[sdiy] Generating a large number of CV outputs

[Richie Burnett]

Thanks for explaining your implementation Tom!

-Richie,

-----Original Message----- 
From: Tom Wiltshire
Sent: Friday, December 8, 2023 4:01 PM
To: mskala at northcoastsynthesis.com
Cc: SDIY
Subject: Re: [sdiy] Generating a large number of CV outputs





On 8 Dec 2023, at 14:41, Matthew Skala via Synth-diy 
<synth-diy at synth-diy.org> wrote:


If PDM means PWM with bit-reversal before the comparison (such as Richie
describes), then it does indeed lock you into a lower sampling rate, and
that's one reason I skipped describing *that* technique.  But PWM with
bit-reversal seems not to be what you mean when you say PDM.


That's not what I meant when I said PDM, certainly.

The way I generated it is using an NCO. The NCO generates a single-shot 
output pulse everytime the phase accumulator wraps.

Now consider what happens with a simple 8-bit NCO. If our frequency 
increment is 2, for example, we get a single output pulse every 128 clocks, 
or 2 pulses per 256 clocks. Notice that they will be nicely spaced apart, 
not next to each other like PWM. The output frequency would be (clock 
frequency / 128) in this situation.
If the increment is 8, we get a output pulse every 32 clocks, 8 pulses per 
256 clocks, and again, they're nicely spaced out. The output frequency is 
now up to (clock /32) so there's been a big improvement, just by getting 
away from those extreme values a little bit.
As the increment climbs, the accumulator wraps more and more often. At 
freq=128, every other clock is an output and we reach our maximum output 
frequency of (clock/2). As the increment goes above half, we start staying 
high for more than a single pulse, and the waveform effectively turns the 
other way up and we get a mirror image of the effect we've seen from 0-128.

HTH,
Tom


==================
       Electric Druid
Synth & Stompbox DIY
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