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Re: How to downsample this signal?

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Piotr Wyderski <peter.pan@neverland.mil> writes:

> 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.
>
> 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.

Hey, you are most welcome. 

"Polyphase filtering" sounds really complicated, but (like many things
in this field) it's just a term for something quite simple: don't
compute values you're just going to throw away. For example, if you are
decimating by 8, you ultimately throw away 7 out of every 8 samples. So
if you're throwing them away, don't compute them in the first place.
Duh!

Armed with this technique, you essentially increase the effective number
of filter taps by M, where M is the decimation factor. So if you could
afforst 200 taps at full rate, you could actually afford 8*200 = 1600
taps when decimating by 8.

>
> 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.

Hmm. Well then you may be up against a wall. Also if you have a
requirement for low-latency, a long FIR is out. But note that even
though CICs can be considered IIR, they also can be considered a simple
boxcar averager so they will have N/2 latency for a length-N boxcar.

I think there are ways to compute polyphase IIRs too but I've never done
that.

PS: Rick has some good material in his book on polyphase filtering,
CIC decimators, etc.

@BOOK{lyonsthird,
  title = "{Understanding Digital Signal Processing}",
  edition = "third",
  author = "{Richard~G.~Lyons}",
  publisher = "Prentice Hall",
  year = "2011"}

Good luck!
-- 
Randy Yates, Embedded Firmware Developer
Garner Underground, Inc.
866-260-9040, x3901
http://www.garnerundergroundinc.com

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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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