Six Sines is a small synth based on audio rate modulation (techniques often called "Phase Modulation" and "Digitial Ring Modulation"). It has an architecture which allows lots of modulation and a synth engine with a couple of tricks which make it sound pretty good.
You can download the latest release or Nightly version of Six Sines here. Six Sines is free and open source software. You can get, study, modify, and re-use the source for it here. And please read the acknowldgements for a list of thanks.
And following in the legacy of other PM and FM synths, it is not exactly easy to program. The sort of 'symmetric/maximal' signal path design approach exacerbates this problem, as does the relatively thin manual you are reading right now. But don't fear! You can have fun with the synth anyway.
Its a 6 operator FM synth, more or less.
Each of the 6 operators can output to audio or can modulate themselves (feedback) or subsequent operators. So operator 3 can feedback its own signal or modulate operators 4, 5, and 6.
Modulation of subsequent operators can be phase modulation (the 'phi' symbol) or digital ring/audio-rate-amplitude modulation (the 'A' symbol).
But the real trick of sound design with the thing is the modulation architecture. Each knob on the screen represents a node with an independent envelope and LFO per voice. So each operator, operator modulation application, operator feedback, and operator level can be indpendently and individually modulated.
Woof. That's work. But its fun!
The Six Sines interface has a patch selector and main menu with a VU meter at the top, then presents sections for each of the core functions. The operators span the top with the knob for each being their ratio, The modulation matrix is below, the mixer to the side, and then the main output above. The macro section participates in modulation.
Each knob has a 'power' button which means the node doesnt run in a voice in any capacity, and subsequently uses no CPU.
Here we show the signal path for operator 3. Yellow lines show audio signals presented for auido output, and green lines show audio signals which are used for modulation.
It starts at the "Ratio" node which sets the frequency of the operator. Then the audio signal travels to the matrix where it first intersects the op3 feedback node and self modulates. The resulting audio travels to the mixer where it is leveled by an independent op3 audio node, and then is mixed into the main output.
The same signal travels further into the modulation matrix providing modulation for ops 4, 5, and 6.
Once you click a knob to select it or edit it the bottom part of the user interface shows the modulation screen for that node. Node 'classes' (ratio, cross matrix, feedback, mixer, main) have subtly different modulation screens but the core idea is an envelope, an LFO, application depth controls, and then performance modulation areas.
The performance modulation area allows you to map MIDI and Macros to the modulation and level beyond the internal modualtors.
The synth has two voicing modes, a Polyphonic and a Monophonic mode. In the polyhonic mode, it additionally has a per-key piano mode. These modes control how and when voices are created.
In polyphonic mode with piano mode off, a voice is created on every key press.
In polyphonic mode with piano mode on, a voice is created on a key press unless there is already a voice sounding on that key, in which case that voice is retriggered.
In monophonic mode, a voice is created only when no voice is playing. If a subsequent key press happens, the voice is moved and retriggered.
Retriggering retriggers envelopes and each enveope can trigger on one of four ways.
- On voice start only. If you use this it can sound like a stuck note if you aren't careful. Its not.
- On voice start or on a voice being re-keyed when not keyed (or "on gate changed" in modular speak)
- On any key press
- On release. In on-release mode, the envelope is gated if the voice is ungated, so resuming a gate gesture on a note will release the OnRelease envelope.
The default for an envelope is 'Patch Default' which is set in the settings screen, but you can override.
Press the COG icon at the top of the UI to get the settings screen, where you can control MPE, Mono mode, pitch bend depth, and more.
The setting screen contains an oversampling control. The Six Sines oversampling strategy has the engine run at a fixed rate mostly independent of the host sample rate which is stored in the patch and you control.
Mostly, because our resampler (a short FIR interpolator) works way better at round multiple downsampling. 2.5x is a lot better than 2.61x or such. So our oversample levels are fixed offsets from either 44.1 or 48khz. If you choose, say, "132.3/144khz" as your engine sample rate, if your host sample rate is a multiple of 44.1 we will choose the lower, and of 48 the higher.
You may need to adjust oversampling in some high feedback cases. It of course burns cpu as it goes up and for most patches the default 2.5x is just fine.
Six Sines supports screen readers and accessible gestures, making the UI and programming model as inscrutable to these assistive technologies as it is to users with a visual display. Since the UI is quite big there's a few extra features for screen reader navigation.
First, standard edit gestures should work on all controls, and I tried really hard to make sure tab order makes sense and labels are reasonable. If you find one which is wrong, please just drop a note on discord or github.
The structure of the UI is that knbos (like "Op3 feedback level") a panel in the bottom 1/4 of the screen to edit the modulators and stuff. This panel arrives when you mouse click or edit the knob. A few features make this easier to navigate for a screen reader.
If on a knob in the top third, `Command-A' will arm that knob (namely select the knob modulation panel in the area below)
If on a knob in the top third, Command-J will jump to the control panel in the
bottom
And finally from anywhere in the UI, Command-N will expose a menu allowing
you to focus any of the focusable top-section knobs or preset manager.
Good luck! Its fun. But tricky. If you want to add to this manual please do send a PR over.