m6561: fix sound implementation

floooh / chips

8-bit chip and system emulators in standalone C headers

zlib License

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m6561: fix sound implementation #26

Open floooh opened 4 years ago

floooh commented 4 years ago

See:

https://twitter.com/petrih3/status/1218081532717543425
http://sleepingelephant.com/ipw-web/bulletin/bb/viewtopic.php?f=11&t=8733&start=210
http://www.pelulamu.net/pwp/vic20/waveforms.txt

nippur72 commented 4 years ago

I'm addressing this issue, I've already implemented the 4 bit DAC and the 8 bit shift registers (aka "viznut" waveforms), and now I am working on the noise generator.

I think I've also spotted another issue on the voice counters, POKE 36874,255 should produce a sound that is lower in pitch than POKE 36874,254. This is a well known behavior of the VIC chip.