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Re: [Discuss-gnuradio] Modulation problem ?

From: irene159
Subject: Re: [Discuss-gnuradio] Modulation problem ?
Date: Fri, 23 May 2008 01:52:50 -0700 (PDT)

Steven Clark-2 wrote:
> hex 00 and 01 are not visible characters, which is why you aren't
> seeing anything. Looking at the data, it looks like unpacked data (ie
> only the bottom bit of each byte is being used). If you convert it
> back to packed data, it may be you'll get your original message back.
> GNURadio has a unpacked_to_packed block for this purpose.

Why does GNURadio dqpsk demodulator code use unpack_k_bits_bb instead of
unpacked_to_packed_bb (since modulator uses packed_to_unpacked_bb)? What is
it used for?

Indeed, in my case it works much better replacing 
                self.unpack = gr.unpack_k_bits_bb(self.bits_per_symbol())
                self.unpack =
gr.unpacked_to_packed_bb(self.bits_per_symbol(), gr.GR_MSB_FIRST)
in the dqpsk demodulator code.

Thanks to WinHex I realised demodulated data is bit shifted (due to the
filter). Implementing simple_framer and simple_correlator should solve this
problem. I have tried using them but I get the following error:
ValueError: source and destination data sizes are different:
unpacked_to_packed_bb simple_correlator

I am connecting my blocks as follows:
Original text file > simple_framer > dqpsk modulator > Modulated.dat file
Modulated.dat file > dqpsk demodulator > simple_correlator > Resultat.txt

I've seen that the simple_correlator expects its input to be oversampled 8
times[1], I think the code honors this requirement : dqpsk_mod
interp_fir_filter_ccf does oversampling and dqpsk.py samples_per_symbol
value is 8. 

I haven't found information for simple_framer/simple_correlator inputs and
outputs and have no idea where my "ValueError: source and destination data
sizes are different" problem comes from. Any hints would be appreciated!

Thank you for your help!


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