Sound card input & output synchronization ?

[address@hidden]

I am contemplating using a sound card as audio-frequency
lock-in analyzer.
To do so, I generate a signal in phase and in quadrature. The 
in-phase signal will go out the D/A audio-out (for some reason
not developed here the opposite signal will go out another channel),
and I record on the A/D audio-in the output of my device under test.

For lock-in detection, I initially contemplated sending the quadrature
output signal (red arrow) to one input of the mixer: the stream becomes 
discontinuous, probably due to unknown latencies between audio out and
audio in.

So we decided to record on one channel the output of the quadrature signal,
and on the other channel the output of the device under test. Mixing these
two channels seems to work this time, but notice how the reference signal (blue,
top) is discontinuous (vertical green lines to guide the eye).

I expect input and output buffers of the sound card to be clocked by the
same oscillator, so since samp_rate is the same at input and output, what 
could be the cause of such discontinuities ? Is there an option in the sound
card configuration/buffer size to make sure such discontinuities do not
appear ? Looking at the oscilloscope at the D/A outputs, they are nicely 
continuous
when activating the Blocing option so the issue is with reading, the output
stream looks good.

Thanks, Jean-Michel

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