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downsampling in c6713

Started by waji...@hotmail.com December 26, 2006
HI EVEYONE.I am trying to implement iir filter on c6713.MY Signal has a bandwidth of 150hz.The minimum sampling rate of c6713 is 8khz.I need to sample at 1khz.This means i have to downsample it.
My code is interrupt based.What i want to ask is whether i will downsample before or after filtering.Any sample code regarding downsampling would be of great help.
Any response in this regard would be of great help.
Thanks .
wajahat.
Wajahat-

> HI EVEYONE.I am trying to implement iir filter on c6713.MY Signal has a bandwidth of 150hz.The minimum sampling rate
> of c6713 is 8khz.I need to sample at 1khz.This means i have to downsample it.
> My code is interrupt based.What i want to ask is whether i will downsample before or after filtering.Any sample code
> regarding downsampling would be of great help.

First apply FIR lowpass filter (maybe something like 450 Hz cutoff frequency), then decimate (take every 8th sample).

One of the tradeoffs is that the closer you want the cutoff to be ideal 500 Hz, the sharper the FIR filter rolloff and
more the number of FIR filter taps. More filter taps means less real-time processing for other algorithms and more
signal delay.

-Jeff
You need do do the filtering prior to downsampling to avoid aliasing. Be sure to have a good filtering in the stopband region (or above the Nyquist rate fc/2). If you want to reduce computation you can use multirate techniques but they are easily applicable only to fir filters. They make it possible to invert the operations of filtering and downsampling (via poliphase decomposition) so the filter operates at the low sampling rate. If speed is not a problem use an iir lowpass filter followed by a decimator if phase distortion is allowed, otherwise use a linear phase fir(higher cost).