JimDrew wrote:Jitter from an index pulse sensor is expected, but it's only measured in tens of nanoseconds. I use standard off the shelf hall sensors in critical flight control system applications for measuring positions with incredible accuracy. I am sure every type of drive is different for accuracy, but I have not seen any accuracy issues that are remotely close to a bitcell duration.
I finally had a chance to perform some authoritative measurements. I hooked a Logic Analyzer to a floppy drive and captured the index and read signals while reading the track. It was taken at the floppy cable. There is no FDC, controller, FPGA, CPU or whatever involved. This is what the drive
outputs. I recorded a special track handcrafted to avoid any write splice issues and to make easy to identify the flux transitions.
These are the captures on two consecutive revolutions:
As you can see, the distance between the index edge and the short flux transition differ in more than 5 microseconds between both revolutions. And the captures clearly show that in one case the first transition after the index is the short one, in the other is a long one.
This is from a high quality (TEAC) 5.25" drive. If I can I'll measure with a 3.25" drive, that I expect to have a smaller index jitter.
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