[sdiy] Novation peak NCOs

[Rutger Vlek]

Hi Richie,

Indeed it concerns FM, mostly.  And perhaps, theoretically, the low end
tightness. I recently read a great article in sound on sound about a new HD
audio codec that aims to improve aliasing as well as low frequency phase
issues and ringing that normally come with a typical anti alias filter in
or post a dac. I assume novation's approach also improves that. Although i
have no idea how therotical this is: if it can be measured, if it makes an
audible difference.

Best,

Rutger

Op 21 apr. 2017 13:13 schreef <rburnett at richieburnett.co.uk>:

Yeah, it's an interesting way to tackle the aliasing issue.  As Tom and
others pointed out, raising the sample rate up to 24MHz doesn't completely
eliminate aliasing for naïve generated sawtooths and pulses, but the high
sample rate means that by the time the harmonics alias back into the audio
spectrum they're at least 60dB down, and probably more like 75dB down for
the highest pitches that people typically play in a music intended for
human consumption (4kHz/24Mhz).

The NCO/DDS technique isn't new.  It was used in the SID, Paul Maddox's
excellent Monowave and 002 synths, and I believe it was used in the "DCO"
chip in Roland's alpha-juno series of hybrid synths back in 1985.  I've had
discussions with Tom W about this off-list and did some tests on my alpha
juno and we came to the conclusion that the "DCO" chip in these synths
generated the waveforms digitally using an NCO outputting at a sample rate
of 6MHz.  So I guess that the increase of sample rate from 6MHz to 24Mhz in
32 years isn't unfeasible!

With the alpha juno you can clearly see the aliased harmonics of very high
pitched notes on a spectrum analyser at about -58dB relative to the
fundamental.  However, I believe there's also a trick where you can apply a
*very tiny* amount of random Frequency Modulation to the NCO that causes
imperceptible pitch modulation in the audio part of the spectrum. However,
it is enough to spread the energy in those upper harmonics around 24MHz
(that alias into the audio band) so that they blend into each other and
produce a lower and more even white noise floor rather than a series of
objectionable inharmonic spikes across the audio spectrum.  I would imagine
that Novation are doing this too, since -60dB alias suppression is nothing
spectacular compared to what can be achieved at a sample rate of 96kHz
using BLIT/BLEP/whatever on a general purpose DSP.

Rutger wrote "I'm particularly interested in how high-frequency
interactions between NCOs are going to sound on this machine. I suspect FM
and RingMod will be pretty smooth, as these interactions make aliasing
effects more pronounced."  I think you are right and this is where the
technique actually shines.  RingMod isn't a problem, but FM produces an
infinite spectrum of harmonics, so whatever sample rate you use it will
alias to some extent, but the higher the better obviously.

The part that I thought was particularly clever was the dithering of the
oscillator waveforms at 24MHz sample-rate down to a single-bit "bitstream"
/ "DSD" output within the FPGA, to mitigate the need for a high-speed
multi-bit DAC that can settle fast enough to handle the 24MHz sample-rate.
Again, the dithering and noise-shaping stuff isn't anything new, but it all
seems to fit well together as a package for their application.

As for the reverb being done in the FPGA, that's almost surely the best
way.  General purpose DSP chips aren't optimised for reverb (except maybe
the FV-1 and Wavefront chips) but you can build the DSP core that you wish
you had with an FPGA and make it optimised for hundreds of allpass filters
or whatever you want for your reverb.

Definitely an interesting time for synth design :-)

-Richie,


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