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Observer stevet
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Registered: ‎10-10-2018

DSP slices reduces when changing filter type

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Hi

 

I am using FIR Complier 7.2 to create a FIR filter prior to decimation.  I can create the filter using using 57 coefficents in a "Single Rate" filter type. Doing this requires 3 DSP slices, when the filter type is changed to "Decimate" the DSP slices drops to 1. I assumed the filter structure inside the FPGA would be the same and the decimate option would just remove values after the filter, could anyone explaine why this is happening?

fir_dsp_slices.png

 

Thanks

 

Steve

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Accepted Solutions
324 Views
Registered: ‎06-21-2017

Re: DSP slices reduces when changing filter type

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The decimating filter only needs to calculate the samples that you are keeping, not the ones you are throwing away.  It only needs to calculate half as many output samples and has twice the number of clock cycles to calculate each output sample.  Decimating FIR filters almost always use a polyphase approach, where the group of filter coefficients are separated into phases, where each output sample is calculated using a different filter "phase".  This also works for interpolating filters and rational ratio resampling filters.  I think this was first described in an IEEE paper in the mid-seventies by Crochiere and Rabiner.  Any good book on multi-rate signal processing will explain it.  The FIR compiler just knows how to do it.  This relieves you of the necessity of learning what is behind it.  That's good for productivity, but I think anybody who wants to do multi-rate processing should code one from scratch, just to understand.

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2 Replies
325 Views
Registered: ‎06-21-2017

Re: DSP slices reduces when changing filter type

Jump to solution

The decimating filter only needs to calculate the samples that you are keeping, not the ones you are throwing away.  It only needs to calculate half as many output samples and has twice the number of clock cycles to calculate each output sample.  Decimating FIR filters almost always use a polyphase approach, where the group of filter coefficients are separated into phases, where each output sample is calculated using a different filter "phase".  This also works for interpolating filters and rational ratio resampling filters.  I think this was first described in an IEEE paper in the mid-seventies by Crochiere and Rabiner.  Any good book on multi-rate signal processing will explain it.  The FIR compiler just knows how to do it.  This relieves you of the necessity of learning what is behind it.  That's good for productivity, but I think anybody who wants to do multi-rate processing should code one from scratch, just to understand.

View solution in original post

Observer stevet
Observer
305 Views
Registered: ‎10-10-2018

Re: DSP slices reduces when changing filter type

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Hi Bruce

 

Thanks for information, you seem to helping me a lot with my filter design at the moment.  You have mentioned some very intresting points that i will look into deeper.

 

Steve

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