Roger Johnstone <rojaws@es.co.nz> writes:
The IBM PC drives are neither MFM or GCR. Most disk drives are really
analogue devices and the disk drive controller in the computer
determines the format.
I know what you meant, but "analog" isn't it. The data signals to and
from the disk drive are entirely digital. However, they are not synchronous
to any clock, so they are free-form in the time domain (within certain
frequency limits).
More specifically, the disk drive electronics, when it sees a digital
pulse on the write data line (and write gate is enabled) will cause a
flux transition to be written to the disk. The pulse is digital, and
the exact voltages of the high and low state have NO effect on what
is written to the disk, as long as the thresholds are met. For instance,
a pulse to a high level of 2.5V will result in the same flux transition
on the disk as a pulse to a high level of 3.2V.
Similarly, in the read path, the disk drive senses the flux transition
on the disk, and generates a pusle on the read data line. The voltage
and duration of the pulse is determined by the drive electronics, though,
and not by any characteristic of the flux transition.