[sdiy] Variable rate waveform playback in NED synclavier
rburnett at richieburnett.co.uk
rburnett at richieburnett.co.uk
Fri Aug 31 16:55:21 CEST 2012
Hi Tom and all,
> 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?
I'd guess it's done with a PLL that has a divider in the feedback path
(divide by N), followed by another divider outside the loop (divide by
M.) The fact that the first divider is inside a negative feedback loop
would make its effect multiplicative on the VCO's output frequency. The
order of the steps might be swapped around, so that the crystal
frequency is first divided by M, then multiplied by N inside the PLL to
keep the intermediate frequency from becomming excessively high!
> This seems a bit dirty for NED.
Why is it dirty? It's a very common technique used to give clean
output signals from PLLs for all sorts of things like radio
transmitters, receivers etc. It may be that the 16-bits worth of
"sample rate generator data" are only changed infrequently like when
changing octave or changing patch so a momentary un-locking of the PLL
for a millisecond or two would likely not be noticed. After all, the
N/M multiplier with integer N & M is quite a course method of frequency
control similar to the 16'/8'/4' octave selection on juno DCOs. Any
fine modulation and pitch-bend would surely be done via the phase
increment to the NCO?
> If not, how? I can't see how this bit works.
Another possible method would be harmonic multiplication of the
original crystal frequency. Since the crystal frequency is fixed and
well known, it would be very easy to generate integer multiples of this
frequency using harmonic doublers, triplers etc. Again, this is a
common technique in RF engineering for frequency multiplication. It
only requires a biased transistor and an LC tuned circuit for each
frequency multiplier stage, and works very well when all of the
frequencies are fixed.
> 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?
It sounds right to me. Save for the weird N/M multipier the rest is
just like a dual-channel DDS (direct digital synthesis) chip with its
NCO, waveform lookup ROM and DAC.
> 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.
It's all good stuff, and you never know when an old trick from the past
will help you solve a new challange with modern parts, or eeek out that
extra bit of performance. Will be interested to hear others replies
about this.
-Richie,
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