[sdiy] Time-varying all-pass filters

Lloyd Abbott lloydabbott94 at gmail.com
Fri Jan 7 13:03:20 CET 2022

I don't have the answer for an APF implementation that avoids the DC
excursion shown but one approach that I think would give something more
like the third plot is placing a HPF after the circuit giving the second
plot ? Provided that you have headroom for the second plot and that the
time-constant of the HPF is usable (i.e. not too long a settling time and
also not cutting the audio you want...).

Just a thought...

On Fri, Jan 7, 2022 at 11:40 AM Rutger Vlek via Synth-diy <
synth-diy at synth-diy.org> wrote:

> 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!
> https://d3i71xaburhd42.cloudfront.net/dd2e6eff1c49d49479c0943615ebc36a19f4fa0d/3-Figure2-1.png
> [image: 3-Figure2-1.png]
> Rutger
> 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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