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Audio Engine

I know pretty well what I want from a live performance audio engine (largely due to years spent with the magnificent Nord G2X).

I don't know exactly how to get it.

In this repository, I hope to articulate exactly what I want and figure out how I might be able to get it.

Requirements

  • Digital (with deep magic analog can be simulated pretty well but analog can never be made as reliable and reproduceable as digital)
  • Hard real-time (guarantee that loaded patches will run without glitching)
  • Autoscale number/complexity of simultaneous patches based on current hardware
  • CPU architecture agnostic (DSP chips are cool but tie you to hardware that stops getting produced)
  • Open source (so the instruments I design could in theory be played in distant future)
  • MIDI support
  • OSC support
  • Support for defining new controller interfaces (if MIDI/OSC aren't enough)
  • Support for arbitrary # of audio inputs/outputs
  • First-class support for defining new DSP audio processing primitives

...I know there are more but that's a start, which is all I'm looking for right now.

How Not To Build It

It's very possible something out there already meets my perceived needs and I just don't know about it.

I've done a fair amount of searching, but the smart thing would be to refine my requirements into a semi-clear articulation, then ask for it in the relevant communities online - electro-music.com, any StackExchange audio site, and maybe a subreddit or three.

If what I want is out there, someone there can tell me about it.

How To Build It

I have not found anything that meets all these requirements. If it doesn't exist and I can't get more talented folks to build it for me, how could I build it?

Write Raw DSP Patches By Hand

I have an okay-ish grasp of how DSP works.

Writing a few basic components from scratch and getting them to generate audio in concert would be a great way to find out just how much I don't understand.

With the knowledge gained from that experiment, I'd be much better suited to start working on the building blocks I need.

Note that these need not be interactive - just render an audio file using nothing but my own code to generate samples.

...as my first few Python experiments show, I'm not requiring that I rewrite standard library tools, like Python's wave or math libraries - just that I write the actual sample generation myself. I think that's how I'll come to understand the DSP theory behind various sound modules better.

Something like:

  • sine wave generator
  • saw wave generator
  • low-pass filter (tested on the saw wave, presumably)
  • LFO (test by modulating oscillator pitches and/or filter cutoff)
  • program combining them into something vaguely resembling 'music'

Write Interactive DSP Patch By Hand

Easy to explain, if not for me to do - repurpose my test hunks of code into things that work interactively on a desktop computer with MIDI reasonably well.

Again, this means teaching it about MIDI entirely myself. That's painful wheel reinvention, to force me to really understand the building blocks myself.

I'm sure it would be a horribly painful experience, and that I'd learn a ton from it.

Bonus Points: find a cheap OSC controller and teach it to speak OSC, too.

Find Real-Time OS That Meets My Requirements

Somewhere out there, a real-time OS that has decent primitives for abstracting the hardware and handling audio has to exist.

...right?

If not, roll up my sleeves and learn enough fundamentals to add whatever's missing to the best candidate.

Genode sounds interesting, though it's not clear to me that it's a hard-real-time OS (the comments in the HN article where I found it made it sound like it was).

TessellationOS is a research project that's precisely about real-time media and apportioning hard resources to meet QoS guarantees; there are some papers about applying it to audio synthesis. It's not clear to me this is or ever will be OSS, though.

ElkOS is a Linux-based audio OS whose website claims it is for "real time audio performances" in its copy. No idea if that's meant to be a claim of a hard real-time kernel.

This Hacker News thread touches on the subject of how to run an RTOS on high-speed multi-core CPUs. Note specifically the claims from namibj on how to get an RTOS that runs open source software on high-speed multicore CPUs:

You should be able to use seL4 as a hypervisor and stuff a GNU/Linux system inside. The actual low-latency work would be done via native seL4 processes. It's proven to have hard latency bounds, thus being suitable for hard-realtime applications (except for modern x86_64 CPUs having special interrupts that can't be disabled, and thus possess the capability to introduce latency spikes of potentially unbounded duration). The HFT community found ways around those issues, however. It wouldn't be good enough to control a manned aircraft, but for entertainment-related audio, it should easily be good enough (those spikes are around a millisecond or so, iirc).

