Raspberry Pi: Brian Eno’s Fat Lady – cheap computation model

June 19, 2014 · Posted in Uncategorized 

Here’s an updated version of Eno’s favourite chubby girl, but rendered with a slimline synthesis model. All in an attempt to get this runnable on a single Raspberry Pi. This uses the non-oversampled synth, so everything is computed at 44.1kHz. It’s using the newer variant of the BPCVO model, in particular the revised bandlimiting code to attack aliasing hard at source, and I’ve redone all the synth presets to remove the filter on everything pitched to eliminate that particular computation burden. There are tricks you can play to bring back a ‘filtery’ sound in the absence of a filter – for example, morphing from sin to saw or sin to square sounds like a filter opening up as harmonics get introduced, and that trick has been exploited to turn the pair of “Brass” into pure 1980s digital filter-free synthesis. But despite the trick (early ramp via EG to morph from sin to saw, giving a harmonic enrichening over the first 100ms or so of the sound) you simply can’t replace a filter with tricks, so the sounds do suffer as a result. But hey, no free lunches here, the goal was to make it burn less CPU and still get a decent sound, so I’m thankful it sounds as good as it does. The “Fat Lady” bass has also gone filterless – with a reduction in its Phase Distortion and a more sin-like wave shape to de-fizz in the absence of actual filtering. Ditto the Warm Jets ‘guitar’, which actually prefers being filterless as it likes to sizzle, ditto Phil’s Wiggly Manzanera which still needs work. The percussive elements remain filtered because they just have to be, they totally rely on the spectral shaping and in most cases the self-oscillating resonance of the filter to generate anything at all apart from ugly noise.

And the result is, after most of the the filter removal and all the new performance tuning, this version runs in exactly 24% of an iPad 2. Only 24% – it may actually run on a Pi in this form. Think about that for just a second – there is a damn good chance that £20 of woefully underpowered educational computer will be able to synthesize this piece in real time, without resorting to a Pocket Orchestra. Bear in mind, ‘this’ is now 14 notes of polyphony, many stereo delays (seven? eight??), a global reverb and 10 separate synths. And I repeat, no samples – every single waveform is computed, constructed by segments of quadratics and cubics, and the waves are being wiggled by the mod matrix, by the 4 LFOs and 4 EGs. My God it’s full of code.

Audio was captured from the iPad and not processed in any way, these are exactly the bytes that I computed. Cool, eh? Plus, big bonus, you get the Eerie Noise – which is actually a combination of a dedicated ‘Eerie Noise’ synth and the pair of brasses, turned way down and pitch bent to buggery. Yay!

Next step – get this codebase rebuilt on a Pi and actually run it.


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