Re: [Discuss-gnuradio] Question about potential application of gnuradio and the USRP

[Eric Blossom]

On Tue, Jul 27, 2004 at 03:11:34PM -0700, Heiko Jones wrote:
> I have a potential application that I want to try with gnuradio and
> eventually the USRP. I have a digital data stream, already down
> converted here in the lab (I can worry about the real RF signal later),
> that is a 0 to 5V level PCM data stream, usually encoded NRZ-L, but
> possible NRZ-M or Biphase-L. Additionally, it can be Viterbi encoded or
> not. The stream of ones and zeros can be up to 1.92 Mbits/sec. First,
> can the USRP read a signal like that and give me a bit stream out to my
> application with the simple raw coax daughter card? I would also like
> to, at the same time, read in a similar data stream of IRIG-B time (sine
> wave, amplitude change makes marks and spaces of slo code) that is
> 1Kbit/sec.

Yes, the USRP can digitize the signal and get it into the PC, though
you'd need to attenuate it a bit before feeding it to us.  Our maximum
input range is 2 V p-p.  The inputs are 50 Ohm.  It could also read
the other signal using the same internal clock.

> Also, potentially, since the digital data stream is actually in frames
> with a frame sync pattern in it, is there room for the USRP to actually
> digitize and frame sync it for me in the fpga, and send the binary data
> already in frames to my application too? Same question for converting
> IRIG stream into a digital time in the fpga.

I suggest that you do the bulk of this on the PC using GNU Radio, not
in the FPGA.  There may be room in the FPGA, but it's a lot easier on
the PC.  You may want to implement a simple slicer in the FPGA so that
the samples sent across the FPGA take fewer bits, giving you net more
samples/sec across the USB.

> Is there any similar examples of an application looking at digital PCM
> data like this in the stuff you have? I am still looking over all the
> docs and code.

No, though what you're trying to do shouldn't be too hard.

> Thanks! Heiko

Eric