Re: [Discuss-gnuradio] question about overruns with simple flowgraph

[Marcus D. Leech]

On 08/05/2013 09:48 PM, Stephen wrote:
> On 8/5/2013 8:17 PM, Marcus D. Leech wrote:
> > On 08/05/2013 09:00 PM, Nick Foster wrote:
> > > A couple of seconds seems pretty fast, however. Make sure you're
> > > resampling the audio to a rate which is actually supported by your
> > > soundcard.
> > >
> > > --n
> > The two-clock problem tends to manifest as the occasional "click" or
> > occasional 'O'.
> >
> > But if the rates between USRP source, and audio sink are significantly
> > mis-matched, you end up with overruns very quickly, since buffers will
> >    start to fill up to exhaustion, since the sink isn't taking samples as
> > fast as they're being produced.
> >
> > There's no "magic" rate-adaptation in Gnu Radio.  If you have a filter
> > block that is decimating down to, let's say, 50kHz, and then feed it into
> >    an audio sink that's expecting data at 48kHz, things will get stuffed
> > pretty quickly.  Just because a (hardware) sink is expect data at
> >    some rate, doesn't mean that Gnu Radio automatically "tweaks" the flow
> > graph to deliver data at that rate.  Gnu Radio is a streaming
> >    architecture, it doesn't actually inherently "know" about sample
> > rates, so you have to take care of decimating/interpolating data streams
> >    so that things "match".
> Ahhhh, now I see. I am getting frequent 'O' and audio clicks and pops. I
> had played with the various rates till I got the final audio decimation
> very close to an integer value but not quite. The integer value used was
> a little lower than needed. I adjusted things some more and got a set of
> rates that resulted in an integer decimation for the audio lp filter and
> that fixed the problem.
>
> The link to the previous discussion was interesting.
>
> thanks, guys
> stephen
I tend to use the fractional interpolator--you can get exact rate 
matching that way.  It doesn't seem that expensive, particularly when I 
use it at the
  "low speed" part of the graph near the audio subsystem.