On Sun, 25 Mar 2001, Olin Lathrop wrote:

> > how are you doing a low pass filter in software ?

<snip>
> >
> > maybe just point me in the right direction with the "extra 2 or 4 bits"
> part
>
> A single pole low pass filter is easy and fast in software.  If NEW is a new
> reading and FILT is the filter value to update with the new reading then the
> algorithm is:
>
>   FILT <-- FILT * (1 - W) + NEW * W
>
> where W is a 0 to 1 weighting fraction for the new value.  This type of
> filter has the same response as an RC low pass filter with NEW as the input
> and FILT as the output.  W determines the time constant of the filter if new
> values come in at regular intervals.

Here's an efficient implementation of that algorithm

http://www.dattalo.com/technical/software/pic/twist.asm

Here's a small snippet of the comments.

;;; --------------------------------------------------
;;; twist
;;;
;;; The purpose of this routine is to evaluate:
;;;
;;;    A*x + B*y
;;;  ------------
;;;     A  +  B
;;;
;;; Such that A+B = 2^N

'x' and 'y' are the "FILT" and "NEW" parameters. The weighting factor is

  W = B/(A-B)

(note:

  1-W = A/(A-B)

Which is the coefficient on 'x' and "FILT".)

My algorithm limits 'x' and 'y' to 8 bits and 'A' and 'B' to 4 bits. The
algorithm is computed with 8-bit arithmetic, so the "2-4" extra bits alluded to
above will not be obtainable. Now, Tony K. has written a similar routine for
24-bit numbers. (See http://www.dattalo.com/technical/software/software.html ).

Scott.

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