Re: [Discuss-gnuradio] Large number of overflows...

[Matt Ettus]

On 04/11/2010 09:22 PM, Ian Holland wrote:
> Hi All
>
> I am trying a modified example of the digital-bert routines, for
> communication between 2 USRP2s, and notice that I am getting a very
> large number of overflows (SSSS....) even with decimation rate at the
> receiver of 20, and 4 samples per symbol (sometimes even with 20
> samples/symbol). If I don't get overflows (as has occurred when I used
> 20 for decimation as well as 20 for samples/symbol in one instance), I
> am able to capture the demodulated bits as 111111111111111111111111...,
> as expected for the example. However, with overruns, which seem to occur
> more for lower samples per symbol and/or lower decimation values, I get
> a large number of bit errors.
>
> My receiver flowgraph is of the form:
>
> USRP2 Source -->  RRC Filter -->  Costas Loop -->  Mueller and Muller Synch
> -->  Complex to Real -->  Binary Slicer -->  Descrambler -->  File Sink.
>
> The transmitter flowgraph uses the same blocks as per
> digital-bert/transmit_path.py, but with a USRP2 sink.
>
> I am transmitting over-the-air, and clocks are not synchronised between
> Tx and Rx.
>
> I have a gigabit Ethernet link, and 2 x 2 GHz CPUs in my PC, which is
> running Ubuntu 9.10.
>
> Can anyone suggest why I am getting so many over-runs, and how I could
> get around this problem?


These overflows indicate one of two things:

- that your flowgraph is too slow to execute in real time on your computer

- You haven't enabled realtime scheduling.

Matt