Re: Handling really large numbers, dynamic ranges....

At 02:42 PM 9/3/02 -0400, Bill Hall wrote:

>hi
>First I must ask what is the expected domain of the resultant y ?
>I ran 12,000,000 thru the equation you gave, and y goes way past 32 bits,
>in the negative direction.
>
>One suggestion is to rearrange the equation as follows:
>
>y = (((a)x + b)x + c)x +d .
>
>Then evaluate it as follows
>[etc....]
>But a is so small that you will need to scale the result UP to
>obtain any
>integer effect from the a term. Which leads me back to the first
>question
>because it is the a term that causes the result to go beyond 32 bits
>negative.

What is the actual resolution of the data you're starting with? That
is ultimately the limiting factor on the _real_ resolution of any result
you get. It's a common fallacy to take, say, 12-bit data and process
it with 32 bits of accuracy; you end up with mostly noise bits.

>You may be able to pick a scaling denominator for the data and use */ ,
>thinking of the data as a fraction of some "full scale" like 16777216.

Better still, use M*/ to multiply your 32-bit intermediate result by
a ratio of 16-bit numbers (w/ extended-precision intermediate
product).

Cheers,
Elizabeth

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Received on Tue Sep 03 2002 - 11:50:23 PDT