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

[Christof Ressi]

> However, what worries me is that if I run a Pd vanilla patch on 
> different Pd versions (i386 or amd64) with the same floating point 
> precision, I expect the same results. 
Generally, you can't really expect that. Even without -ffast-math, 
different CPU architectures or instruction sets will give slightly 
different results (unless you jump through various hoops).

Here's an interesting article by a game developer: 
https://gafferongames.com/post/floating_point_determinism/

Usually, these errors are very small, but with (suboptimal) recursive 
algorithms they can get amplified significantly. Your patch is a very 
striking demonstration of the problems with the direct 2 form of the 
biquad filter :-)

Christof

On 11.02.2022 23:11, musil at iem.at wrote:
>
> Zitat von Christof Ressi <info at christofressi.com>:
>
>> That's only half the story. Generally, one shouldn't expect floating 
>> point computations to yield the exact same result with different 
>> compilers/machines. The rounding errors themselves are very small, 
>> but they can accumulate over long periods of time or get amplified by 
>> recursive algorithms.
>> I've made an alternative implementation [biquad2~] that uses direct 
>> form 1. It shows significantly less noise than the [biquad~] 
>> examples. See attachments.
>
> That's right what you wrote.
> However, what worries me is that if I run a Pd vanilla patch on 
> different Pd versions (i386 or amd64) with the same floating point 
> precision, I expect the same results.
>
> e.g. the test patch running on Pd-0.52-1-xxx-i386 produces -60dB low 
> frequency spikes, the identical patch running on Pd-0.52-1-xxx-amd64 
> produces -34dB high frequency bursts. And that's a realistic simple 
> situation, a microphone signal feeds a 3rd order high pass filter.
>
> Thomas Musil
>
>
>
>
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