[Discuss-gnuradio] USRP Streaming Latency

[Tom Hendrick]

Hi all,

I'm having some issues with the USRP and some latency in the output signal.  I am using a C++ program which modulates a signal and writes to a file continuously.

When I use a normal file, the C++ program output looks perfect.  When I use a script made in GRC to read the C++ output file, the USRP generated signal also looks perfect with an oscilloscope.

Now to get the process real-time, when I use mkfifo to tie the C++ and GRC script together, I notice a problem.  The C++ program constructs a packet and then flushes and repeats continously.  I'm sure this is less than the 64KB buffer size with FIFOs.  The GRC script reads from that file and I see the signal output showing the modulated blocks, but I notice it is not continuous and there seems to be 1-2 seconds of zero signal, every 5 seconds or so. 

I also see uU from the GRC script output about every 5 or so seconds.  Does anyone have any idea if this is a FIFO problem, or is it a USRP/GRC issue?  I am using a Duo core SU7300 laptop so I would think it should run OK real-time unless the C++ and GRC script demand much more processing power when used at the same time.  Does anyone know any tricks I can try?

Thanks,
Tom