[USRP-users] A few questions about RFNoC streaming

Rob Kossler rkossler at nd.edu
Thu Mar 28 14:40:59 EDT 2019


Thanks EJ and Nick,
I hadn't thought about the propagation of the streaming command occurring
in UHD rather than on the FPGA.  I was thinking that after issuing the
rx_streaming start command to the downstream-most block in the chain, some
magic was happening on the FPGA to get the upstream blocks started.
However, I think this makes more sense that there is some magic happening
in UHD such that the upstream blocks get the command to start streaming.  I
will let you know if I figure anything out.
Rob

On Mon, Mar 25, 2019 at 12:37 PM Nick Foster <bistromath at gmail.com> wrote:

> I can tell you the answer to #3 off the top of my head: the two streams
> will be sample-aligned, and if you use timed start commands, they will be
> time-aligned.
>
> The other two are probably best answered by trying it out. Maybe someone
> from Ettus can chime in.
>
> Nick
>
> On Fri, Mar 22, 2019 at 7:09 AM Rob Kossler via USRP-users <
> usrp-users at lists.ettus.com> wrote:
>
>> Any suggestions?
>>
>> On Wed, Mar 20, 2019 at 9:49 PM Rob Kossler <rkossler at nd.edu> wrote:
>>
>>> 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
>>>>> _______________________________________________
>>>>> USRP-users mailing list
>>>>> USRP-users at lists.ettus.com
>>>>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>>>>
>>>> _______________________________________________
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>
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