[bdbarnett]

Two nodes for synthesizers built from audio-rate parts.

audiooscillator.Oscillator is the oscillator synthio doesn't have: saw, pulse, triangle and sine from one phase, with pitch, linear FM, pulse width and hard sync read from audio streams every sample.

osc = audiooscillator.Oscillator(frequency=220.0, waveform=audiooscillator.SAW, fm_depth=300.0)
osc.modulate(sync=audiooscillator.Oscillator(frequency=110.0, waveform=audiooscillator.PULSE), fm=lfo)
audio_out.play(osc)

waveform is any sum of SAW, PULSE, TRIANGLE and SINE, and a two-channel node puts waveform_right on the right channel from the same phase. Steps are band-limited with PolyBLEP and the triangle's corners with PolyBLAMP. A sync edge is placed between the two samples it fell between and band-limited like any other step, which costs one sample of latency. It builds for MicroPython, the CPython extension and CircuitPython (circuitpython.mk), and takes buffer_size like the effects.

audioladder.Ladder(buffer_size=...) sets the block size in bytes, as the CircuitPython effects do, from one frame up to the default 256. A host running 64-frame blocks gets the filter with no latency and a cutoff that can move every 64 frames. The filter is per sample, so the audio doesn't change: ladder_probe.py renders byte-identical to main on MicroPython and CPython, and the new ladder_frames_probe.py shows 64-, 1- and 200-frame renders equal to the default one.

Evidence

• Aliasing, a 4567 Hz note at 48 kHz, energy outside the harmonics below 10 kHz relative to the harmonics: saw -50.7 dB (naive -13.5), pulse -50.6 dB (naive -15.2), triangle -69.5 dB (naive -36.3). An additive band-limited saw reads -93.6 dB, which is the int16 floor. The tests hold saw and pulse to -45 dB and the triangle to -60 dB, and check that the naive versions fail those bars.
• Sync: a saw synced to a pulse Oscillator matches the analytic synced saw to within 0.003 away from its edges (the same comparison without sync is off by more than 0.1).
• FM, PWM and pitch are read every sample: a saw's per-sample slope tracks a 750 Hz FM input to 2e-4, and duty cycle and period follow a step that lands mid-block.
• Without its inputs, the oscillator matches a Python PolyBLEP saw and pulse sample for sample, to float precision.
• Planted faults, each restored afterwards: sync detection disabled (sync tests fail), the BLEP kernel made a no-op (band-limiting and sync tests fail), buffer_size ignored in the CPython Ladder (block-size tests fail) and in the MicroPython binding (ladder_frames_probe.py reports all six renders unequal).
• verify_dsp.py: 58 comparisons, 0 failures, CPython against a MicroPython unix build of this branch. The deinit-surface and host-reset probes cover the new node on both.