On 07/04/2023 08:26, Jason Matusiak wrote:
I have been trying to work on a gnuradio block to fill in the dropped
samples in a USRP stream. While testing things out, I came across
something odd. I am using an X310 as my test device and am changing
the sample rate. I was at 100Msps, let it run for some time, and then
changed it to 10Msps; sometimes this works, sometimes I need to change
a few times to get dropped samples. Sometimes, if I look, the time
between the last tag and the tag when I changed the sample rate was
37.4225 seconds (for example). If I count the number of samples
between those two tags, I get 374232036 samples. That is more samples
than should have happened in that time at 100Msps.
I am guessing that tweaking sample rates does something weird to the
sample buffers and probably means that my sample packing approach will
only work with lost samples caused by freq changes or other driver
slowdowns.
Does anyone have any insight to what might be going on here? I know
that it is wasted cycles for most people, but it would be nice if the
USRP/UHD would report the number of lost samples when it retags at the
beginning of good samples.
The command stream is entirely asynchronous to the sample stream. So
when you change sample rates, AFAIR, you don't
know which samples correspond to the sample-rate change.
The underlying UHD API has timestamps on every frame of samples from the
hardware. But the GR interface
only issues a new timestamp tag when something requiring a new tag is
required--like overrun.
I don't think the case of rate-change+overrun was a use case that was
particularly explicitly addressed in the GR interface
into UHD.
Another issue is that if you're changing *master clock* in the middle,
that's not supported on X310, and the master-clock
rate can only be specified reliably when you create the session, not
in the middle of streaming.
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