On 09/04/2010 08:56 AM, ikjtel wrote: > > >>/ In the case of a "real" flow-graph, taking real data in at/ > >/> 4800symbols/second, going to a real USRP transmitter, will it still/ > >/> run in "fits and starts" or will it "do the right thing"??/ > > > > It will do the right thing, assuming that all blocks "do the right > > thing" and compute as much output as they are asked to. [snip] > > There are of course complexities. When both ends of the flow graph are > connected to hardware, if the clocks aren't synchronized, you get the > well-known "multiple clocks" problem. This can cause data to either > overrun or underrun the sink. This multiple-clocks problem has > been discussed previously on this list. The op25 TX app has its > "stretch" buffer, Asterisk (PBX) has its "jitter" buffering, > etc. > > The multi-clocks problem is nothing new. When our packet-radio group built a bit-regenerating 56KBPS repeater in the 1980s, we had to build in a hardware elastic buffer to compensate for clock-skew between Tx and Rx.
> A similar kind of issue you can run into is when local clock drift can > cause the *apparent* receive rate to differ somewhat from the nominal > value of 4800... > > More generally, for example in the case where RF transmissions are only > intermittent, we have the question of how to keep the blocks happy > when *no* data is flowing. Very briefly, one place this manifests > is in GR's UDP stuff (used in our remote TX). If you have a GR > udp source and it's not receiving a *constant* flow of input, it > will give up and shutdown the entire graph. This is also something > I've seen mentioned on the GR list. It's *not* a complaint or a > request to fix anything, just an observation.... > > Best Regards > > Max > > p.s. another interesting thing on which to speculate is the > effects of the dreaded USRP "underrun" on the overall integrity > of the transmitted spectrum. I've a suspicion that one could see, > shall > we say, "undesired" spectral components at the instant in > time when the underrun occurs? > > > If those underruns happen only very occasionally, they'll be no different than analog noise in a traditional analog Tx chain, I'm thinking. -- Marcus Leech Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
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