[sdiy] Novation peak NCOs

[rburnett at richieburnett.co.uk]

Which issue of SOS?

On 2017-04-21 13:02, Rutger Vlek wrote:
> 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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