[sdiy] Time-varying all-pass filters

Rutger Vlek rutgervlek at gmail.com
Fri Jan 7 12:37:20 CET 2022

Thanks for the suggestion Ulfur! Do I assume correctly that it comes down
to a mathematical equivalence of wave folding and phase modulation?

In reply to others: I see that my original question may not have been
phrased clearly enough. It's not about making the frequency of an all-pass
filter change in time, which I know how to do. The problem is when you
change the frequency at audio rate. The figure from the paper I referred to
in my original post describes the problem well. It shows how a rapid
modulation of the APF frequency (top graph) leads to a low-frequency
component in the output (middle graph), which could be avoided (bottom
graph). The analog APF I am using acts identical to the middle graph,
leading to (unnecessary?) headroom issues, while true PM via the oscillator
directly would never cause this.

Despite headroom issues, the audio-rate modulated analog APF still sounds
cool, by the way!

[image: 3-Figure2-1.png]


Op do 6 jan. 2022 om 22:58 schreef ulfur hansson <ulfurh at gmail.com>:

> hey rutger!
> i have had amazing results with openmusiclabs sinecore design. I've
> changed it a lot for my own personal needs, but the basic circuit is all
> there and presented in a very informative way :) recommend checking it out!!
> very best,
> -úlfur
> fim., 6. jan. 2022 kl. 14:16 skrifaði Rutger Vlek via Synth-diy <
> synth-diy at synth-diy.org>:
>> Hi Neil,
>>  Yes that schematic on page 22 describes very well what I'm doing, but
>> with an audio rate modulation signal added to the tracking cv of the all
>> pass filter. The problem occurs when input signal peaks and is added to
>> twice the output signal of the low pass filter when IT peaks. That can
>> happen when receiving a sudden change in the modulation CV.
>> Rutger
>> Op do 6 jan. 2022 13:08 schreef Neil Johnson <neil.johnson71 at gmail.com>:
>>> Hi Rutger,
>>> > Then it occurred to me that, assuming the input is a sine wave, a
>>> phase shifted version up to 180 degrees could be obtained from modulating
>>> an all-pass filter that is also tracking along with the pitch of the
>>> oscillator.
>>> See page 22:
>>> https://www.soundsemiconductor.com/downloads/ssi2130datasheet.pdf
>>> Implementation here:
>>> https://www.youtube.com/watch?v=bq-2UQ6zkSU
>>> That was modulated at a slow (manual) rate, so no idea what it does at
>>> faster rates.  Interesting question though....
>>> > ps. Happy healthy new year!
>>> Happy new year to you too!
>>> Cheers,
>>> Neil
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> --
>> ulfurhansson.com
> instagram.com/ulfur_
> .•°~ Úlfur - Arborescence on Spotify
> <https://open.spotify.com/album/118CEJliIg47Jcr70y7Kww>
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