[alsa-devel] Variable Delay through ALSA driver

[Pierre-Louis Bossart]

On 9/17/13 3:07 PM, Brandon Yates wrote:
> I have an application using the ALSA API that writes to playback device
> and simultaneously reads from capture device.  My processing loop is :
>
> 1)Read from microphone (blocking)
> 2)Process Data
> 3)Write to speaker
>
> I am measuring the delay from the time I play a series of frames to the
> speaker until I detect the echo of this coming back in through the
> microphone.  This delay seems to be changing every time I initialize the
> devices with ALSA. I have measured in the range of 60-160ms delay.  I
> know things like period size effect delay, but using the exact same
> software and hardware configuration I would expect this to be constant
> across runs.  What are some parameters that can help me make the delay
> deterministic?

snd_pcm_link can help make sure your playback and capture devices start 
at the same time, if supported in hardware (HDAudio works well). 
Otherwise you need a cross-correlation to find out the delay and 
compensate for it in software.
-Pierre