Talk:Group code recording

1541's pioneering scheme of CAV + varying data rate

Perhaps it's worth noting that while the 1541 used constant angular velocity, it did vary its data rate to keep the data density near the maximum across all tracks. Compare this with the PC, which really did space bits further out on outside tracks, and CD-ROM drives, which started out Mac-style and changed to 1541-style in the mid/late 90s. --Anonymous

GCR Didn't start with CBM. GCR was being developed before Commodore deployed it on their drives. The Durango Systems F-85 (introduced in late 1976) boasted 500K per 100 tpi floppy using 4/5 group code; eventually expanded to a double-sided option boasting almost 1MB per diskette. A few years before this, Sperry ISS was working on large hard drives for the mainframe business using group coding. --CG

GCR being thought of as originally a floppy format says something about which generation many Wiki editors (including me) belong to. It was around as early as 1973 (with the IBM 3420/3803 models 4,6, and 8), as a tape format for 6250 BPI drives. Wozniak re-invented a much simpler form for the Apple II with 5-and-3 and later 6-and-2 codes. I don't know where CBM got their code, which had 5 disk bits for 4 data bits; perhaps they didn't have an 8-bit data latch as a constraint.

Jackpot-- I found one of IBM's papers on the subject: http://www.research.ibm.com/journal/rd/186/ibmrd1806Q.pdf. IBMs GCR is way more complex than the CBM or Apple versions, as it not only satisfies physical constraints but provides error correction as well.

Nybbler 23:20, 25 February 2006 (UTC)

Reference for codes?

I wish I had my ProDOS and Copy II Plus manuals... they have all the codes used. It would be handy to have these on Wikipedia since it's rather hard to come across now without reverse engineering the ROMs and OS disks.

I don't know if I ever saw the other drive's encoding documented, but the Commodore drives would be nice to have as well. One table for each drive format. There's only a few types, actually.

Apple 2: 4/4,5/3,6/2

Commodore: 1541(encoding name?)

The double-sided drive for a Commodore was actually the same format repeated on the opposite side, so the encoding was the same. Proof of this would involve loading up "Locksmith" on a single-sided drive and flipping the disk over.  ;) Any references to back this (foggy) memory up? 24.162.138.110 (talk) 01:00, 2 December 2009 (UTC)

THis is actually me... Sorry, forgot to login. JWhiteheadcc (talk) 01:03, 2 December 2009 (UTC)

Found some references, including an online copy of the Central Point Software manual.

http://cps.applearchives.com/ has the manual that came with Copy II Plus v9.1. It was my favorite reference book but the next two were also popular.

"Beneath Apple DOS" has a nice set of tables in the back. http://www.1000bit.it/scheda.asp?id=41

See chapter 9 of "Understanding the Apple II" for low-level programming information include the bit patterns allowed on the disk (including some not normally used).

(Can someone verify if these are authorized? If not, the reader will have to look for these titles themselves after I remove these links from this comment. It is common for some things to be legally available for the Apple II community on the Asimov FTP server for example, but I want to be sure before adding them to the article. Symantec bought CPS, so asking Norton/Symantic is a start. )

A blog that references this article plus some of the books I mentioned. http://blog.naver.com/PostView.nhn?blogId=drroach&logNo=30036296938&widgetTypeCall=true

It appears that the 4/4 encoding was in fact used but only in the sense that values like 55AA were used for timing since Apple II's used soft sectoring. I won't even get into the issue of copy protection and custom formats.

JWhiteheadcc (talk) 19:19, 21 December 2009 (UTC)