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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
>>
>>
>> ------------------------------------------------------------------------
>>
>> _______________________________________________
>> Discuss-gnuradio mailing list
>> address@hidden
>> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>>
>
>
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff, (continued)
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff, Dawei Shen, 2005/11/21
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff, Robert McGwier, 2005/11/21
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff,
Martin Dvh <=
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff, Robert McGwier, 2005/11/21
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff, Eric Blossom, 2005/11/21
- [Discuss-gnuradio] EETimes article on SDR, Ed, 2005/11/22
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff, Dawei Shen, 2005/11/30
- Re: [Discuss-gnuradio] GMSK at 768kb/sec and other good stuff, Eric Blossom, 2005/11/30