Well, some questions first:
Is it a continuous signal, changing only the frequency or is a signal with
start and end, maybe different lengths also (bursts)?
Does the midi have to be 'realtime'?
If the midi has to reflect the length if the signal, you will always lag
behind, because you would need to wait till the end of the audio burst to
determine the length...

Richard




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> A couple of friends of mine have built a theremin-like device.
> The circuit outputs a simple square signal, wich is then connected to a pc
> audio input for processing.
> They want me to convert that to MIDI and i have no clue where to start.

A Theremin outputs a continuous tone whose pitch can change over a wide
range. This is difficult to model in MIDI.

I'm assuming you can use some sort of audio-to-pitch conversion to figure
out the current pitch -- that may be the easy part. The hard part is
describing the pitch in MIDI terms.

If you output a single note (a note-on message) and then use pitch-bend data
to adjust its pitch up or down as needed, you'll certainly need to use all
14 bits of the pitch-bend message. And you'll need to know in advance the
maximum amount of pitch deviation that the performer will employ -- perhaps
two octaves up or down from some central starting point. 14 bits may or may
not be enough precision over a 4-octave range; I'll let someone else do that
calculation.

The issue is, you'll also need a synth to receive the MIDI data that (a) can
make use of all 14 bits of the pitch-bend messages, and (b) allows
pitch-bend range to be set to 2 octaves up or down. If the receiving
instrument lacks either of these features, your effort in creating the data
transforming software will be wasted.

--JA



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Hi,
Perhaps relevant, I did an audio to midi instrument in Csound recently
(attached).
It is intended to translate speech to midi notes, so it uses spectral
analysis and each spectral component gets it's associated midi note.
For your purposes perhaps a simpler pitch to midi converter would work
better, the plltrack opcode should do a nice job with a Theremin audio
signal. In that case the attached csd could be simplified a great
deal.
But, since you mention doing custom processing of the audio signal,
perhaps you should just pitch track it and synthesize something nice
directly in Csound? plltrack (or ptrack) should work nicely, and you
can use rms or follow2 for the amplitude envelope.

best
Oeyvind

Maybe it is worth considering the OSC protocol if that is an option. That
might be more easy and flexibale than midi in this case...

Richard



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Thank you, Oeyvind for this very nice and so well commented example!


tarmo

On Friday 20 September 2013 00:46:18 Oeyvind Brandtsegg wrote:
> Hi,
> Perhaps relevant, I did an audio to midi instrument in Csound recently
> (attached).
> It is intended to translate speech to midi notes, so it uses spectral
> analysis and each spectral component gets it's associated midi note.
> For your purposes perhaps a simpler pitch to midi converter would work
> better, the plltrack opcode should do a nice job with a Theremin audio
> signal. In that case the attached csd could be simplified a great
> deal.
> But, since you mention doing custom processing of the audio signal,
> perhaps you should just pitch track it and synthesize something nice
> directly in Csound? plltrack (or ptrack) should work nicely, and you
> can use rms or follow2 for the amplitude envelope.
> 
> best
> Oeyvind
> 
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