Re: [Discuss-gnuradio] Overflow/Underflow streaming problem (due to rate & type conversions?)

Hi list,

So, I give this another try:
- Can anyone explain why the USRP source start to cause overruns (uOuOuO...) after about 1 min when connected to a USRP sink for my real time signal-streaming application?

In my transmission chain some necessary rate and type conversions are performed on the stream in between the USRP Source and the USRP Sink, see it depicted in the attached GRC figure (difficult to describe the issue accurately without it).

When the corresponding Signal Source (which is disabled, "grayed out", in the figure) with the same sample rate (=1 MSa/s) is used in place of the USRP Source there are NO problems, which is quite illogical.

Please note that the USRP Source overruns occur although the total processor load is quite low, in the range of about 10 to 30 %. I have tested this on different platforms running Linux (Ubuntu 10.04) and Mac OS X (10.6) with the following processors: Core 2 duo @ 2.8 GHz, Core i3 @ 2.27 GHz, Core i5 @ 2.66 GHz. Overruns occur each time on all platforms, no difference, and the processor load is quite low (< 30%) each time. Real time scheduling does not change this behavior either. Hence, processor overload seems to be a very unlikely reason.

I have a very similar issue on on the receiver side (not shown, but essentially with reverse operations) which outputs the signal via a LFTX daughterboard after receiving it over-the-air (via the RFX2400 daughterboard). At the receiver side I get instead underruns (uUuUuU...) reported from the USRP Sink which feeds out the DC-signal at the receiving end (via LFTX).

The attached figure depicts a baseband (DC) signal acquisition and radio-transmitter for my simple real time NRZ-signal streaming link. The USPR Source uses the LFRX daughterboard (to acquire the source signal) and a USRP Sink with the RFX2400 to transmit the rate reduced source signal over-the-air (bottom right in the stream).

- Any ideas what is causing these overruns (and underruns with probably the same root cause) and suggestions how to fix them?

/ Rickard