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furnace/papers/doc/7-systems/vrc6.md
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Konami VRC6

the most popular expansion chip to the NES' sound system.

the chip has 2 pulse wave channels and one sawtooth channel. volume register is 4 bit for pulse wave and 6 bit for sawtooth, but sawtooth output is corrupted when volume register value is too high. because this register is actually an 8 bit accumulator, its output may wrap around.

For that reason, the sawtooth channel has it's own instrument type. Setting volume macro and pattern editor volume setting too high (above 42/2A) will distort the waveform.

pulse wave duty cycle is 8-level. it can be ignored and it has potential for DAC at this case: volume register in this mode is DAC output and it can be PCM playback through this mode. Furnace supports this routine for PCM playback, but it consumes a lot of CPU time in real hardware (even if conjunction with VRC6's integrated IRQ timer).

effects

these effects only are effective in the pulse channels.

  • 12xx: set duty cycle (0 to 7).
  • 17xx: toggle PCM mode.