----- Original Message ----- From: "William Levak" <wlevak@SDF.ORG> To: <email@example.com> Sent: Wednesday, October 26, 2011 12:19 AM Subject: Re: CBM-900 floppy disk format/encoding > Quad density has the same capacity as HD, but supports the same magnetic > coating used in DD, except finer grain particles. HD uses a magnetic > coating with a stronger magnetic field, and therefore, not campatible with > DD. --------------- Actually, that's a little misleading; so-called quad density disks/drives use exactly the same magnetic medium as DD disks/drives and, as mentioned previously, are recorded at the same linear density as DD disks but with twice the number of tracks, fixed sector MFM formats giving roughly 720K compared to DD's ~360K. Although they were the same disks, in the early days of poor manufacturing yields QD disks were indeed QA selected for the higher quality required for the narrower tracks. High density disks on the other hand do use a higher coercivity medium and rotate at a higher speed, which permits a greater linear density and a greater number of sectors/track, yielding 1.2MB per disk compared to QD's ~720K; by switching the strength of the write current and/or double-stepping most HD drives are actually capable of writing (and reading) DD and QD diskettes, although there will almost certainly be errors if the different formats are used on the same disk. A good overview of the 'industry-standard' formats and capacities: http://www.3480-3590-data-conversion.com/article-floppy-disks.html Cyl Sides Density oe RPM Capacity 40 1 SD 300 300 80 KB 40 1 DD 300 300 180 KB 40 2 DD 300 300 360 KB 80 1 DD 300 300 360 KB 80 2 DD 300 300 720 KB ("QD") 80 2 HD 600 360 1200 KB With the possible exception of the CBM-900 disks that started this discussion, Commodore and some other manufacturers of the day only used DD diskettes but took advantage of their 'intelligent' drives to squeeze more capacity out of the higher capacity disks than the 'standard' fixed sector/track MFM disks by using GCR encoding and variable number of sectors/track (more sectors/data on the longer outer tracks); this is how they managed to get >500 KB per side on 8050/8250/SFD1001 disks compared to the 'standard' ~360KB/side. http://www.commodore.ca/manuals/commodore_1541_4040_8050_8250_comparison.htm Message was sent through the cbm-hackers mailing listReceived on 2011-10-26 08:00:03
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