Re: [fluid-dev] Akai EWI-USB, Raspberry-Pi, and FluidSynth

[Marcus Weseloh]

Hi Ben,

2015-11-14 21:19 GMT+01:00 Ben Gonzales <address@hidden>:
> 1. How does one measure latency for a wind controller?

An easier and more accurate way to measure the overall system latency
could be to measure it while changing from one note to another:

Use a sound card with two channel (i.e. stereo) input. One channel you
attach directly to the ouput of your RPi system, the other channel you
attach to a microphone. The microphone you place very close to the
fingers on your wind controller. The idea is that you tap very hard
when pressing a key on your controller, this tapping should be picked
up by the microphone. If you record the stereo signal and then measure
the time between the tap and the note changing, you get a fairly good
idea of the latency in your system. It might be good to use an
interval with very different harmonics, that way you can easily see
the change in frequency (in Audacity you could use the "Sectrogram"
view).

If you want to measure system internal latencies, for example the ALSA
latency introduced by your buffer and period size, you could use the
"latency" tool from the alsa-utils.

> 2. Does anyone have a HOWTO for a low latency implementation on a R-Pi?

For more information about low latency audio on the RPi, maybe this
page will help: http://wiki.linuxaudio.org/wiki/raspberrypi

If you set up a Preempt-RT kernel, then tuning the IRQ priorities
takes some time and experimentation. This page helped me a lot (from
archive.org, as the main site is currently down):
https://web.archive.org/web/20150223092644/http://subversion.ffado.org/wiki/IrqPriorities

Best regards,

   Marcus