[USRP-users] A few questions about RFNoC streaming

Rob Kossler rkossler at nd.edu
Wed Mar 20 15:45:21 EDT 2019

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)?

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