[iiwusynth-devel] Latency measurement results

[M. Nentwig]

Hello again,

as I mentioned before, I'm really interested in getting the latency down
as much as possible (and keep it as constant as possible), because I'm
keen on using iiwusynth 'realtime' in a band.
Therefore I went to the lab and did a couple of measurements.
What I have measured is the time between the audio output of my MIDI
controller and the audio output of iiwusynth - that is the latency. I
repeated this for different fragment settings: for 16 fragments (the
only setting that works without tricks on my own PC) and 3 fragments
(soundcard set to 16 fragments, and a loop delays the synth until there
are 13 or more buffers empty).
The problem with 16 frags is, that the latency varies a lot.
For 3 fragments, it is stable and short (8 ms of latency is equivalent
with the monitor speaker being two-and-a-half meters away, that's
tolerable).

A typical printout of the oscilloscope:
http://millisampler.sourceforge.net/3frag.gif (see also 1frag.gif and
2frag.gif)

All results summed up in a plot:
http://millisampler.sourceforge.net/plot.gif

It is obvious, that using 16 fragments without additional 'tricks'
results in quite sloppy timing. But I have a feeling, that the
performance with a proper '3-fragment-capable' sound card can compete
with any commercial product. For example, in the keyboard review of
Native instrument's latest B4 softsynth, 'Keyboards' magazine wrote,
that the latency is 'tolerable'.
Now concerning iiwusynth, 8 +/- 2 ms is better than 'tolerable', if you
ask me :-)

Regards

Markus