Re: [Discuss-gnuradio] fast parallel filtering

[Marcus Müller]

Hi Dirk,

this is more of a general DSP question than a GNU Radio question:

How do these filters relate to each other?

My gut feeling is that this gets a lot easier as soon as you tell us why
you're doing this, i.e. for what purpose :)

Best regards,
Marcus

On 11.03.2017 19:28, Dirk Gorissen wrote:
> Hello all,
>
> Given a stream of samples I would like to apply n slightly different
> filters to it with n being able to be chosen at runtime. Then combine
> the results back to a single stream.
>
> As a test I built a flowgraph with the following chains in parallel for n = 6
>
>                 | ->  decimating fir filter 1 -> complex to mag ->    |
> stream -> | ->  decimating fir filter 2 -> complex to mag ->    |  -> Max -> 
> ...
>                 |                                  ....
>                         |
>                 | ->  decimating fir filter n -> complex to mag ->    |
>
> So the same stream is sent to each chain (decimation is 1) and the
> output of each chain is pushed through a big Max block with 6 inputs.
>
> This works but not particularly elegant and a bit annoying to change
> if I suddenly decide I want to change n. In particular it also does
> not seem computationally efficient.
>
> What I would like is to replace the above by a single block that
>
> - replicates the input n times
> - applies each filter on each replica
> - combines the output again to a single stream
> - have a tunable n parameter
> - is fast
>
> I did this with an Embedded python block doing essentially this:
>
> for i in range(n):
>      out[i] = scipy.signal.lfilter(taps[i], 1, input)
>
> This is using exactly the same taps as in the chain case. This works
> but the output is different and worse than what I get with the
> separate chains. As a test, instead of lfilter I tried:
>
> gnuradio.filter.fir_filter_ccc(1,taps[i]).work(input[0],output)
>
> Thinking perhaps that is a closer replica. But couldnt get it to work..
>
> I suspect there should be an easy / natural way of doing this in
> gnuradio. Looked at the filter bank / channelliser blocks but failed
> to get anywhere.
>
> So what is the best way forward to do this?
>
> Many thanks
> Dirk
>
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