It was using the DAC value as a positive offset, which makes a slight
click at the start and end of the SEGA voice at the start of a game.
now subtracts half the value and amplifies it a bit so that it
crosses the 0 value
I wanted to make this a bit more modular, so it's easier in theory to
write external crates that can reuse bits, and selectively compile in
bits, such as adding new systems or new cpu implementations
