I was curious as to how the data on an Apple II floppy was related from track to track on a commercially generated disk. As the Apple drives do not have an index sensor, sector zero does not need to be positioned relative to the index hole.
As my software records the time of each index pulse, it is relatively easy to line up the data from each track in graphical format.
To the right, you will see a visualization of each of the half-tracks of data from a commercially produced original copy of Choplifter. The x-axis is the pulses from left to right, and the y-axis for each track is the time it took for the pulse to arrive. Each track is stacked with track zero on top and track 79 on the bottom.
It is easy to see the data which falls into standard time intervals, and the “hash” that results from the random fluctuations as the heads pass over unrecorded spots on the disk.
Below is an image of track zero. The beginning portion on the left is an unrecorded portion of the disk. Then there is a sequence of 4×4 encoding, which is followed by the boot sector encoded in standard encoding as shown by the third line indicating 12 microsecond intervals. A short burst of unrecorded data is followed by a large number of 4×4 encoded bytes. Lastly, the remaining space on the track is unrecorded.
Interestingly, this disk is entirely encoded in 4×4 format, other than the boot sector.
Information regarding the Choplifter boot process can be found at the below links:
http://www.hackzapple.com/phpBB2/viewtopic.php?p=4836
Computist Issue 23 pages 10-12:
http://computist.textfiles.com/ISSUE.23/
And for a visualization of how the heads read the above data: