Bill Pechter <pechter_at_monmouth.com> wrote about 1/2 mag tapes and drives:
> Tape Density Encoding method DEC Tape Drive
>
> 200BPI (NRZ?)
> 556BPI (NRZI?) TU10?
200 and 556 were only used for 7-track. Even from a single vendor such
as IBM, some 7-track systems used odd parity and others used even. I'm
not sure what recording mode the even parity systems used, as NRZI wouldn't
work with even parity unless you could otherwise guarantee that there are
no all-zeros data conditions. See below.
> 800BPI NRZI (Non-Return to Zero Indescrete) TU10/16/TE10/16
800 BPI was used for 7-track and 9-track. NRZI is Non Return to Zero
Inverting. On a 9-track, each "line" on the tape consists of 8 data bits and
an parity bit. The "I" (Inverting) means that a flux transition is recorded
on the tape if a data bit in the line is different than the same bit position
in the previous line. So unless the data is all zeros, each line will have at
least one flux transition. Unlike DECtape, there is no "clock" track, so it
is essential that there is always at least one track with a flux transition,
from which a read clock can be recovered. The odd parity guarantees that if
all 8 data bits are zero, the parity bit will be one, so there is still one
flux transition.
If the head isn't aligned properly, the flux transitions on multiple tracks
may not occur at exactly the same time. The drive electronics is designed
to deal with a certain amount of skew.
> 1600BPI PE (Phase Encoded) TE16/TU45/TU77/TU78
> TA78 TS11
> 3200BPI
I think 3200 also uses PE, but I'm not certain. It is much less common
than 800, 1600, and 6250.
> 6250BPI GCR (Group Coded Recording) TU78/TA78
HP also has a proprietary 6250XC format that adds data compression. This
was the forerunner of DDS-DC, the data compression used on 3.81mm DDS (DAT)
cartridges.
Received on Wed Jan 27 1999 - 16:49:10 GMT