24 Jan
2013
24 Jan
'13
7:39 p.m.
SD and DD use the same transition rate. It is how the datais encoded that makes the
difference. It wasn't denser transistionsor closer tracks, it was FM to MFM.Quad
density was hight transition rate per track but stll MFM.Dwight
Date: Wed, 23 Jan 2013 22:00:11 -0800
From: cisin at xenosoft.com
To: cctalk at classiccmp.org
Subject: Density (Was: Need Kermit for Xerox 820-II in .IMD format
On Wed, 23 Jan 2013, Dave Land wrote:
Well now, that's an interesting twist.
They're definitely soft-sectored.
I was under the assumption that the physical media was different between
the types of disks. So the DSDD are the same stuff, just a little higher
quality oxide to support more and closer tracks?
close.
8" SD V DD are NOT more NOR closer tracks. They are both 77 tracks at 48
tracks per inch. The difference is how much data is crammed onto each
track. The oxide is nominally the same, but you are right that DD relies
on high quality.
SSSD holds a total of about 250K
SSDD holds a total of about .6M, or about 1.2M for DSDD.
DSSD existed, but was not very commonly used.
However, on 5.25", there are both 48tpi and 96 tpi, (more and closer
tracks),
AND on both 5.25" and 3.5" there are two different oxide formulations
available for each for different "densities" of recording on each track.
5.25 is available with 300 Oersted 5.25" for FM (typically about 100K)
or MFM (typically 160K or 180K per side ("360K"))
and
600 Oersted for MFM "high density" ("1.2M")
(same recording format as "DD MFM", but twice the data transfer rate)
3.5" is available in 600 oersted ("720K") and
700- 750 oersted ("1.4M")
Same number of tracks (80), track spacing (135 tpi), but two different
densities on the tracks.
(same recording format, but twice the data transfer rate)
Besides the common forms, there also exist numerous variants, such
as 100 tpi 5.25", 67.5 tpi 3.5", GCR, etc.