[PD] rpole bug?

Christof Ressi info at christofressi.com
Sat Feb 15 18:24:48 CET 2020

This expected behavior due to limited precision of floating point 
numbers. When a float gets larger and larger, it gradually loses 
precision in the lower bits, because the mantissa has a fixed size (see 

There will be a point where the precision loss exceeds the size of your 
input, so the filter will just stop accumulating. In your case, the 
input is "1". Floating point numbers have a mantissa of 2^23 (without 
the sign bit), so the largest whole number you can represent without 
truncating lower bits is 2^23 = 8,388,608. This is the limit you've 
experienced in your patch.

BTW, this is also the reason why you get artifacts when indexing an 
audio buffer with a large float index.

Interestingly, I couldn't immediatly reproduce this behavior on my 
system (Pd 0.50.2 Windows 32-bit), the limit would be about 8 times as 
large as yours. I think the reason is that the code keeps the filter 
state in an intermediate higher precision register, so the accumulation 
is not immediately lost. When I run your patch with [block~ 1], each 
accumulation step has to be written to an actual 32-bit float, and I can 
indeed reproduce your observation.


On 15.02.2020 17:36, Simon Iten wrote:
> i have a strange behaviour with rpole, see attached patch.
> basically it stops accumulating at a certain point (depending on the input value)
> ist this a “rounding” problem?
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