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Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff


From: Martin Dvh
Subject: Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff
Date: Mon, 21 Nov 2005 20:44:08 +0100
User-agent: Debian Thunderbird 1.0.2 (X11/20050817)

Robert McGwier wrote:
> 
> Dawei:
> 
> You have understood it.  Can you suggest a less expensive way than a
> single pole iir filter (integrator) to compute the running average of
> the data so we can subtract that mean?
You could use a decimating fir with no window (use all samples)
and taps (1.0/ntaps,1.0/ntaps,1.0/ntaps,1.0/ntaps, .....)
and decimation==ntaps
followed by a single pole IIR (running at a much lower rate)
What you win by this is the more efficiant SIMD implementation of FIR versus 
IIR.
You could also make a block which just adds up a whole block of samples
Then divide the result by the number of samples, followed by a single pole IIR
This way you avoid the tap multiplication.
(I used this once for DC-removal in a test gnuradio ADC frontend)

Note however that this is mathematically not exactly the same as only a IIR, 
because you work in blocks and only do the feedback after every block.

Greetings,
Martin
>  That was the only thought. We
> needed a very low frequency cut off and this is computationally cheap.
> 
> Bob
> 
> 
> Dawei Shen wrote:
> 
>> Hi Eric and other dudes
>>
>> I am studying the GMSK2 code now. The code is very clean and obviously
>> designed sophisticatedly. I feel excited to read the code. Could you
>> please help explain the following code to me?
>>
>>     alpha = 0.0002   # FIXME, maybe 0.0008/spb
>>     self.freq_offset = gr.single_pole_iir_filter_ff(alpha)
>>     self.sub = gr.sub_ff()
>>
>>     fg.connect(self.fmdemod, (self.sub, 0))
>>     fg.connect(self.fmdemod, self.freq_offset, (self.sub, 1))
>>
>> This part is following the FM demodulator.  My guess is you are going
>> to remove the DC offset, is that right? But why do you use an IIR filter?
>>
>> I plan to add comments to the codes and put them on my website once I
>> finish studying them.
>>
>> Thanks for your time
>>
>> Dawei
>>
>>
>> ------------------------------------------------------------------------
>>
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> 
> 





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