I imagine the 0xFF might be converted to 0x00 due to DC blocking capacitors/high-pass filtering, audio circuits aren't really suited for non-audible content.
He might get better capture accuracy with a digital oscilloscope hooked directly at the chip's audio outputs if he's lucky, but it's still pretty impressive he got a bootable image out of that.
From what I remember reading in the comments of his youtube video, the point of the exercise was to do it with as basic equipment as possible.Hence the hours of multiples passes to average the error out instead of a proper data-logging oscilloscope.
> I imagine the 0xFF might be converted to 0x00 due to DC blocking capacitors/high-pass filtering, audio circuits aren't really suited for non-audible content.
Yeah, you want to generate a signal with no DC bias, something as simple as Manchester encoding will go a long way. If that's not good enough, there's NRZ or even a convolutional encoding. You also want to make sure you either send a sin wave, or if you can't do that, at least make sure your square wave frequency is high enough that it doesn't get eaten by the AC coupling capacitors.
Would it be possible to get a higher quality read from using something like an Arduino's I/O pins and some bit-banged C code? I'd be curious to see what would be possible using cheap, off-the-shelf tools since a lot of people don't necessarily have an oscilloscope laying around. :P
Without looking up datasheets, just form the top of my head: the Arduino DAC most likely has 12 bits resolution (as common for cheap uCs), and maybe even slower sampling than a soundcard. A sound card was probably better than that even in the 1990s (say a Sound Blaster).
He might get better capture accuracy with a digital oscilloscope hooked directly at the chip's audio outputs if he's lucky, but it's still pretty impressive he got a bootable image out of that.