Putting this here so it doesn't get scrolled off the shoutbox, additions / corrections welcomed.
Saturation magnetic tape and disk recording works in essentially the same way. A bulk-erased piece of media is magnetized with an AC electromagnet, resulting in a random arrangement of flux transitions on
the surface. When a track is recorded, a DC erase magnet straddles the width of a track, putting the media in a known magnetic state. The write head produces alternating polarity magnetic fields which are
transferred to the magnetic surface of the media through a gap in the write head. The flux reversals in the media are later detected by using that same write head as a transformer through a differential amplifier.
That signal is low-pass filtered then passed through a differential active filter, which applies a 90 degree phase shift to the signal. That differentiated signal goes to a comparator which detects the peaks of the flux
transitions (maximum dv/dt). That pulse stream is sent to a time-domain filter which attempts to reconstruct the original FM/MFM/GCR encoded bitstream as a pulse train with fixed pulse width, which is then
further decoded into the actual stream of 1's and 0's originally recorded. Typically in recording formats with embedded clock information, a bit clock will be derived from the bitstream in the digital data separator
which often has an analog phase-locked loop in the clock generation circuit to attempt to correct for variations in the bitstream rate due to speed variations on the media.
At least, in theory, that's how it works. But the physical medium may have problems like dropouts, speed variations, and in the case of random-access devices like disks, spots where new data is recorded over
the previously laid down format for the sector, resulting in non-deterministic 'write splices'. Or, it is deliberately tampered with, as is the case when copy-protected media is produced.
This also points out the fact that a lot of analog processing has been applied to the signal coming off of the head by the time a digital 'flux-transition' timing analyzer ever sees it. In particular, the time domain filter.
Len Shustek and I did a bunch of work a few years ago recovering old magnetic tapes using analog digitization. Len made a video about it for VCF West in 2020, and the code is on gitHubhttps://github.com/LenShustek/readtape
-- to be continued