> Anybody got a good averaging algorithm?

There's some literature on this in the networking world.  A non-worthless
TCP implementation keeps track of an average "round trip time", as well
as an average variance, and a filter of sorts to prevent assorted erronous
results from being used in the caclulations.

The basic algorithm is similar to what's already been discussed.  First
you note that an alternate formula for an average:

        avg = samp1/N + samp2/N + samp3/N + ... sampN/N

You can rearrange this to:

        avg = sampN/N + (samp1/N + samp2/N...)

The second part of this is VERY CLOSE to what you'd get by using N-1
"average" samples:

        ((N-1) * avg)/N

Or      (1 - 1/N) * avg


So you have:

   new_avg = sampN/N + ( (1 - 1/N) * avg )

Of course, you can now pick N rather arbitarilly and keep
a "running average" by calculating:

        avg = avg * (1 - 1/N) + sampN/N

Now, this would normally be rather problematic to calculate without floating
point, and slow to do WITH floating point, so we observe that the smallest
fractional value we need to deal with is 1/N, pick N to be a nice power of
two, and use what is essentially FIXED POINT fractional numbers, but we'll
call keep an integer perspective by modifying the equation.
Multiply both sides by N:

        N*avg = avg * (N-1) + sampN

Or
        N*avg = N*avg - avg + sampN
        N*avg = N*avg + (sampN - avg)

If we store N*avg, we now have one division (shift) and a couple adds rather
than anything more complicated, and we keep log2(N) bits of fraction around.
Of course, you have to remember to also divide by N whenever you want to
access the actual average...

Typically, N=8 and you have (c code now!):

        sampN -= scaledavg>>3;
        scaledavg += sampN;

On an 8 bit microcontroller, it might make sense to use N = 256, so you
can do your division by simply selecting the high byte of scaledavg instead
of shifting.

Hope this makes sense...

References:

    "Round Trip Time Estimation," P. Karn & C. Partridge, ACM SIGCOMM-87,
         August 1987.


    "Congestion Avoidance and Control," V. Jacobson, ACM SIGCOMM-88,
        August 1988.

BillW