Quote:
Originally Posted by zero
So maybe I misunderstood how they were creating these weak bits.
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Some main ways to generate weak bits :
- Unformatted surface : The drive's AGC circuit goes very quickly to its maximum gain. This generates electronic noises and finally generates random pulses on the read pin.
(This can also be done by removing a part of the magnetic surface)
- Long timing between flux reversals : Very similar to the previous process: The drive's AGC circuit goes very quickly to its maximum gain. This generates electronic noises and generates random pulses on the read pin.
- Modulated flux reversal : Modulate out of spec flux reversal to disturb the floppy controller : This generate random decoding errors. (Example : Dungeon Master :
)
Some analogues captures at the oscilloscope :
[ Show youtube player ]
Video details :
Drive : YD-480 5"1/4 - 40 Tracks DD Floppy disk .
Channel 1: Preamplified head Analog signal (HA16642MP Diff in)
Channel 2: Drive Read Data output (/RDATA pin 30)
Oscilloscope synced on the index signal (Drive Pin 8).
00m00s : Track write splice. (see the random drive data output in the write splice area)
00m07s : Head moved between the disk track 0 and 1.
00m15s : Track 1 (with a write splice again).
00m23s : Head moved between the disk track 1 and 2.
00m35s : Head moved to the unformatted track 40 ( see the total random output data : the drive AGC's gain is set at its maximum value : The random data are generated directly from the AGC circuit).