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.
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).
