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

[Robert McGwier]

Your latter suggestion (do it in blocks) with a once per block 
integrator makes perfect sense and would save many ticks.  
Mathematically, it is nearly identical.  Since we are taking a perfect 
average for the block, we would have to change the integration time 
constant on the IIR to accomodate varying block sizes, but I see this as 
no problem would be a computational win.

Thanks,
Bob



Martin Dvh wrote:

> 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
>  


--
Laziness is the number one inspiration for ingenuity.  Guilty as charged!