On Donnerstag, 3. Januar 2019 18:20:31 CET Francesco Messineo wrote: > >> > I really think the better Kbps rating is due to the use > of the more > > > efficient GCR code instead of the MFM. > > > > Why wouldn't Commodore had used that in the > > 2040/3040/4040/2031/2031LP/1540/1541/1551/1570/1571 drives too then? > > In facts, CBM used the more efficient GCR in their drives :) GCR is NOT more efficient than MFM using the same bit frequency. In fact MFM is more efficient (writing 8 cells @ 250kHz for MFM vs. 10 cells @ 250kHz for CBM GCR). The reason that MFM can do this is that they actually use "half cells" at 250kHz, but still ensuring that flux transitions are at least 4us (i.e. a 250kHz cell) apart. > LSX-3005 QD MFM drives stored 720Kbytes (double side), using 80 tracks > per side, while the 8250/SFD-1001 GCR drives stored 1Mbyte on the same > disks and using only 77 tracks per side :) > MS-DOS stored 360Kbytes on 40x2 tracks using MFM, 4040/1541 GCR stores > 340Kbytes (on two sides) but only on 35x2 tracks (so, using 10 less > tracks!!) One feature Commodore made more efficient is zoned recording, i.e. increasing the recording frequency on the outer tracks. > Yes, of course CBM drives also use variable bitrate to store more > sectors in the larger tracks, but this is possible because of GCR > encoding. No, this is not due to GCR encoding. You could even increase recording frequency on MFM on outer (i.e. longer, i.e. faster) tracks, as long as you don't get over the bpi rating of the media. The MFM controllers did not support it though, so it was not used. > All this on 300 Oersted media. Yes Regards AndréReceived on 2019-01-03 19:01:27
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