Recent Finds & Thoughts

From: Allison J Parent <allisonp_at_world.std.com>
Date: Fri Nov 27 22:15:40 1998

< Actually, if you set the azimuth of the replay head to be the same as th
< recording head, you do improve the signal and the HF response. It doesn'

Not much at all. We are talking halftrack heads or wider and you get
phase cancelation if the gap is not perpendicular to the tape travel
on record. if the play back head is narrower say using a stereo
playback or one of the quad decks then you can tweek some but it still
only buys you a little.

Keep in mind that impats the upper limit of the HF response and most
tape systems were designed to stay way clear of that. The best upper
freqency response you could count on was 9-10khz from your 35-50USD
portable that was sold for or commonly used. Most modulations schemes
were either modem high pair (~2200hz) or phase encoded data (FM)
if you FM data using a data rate of TARBEL (187char/SEC) you were using
tones that were around 3000hz and 1500hz. TRS80 was in the same range.
So the actual bandwidth needed was far less than 5khz. Kansas CITY
format was exactly 8cycles of 2400 or 4cyles of 1200 for one or zero.

FYI I droped using audio cassette save for when I needed to get some neat
software from one format to mine. I went direct saturation where the
NS or SN polar flux change carried the information. The same heads at
audio that might work at 9khz audio did respectable 9600 baud using FM
encoding. LAter I would find some low impedence heads that would allow
me to hit a solid 1600frpi (800 bits/inch). This fixed some of the ills
that plague audio schemes.

< become perfect, but it does get better. VCRs (certainly UK VHS ones) use
< this trick to reduce crosstalk between adjacent video tracks (which are
< read by different heads on the head drum) and to allow a stereo audio
< signal to be recorded under the video track and then replayed by a
< separate head at a different azimuth setting.

Different world and heads. Heical scan systems the path across the tape
is different from linear travel and since the tape effectively moves in
two directions operation is different than a portable audio cassette.

Allison
Received on Fri Nov 27 1998 - 22:15:40 GMT