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Re: How to downsample this signal?

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On 5/14/2017 6:42 PM, Piotr Wyderski wrote:
> Thank you all for your input.
>
> Randy Yates wrote:
>
>> What I don't understand is why, with the sledgehammer processor
>> you have, you don't do a decent polyphase filter with a good
>> FIR?
>
> It is a hobby project and I had 15 years of break with any form
> of DSP, at least understood as *signal* processing. I believe
> I still understand the math behind it, but I know only a limited
> number of "tricks". Polyphase representation is totally new to
> me, so are its applications to decimation/interpolation.

A polyphase filter *is* a FIR filter.  It is what you end up with by 
factoring the decimation and only calculate the outputs you need rather 
than calculating every output and *then* doing the decimation.

It's been a long time since I wrote one of these, so the details are 
rather fuzzy.  Google is your friend, at least much more accurate than 
my hazy recollections.


> The FIR filter has many advantages, e.g. maintaining linear phase
> will be easy, but the "direct" approach implied a FIR of insanely
> high order, so I didn't pursue the idea and drifted towards IIR,
> with which I had some positive experience, especially with CICs
> used in my SDR project.
>
> As far as I can see, the polyphase FIR approach should be the right
> choice for me. I am still not sure whether I am able to design a
> filter in this topology correctly, but well, it's what we call
> "learning". Thank you for bringing it to my attention.

As it was a learning experience for me.


> BTW, the processor is not as sledgehammer as it may sound, especially
> due to the very limited RAM resources available to the DSP unit (2 banks
> of 128 24-bit words each, period.). The main processor (an ARM) has
> everything needed to the job, except of low latency guarantees, which
> is enough to kick it out of the scene. Whatever I can do, I must do
> using the DFB block. Currently the unknown is the definition of
> "whatever", but I'm aiming at saturating the block.

Are the 248 words for data or also program?  That should be plenty of 
room for coefficients.  The data can be processed on the fly.

-- 

Rick C

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Thread

How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-13 09:23 +0200
  Re: How to downsample this signal? Evgeny Filatov <filatov.ev@mipt.ru> - 2017-05-13 15:42 +0300
  Re: How to downsample this signal? bitterlemon40@yahoo.ie - 2017-05-13 07:53 -0700
  Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-13 11:48 -0400
  Re: How to downsample this signal? Randy Yates <yates@digitalsignallabs.com> - 2017-05-13 12:58 -0400
    Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-13 16:04 -0400
      Re: How to downsample this signal? Randy Yates <yates@digitalsignallabs.com> - 2017-05-14 14:19 -0400
    Re: How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-15 00:42 +0200
      Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-15 00:46 -0400
        Re: How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-15 07:21 +0200
          Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-15 10:51 -0400
            Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-15 11:49 -0400
              Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-15 17:20 -0400
              Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 02:59 -0400
                Re: How to downsample this signal? eric.jacobsen@ieee.org - 2017-05-16 15:59 +0000
                Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 15:36 -0400
                Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 13:11 -0400
                Re: How to downsample this signal? eric.jacobsen@ieee.org - 2017-05-16 18:08 +0000
                Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:35 -0400
                Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:45 -0400
                Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:50 -0400
                Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 21:36 -0400
                Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 15:35 -0400
                Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:36 -0400
                Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-16 15:40 -0400
                Re: How to downsample this signal? rickman <gnuarm@gmail.com> - 2017-05-16 22:06 -0400
                Re: How to downsample this signal? Randy Yates <yates@digitalsignallabs.com> - 2017-05-17 01:25 -0400
      Re: How to downsample this signal? Evgeny Filatov <filatov.ev@mipt.ru> - 2017-05-15 14:51 +0300
      Re: How to downsample this signal? Randy Yates <randyy@garnerundergroundinc.com> - 2017-05-15 10:37 -0400
  Re: How to downsample this signal? makolber@yahoo.com - 2017-05-15 06:27 -0700
    Re: How to downsample this signal? Piotr Wyderski <peter.pan@neverland.mil> - 2017-05-15 20:39 +0200

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