Port Interactive DSP Patch To Chosen OS

Again, this should teach me all sorts of horrible lessons I don't yet know I don't know.

Port Three Of My G2 Patches To Chosen OS

This, again, would be horribly painful.

But, once I've done it, even if they sound nothing like the original and barely work at all, I will have a bunch of shoddy code that taught me painful lessons I can then apply for the next step:

Hack Out POC For Building Interactive Patches On Chosen OS

This is the "miracle occurs" step.

at the moment I think the essence of this is:

  • DSP primitive units know nothing about hardware
  • Defining new DSP primitives is semi-feasible
  • MIDI (and OSC?) are handled for you
  • Layering, splitting, and inter-patch communication all work
  • Engine can tell you how much load this machine can handle

???

There are ten billion things I'd want to layer on top of this if I got that much working.

For the moment, that's way more planning than I need for a project I have no time to work on.

If I actually managed to get it this far, I might actually consider trying to apply my business plan.

How To Finance It

I can't build this for free, really. No way I'll ever have enough time, even if I retire at 55 and live into my 90s.

A business model could be to build a killer performance instrument around the core audio engine, not unlike the G2 (so freaking playable).

The mistake that killed the G2, IMO, was not having patches stock on the shipping instrument that blew peoples' minds. The stuff that came in later years many regular keyboard players would have died to have, if only they'd known it could be done.

By the same token, regular keyboard players will pay through the nose for an instrument with killer patches. The fact that the software is OSS doesn't matter - they want to buy a working instrument that plays beautifully, not spend hours sourcing and assembling their own hardware.

So, unlike many OSS projects, I think this one could actually be a viable business, supported by hardware sales - if you could build a really amazing instrument(s?), which is a gigantic if.

Prior Art / Useful References

There are an unbelievable number of open source synthesizers in the world.

I have not been able to find any that are hard-real-time and hardware-agnostic (perhaps for very good reasons I am as yet too naive to understand).

Regardless, reading their code could be very educational, and some of it could probably be adapted for use with anything I might wind up writing.

Without further ado, a list:

  • SuperCollider is another audio programming language. It, too, is aimed at desktop general-purpose OSes, so again a port to behave like I want it to could be a massive undertaking. However, someone did write Supernova, which is a version of SuperCollider that supports parallelism, so it may not be as hard as I think (and that paper has tons of useful insight into the problem of writing real-time audio applications without depending on a specific CPU architecture).

  • nova.simd might be a useful study in how to use SIMD to parallelize some audio computations.

  • Wren is a software modular by one of the G2 gods of electro-music.com. Looks a lot like the old G2 editor (not by accident, I imagine).

  • ChucK is a programming language for real-time DSP music work, and very interesting. It's aimed at Mac OS, Windows, and Linux, though. Porting it to a real-time OS and implementing an algorithm to verify loaded patch computability could be a herculean effort (I have no understanding of the language's underpinnings), but that might still be a smarter route than trying to build my own. A quick glance at the code suggests it's written in C++, which I shy away from.

  • CSound is a venerable old sound programming language. I know less about it than the other two, but I believe it's aimed more at composition historically rather than interactive instrument design. Nonetheless, I believe it still can be used for the latter, and is something that would be worth understanding.

  • Here is a collection of open source Eurorack hardware modules. If nothing else they could provide a basis for software variations on the modules. Apparently they've been ported to VCV Rack?

  • Here are some open-source VST / AU plugins. I believe some of them may even work in Audacity.

  • VCV Rack is a virtualized Eurorack DAW. Pretty cool, and yet again OSS.

Useful Educational Resources

I have a few audio DSP books that have helped me learn the basics.

I recently stumbled across this MIT-published volume and thought it seemed relevant:

https://mitpress.mit.edu/books/audio-programming-book

This Hacker News discussion of how to learn audio programming is interesting:

https://news.ycombinator.com/item?id=27273706

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Some design notes on my ideal audio DSP performance engine

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