KLUDGE@AGCB1.LARC.NASA.GOV (12/06/90)
[Yet another article from a resident guru. -tjk]
Okay, this is just roughed out as I sit at the terminal, so it may need
some work later on:
1. Get a tape. Make sure the tape you use to bias the deck is the same type
of tape you intend on using, because no matter what the manufacturers
say, all tapes have different bias. In fact, it might be a good idea to
rebias the deck for each new brand of tape.
2. Find where the bias controls on the deck are.
3. Get yourself a white noise generator. If you don't have access to a
laboratory noise generator, use your FM tuner to generate the noise.
It doesn't have to be completely flat in response, because you'll be
comparing the recorded signal with the original reference. This means
you can get by with an FM tuner and still get excellent results.
4. Get a speaker or a pair of headphones with as clean response as possible.
Since you'll be using your ears for the comparison, the quality here is
crucial. Get that howling dog out of the room and clear the area of
distractions. You'll be here for a while.
5. Turn down one channel. On the other channel, record white noise, and play
it back.
6. Compare the sound played back with the sound from the tuner. If it's
higher in pitch, reduce the bias. If it's lower, increase it. Now
rewind the tape.
7. If you are satisfied, go back and do the same on the other channel.
Otherwise, record more noise and try again.
With enough iterations, you can get the playback to sound quite a bit like
the recorded noise. It'll take some time to get used to how much motion of
the bias control is required to get a given pitch change, but once you get
the hang of it, you'll never lose it. (You may decide to purchase a three
head deck after a short amount of doing this, though).
--scott
(OOOPS! I forgot to point out to:
-- demagnetize the tape path before doing anything
-- clean the tape heads (every afternoon if possible)
-- turn off ALL noise reduction before doing this (actually, you should
adjust the noise reduction system levels after doing the bias check,
but you can get steve graham to write about that :-).
There are two other methods I know of. One requires a tone generator (and
I suspect it's no more accurate, and a lot more trouble). The other requires
a gaussmeter (and relies on the manufacturer's recommendations printed in
webers/meter on the tape datasheet)