[Discuss-gnuradio] Problem with FIR filter

[John Wilson]

Hello,

I've got a problem with the FIR filters in GNU radio. Basically, at the moment I'm using it as a platform to run some simulations on (as I've already written a load of code on it for implementing a digital radio), and one of the tests I'm doing is trying to find the BER through an equaliser. To do this I'm loading a vector source up, running my flow graph with run() and upon completion I'm checking a vector sink for errors. This works fine for a fixed channel. The channel model I'm now using (a Watterson model which I've coded into the flow graph), however, uses a FIR with a large number of taps. Now the problem is that every time I run my flow graph with run(), which presumably also calls wait(), the FIR seems to start from a reset state. Is there any way to keep the contents of the FIR's shift register in situ through calling run() multiple times? Every other signal processing block I have maintains its state through this. For the record I'm not calling stop() at all in the code, which I would expect to reset everything.

I've set up a quick test case to describe 
this problem,

So if I set up a little top_block like this, which 
just sends a string of complex symbols valued +1+0j through a FIR 
filter, and receives the result in block_out;

------------------------------

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class my_top_block(gr.top_block):
  def __init__(self): 
    gr.top_block.__init__(self)
       
    self.data_rate = 1000; 
    self.totbits = 0;
    self.toterrors = 0;
   
    grey_const = ([1, -1])
   
    length = 2
       
    blank = [0x0,] * (length)
    taps = [1,] * 10
       
    self.transmit_source = gr.vector_source_b(blank, False, length);
   
    self.unpack_for_mod = gr.packed_to_unpacked_bb(1, gr.GR_MSB_FIRST)
   
    self.symbol_mapper = gr.chunks_to_symbols_bc(grey_const)
   
    self.fir = gr.fir_filter_ccc(1, taps)
   
    self.v2s = gr.vector_to_stream(1, length)
   
    self.connect(self.transmit_source, self.v2s, self.unpack_for_mod, self.symbol_mapper, self.fir)

    self.connect(self.fir, block_out)
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Then if I run it from main() like this;

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def main ():

        tb = my_top_block();
       
        for a in range(20):
           
            tb.run()
           
            for n in block_out.data():

            print n

        block_out.clear()

        tb.transmit_source.rewind()

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I get as output;

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>>> gr_fir_ccc: using SSE
(1+0j)
(2+0j)
(3+0j)
(4+0j)
(5+0j)
(6+0j)
(7+0j)
(8+0j)
(9+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(1+0j)
(2+0j)
(3+0j)
(4+0j)
(5+0j)
(6+0j)
(7+0j)
(8+0j)
(9+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(10+0j)
(1+0j)
(2+0j)
(3+0j)
(4+0j)
(5+0j)
(6+0j)
(7+0j)
(8+0j)
.
.
etc
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Which is wrong (after the first 10 outputs the output should always be (10+0j). Is this a bug? Also does anyone know of any work around?

Cheers,

John