Re: [Discuss-gnuradio] Understanding the c++ interface

[Eric Blossom]

On Tue, Nov 14, 2006 at 05:18:43PM -0800, Dan Halperin wrote:
> Hi,
> 
> I'm trying to understand the C++ interface to the USRP. I've read the
> library files (usrp_{basic,standard}.h) and the test_usrp_standard_tx
> and test_usrp_standard_rx scripts.

OK.  Note that these C++ programs don't know how to control any of the
daughterboards.  You're probably OK with the basic tx and rx, since
there's nothing to control, but you do lose the easy to use "tune"
function, easy handling of any daugtherboard in any slot, etc...

> My current mission is to modify (or use?) these scripts to get the same
> information out of the USRP that I put in. (Using Basic TX and Basic RX
> on side A with a coax cable between them).

Any particular reason you're not working Python?

> I run [test_usrp_standard_tx -f -F 0 -I 32], and then pull up
> [usrp_oscope -d 32 -f 0] and the output on the scope looks good. I then
> run [test_usrp_standard_rx  -F 0 -D 32 -o usrp_rx_dump -M 1] to capture
> 1 MB of data from the USRP, but the file (seems to be) wrong - it only
> contains the (signed short) 0 and -1=0xffff. I thought maybe there is
> something wrong with the mux parameter (-1 seems to indicate all zeros
> input?) and changed it to 0x33333333 but that didn't work either.

The rx_mux value of -1 means "do the right thing".  It ends up setting
the mux to 0x32103210.

The tx_mux value of -1 means "do the right thing".  For the single
channel case it sets it to 0x0098.

Are the tx and rx daughterboards both on the "A" side?

> I've also tried to mimic usrp_oscope.py and usrp_rx_cfile.py; I set the
> gain to the average of the max and min gains (10.0 db). What am I
> missing?

The basic_tx has _very_ low output power.
Be sure to set the rx gain to its maximum value.

> Do I need to convolve the buffer returned by urx->read() with
> another signal, for instance?

No.

> As an aside, is there a good reason the decimation/interpolation
> arguments are not standardized across the various scripts? It's hard to
> remember whether the argument is -I, -i, -D, or -d... similar questions
> apply to frequency arguments (-f vs. -F).

The arguments are standardized across the python code.  Please feel
free to submit a patch that makes these match the python code.  (You
happen to be using two pieces of C++ code that were written at the
beginning of time to test early usrp functionality.  They don't get
used much, and were almost dropped from the distribution because they
don't know how to talk to the daugtherboards.)

> Thanks,
> 
> -Dan

Eric