With regards to DAC vs. PCM, the YM2612's DAC simply puts whatever value you write to it on the audio output, effectively converting the digital value to analog. It doesn't handle playing PCM samples as-is. You have to write a program to send the DAC the correct samples from the input source at the right time. The source data could be PCM, PWM, some form of ADPCM, or even MP3 (assuming you can write a decoder that runs fast enough on the Z80 or 68000). On the other hand, there's the SegaCD PCM chip, which plays PCM audio data from a RAM bank directly without software intervention (other than loading the data and telling it what to play). The PCM chip only understands specific formats, so there isn't as much flexibility as with the YM2612's DAC, but it's easier to program.
