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Sun Sep 19 15:01:13 UTC 2010

"The Commodore 64 has a sound chip that wasn't designed for
playing samples. Since there's not much available memory,
they did not intend the SID chip to play samples - 64kB with
8kHz sample rate will give you a some 8 seconds of sound to
play. There was no need for sample playback.

So, we have to fool the SID chip to play samples, even
though it only has the means of playing either a continuous
triangle waveform, sawtooth waveform, pulse-width waveform
or noise waveform. This is done by using the triangle
waveform, resetting the oscillator with an undocumented testbit
originally implemented for factory testing, setting the
accumulator frequency to change the increment speed of the
accumulator, and then after an exact number of clock cycles
enable the triangle waveform output just briefly, practically
emulating a sample-and-hold filter that will keep the analog
output fixed at a certain voltage."


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