Hi,

About a year ago I started to learn a bit pure data in order to create a patch that would act as a groovebox and that should perform on limited cpu resources since I want it to run on a raspberry pi. First I tried to make somekind of fork of the Martin Brinkmann groovebox patch, even if it allowed me to learn a lot about data flow I didn't went to the core of the patch tweaking with sound generation. This led me to end this attempt at forking MNB groovebox patch because even if I could seperate GUI stuff from sound generation and run it on different thread ect... I couldn't go further in optimization in order to reduce the cpu use.

Then a few weeks ago I decided to start again from scratch my project and this time I wanted to be more patient and learn anything needed in order to be capable of optimizing my patch as much as possible. After making a functional drum machine which runs at 2/3% of cpu with 8 different tracks, 126 steps sequencer, a bit of fx ect... I tried to find synths that would opperate well aside the drum machine. And I basicly didn't find any patch that wouldn't use massive amount of cpu time. So I created my own synths, nothing incredible but I'm happy with what I got, though I noticed some aliasing. I read a bit the floss manual about anti aliasing and apply the method used in the manual(http://write.flossmanuals.net/pure-data/antialiasing/), it work well but my synths almost trippled their cpu use, even if I put all my oscilators in the same subpatch in order to use only one instance of oversampling.

I didn't tried to oversample it less than 16 time but since oversampling is so cpu intensive I'm wondering if there's no other option in order to get a good sound definition at a lower cpu cost. I'm already using banlimited waveform so I don't know what I could do in order to limit the aliasing, especialy for my fm patch where bandlimited waveform isn't very useful in order to reduce aliasing.

Since I want to have at least 4 synth track with some at least one synth having 5 voice polyphony I want to know what the best thing to do. Letting FM aside for this project and use switch~ for oversampling 2 or 4 time my synths that use bandlimited waveform ? Or should I try to run different instances of pd for each synth and controling it from a gui/control patch with netsend(though it wouldn't bring down the cpu use at least it would provide somekind of multithreading for my patch) ? Or is there another way to get some antiliasing ? Or should I review lower my expectation because there is no solution that could provide a decent antialiasing for 4 or more synth running at the same time with a low cpu use in pure data in 2021.

Thanks to everyone that would read my topic and try to give some advice in order to get the best antialising/low cpu use solution.

@lysergik
I've done some stuff with Pd/oversampling, although not related to multichannel synths (mono audio streams).

I don't think you're wrong describing the CPU hit -- and that's not really a knock on Pd. It's the nature of all that extra data.

However, definitely try a lower sampling rate; 4X or 8X, and see how that impacts your machine. The J07.oversampling.pd patch ref'd in the manual also illustrates the butterworth.pd coefficients patch, and dropping the low-pass cutoff freq will reduce antialiasing.

Any low-pass filter will help (even lop~), though some will impact your sound more than others. I found biquad~ to be quite useful with incoming audio signals. Sample-resolution tools like fexpr~ will hit the CPU in a major way, but the expr~ obj not so much (it's unfortunately not as useful for filtering). I tend to be limited to vanilla Pd; you may not be.

Working with synth-generated signals within the upsampled block at least reduces some of the overhead, verses upsampled/downsampled streams. It also lends to some sculpting of the sounds without oversampling (the second half of your manual link).

There's much to be said for really targeted and tailored EQ. I like to think much of the 'magic' of commercial software is carefully working around limitations.

I'm unclear on how (or whether) Pd distributes any processes. It might be interesting to try the pd~ obj, and explore "Embedding Pd in Pd" as outlined in Puckette's article on multiprocessors & Pd.

@lysergik you might want to mix some oversampling with an antialiasing method (like the BLEP method in J09.bandlimited.pd in the audio examples, or the wave table method of the bandlimited oscillators in the mmb library https://github.com/dotmmb/mmb. Incedentally it seems like serum does something similar but with mip-mapping http://www.mp3-tech.org/programmer/docs/resampler.pdf)
Unfortunately getting bandlimited fm is pretty difficult, I'd imagine you'd have to upsample at least 2x. I did find this when looking up BLEP tho http://www.willpirkle.com/Downloads/AppNote AN-11 PDSynthesis.pdf

Thanks guys for the tips and links I will look into that in order to figure out the best trick to get a cleaner sound with the less CPU usage possible. I've actualy understood why my soustractive/additive bandlimited oscilators had some noise/clicking and it has nothing to do with aliasing but bad signal use in my design that I could fix easily.

Then while running my osc's with antialising/oversampling I did'nt notice an audible difference with or without antialising/oversampling, at least for the soustractive and additive synthesis. For FM synthesis I ran different test and got a good CPU use/antialising solution when oversampling two times. In order to get the best performance possible I could only apply the antialising method when using my FM osc and not applying it to my banlimited oscilators.

I've also tested inscreasing my block values and the result are interesting though I've heard that doing so leads to increase latency and since I want to make a patch meant for live performance it could became an issue if I rely on that to lower my CPU use. Though I might find a solution to the aliasing within use of a low pass filter which could offer a good alternative to the antialising method I used.

@gmoon I've used once pd~ and I don't know if I poorly implemented it or if the object isn't ready yet to deliver an interesting use of multiple cores but when I used it pd~ managed to multiply by four the CPU use of the patch I was working on. From my experience I won't recomend to anyone using pd~ for CPU optimization but maybe there's someone out there that knows how to use it properly and had succesfully devided his sound processing within pd.