Talk:Group coded 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.

--2004-11-27T20:17:31‎ 84.43.5.61


 * 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 (2004-12-10T22:09:54‎ 65.102.127.71)


 * 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)


 * Chuck, just a small sidenote, based on magazine announcements and George Comstock's interview, the Durango F-85 was introduced in September 1978, not 1976. Based on the same sources, Durango was started in 1977. It is still possible, however, that Comstock left Diablo in 1976, as stated on your web-site. As several GCR-floppy related products were introduced around 1977 to 1978, the dates may become important if we want to establish something like a GCR timeline or chronology. --Matthiaspaul (talk) 15:39, 21 June 2017 (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)

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Durango, Brother and Sharp GCR codes missing
For historical completeness it would be great if we could add the actual GCR codes used by the Durango, Brother and Sharp floppy controllers. Unfortunately, I could not find any documents listing these codes so far. Are the Durango and Micropolis GCR codes the same?

Regarding the Sirius/Victor codes, I have found references indicating that they are byte-wise identical to the Commodore codes and others indicating that they are "almost" the same but differ in nuances. To settle this once and for all, I'm still looking for a Sirius- (not Commodore-)related RS listing the codes explicitly.

Have there been any other (possibly obscure) floppy formats using some form of GCR?

More info on GCR coded hard disk formats would be appreciated as well.

--Matthiaspaul (talk) 13:56, 19 April 2017 (UTC)

GCR and Potter Instruments
Several years ago, the following comment was added to the Talk:Magnetic_tape_data_storage page. I'm copying it to here as it may give some pointers to further research the early history of GCR encoding: --Matthiaspaul (talk) 15:26, 21 June 2017 (UTC)


 * It is rather odd and sad that Potter Instruments, with its many, many contributions to magnetic tape (and disk) recording technology is all but forgotten. The company folded in the seventies. They held hundreds of patents including 6250 GCR which was done for Burroughs. They were another of the "plug compatable" tape, disk and printer manufacturers during the sixties, early seventies. IBM used (under license) many of those patents. Woodym1 (talk) 00:29, 6 October 2008 (UTC)

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""6 and 2" encoding" listed at Redirects for discussion
A discussion is taking place to address the redirect &. The discussion will occur at Redirects for discussion/Log/2020 September 14 until a consensus is reached, and readers of this page are welcome to contribute to the discussion. Steel1943 (talk) 17:48, 14 September 2020 (UTC)