Bias level question

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regebro

regebro

Insane Genious!
I calibrated my Fostex A-8LR 8-track yesterday, and I did what Quantegy recommended for the 456 tape, and used a 3.0 db overbias at 10kHz. But this actually lowers the frequency response at 10kHz to about -2db, as compared with 1 kHz. Hummm...

My gut reaction here is to set the bias so that the frequency response is flat. What is your recommendation?

Ok, I see now that they recommend the bias setting should be done with an input level of -10db below reference, and I did it at reference level, so maybe that will change things, but if they don't? Should I instead set the bias so that I get a flat frequency response?
 
Check out Eddie Ciletti's column from Mix:
http://www.digitalprosound.com/Htm/TechStuff/2000/Sept/AnalogPt2-1.htm

Also...

From the rec.audio.pro FAQ (http://recordist.com/rap-faq/current):
6. Bias adjustment. There are a lot of ways to do this. The common way to
do this is to record a high frequency to the tape, and adjust the bias until
the signal peaks, then increase the level of the bias until the signal
coming off the tape drops by a little bit. That little bit is called the
overbias, and you should check on the data sheet of the tape you are using
and see what level of overbias is recommended for that tape. Usually this is
done at 10 kHz for 15 ips tape.

Other folks will use a 1 KHz tone and adjust for a peak with no overbias,
and this is the method recommended in the manual for many older tape machines.
However, it's not optimal for most of the newer tape formulations. Some other
folks record a low frequency tone and adjust for lowest modulation noise, a
method which also has adherents. Previous copies of this FAQ recommended
adjusting for flattest response, a method I no longer recommend.


Hope this helps...

Bruce
 
OK, thanks. I have some more experimenting to do this evening, but it sure seems like I'm stuck with having a non-optimal freqency response. Gggrmbl.
 
A happy man!

Well, whaddayouknow! Redoing the bias calibration with 10kHz at -10db instead of 10kHz at 0db fixed it! A -3db overbias then actually ended up giving a flat frequency response for 10khz! My Fostex now has a nearly flat response over most of the spectrum, except the bass, which is 3-4dbs too loud at its largest. Yihaaa!

From http://www.tangible-technology.com/media/media_2.html :
"If you are not sure how much overbias is required, record a low frequency sine wave no higher than 40 Hz. A course adjustment will deliver maximum output. Listen to what happens as the bias is varied and adjust until the fuzz and harmonic distortion are minimized"

This seems to be bogus, at least on my Fostex with 456 tape. The more bias you have, the less distortion you'll get, and the harder you can hit the tape with that sine wave. And also, the more bias you have, the worse the high frequency response you will have. The bias adjustment is basically a compromise between these two, AFAIK.
 
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