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

From: Nick Foster
Subject: Re: [Discuss-gnuradio] question about overruns with simple flowgraph
Date: Mon, 5 Aug 2013 18:00:35 -0700

On Mon, Aug 5, 2013 at 5:53 PM, Stephen <address@hidden> wrote:


I have a simple FM broadcast receiver flowgraph in c++ that causes
overruns. I'm using a B100 with a wbx daughter board. The flowgraph is

usrp source -> lowpass filter -> quad demod -> deemphasis -> audio lp
filter -> constant multiply -> audio sink

The sample rate is 250K. It works but after a couple of seconds the
overruns start. I've used much more complicated flowgraphs than this
with much higher sampling rates with out overruns. Looking at the cpu
utilization shows each core at less than 5%. Would anyone have any
suggestions for what to look at that could be causing the problem?

It's been called the "two-clock" problem. Basically, your soundcard's clock is decoupled from the USRP's clock, and neither is ideal -- they have error. As they drift apart either the audio clock will run slower (producing overruns) or faster (producing underruns).

Here's an older discussion of the same issue.

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.



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