1.2M HD / 300kbps / single-density

[Allison]

>
>Subject: 1.2M HD / 300kbps / single-density
>   From: "Dave Dunfield" <dave06a at dunfield.com>
>   Date: Sun, 18 Mar 2007 20:36:40 -0500
>     To: cctalk at classiccmp.org
>
>Today I stumbled across something that may explain why some
>people I've corresponded with have had such a hard time
>getting single-density to work with disk imageing.
>
>For my own setup, I have a couple of trusty Aopen AP5T
>mainboards which do single-density nicely, installed in
>cases with an internal 3.5" HD (1.44M) drive as A:, and
>an external cable allowing me to connect any of the
>following as drive B:
>  - 5.25" DD 40 track (Teac 55-G)

Dave,

Don't you want to be using a 55B whish is a real 40 track drive?

>  - 5.25" DD 80 track (Teac 55-F)
>  - 5.25" HD 80 track (Teac 55-G)
>  - 8" DS (Qumetrack 242)
>
>I'm also quite a stickler for using the proper drive when
>I read/write images - so if it's 5.25" 40 track, the 55-G
>gets connected etc. This has worked very well for me.
>
>Today I was setting up another system to do 5.25" disks
>only, and I wanted it to be self contained within one case.
>To minimize the drives, I decided to modify a Panasonic
>JU-475 with a switch on the front bezel to force it to 300
>rpm to serve as both the DD/80 and an HD/80 drive.
>
>To verify that the drive was good, I hooked it up and tried
>to read/write some disks - to my surprise I could not do
>single density (at 300kbos). Tried a couple other HD drives
>with the same result. On a hunch, I modded it to 300rpm, and
>sure enough, I can read/write single-desnity fine at
>300rpm / 250kbps.
>
>(Yeah, I know the SD rate is really only 1/2 - by 250/300kbps,
>I am referring to the MFM settings for the AT controller data
>rate select register).
>
>To rule out some odd ImageDisk quirk, I tried several versions
>of TeleDisk - with the drive at 360rpm and configured as a 1.2M
>HD drive, none of them could read a single-density disk either.
>
>So at least for my AP5Ts, it appears that the internal controller
>can do single or double density at 250kbps, but only double-
>density at the "at compatible" 360rpm rate of 300kbps. My guess
>is that the data separator does not work at that rate (works
>fine at 500kbps however).
>
>I'm curious to know if this is a charactistic specific to the
>machines I am using, or if it is common among PCs that do single-
>density to not work at 300kbps. Has anyone here read and/or
>created single density 5.25" disk on a 1.2M HD drive spinning
>at 360rpm?

There is at least one reason not to do this.  The bit shift is greater
due to write speed.  Also some integrated version of the  765 (superchips)
select precomp based on clock speed and that may be less than optimum.
Finally the drive used may shift write current based on media speed. 
this also ignores the 96tpi and 48 tpi issues that are worse when writeing
an already formatted  disk.  Any or all of those can undermine relaibility.

Allison