matches the 400 Hz playback level.
   This may take a few tries.

5. Once you get a match, go back and adjust the 400 Hz level to get
   the same output as input level. This may be required if your decks
   bias was way out, and the tape's sensitivity has been affected
   by your tuning procedure. Then re-do the bias adjustments.

At the end of this, you deck will perform better with that tape.
I do this with every tape that I record on my machine, as it has 
front panel Dolby level and bias adjustments, with built in
400 Hz and 15kHz sine wave generators. Even on an older Sony TC-177 
(I think), I used to follow the same procedure, and I bought a 
simple sine wave generator to do the job.

Hope this has helped...

+------------------------------+--------------------------------------+
| Witold Waldman               | Aeronautical Research Laboratory     |
| Senior Professional Officer  | 506 Lorimer St                       |
| Airframes & Engines Division | Fishermens Bend VIC 3207             |
| Tel: +61 3 647 7488          | Melbourne, Australia                 |
| Fax: +61 3 646 7771          | eMail: witold@hotblk.aed.dsto.gov.au |
+------------------------------+--------------------------------------+
Newsgroups: rec.audio.pro