[sdiy] In praise of the ATM STM32F303

[Loscha]

The manual is here:
Around page 21 is the MIDI spec, which tells you all the controls it has.
Quite impressive given the chip

http://waldorf-music.info/downloads/Streichfett/Streichfett%20Manual%20EN.pdf

Warning: "Do not leave your Streichfett near children, mothers-in-law or
pets. This could lead to critical interactions"



On Fri, Feb 12, 2016 at 8:11 PM, Olivier Gillet <ol.gillet at gmail.com> wrote:

> So many of these string machines have a terrible noise floor! I'm not
> surprised that a 12-bit DAC is good enough, and could still be
> perceived as an improvement in terms of stability/noise compared to
> the original machines.
>
> The most impressive thing for me is how they did with so little
> computational power - given that Waldorf claims this thing has full
> 128-note polyphony!
>
> I wonder how it's done. Here is my theory...
>
> * a big bank of 128 sawtooth oscillators from 8 Hz to 12kHz (all MIDI
> notes).
> * an array of 128 amplitudes for each of these.
>
> The amplitudes array depends on the currently played notes and the
> octaves and registration knob. The CPU load is constant - the
> oscillators are always rendered even if they have an amplitude of 0.
>
> Assuming the MCU is clocked at 72 MHz, the audio at 32kHz, 5% of the
> computational budget is for the filter/FXs and another 5% for
> bookkeeping (MIDI decoding, envelopes, modulations, but most
> importantly keeping track of the 128 amplitudes), this leaves about 16
> CPU cycles per oscillator, including mixing.
>
> That's quite low for a band-limited sawtooth oscillator!
>
> But maybe there's a way of coding a polyBlep oscillator (the simplest
> band-limiting scheme I can think of) that makes clever use of the fact
> that the frequency is fixed, and known in advance. Firstly, we don't
> have to do the costly division by frequency when evaluating the
> polyblep function - we can multiply by its reciprocal which would be
> precomputed. Secondly, after a phase reset has occurred, we will know
> for sure that the next fs/f0 samples will be identical to a naive
> sawtooth and won't have any discontinuity, so we can jump to a branch
> of code that is specialized for rendering long chunks of samples
> without any phase counter overflow check, and no reset handling. This
> trick would bring a huge computational gain for the lowest octaves: if
> the phase counter is currently at 0.2, the period is 200 samples
> (phase increment of 0.005), and we have 64 samples to render, that's
> 64 samples that can be rendered with naive code, simple enough for an
> unrolled loop. Load a few registers, and then go for a long sequence
> of two instructions: 1/ increment phase counter, 2/ multiply by
> amplitude and store in audio buffer. Now that's fast!
>
> This trick wouldn't allow vibrato/pitch bend. But assuming that the
> vibrato rate and amount are sufficiently low - only small wobbles in
> pitch - vibrato/bend could be done simply at a global level by messing
> with the DAC frequency (easy to do with a built-in DAC, less so if you
> use an external audio DAC - another argument for keeping it simple and
> do everything with the MCU peripherals).
>
> Olivier
>
> On Fri, Feb 12, 2016 at 7:43 AM, Loscha <loscha at gmail.com> wrote:
> > Markus Fuller just uploaded another of his great series of teardown
> videos.
> >
> > Today's victim was the Waldorf Streichfett
> > https://www.youtube.com/watch?v=umXnCuVBwf0
> >
> > The entire synthesizer string machine is a single ARM STM32F303 chip.
> > There is a power supply chip, an op amp for the audio outs, crystal, but
> the
> > only computational device is the ARM
> >
> > http://www.st.com/web/en/catalog/mmc/FM141/SC1169/SS1576/LN1531
> >
> > Notable for Audio considerations is that the DACs are 12 bit. Given the
> > amount of beloved instruments that use 12 bit DACs, and the quality of
> this
> > machine, it hardly seems a limitation.
> >
> > /Edward
> >
> > _______________________________________________
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> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> >
>
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