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

From: Nowlan, Sean
Subject: Re: [Discuss-gnuradio] Transmitting bursts with GRC by inserting SOB and EOB
Date: Tue, 21 Oct 2014 17:29:24 +0000

I'm concerned that the problem Frederik is observing has to do with the very 
short burst he is sending, something like 5 samples. I suspect this requires 1 
call each to work and tag_work per 5 sample burst, which seems like an awful 
lot of context switching and overhead.

-----Original Message-----
From: Marcus Müller [mailto:address@hidden 
Sent: Tuesday, October 21, 2014 1:24 PM
To: Nowlan, Sean; address@hidden; Martin Braun
Subject: Re: [Discuss-gnuradio] Transmitting bursts with GRC by inserting SOB 
and EOB

Hash: SHA1

Hi Sean,

aaah good catch! Yes, that's right; sob is safe.


On 21.10.2014 19:19, Nowlan, Sean wrote:
> From Marcus:
>> ... and that (wut) might be a bug, because it implies that, if the 
>> stream has both a time tag and a sob tag, the question whether the tx 
>> metadata has a time tag depends on in which order these tags are 
>> sorted on the the tag storage multimap. Which might be random, 
>> because tags are sorted only by tag offset.
>> @Martin: is there a reason that this is explicitely set to false 
>> here, or can one just fix this by deleting a line?
> This appears to be handled correctly. Given the same tag offset, the 
> order of tx_time vs. tx_sob tags should not matter. If tx_time is 
> found first, it sets found_time_tag=true and 
> _metadata.has_time_spec=true (lines 652 & 653). Then 
> _metadata.has_time_spec is overwritten in the tx_sob check (line
> 665) with 'false', but is set back to 'true'  in line 743 due to 
> found_time_tag being true.
> if (found_time_tag) { _metadata.has_time_spec = true; }
> If instead tx_sob is found first and tx_time second, then the time 
> spec will also be set in line 743. So the question is whether setting 
> _metadata.has_time_spec=false is really necessary. I'd say it's worth 
> keeping in case the user doesn't always want to use tx_time stamps. 
> The user may want to schedule some bursts but force others to transmit 
> as soon as possible while still using the ATR functionality of the 
> I know all this logic can be hard to follow, but it has to handle so 
> many different cases and possibly span many calls to work and 
> tag_work.
> From Frederik:
>> 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.
> It should be mentioned that there are two burst tag interfaces that 
> cannot be mixed. if a Length Tag Name is specified to the constructor, 
> all tx_sob and tx_eob tags will be ignored in tag_work to ensure the 
> interfaces are mutually exclusive. If no Length Tag Name is specified, 
> then tag_work will search for tx_sob/tx_eob tags and won't look for 
> length tags.
> Sean
> On 21.10.2014 15:53, Frederik Wing wrote:
>> Hi Marcus,
>>>>> I cannot believe that there is no solution to it since the  
>>>>> "tags_demo" application shows that it is indeed possible.
>>>>> :-/
>>> that makes the two of us! I didn't get that when using tags_demo, 
>>> you're not seeing the carrier that you use tags_demo; as far as I 
>>> understood, your application does exactly the same, sending bursts 
>>> with sob/eob tags?
>> That's right. tags_demo works perfectly. No carrier in between the 
>> bursts. The flow graph I posted before sends exactly one burst with 
>> SOB and EOB tags. The only difference to tags_demo I could recognize 
>> so far is that I don't assign time stamps to the samples. But this 
>> shouldn't be a problem, should it?
>> Frederik
>> _______________________________________________ Discuss-gnuradio  
>> mailing list address@hidden 
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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