rcd@ico.UUCP (Dick Dunn) (08/21/87)
This stuff about speed of sound vs disk speed seems to be getting out of
hand. Reality-check time.
The largest disk platter in common use is 14 inches in diameter. The
most common max rotational speed is 3600 rpm, which is 60 rps. This gives
a maximum (at OD) head:disk velocity of
14 * pi * 60 / 12 feet/sec
which is about 220 feet/sec, or about 0.2 of the speed of sound.
So would someone explain why we're worrying about hypersonic disk heads?
If we went to 10000 rpm, we'd still be just over Mach 0.5.
Another way to look at it: If we can realize a bit density of, say 800 bpi
(the effective bit density of 6250 bpi, 9-track tape) on a disk, the
effective data rate at mach 1 is somewhat over 10 Mbit/sec.
--
Dick Dunn {hao,nbires,cbosgd}!ico!rcd (NOT CSNET!) (303)449-2870
...Never offend with style when you can offend with substance.rcd@ico.UUCP (Dick Dunn) (08/24/87)
Well, after I showed (correctly, I think) that the head at OD of a 14" disk at 3600 rpm is only doing about mach 0.2, my brain disengaged and my fingers went on to say: > Another way to look at it: If we can realize a bit density of, say 800 bpi > (the effective bit density of 6250 bpi, 9-track tape) on a disk, the > effective data rate at mach 1 is somewhat over 10 Mbit/sec. The divide-by-8 implied there is kind of silly (also wrong!)...yes, I know that data is not put on tape in bit-serial fashion! Leave it at 6250 bpi and the data rate at mach 1 (call it 1100 ft/sec) is 1100*12*6250 bits/sec which is about 80 Mbit/sec or 10 Mbyte/sec (per surface, of course:-) Sorry 'bout that. -- Dick Dunn {hao,nbires,cbosgd}!ico!rcd (NOT CSNET!) (303)449-2870 ...Never offend with style when you can offend with substance.