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Re: [Discuss-gnuradio] Transmitting bursts with GRC by inserting SOB and

From: Frederik Wing
Subject: Re: [Discuss-gnuradio] Transmitting bursts with GRC by inserting SOB and EOB
Date: Tue, 21 Oct 2014 13:47:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

Hello Marcus,

thank you very much for your hints!
- You're using an ancient UHD version. Is there a reason you can't use a
newer one?
I am using Ubuntu 14.04 LTS and just pulled GNU Radio and UHD from the
packet sources. There is no newer version available. And I don't like to
compile myself unless it is absolutely necessary.
Consider it outdated. Ubuntu's packet sources are most of the time horribly out of pace compared to GNU Radio's development process. For UHD it's the same. Note that Ettus ships its own Ubuntu packages for UHD *and* GNU Radio, so you can get a non-ancient version of both without even compiling it. By the way: GNU Radio is a software radio developer's framework. You can do great things with it without touching a compiler, but at many points it becomes helpful when one can write his own C++ block, so you definitely need to lose your fear of compilers if you want to use GNU Radio to its fullest potential :)

After some hours of compiling time I could finally update GNU Radio to version 3.7.6 and UHD to 003.008.

I don't think it's because of an outdated version of UHD as the
"tags_demo" application is working perfectly with my current version.
... Well, my old tube radio also still works ;)

Unfortunately, even with the newest version the USRP is still transmitting its carrier over the air. I tried both with the Message Burst Source as well as with the Stream to Tagged Stream Block combined with setting a Length Tag name in the USRP Sink. With the Tag Debug Block I see tx_sob+tx_eob and the Length Tag, respectively. They all seem to be at the right place and have the right value.

The Length Tag should arrive properly at the Sink. I checked by changing the tag's name at the Stream to Tagged Stream Block to something stupid and finally got an "tG" printed out.

Another interesting observation:
I don't get the "U" underrun output if I use a larger number of samples for my burst (in Vector Source). Even with only Vector Source and USRP Sink (no blocks in between) I don't get any "U" printed. I tried that with 500 samples.

Has anyone ever succeeded in transmitting proper bursts with a GRC flow graph? Or at least on Python level? I really feel desperate right now...

- In current GNU Radio, you can use 'length tags' to mark bursts, if
that's any help.
I tried using length tags by inserting an "Stream to tagged Stream"
block and adding the length_tag_name property to the USRP Sink. Here is
my sample code, now with a burst of 5 samples:
The GNU Radio doxygen contains documentation for the current GNU Radio build, containing the newly available message passing functionality.
Maybe that's of interest to you!

That's definitely an interesting feature. But first of all I'd like to make sure that transmitting bursts works as expected before making other improvements in my transmitter chain.


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