Max Schmidt wrote:
I've got a BeagleBone Black (ARM Cortex A 8 µP) with an Audio-Cape (using McAsp and ALSA-Davinci-Drivers) and wrote an mmap based playback-capture application (both devices "hw:0,0"). The important thing is that the application needs a constant delay (not necessarily small) between the start of the playback and the capture stream. To realize that I use the ALSA API function "snd_pcm_link(c_handle, p_handle)".
- Looking at a plot of multiple measurements of looped back and
captured square waves (or sines) there still is a jitter of about two to four samples (at 48 kHz; which is about 40 to 80 ms), no matter if I run it as RT-app (energy save modes etc. disabled, just important Processes have a higher Priority e.g. EDMA) or normal app. Please correct me if I'm wrong, but as far as I understand when the streams are linked, at start the processor is going through a linked list triggering all linked streams. And the trigger start is an atomic process so shouldn't get interrupted.
Yes.
Shouldn't it always take the same time between starting the playback and capture stream then? And if yes, where could the variable start delay come from?
Hardware accesses can be quite nondeterministic. And if the driver is not written properly, the start trigger will do all the DMA stream initialization that should have been done when preparing the device. (mcasp_start_tx() has a busy loop waiting for the device.)
There is hardware that actually supports starting two streams at exactly the same time (by setting two bits in the same register). Obviously, this hardware or its driver doesn't support this.
Regards, Clemens