25 Feb
2011
25 Feb
'11
12:08 p.m.
I'd always assumed that the reason a relative
shallow spool was used
is because the tape is reel-driven, sans capstans to assure a more-or-
less constant speed. Some sort of clocking mechanism (either with a
clock track or by modulation methods) would seem to be a necessity.
And it's also why you don't see 2400' DECtape reels...
Cheap Japanese portable tape recorders of about the same time also
used reel-drive; some even had rheostats to tweak the motor speed.
The HP9865 (I assume), internal cassette drive of the HP9830 (and I
asusme 9821) and other machines using a simialr mechanism (such as the
Racal P72 Digideck) were spool-drive only. The Racal manual claims this is
a beneifit since it reduces tape wear. Hmmm...
Anyway, these units use something called BMS encoding (Bit-Mark Sequence,
I believe). There are 2 tacks on the tape. A '1' bit is a pulse on one of
the tracks. A '0' bit is a pulse on the other track, A byte marker is a
pusle on the 2 tracks simultaneuosly. Obviously this is self-clocking, so
tape speed doesn't matter too mach.
AFIk, HP put a marker at the start of every (9 bit) word on the tape,
Racal put one on the start and end of every word. So words on a Racal
tape have 2 markers between htme, on an HP tape only one.
Th 9 bit words is the 8 bits of the'user byte, and a flag bit. The latter
is in the middle of the worf, and is set to indciate file markes and the
like. Putting it in the middle means it's in the same position relative
to the amrkers no matter which way yhr tape is movign, so you can scan
for file markers in either direction.
-tony