[USRP-users] A few questions about RFNoC streaming

Nick Foster bistromath at gmail.com
Wed Mar 20 19:48:50 EDT 2019


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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