On Thu, 3 Jan 2019, Mia Magnusson wrote: > Den Thu, 3 Jan 2019 15:15:43 +0100 skrev Francesco Messineo > <francesco.messineo@gmail.com>: >> On Thu, Jan 3, 2019 at 2:51 PM Mia Magnusson <mia@plea.se> wrote: >>> >>> Den Wed, 02 Jan 2019 15:34:24 +0100 skrev André Fachat >>> <afachat@gmx.de>: >>>> Hi there, >>>> >>>> I was looking at floppy disk recording schemes and I am wondering >>>> if the 8050/8250/1001 floppy disk format with over 500kB per side >>>> was actually out of spec of even the Quad Density disks? >>>> >>>> The recording frequency was increased from 250kHz to 375kHz (× >>>> 1.5, for the innermost i.e. most critical track/speed zone). That >>>> resulted in a much increased number of bits per inch. See here: >>>> https://extrapages.de/archives/20190102-Floppy-notes.html >>>> >>>> What do you think? >>> >>> Afaik the 8050/8250/1001 drives are supposed to use "QD" disks, >>> which seems to be a format that's supposed to handle a higher >>> density than DD. >> >> "QD" disks have the very same 300 oersted magnetic media as SD/DD >> disks, it was only the mechanics and R/W heads that allowed to use >> more effectively the storage media. >> Later HD drives used different R/W heads (or different currents) and >> required 600 oersted media, doubled bitrate to 500 Kbps and changed >> speed to 360 rpm. >> >> At the university, we had a few Olivetti LSX-3005 that were equipped >> with 96tpi 5 1/4" drives (QD, not HD obviously), >> I remember nobody ever tried to find "QD" disks, normal DD 48TPI disks >> were used (albeit I remember good quality brands were purchased >> usually, like 3M, Olivetti). > > As I recall, some 48 TPI disks actually caused problems when used as 96 > TPI. > > A qualified guess is that once 96 TPI DD disks became rather common, > they just made that kind of disks and labeled some of them as 96 and > some as 48 TPI for market / pricing purposes. What they were labeled as was determined by what they were tested as. The testing is the most expensive part of the manufacturing process. > Later when the market settled for 40 TPI DD and 96 TPI HD the > manufacturers could well have switched back to media that only support > 48 TPI, if there were ever any kind of issue with track-to-track crosstalk. > >>> It seems common for people to think that QD was a marketing thing >>> used for 96TPI DD disks, but I've seen so many 96TPI disks marked >>> DD and >> >> again, 96TPI DD is just the same media as 48TPI DD, just maybe tested >> better (or just advertised as 96TPI, who knows). > > Well, with narrower tracks, the signal to noise ratio will be worse > with all other parameters the same, so 96 TPI disks might actually > differ from some 48 TPI disks. > >>> only a few (like one or two, and it was last summer that I first saw >>> them) disks actually labeled QD. (They contain a book keeping >>> software package, in Swedish, from the Swedish Commodore importer >>> Datatronic. Will be preserved as soon as I get my 8050 up and >>> running, which has been waiting a while for me to find my stash of >>> IEEE cables :) )). >>> >>> It would be really strange if floppy media didn't evolve the same >>> way as magnetic tapes did. With media good enough for 250kHz at >>> track 35 when the 5.25" floppys were new, and soon good enough for >>> 250kHz at track 40, it seems reasonable that some years later the >>> media used for those drives were actually good enough for 375kHz at >>> the 48TPI equalient of track 35, which almost is where the highest >>> track number on a 77 track 100 TPI drive will end up. >> >> I really think the better Kbps rating is due to the use of the more >> efficient GCR code instead of the MFM. It's not more efficient. It's more accurate. In MFM, if you have a long string of zeros, reading accuracy is limited by the accuracy of your clock. GCR eliminates this problem. > Why wouldn't Commodore had used that in the > 2040/3040/4040/2031/2031LP/1540/1541/1551/1570/1571 drives too then? > >>> (At some point in time a market for cheap rather crappy disks seems >>> to have evolved though, but those were probably anyway nothing >>> people used in their 8050/8250/1001 drives). >>> >>> (Everyone who's been around long enough to remember cassette tapes >>> from the 70's and the 80's remember that before tapes like Maxell >>> UD and similar the standard / ferro / type I tapes did really sound >>> crap with a high noise level and muffled treble. Then something >>> happened in the late 70's and early 80's, resulting in more and >>> more kinds of tapes getting a lot better, and at the start of the >>> 90's basically almost all tapes had a decent sound even though >>> there were of course still differences between them). >> >> There're two different issues on compact cassette: >> 1) different (really much different) magnetic media, type I (Fe2O3, >> iron oxide), Type II (CrO2), type III (FeCr), type IV (metal), these >> media required different equalization and different recording >> currents. Type I are usually very bad sounding and noisy, type IV have >> the best quality, but the recorder really NEEDS to know what type of >> tape it's trying to record into, otherwise you wouldn't get much >> better results, unless maybe a bit less noise if you use a type IV >> tape on a old, low quality recorder. >> 2) Dolby NR pre/de-emphasys. These noise reduction techniques have >> been introduced starting from 1965, last one afaik was introduced in >> 1986. Not all recorder were equipped with these circuits. A type I >> with the best NR circuit could sound really better than a type II with >> no Dolby. >> So, all in all, nothing in common with floppy disks :) > > In practice the cassettes did differ rather much between models and > manufacturers. There were even cassette decks like AKAI CS-707D which > had two different tape positions for "type I" tapes, called LN and LH, > which were intended for usage with older/"European" (Usually > Philips, Agfa, BASF and similar from the 60's and 70's, and the crappy > American tapes like Ampex, Scotch/3M and similar) v.s. > newer/"Japanese" (usually Maxell UD and similar from the 70's, and most > types from the 80's and newer) tapes. > > Have a look at any decent cassette tape test in some serious consumer > electronics magazine from back in the days, and you'll find that the > tapes differed a lot within each type. > > This must surely have happened on diskettes also, but as the media is > used in a different way the only important things would be that the > noise is under a certain threshold and the "treble response" is good > enough so data won't get lost at higher bit rates, and of course drop > outs. > > > > -- > (\_/) Copy the bunny to your mails to help > (O.o) him achieve world domination. > (> <) Come join the dark side. > /_|_\ We have cookies. > > > wlevak@sdf.lonestar.org SDF Public Access UNIX System - http://sdf.lonestar.orgReceived on 2019-01-04 07:00:03
Archive generated by hypermail 2.2.0.