Re: RX/TX switching

Hi Fabien,

Personally, I use PDUs to control when TX starts. GR 3.10 integrated a
lot more of these.
https://github.com/gnuradio/gnuradio/tree/main/gr-pdu
Besides, the gr-pdu_utils OOT might be a good reference.
https://github.com/sandialabs/gr-pdu_utils
It mentions SOB/EOB in their docs.
EOB is required to signal that a burst ends. I'm not sure if a SOB is
really required or if a USRP would just "wake up" and start transmitting.

I suggested the SOB/EOB mechanism because it sounds like you transmit
long bursts. Short bursts might be handled via tagged streams. The UHD
sink would handle these cases. By short bursts I mean <=4k complex samples.

Generally, the idea is: trigger transmission with a PDU block, use a PDU
to tagged stream block to fead you data into the GR streaming interface.
Then make sure to signal to the UHD sink when a burst ends.

Cheers
Johannes

On 04.03.22 12:50, Fabien PELLET wrote:
> Hi Johannes,
>
> Yes this is exactly my problem : after unlocking the flowgraph, I get
> some underruns (only, no overrun).
>
> Do you have an example of code using EOB/SOB ? Ok for sending EOB before
> locking but is it necessary to send a SOB also and when ?
>
> Your suggestion of idling the TX flowgraph is something I looked for
> during several days but I never found how. Indeed, if I do not connect a
> source and a sink at each ends, gnuradio will fail to run. How to stop
> feeding one branch of a flowgraph ? Do you have examples of codes for
> doing that ?
>
> Thanks for your help,
>
> Fabien, F4CTZ.
>
> Le 04/03/2022 à 12:03, Johannes Demel a écrit :
>> Hi Fabien,
>>
>> do those underruns occur after you lock/unlock and switch from TX to
>> RX or vice versa? Do you see overruns as well?
>>
>> I'd assume the USRP expects a constant sample flow and even a short
>> interuption, like your lock/unlock task interrupts that flow.
>> Still assuming this is the root cause, you might fix this by sending
>> an EOB signal to the USRP before you lock the flowgraph.
>>
>> Besides, the whole SOB/EOB mechanism might help you with your issue.
>> You can just stop the TX flowgraph portion while you don't need it. By
>> stop I mean, the flowgraph is not fed any new samples and just idles.
>> As an added bonus, you minimize LO leakage which might be an issue for
>> your RX flowgraph. I know this is a more intrusive change.
>>
>> Cheers
>> Johannes
>>
>> On 04.03.22 10:45, Fabien PELLET wrote:
>>> Hello again,
>>>
>>> I build a flowgraph in C++ with an RX chain and a TX chain. I'm using
>>> a USRP N210. When I'm in RX, the TX chain is connected to a constant
>>> zero source and feed a null sink and the RX chain is connected to
>>> USRP source and sink. When I'm in TX, it is the opposite.
>>>
>>> I have to keep the two chains running at the same time for switching
>>> speed purpose and do not have to stop the flowgraph and restart it.
>>> So for my switching, I just lock the top block and unlock it after.
>>>
>>> If I always stay in RX, I have no underrun. Same thing in TX. My
>>> problem is that when I lock and unlock the flowgraph, I get a lot of
>>> underruns.
>>>
>>> As a test, I remove the TX part of my flowgraph and only keep the RX
>>> part and I try do some lock/unlock sometimes : same problem with
>>> underruns.
>>>
>>> How to do lock/unlock safely in C++ to avoid underruns ?
>>>
>>> Thanks for the help,
>>>
>>> Best regards,
>>>
>>> Fabien, F4CTZ.
>>>
>>>
>>
>