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Copy file name to clipboardExpand all lines: docs/source/techspecs/floppy.rst
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@@ -66,7 +66,7 @@ The data is structured at two levels: how individual bits (or nibbles, or bytes)
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All floppy controllers, even the wonkiest like the Apple II one, start by dividing the track in equally-sized cells. They're angular sections in the middle of which a magnetic orientation inversion may be present. From a hardware point of view the cells are seen as durations, which combined with the floppy rotation give the section. For instance the standard MFM cell size for a 3" double-density floppy is 2us, which combined with the also standard 300 rpm rotational speed gives an angular size of 1/100000th of a turn. Another way of saying it is that there are 100K cells in a 3" DD track.
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In every cell there may or may not be a magnetic orientation transition, e.g. a pulse coming from (reading) or going to (writing) the floppy drive. A cell with a pulse is traditionally noted '1', and one without '0'. Two constraints apply to the cell contents though. First, pulses must not be too close together or they'll blur each-other and/or be filtered out. The limit is slightly better than 1/50000th of a turn for single and double density floppies, half that for HD floppys, and half that again for ED floppies with perpendicular recording. Second, they must not be too away from each other or either the AGC is going to get wonky and introduce phantom pulses or the controller is going to lose sync and get a wrong timing on the cells on reading. Conservative rule of thumb is not to have more than three consecutive '0' cells.
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In every cell there may or may not be a magnetic orientation transition, e.g. a pulse coming from (reading) or going to (writing) the floppy drive. A cell with a pulse is traditionally noted '1', and one without '0'. Two constraints apply to the cell contents though. First, pulses must not be too close together or they'll blur each-other and/or be filtered out. The limit is slightly better than 1/50000th of a turn for single and double density floppies, half that for HD floppies, and half that again for ED floppies with perpendicular recording. Second, they must not be too away from each other or either the AGC is going to get wonky and introduce phantom pulses or the controller is going to lose sync and get a wrong timing on the cells on reading. Conservative rule of thumb is not to have more than three consecutive '0' cells.
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Of course protections play with that to make formats not reproducible by the system controller, either breaking the three-zeroes rule or playing with the cells durations/sizes.
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@@ -108,7 +108,7 @@ Other encodings exist, like M2FM, but they're very rare and system-specific.
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2.3.1.6. Reading back encoded data
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Writing encoded data is easy: you only need a clock at the appropriate frequency and send or not a pulse on the clock edges. Reading back the data is where the fun is. Cells are a logical construct and not a physical measurable entity. Rotational speeds very around the defined one (±2% is not rare), and local perturbations (air turbulence, surface distance…) make the instantaneous speed very variable in general. So to extract the cell values stream, the controller must dynamically synchronize with the pulse train that the floppy head picks up. The principle is simple: a cell-sized duration window is built within which the presence of at least one pulse indicates the cell is a '1', and the absence of any a '0'. After reaching the end of the window, the starting time is moved appropriately to try to keep the observed pulse at the exact middle of the window. This allows the phase to be corrected on every '1' cell, making the synchronization work if the rotational speed is not too off. Subsequent generations of controllers used Phase-Locked Loops (PLLs) which vary both phase and window duration to adapt better to inaccuarate rotational speeds, usually with a tolerance of ±15%.
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Writing encoded data is easy: you only need a clock at the appropriate frequency and send or not a pulse on the clock edges. Reading back the data is where the fun is. Cells are a logical construct and not a physical measurable entity. Rotational speeds very around the defined one (±2% is not rare), and local perturbations (air turbulence, surface distance…) make the instantaneous speed very variable in general. So to extract the cell values stream, the controller must dynamically synchronize with the pulse train that the floppy head picks up. The principle is simple: a cell-sized duration window is built within which the presence of at least one pulse indicates the cell is a '1', and the absence of any a '0'. After reaching the end of the window, the starting time is moved appropriately to try to keep the observed pulse at the exact middle of the window. This allows the phase to be corrected on every '1' cell, making the synchronization work if the rotational speed is not too off. Subsequent generations of controllers used Phase-Locked Loops (PLLs) which vary both phase and window duration to adapt better to inaccurate rotational speeds, usually with a tolerance of ±15%.
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Once the cell data stream is extracted, decoding depends on the encoding. In the FM and MFM case the only question is to recognize data bits from clock bits, while in GCR the start position of the first group should be found. That second level of synchronization is handled at a higher level using patterns not found in a normal stream.
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