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Re: [Discuss-gnuradio] obtain data before FFT

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] obtain data before FFT
Date: Sun, 5 Dec 2010 22:18:01 -0500

2010/12/5 lishan_wh <address@hidden>:
> Hi, all
> I want to plot a figure with the samples from usrp, but I have some
> questions. Here is the flowgraph:
> usrp.source_c --> gr.streams_to_vector(gr.sizeof_gr_complex*1, 1) -->
> gr.complex_to_mag(1)
> --> gr.message_sink(gr.sizeof_float*1, self.msg_sink_msgq_out, True)

Why are you using streams_to_vector that way? It looks like you're
creating vectors with a single element in it.

> Here, self.msg_sink_msgq_out = gr.msg_queue(2)
> In main function, I did this:
>         msg = tb.msg_sink_msgq_out.delete_head()
>         a = int(msg.arg1())
>         b = int(msg.arg2())
>         s = msg.to_string()
>         float_data = numpy.fromstring(s, numpy.float32)
>         for bin in float_data:
>                 print bin
> Questions are sa follows:
> (1) Is my code correct? If it isn't, how should I modify it?
> (2) What is the return value of gr.msg_queue(2)? I mean what's the meaning
> of "a","b","s" in this code?
> (3) Is what stored in "s" the amplitude of the samples? If I plot the figure
> with s, can I get the envelop of the signal? (4) How to calculate the time
> interval between each sample? Can I set the collecting time for USRP? For
> example, I want to collect the signal for 1 second.
> Thank you! Any help will be appreciated!

Why are you using a message sink? If you want to plot, why not use
either the WX or QT plotting tools? Use the gnuradio-companion and
drop down a graphical sink.

If you just want to run some data through the graph and plot it
afterwards, store it to a vector or a file_sink and then plot it


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