BLuRry wrote:
And what's more, the emulated applewin sound doesn't quite match the real thing -- CiderPress can't recognize it even if I comb out only the 770hz, 1khz and 2khz audio bands. So I'll have to abuse a real apple machine for this experiment I think. Following what Andy McFadden wrote about the basics on cracking apple casette programs, is it exactly 1khz and 2khz tone or is that just a close approximation? I think I can pretty easily generate very clean apple tape data after reading the book of McFadden. :-)
To refer to the signals as "tones" is a bit (!) misleading, since only a single cycle of each frequency is present, in general, so the spectrum is considerably wider than "tone" would suggest. The signal is processed in the time domain, not the frequency domain. The Apple read routine times the number of iterations of a sampling loop between two zero crossings (starting a fixed time after the previous zero crossing) and then compares the iteration count to a value intermediate between the "0" timing and the "1" timing to decide which bit to shift into the data. Full cycles are used so that the DC level of the signal does not shift during runs of zeroes or ones, so that zero crossing detection is more stable. -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."