[sdiy] Novation peak NCOs

[Scott Gravenhorst]

Interesting discussion.

My first FPGA subtractive synth was a 4 osc with naive waveforms.  I did that because the
DAC was capable of 1 MHz sample rate which I used.  Harmonics above 500 kHz are so low in
amplitude as to be negligible.  I could portamento without siren fear.  The drawback was
that it's 1 MHz DAC was also only 12 bits (the Spartan 3E Starter Kit board).  However, the
12 bit-ness didn't really seem to degrade it's sound.  But still, aliasing is aliasing and
a good high sample rate is one of the cures.  It allows the simplicity of generating
waveforms in a naive manner.

Tom Wiltshire <tom at electricdruid.net> wrote:
>Ok, so let's just have a look at that for a moment.
>
>Assume a highest output frequency of 20KHz (I wouldn't even be 
>able to hear this). We've got a nyquist frequency of 12MHz. That 
>means that only anything that goes far enough over 12MHz to alias 
>all the way back down below 20KHz is actually going to be 
>audible. So that's 23.98MHz before we get audible aliasing. 
>That's equivalent to the 1199th harmonic of 20KHz ramp wave. 
>
>It's a high enough frequency they can ignore aliasing, even for 
>naive waveforms, and even for tough cases like ramps and squares. 
>
>It does seem like overkill, but FPGAs are cheap these days, and 
>running a few (or a lot of) NCOs isn't even going to make one 
>break a sweat. 
>
>Yeah, why not? I like it. Simple, direct, does the job.
>
>Tom
>
>On 20 Apr 2017, at 18:20, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>
>> An interesting direction Novation are taking with the use of 
>FPGA based NCO oscillators on their new hybrid polysynth. There's 
>a video from Superbooth about it on sonicstate. It uses a sample 
>rate of 24MHz to generate classic analog saw, pulse, tri waves, 
>etc, then feeds them through conventional analog VCF, VCA, etc. > 
>> Seems like a bit of a brute force way to crack the old aliasing 
>nut, but I guess it gives them the ability to do some wacky audio 
>rate modulation stuff with reduced aliasing too. > > -Richie, > > 
>Sent from my Xperia SP on O2 > > 
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- http://scott.joviansynth.com/
-- When the going gets tough, the tough use the command line.
-- Matt 21:22