150 Mb tape drive question

[Wolfgang S. Rupprecht]

James.Reilly at tel.vtt.fi (Jim Reilly) writes:
>I have a question about our Sparc stations (150 Mb) tape drive, well
>actually about the tapes.  The tapes (Sony QD-6150) are 620 ft, 12500
>ftpi.
>What does ftpi stand for ? 

Flux Transitions Per Inch

> I've assumed it means bpi, and that the calculation for how much can
> be stored on it is:

Well, the asymptotic maximum is one bit per flux transition.  Normal
encodings of data range anywhere from 1 bit per 2 transitions (MFM) to
over 0.9 bits / flux transition for some of the RLL encodings.

The reason you can't get exactly 1 bit / flux reversal is that you need to
guarantee some transitions so that you can keep your recovery clock
sync-ed to the data stream.  If you get a string of thousands of 1's or
0's recorded with a direct recording scheme, its hard to determine at
recovery time where each bit starts and ends.  Your recovery clock would
slowly wander and you'd end up getting a few more or less bits than were
really recorded.

On the other end of the scale, its often advantageous to not try to record
a very high frequency 0-1-0-1-0-1-0-1 pattern.  High frequency patterns
require very closely spaced bits on the tape.  Tape heads will tend to
average adjacent bits, so its best to avoid this pattern also.  

Something like RLL 2,7 will make sure that there are no "runs" less that 2
bits wide, or more than 7 bits wide.  This neatly solves both of the above
problems.

Wolfgang Rupprecht    wolfgang at wsrcc.com (or) uunet!wsrcc!wolfgang
Snail Mail Address:   Box 6524, Alexandria, VA 22306-0524