Inconsistent/variable delay between USB and onboard alsasrc

[Michiel Konstapel]

I am recording audio from two alsasrc elements: an analog line input 
from the onboard sound card, and a USB audio device (USB microphone). I 
need the recordings to be synchronized, so playing them back together 
doesn't produce a nasty "tiled bathroom" sound. I expected the different 
sources to have different internal delays, which I can adjust for with a 
probe that adjusts the buffer timestamp.

However, there seems to be no consistency in the time difference: in one 
run, the analog input is 3 ms behind the USB audio, the next time it's 
44 ms, then it's 20 ms... I measured this by placing the microphones 
side by side, producing a clap or tick and comparing the tracks in Audacity.

I've tried
- always choosing the onboard card as the clock source
- choosing neither and using the system clock
- all the options for alsasrc slave-method except "none"

How should I set things up so the sources are reliably synchronized?

Kind regards,
Michiel