Re: [Discuss-gnuradio] Detecting underflows with uhd_usrp_sink

[Sean Nowlan]

Do late packets always get dropped by the USRP? What happens if its buffers get 
filled up with samples, all of which are late?

"Marcus D. Leech" <address@hidden> wrote:

>> L = late packet, there was a time on the packet which was>  time on
>> device when
>>
>>
>There are two different "cases" for late packets happening.
>
>The first is that you haven't sent your packet far enough in advance to 
>account for latency variations on the host.  Unfortunately, on a 
>general-purpose
>   OS like Windows or Linux, latency variability can be extreme, and for 
>long-running flow-graphs you might need to develop a good model to determine
>   what the worst-case is and account for that.
>
>The second is that the clock on the USRP and the clock on the host will 
>tend to drift apart over time, particularly if both of them are "free 
>running".
>   So when you schedule timed bursts far enough in advance during the 
>start of a "session", it's entirely possible that after quite some time, the
>   two clocks have drifted apart unfavourably in terms of allowing you 
>to schedule things far enough in advance, relative to the USRP clock.
>   PC clocks are *terrible* by themselves.  They'll drift significant 
>fractions of a second on a daily basis without any outside steering. 
>  The USRP
>   clock, even free-running, is typically much, much better.
>
>
>-- 
>Marcus Leech
>Principal Investigator
>Shirleys Bay Radio Astronomy Consortium
>http://www.sbrac.org
>
>
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