[Hackrf-dev] sources of audio underrun?

[Kevin Reid]

On Thu, Dec 22, 2016 at 1:24 AM, Chuck McManis <chuck.mcmanis at gmail.com>
wrote:

> Hi all,
>
> I'm building a more sophisticated FM receiver in grc and I find I'm
> getting regular audio underruns (aUaUaU ... ) they come in bursts. My CPU
> utilization is like 33 - 40% for 3 of the 4 cores, maybe 70% for the fourth
> one. What techniques can I use to avoid audio underruns?
>

Underrun or overrun is inevitable, because there are two different hardware
clocks involved with therefore different definitions of the passage of
time: the HackRF's sampling clock and the audio device's sampling clock.
(There was recently some discussion on the GNU Radio mailing list about
developing a feature to handle this clock skew by adaptively resampling the
audio, but that project has not even been started.)

If CPU utilization were the problem, then you would also be seeing "O"s
from the HackRF driver not being able to offload its samples into the
flowgraph fast enough. I think.

If you changed your demodulator design and are now getting more
underrun/overrun than you used to, then you should look at your
demodulator's sample rate conversions and check if you are doing something
imprecise (e.g. rounding a non-integer ratio to an integer one).
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