[USRP-users] A few questions about RFNoC streaming

Rob Kossler rkossler at nd.edu
Wed Mar 20 21:49:21 EDT 2019

Yes, the example was for illustration only.  You can throw a couple of DDCs
in between the radio and add/sub block to slow the rate down.  But, the
questions are still the same.

On Wed, Mar 20, 2019 at 7:49 PM Nick Foster <bistromath at gmail.com> wrote:

> First things first. The flow graph you're describing don't work because
> the two radio blocks will saturate the bus going into the addsub block. You
> will need to decimate the streams going into the addsub block.
> I don't have a ready answer to your question about the streamers, but I'd
> suggest using timed commands to align the two radio streams, if UHD isn't
> smart enough to recognize the two radios and propagate the stream command
> accordingly.
> Nick
> On Thu, Mar 21, 2019, 6:46 AM Rob Kossler via USRP-users <
> usrp-users at lists.ettus.com> wrote:
>> Hi,
>> I am starting to develop more complicated RFNoC graphs and several
>> questions occurred to me. I am using my own C++ application with the UHD
>> RFNoC-enabled library.
>> Consider a receive-only RFNoC graph with 2 radio blocks feeding a
>> 2-input, 2-output Add/Sub block.  Also, assume there are two rx_streamers
>> connected to the 2 output ports of the Add/Sub block.  Note that these
>> streams are no longer independent or one-to-one matched with the radio
>> channels.
>> 1) How does an "issue_stream_cmd()" to one of the Add/Sub block ports
>> propagate back to the radio block?  Actually, it would need to propagate
>> back to both Radio blocks no matter which rx_streamer I used since they are
>> no longer independent streams.  Does this make sense?
>> 2) What happens if I only call "issue_stream_cmd()" for one of the
>> rx_streamers instead of both? Perhaps since the other streamer isn't
>> running, it backpressures the streaming such that it eventually quits and
>> thus quits for the other port as well?
>> 3) Do I have to do anything in the Add/Sub block to sync up the streams
>> or can I rely on the first sample from Radio 0 being time-aligned with the
>> first sample from Radio 1 (assuming I issued timed start commands)?
>> Rob
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