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)
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!
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