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Re: How to downsample this signal?

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rickman <gnuarm@gmail.com> writes:

> On 5/13/2017 12:58 PM, Randy Yates wrote:
>> Piotr Wyderski <peter.pan@neverland.mil> writes:
>>
>>> Hello,
>>>
>>> I have the 50Hz mains voltage signal sampled at 12bit/100kHz. This
>>> high frequency is for rapid detection of overvoltage conditions.
>>> However, for
>>> further processing this sample rate is way too high, so I'd like to
>>> change the sampling rate by a factor ~100 (not a very critical value,
>>> just "sensibly low").
>>>
>>> The thing will be implemented on a PSOC5LP, which has a dedicated
>>> digital filter block capable of ~32e6 FMACs per second with fixed
>>> 24 bit resolution. I'll use half of that for other purposes and would
>>> like to retain data accuracy at least 12 bit. The dominant signal
>>> is around 50Hz and its quality should remain high, especially the
>>> phase relations.
>>>
>>> Given that constraints, what architecture would you suggest to
>>> do that? I was thinking about not using multiplication at all
>>> (at least at the high freuency stage) and implement two cascaded
>>> order 4, decimation ratio 8 CICs, which fit in the 24-bit registers
>>> and provide combined decimation ratio of 64, which is fine. Then
>>> clean up the output with a FIR. This combo worked years ago in
>>> the FPGA world, where multiplication was a problem. But I think
>>> I loose something important by not using the available MAC unit,
>>> so could you please suggest me a better approach?
>>
>> Piotr,
>>
>> CICs are not really very good decimation filters, in general. That
>> is, they alias, and there is no way to "clean up" the signal
>> afterwards.
>>
>> 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?
>
> They can be used without aliasing into the pass band if they are used
> only for the higher stages of decimation.  They have nulls that are
> aliased to baseband.  As long as that attenuation and width is
> sufficient they work great.

They can be, depending on the situation, as you say. Conversely there
appear to be situations in which the aliasing would be too significant,
or the passband response would be too unflat because of the high order
required to get the aliasing down. Of course you can compensate for
that. 

One would have to consider all these things if you were short on
computational resources. It appears computational resources in Piotr's
case are plentiful, and his application requires high accuracy (i.e.,
low aliasing and a flat bandpass), hence my suggestion.
-- 
Randy Yates, DSP/Embedded Firmware Developer
Digital Signal Labs
http://www.digitalsignallabs.com

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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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