[sdiy] Generating a large number of CV outputs

[Mike Bryant]

That's one way of doing it, but still locks you into a lower sampling rate.  Often better to just sample at a very high rate (as fast as your logic allows) and just decide do I need to output a '1' now or not.

________________________________
From: Richie Burnett <rburnett at richieburnett.co.uk>
Sent: 08 December 2023 10:47
To: Mike Bryant <mbryant at futurehorizons.com>; mskala at northcoastsynthesis.com <mskala at northcoastsynthesis.com>
Cc: SDIY <synth-diy at synth-diy.org>
Subject: Re: [sdiy] Generating a large number of CV outputs

I forget, is PDM equivalent to PWM but with the ramp counter bit-reversed
before it gets compared with the reference?  So the overall ratio of 1's to
0's in one period of the bit sequence is the same as PWM, but the 1's and
0's are scattered around the period, instead of having all of the 1's
grouped at the start of the period and all of the 0's grouped together at
the end of the period.  Am I right?

-Richie,



-----Original Message-----
From: Mike Bryant
Sent: Friday, December 8, 2023 9:34 AM
To: mskala at northcoastsynthesis.com
Cc: SDIY
Subject: Re: [sdiy] Generating a large number of CV outputs


Yes the key thing about PDM over other methods is it doesn't have to have a
fundamental fairly low sampling rate like PWM or D-S.  This gives it much
faster response time to requested changes.  One way to think of it is that
PWM and D-S have a low pass filter on their feedback loops, which D-S
actually does have, whilst PWM sort of has in the lag of changing the M-S
ratio.


This, and the fact that you get far less energy at the sampling rate, is why
many digital microphones have moved to using PDM, and of course most new
MCUs now support this mode.




From: mskala at northcoastsynthesis.com <mskala at northcoastsynthesis.com>
Sent: 08 December 2023 03:30
To: Mike Bryant <mbryant at futurehorizons.com>
Cc: Danjel van Tijn / intellijel <danjel at intellijel.com>; SDIY
<synth-diy at synth-diy.org>
Subject: Re: [sdiy] Generating a large number of CV outputs


On Fri, 8 Dec 2023, Mike Bryant wrote:

> I just had a read of this and generally very well put.  However you seem
> to
> miss out mentioning PDM  between the sections on PWM and Delta-Sigma.
> This
> gives faster settling time of DC voltages than PWM, as well as other
> advantages when producing audio as its easier to push the noise out of
> band.

I'm not sure PDM is meaningfully different from delta-sigma at the level
of detail I included in the posting.  The things I would want to say about
it would be the same things I did say about delta-sigma.  Is there an
important example of a PDM scheme that is not a form of PWM nor
delta-sigma modulation?

--
Matthew Skala
North Coast Synthesis Ltd.






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