Fred, There was a project someone did years ago where you can read GCR disks in an unmodified PC drive by first inserting a PC formatted disk to get synced and then swapping in a GCR encoded disk, then it can actually read the raw pulses and they get decoded in software. I forget the website where the project can be found but a web search will hopefully turn it up.
Sellam On Thu, Nov 3, 2022 at 4:23 PM Fred Cisin via cctalk <cctalk@classiccmp.org> wrote: > >> Also GCR, not MFM. NOT readable with a PC FDC. > > On Thu, 3 Nov 2022, Grant Taylor via cctalk wrote: > > Please expand "GCR". > > Sure, . . . (GROSSLY OVER-SIMPLIFIED, such as "pulse" instead of flux > transition) > FM is "frequency modulated". Well, it is actually a regular clock pulse, > with data bit pulse, or no pulse, between each of the clock pulses. > There is, of course, a limit to how densely packed that can be on a track. > A signal with all zero bits of the data, and a signal with all one bits of > the data therefore are two different frequencies. > > > MFM is "Modified Frequency Modulated". Clock pulses really aren't > necessary when they fall between two consecutive data pulses. If we leave > those out, we end up with a much less dense pattern of pulses. > (Over-simplified: MFM is FM without any clock pulses deemed "unnecessary") > We can get away with a higher data transmission rate, even TWICE, and > still not be much too overcrowded on the track. Therefore, twice as much > data per track. The marketing people called that "DOUBLE DENSITY", and > immediately started calling FM, "SINGLE DENSITY", although some engineers > would argue that FM was "half density" and MFM would be "about single > density". If you do historical research, you will find the term "double > density" was used in the literature BEFORE the term "single density" was > (Just like the phrase "WORLD WAR TWO" was used in newspapers before "WORLD > WAR I" was ever applied to the "great war") > > > But, going back to FM, . . . > if you look at all of the patterns of pulses, you'll see that not ALL of > them are dense. In fact, of the 256 possible patterns for an 8 bit byte, > you can find 32, or even 64, that are low enough density that they could > be compressed. We can use 5 or 6 bits to represent those patterns. But, > having only 5 or 6 bits usable to only use the > specific patterns that were low enough density means that we can't use 8 > bit bytes directly. but, we COULD recombine, to store 5 8 bit bytes as 8 > 5 bit patterns, or 3 bytes as 4 6 bit patterns. THAT produced low enough > "density" of the signal that by upping the data transfer rate, about one > and a half times as much data scould be stored on a track, admittedly with > some additional processing overhead. Thus, the Apple2 got about 140K on a > disk, when the TRS80 got about under 100K (89,600). (Both were originally > 35 track, using Shugart SA400 and SA390 drives) > "Beneath Apple DOS" has a decent description) > > > The FDC of PC can only directly handle WD/IBM sector and track structure, > so reading GCR, such as Apple (prior to 1.4M) Victor/Sirius, Commodore, > etc. calls for different hardware. > http://www.xenosoft.com/fmts.html has a list of a few of the different > machines that use formats that CAN be done using the PC FDC. They do > still have different file systems, with various sector sizes, and > directory structures. > > > -- > Grumpy Ol' Fred ci...@xenosoft.com
