[sdiy] Variable rate waveform playback in NED synclavier

[Tom Wiltshire]

Hi All,

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I've been reading about the NED Synclavier. It's still a great design all these years later, and some aspects of it make you gasp.

The best resource I've found is this:

	http://www.500sound.com/uniquesync.html

The second half of this page ("The blank sheet of paper") I found fascinating.

On the same site there's a nice diagram of the additive/FM voice structure:

	http://www.500sound.com/SyncII/synthschem.jpg

There were plenty of versions of the Synclavier hardware, but this shows 8-bit wave memory and envelopes. This still sounded really good because it used multiplying DACs for the output, so there was no loss of resolution - as the signal level goes down, so does the quantisation noise. It's all simple but clever stuff.
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Going back to the voice diagram:

	http://www.500sound.com/SyncII/synthschem.jpg

There's one bit I don't understand. On the left hand side of the diagram are the two Sample Rate Generators, which seem to take a clock signal Fclk and multiply it by a fraction N/M, where N and M are 8-bit numbers. How is this done? Doing arbitrary division is easy enough (just count M clock pulses) but how is the multiplication performed? Would it have to be a PLL with a divider in the feedback loop? This seems a bit dirty for NED. If not, how? I can't see how this bit works.
Secondly, once this new sample rate is generated from the Fclk, it's fed to a Phase Angle Incrementer, which I take it is what we'd call an NCO. This seems to have a 8-bit frequency increment. So the final frequency is some weird combination of the variable sample rate Fclk*N/M and the freq inc - does this seem right or am I miles off here?

This is just a case of me being curious about famous instruments from the past. These things are still revered by many, so I'd like to know how they did what they did.

Thanks,
Tom