Monday, July 13, 2009

Sega SN76489 in MD2 vs SMS2

The Sega Mega Drive 2 implementation of the SN76489 PSG displays some interesting sonic characteristics, when compared to a Sega Master System or Sega Master System 2.

To my ears, there is some sort of timbral change associated with volume change, especially if only one channel is playing. I have no idea what is the cause of this phenomenon, but it sounds to me like it might be something to do with a DAC portion of the VDP or some sort of quantisation or aliasing effect. If anyone knows why this exists, I would love to learn about it.

Let's have a look at some waveforms from the two audio outputs.

The Sega Master System 2 output, taken directly from the chip.

The Sega Mega Drive 2 output, taken from the AV output port.


Maxim said...

The SN76489 doesn't have a DAC as such - it has 1-bit output (literally 0 or 1) that goes through an attenuator, which I suppose is kind of like a non-linear DAC. This then goes through various filtering caps to remove the DC offset and lots of high frequencies.

You will therefore get odd things happening on volume changes, because that effectively changes the DC offset.

Of course, it's possible the MD2 had a different implementation of the chip, but I'd doubt it; redesigning chips isn't cheap.

The timbral qualities are more likely due to the stuff outside the chip - mainly the smoothing caps - and their state of repair. If you gutted the system, installed good-quality new caps everywhere, isolated the chip outputs and sampled them with a very low-load device you'd probably get much cleaner squares.