AVR-Chat

On Jan 15, 2009, at 10:25 AM, wagnerj@proaxis.com wrote:

> Heard that this is a common technique.
>
> Mean of N samples is (1/N) sum(Xi) where Xi is one of the samples  
> X0, X1,
> ... XN-1
>
> Now if you are doing a total mean rather than a windowed mean (most  
> recent
> N samples), then the new mean value is
>
> Mean = (N*MeanPrevious + Xnew)/(N+1)

I'd keep a what I'd claim to be the Sum of X samples laying around.  
Then I would say that Sum(N) = Sum(N-1) - Mean(N-1) + Sample(N); and  
then Mean(N) = Sum(N) / X.

If you kept a history of X samples then you could keep a running sum  
subtracting the old out and adding the new in. But without a history  
to draw on you could subtract out the previous average as a good  
approximation before adding the new.

The difference between what I suggest and what you proposed is that I  
wouldn't multiply the old mean by the number of samples, I'd keep the  
raw sum laying around as it has a tad more precision and this skips a  
multiply step.

Make sure that the precision of Sum is great enough to hold X Samples  
at max values.

When using integer math you might consider adding 0.5X to Sum before  
dividing by X so that it rounds "normally".

--
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.