Sointu

A cross-platform modular software synthesizer for small intros, evolved from 4klang.

Summary

Sointu is work-in-progress. It is an evolution of 4klang, a modular software synthesizer intended to easily produce music for 4k intros-small executables with a maximum filesize of 4096 bytes containing realtime audio and visuals. Like 4klang, the sound is produced by a virtual machine that executes small bytecode to produce the audio; however, by now the internal virtual machine has been heavily rewritten and extended to make the code more maintainable, possibly even saving some bytes in the process.

Implemented features

• Per instrument polyphonism. An instrument has the possibility to have any number of voices, meaning in practice that multiple instruments can reuse the same opcodes. Done, see here for an example and here for the implementation. The maximum total number of voices will be 32: you can have 32 monophonic instruments or any combination of polyphonic instruments adding up to 32.
• Any number of voices per track. For example, a polyphonic instrument of 3 voices can be triggered by 3 parallel tracks, to produce chords. But one track can also trigger 3 voices, for example when using arpeggio. A track can even trigger 2 voices of different instruments, alternating between these two; maybe useful for example as an easy way to alternate between an open and a closed hihat.
• Easily extensible. Instead of %ifdef hell, the primary extension mechanism will be through new opcodes for the virtual machine. Only the opcodes actually used in a song are compiled into the virtual machine. The goal is to try to write the code so that if two similar opcodes are used, the common code in both is reused by moving it to a function.
• Take the macro languge to its logical conclusion. Only the patch definition should be needed; all the %define USE_SOMETHING will be defined automatically by the macros. Furthermore, only the opcodes needed are compiled into the program. Done, see for example this test! This has the nice implication that, in future, there will be no need for binary format to save patches: the .asm is easy enough to be imported into / exported from the GUI. Being a text format, the .asm based patch definitions play nicely with source control.
• Harmonized support for stereo signals. Every opcode supports a stereo variant: the stereo bit is hidden in the least significant bit of the command stream and passed in carry to the opcode. This has several nice advantages: 1) the opcodes that don't need any parameters do not need an entire byte in the value stream to define whether it is stereo; 2) stereo variants of opcodes can be implemented rather efficiently; in some cases, the extra cost of stereo variant (e.g. most filters) is only 6 bytes. 3) Since stereo opcodes usually follow stereo opcodes, and mono opcodes follow mono opcodes, the stereo bits of the command bytes will be highly correlated and if crinkler or any other compressor is doing its job, that should make them highly predictable i.e. highly compressably. Mostly done.
• Test-driven development. Given that 4klang was already a mature project, the first thing actually implemented was a set of regression tests to avoid breaking everything beyond any hope of repair. Mostly done, using CTests. Tests for new opcodes / opcode variants implemented since 4klang are not done.

Future goals

• Cross-platform support for win / mac / linux. The build is already based on CMake and compiles on Windows. Cross-platform YASM macros have been drafted and remain to be tested. Once the project is more mature, I will try compiling on other platforms.
• New opcodes. At least: bit-crush, compressor (with side-chaining), change bpm. Maybe also equalizer.
• Support for 64-bit targets.
• Browser-based GUI and MIDI instrument. Modern browsers support WebMIDI, WebAudio and, most importantly, they are cross-platform and come installed on pretty much any computer. The only thing needed is to be able to communicate with the platform specific synth; for this, the best option seems to be to run the synth inside a tiny websocket server that receives messages from browser and streams the audio to the browser. The feasibility of the approach is proven (localhost websocket calls have 1 ms range of latency), but nothing more is done yet.

Nice-to-have ideas

• Sample import from gm.dls. This is Windows only, but implementing it should be easy and the potential payoffs pretty high for Windows users, so it is a nice prospect.
• Tracker. If the list of primary goals is ever exhausted, a browser-based tracker would be nice to take advantage of all the features.

Anti-goals

• Ability to run Sointu as a DAW plugin (VSTi, AU, LADSPA and DSSI...). None of these plugin technologies are cross-platform and they are full of proprietary technologies. In particular, since Sointu was initiated after Steinberg ceased to give out VSTi2 licenses, there is currently no legal or easy way to compile it as a VSTi2 plugin. I downloaded the VSTi3 API and, nope, sorry, I don't want to spend my time on it. And Renoise supports only VSTi2... There is JUCE, but it is again a mammoth and requires apparently pretty deep integration in build system in the form of Projucer. If someone comes up with a light-weight way and easily maintainable way to make the project into DAW plugin, I may reconsider.

Design philosophy

• Try to avoid %ifdef hell as much as possible. If needed, try to include all code toggled by a define in one block.
• Instead of prematurely adding %ifdef toggles to optimize away unused features, start with the most advanced featureset and see if you can implement it in a generalized way. For example, all the modulations are now added into the values when they are converted from integers, in a standardized way. This got rid of most of the %ifdefs in 4klang. Also, with no %ifdefs cluttering the view, many opportunities to shave away instructions became apparent. Also, by making the most advanced synth cheaply available to the scene, we promote better music in future 4ks :)
• Size first, speed second. Speed will only considered, if the situation becomes untolerable.
• Benchmark optimizations. Compression results are sometimes slightly nonintuitive so alternative implementations should always be benchmarked e.g. by compiling and linking a real-world song with Leviathan and observing how the optimizations affect the byte size.

Background and history

4klang development was started in 2007 by Dominik Ries (gopher) and Paul Kraus (pOWL) of Alcatraz. The write-up will still be helpful for anyone looking to understand how 4klang and Sointu use the FPU stack to manipulate the signals. Since then, 4klang has been used in countless of scene productions and people use it even today.

However, 4klang is pretty deep in the %ifdef hell, and the polyphonism was never implemented in a very well engineered way (you can have exactly 2 voices per instrument if you enable it). Also, reading through the code, I spotted several avenues to squeeze away more bytes. These observations triggered project Sointu. That, and I just wanted to learn x86 assembly, and needed a real-world project to work on.

Credits

The original 4klang was developed by Dominik Ries (gopher) and Paul Kraus (pOWL) of Alcatraz.

Sointu was initiated by Veikko Sariola (pestis/bC!).

PoroCYon's 4klang fork inspired the macros to better support cross-platform asm.