[PD] different float accuracy betw. Pd-0.52-1-msw-i386 and Pd-0.52-1-msw-amd64

[musil at iem.at]

ISSUE: floating-point-inaccuracy of low frequency filters with low  
frequency signals in pd vanilla

The patch "Test_HP3_butterworth_ at _20_Hz.pd" is part of the Live  
Electronic of K.H.Stockhausens piece Mikrophonie I.
My colleague David P. told me, that there occur hearable noise,  
distortion, oscillations and rustling during perfoming this piece.

The test-patch is a 0.1 Hz oscillator sourcing a highpass filter  
3.order with butterworth characteristic at 20 Hz.

Try this patch with pd-0.52-1-msw-i386 and amd64 aka 32-bit and 64-bit  
and older pd versions.
The 32 bit version has 20 dB less noise than the 64 bit version (and  
no oscillations).

Double precision filters of iemlib ("hp3_butt_dp~.pd") have less noise  
than single precision filters.

IOhannes z. and Chrostof R. figured out, it depends on compiler  
options -ffast-math and / or -fassociative-math.

Some screenshots are added.

have fun

Thomas Musil
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