Re: [Discuss-gnuradio] Question about the USB of USRP1

[Andrew Lutomirski]

On Thu, Jun 4, 2009 at 7:20 AM, Sebastiaan Heunis <address@hidden> wrote:
> Hi all
>
> I've got a question about the bandwidth that the USRP1 can support
> across the USB.  We have 32MB/s across the USB corresponding to 8Msps
> for a single I-Q stream.  When we have two I-Q streams, this becomes
> 4Msps for each stream.
>
> This means that when I have two I-Q sampled channels, I should be able
> to set the decimation for each channel to 16.  This does however not
> work.  With two I-Q channels, if I set decimation to 16, I get about
> 40 of those 'uO' s printed on my screen.  With a decimation of 32, I
> get about 4.  With a decimation of 64, I get none.  So, currently I am
> only able to support 8MB/s accross my USB without losing any samples.
>
> I ran the usrp_benchmark_usb.py and got the following:
>
> Testing 32MB/sec... usb_throughput = 32M
> ntotal    = 16000000
> nright    = 15999965
> runlength = 15999965
> delta     = 35
> OK
>
> So here I'm also losing 35 samples?
>
> I have an HP Pavilion dv6500 laptop with a 1.5GHz Core2Duo CPU and 2GB
> of RAM.  I'm also using a CPU frequency scaling utility to make sure
> that my PC is running at full blast.  Might it be that the USB
> controllers of laptops aren't that great, or is it normal to lose some
> samples at 32MBps?

I have 12 USRPs running at a decimation of 16 with 2 I/Q channels.  On
the current run, I've collected 2 *trillion* samples on each without
any overruns.  I'm doing this on Celeron 220's, which makes your
laptop look positively fast.

I'd check if you have realtime scheduling enabled and maybe try with
C++ code instead of Python -- I've had good luck with
usrp/host/apps/test_usrp_standard_rx.cc, although I haven't had much
trouble with the Python code either.

--Andy