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From: scotty@l5comp.UUCP (Scott Turner)
Newsgroups: comp.sys.amiga
Subject: Re: Reading MacDisks on Amiga
Message-ID: <122@l5comp.UUCP>
Date: Tue, 12-May-87 07:48:12 EDT
Article-I.D.: l5comp.122
Posted: Tue May 12 07:48:12 1987
Date-Received: Sat, 16-May-87 01:22:49 EDT
References: <8705080543.AA18112@cory.Berkeley.EDU> <112@l5comp.UUCP> <2112@hoptoad.uucp>
Reply-To: scotty@l5comp.UUCP (Scott Turner)
Organization: L5 Computing, Edmonds, WA
Lines: 43
Summary: Say what?

In article <2112@hoptoad.uucp> farren@hoptoad.UUCP (Mike Farren) writes:
>News to me - could you please specify what you're talking about?  I
>just put down the Hardware Ref. Manual and couldn't find such a beast.
>There IS a bit for selecting 2 or 4 microseconds/bit cell, but that
>isn't going to buy you a lot of flexibility.
Yeah but it DOES change the clock rate! Which is what we were talking about.

>No, it just makes it more brain damaged.  If the extra speed thing is
>true, then they should have been able to get a LOT more data onto the
>disk than the 400/800K that they did.  Twice the rotation speed ==
>twice the data rate, after all.  (Is this how IBM is getting 1.4M
>3-1/2" drives, by the way?)
Uh, twice the rotational speed at the same data rate := half the data at a
1X rotational rate. The laws of nature win out, grin. Take for example a
phonograph record. Which record stores the most music: 33 1/3, 45, or 78 RPM?
The faster rotational speed does one thing, decreases rotational latency.

And actually considering that Apple is using SINGLE DENSITY data rates I think
they do a rather nice job of packing the data on the disk.

Data density is a function of three things: Media's ability to record X number
of flux changes per inch. Drivehead's ability to produce X number of flux
changes per inch. And the drives ability to position the head in tracks
per inch.

Rotational speed has very little to do with anything except rotational latency
and the EFFECTIVE fci (FluxChangeInch) being laid down. If a constant speed
is used for rotation and fci being sent to the drive, the actual fci will vary
from the inner most track to the outer most track. This is because no matter
which track you are you always go round in the same amount of time, but the
length of track that goes round varies from the inside to the outside. Thus
on the outer tracks the bits are "fatter" because more track goes under the
head during a fluxchange in the head. So what Apple did was have the drive
change speed so that the effective fci laid down is pretty much the same from
the inside track to the outside track.

The trouble with Apple is that they've never gone double density :-).

Scott Turner
-- 
L5 Computing, the home of Merlin, Arthur, Excalibur and the CRAM.
GEnie: JST | UUCP: stride!l5comp!scotty | 12311 Maplewood Ave; Edmonds WA 98020
If Motorola had wanted us to use BPTR's they'd have built in shifts on A regs
[ BCPL? Just say *NO*! ] (I don't smoke, send flames to /devA SOe
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