[Stk] MIDI event timing relative to audio stream

[Arthur Benilov]

Hi Tim,
You can timestamp the MIDI events relative to the next audio frame using
the system steady clock. The very first MIDI event received will have delta
time zero, all following events will have delta time measured relative to
the first event. By relating these to the system clock and the system time
recorded at the audio callback you can estimate for a particular MIDI event
its time within the next audio frame. Also you don't need to be
sample-perfect, all commercial software synths I know will discretize the
MIDI events processing within an audio frame to 16 or 32
samples @44.1-48kHz. You may also let the jitter be and just decrease the
audio frame size to minimize it (Raspberry Pi will be able to do 256
samples per frame @44.1kHz with its onboard audio), it may not be that
critical for an instrument you are developing.

Cheers,

Arthur.


On Tue, Apr 12, 2022 at 10:06 PM Tim Grant <tim at rootdown.net> wrote:

> Hello!
>
> I've recently started prototyping a synthesizer project using RtAudio and
> RtMidi. I'm doing the initial development in Windows and I plan to
> eventually port the project to something like a Raspberry Pi if the
> prototype goes well.
>
> I have the audio output and MIDI input working, and I'm starting to think
> about triggering the synth voices with MIDI notes. I would like to keep the
> jitter between the notes and the triggered sounds low, but I'm not quite
> sure how to achieve this with these two APIs.
>
> A diagram might be helpful here:
> MIDI      -----N----------------------N-------------------------->
> Input          |                      |
>                |                      |
> Audio     |----*-----|----P-----|-----*----|-----P----|---------->
> Stream       Frame 1    Frame 2    Frame 3    Frame 4    Frame 5
>
> My thinking is that I need to know at what point in the current audio
> frame an MIDI note arrives. From the diagram, assume a note (N) arrives at
> the moment corresponding to the 5th sample in Frame 1; knowing this, I can
> begin playback at the 5th sample on the next frame (P). This way, the
> latency is held constant at 1 frame and the amount of jitter is minimized.
>
> So with that long-winded introduction out of the way, my question is:
>
> Is it possible to recover the timing of MIDI events relative to the audio
> stream? I can't see how to do this using the delta time provided with the
> MIDI note and the stream time provided by RtAudio, but my hope is that I'm
> missing something.
>
> Thanks!
>
> Tim
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