Re: [Discuss-gnuradio] How to receive a finite number of samples at a future time using UHD/GNURadio?

Hi Eric,

this mail kind of fell through the cracks. Have you found a solution to this, yet? 

It's not overly clear from the documentation, but the finite_acquisition method really bypasses parts of what you configure as behaviour for the USRP_source. So, ok, that won't work.

The second one might be slightly off, syntaxwise. You'll need to create a stream_cmd_t (of the right type), not use one of the type constant fields of stream_cmd_t (just as you do in the next approach).

The third might work, but really breaks GNU Radio idea of processing streams of samples; so, I'm not 100% sure what is appropriate for you :(

Best regards,

Marcus

On 04/24/2017 09:30 PM, Eric Martin wrote:

I'm using the GNURadio python interface to UHD, and I'm trying to set a specific time to start collecting samples and either collect a specific number of samples or stop the collection of samples at a specific time. Essentially, creating a timed snapshot of samples. This is something similar to the C++ Ettus UHD example 'rx_timed_sample'.

I can get a flowgraph to start at a specific time, but I can't seem to get it to stop at a specific time (at least without causing overflows). I've also tried doing a finite aquisition, which works, but I can't get it to start at a specific time. So I'm kind of lost at what to do next.

Here is my try at the finite acquisition (seems to just ignore the start time and collects 0 samples):

num_samples = 1000

usrp = uhd.usrp_source(

    ",".join(("", "")),

    uhd.stream_args(

        cpu_format="fc32",

        channels=range(1),

    ),

)

...

usrp.set_start_time(absolute_start_time)

samples = usrp.finite_acquisition(num_samples)

I've also tried some combinations of following without success (TypeError: in method 'usrp_source_sptr_issue_stream_cmd', argument 2 of type '::uhd::stream_cmd_t const &'):

usrp.set_command_time(absolute_start_time)

usrp.issue_stream_cmd(uhd.stream_cmd.STREAM_MODE_NUM_SAMPS_AND_DONE)

I also tried the following in a flowgraph:

...

usrp = flowgrah.uhd_usrp_source_0

absolute_start_time = uhd.uhd_swig.time_spec_t(start_time)

usrp.set_start_time(absolute_start_time)

flowgrah.start()

stop_cmd = uhd.stream_cmd(uhd.stream_cmd.STREAM_MODE_STOP_CONTINUOUS)

absolute_stop_time = absolute_start_time + uhd.uhd_swig.time_spec_t(collection_time)

usrp.set_command_time(absolute_stop_time)

usrp.issue_stream_cmd(stop_cmd)

For whatever reason the flowgraph one generated overflows consistently for anything greater than a .02s collection time.

I posted this question on stack overflow without any success:

http://stackoverflow.com/questions/43420300/how-to-receive-a-finite-number-of-samples-at-a-future-time-using-uhd-gnuradio

Any help would be greatly appreciated!!

Thanks.

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