[PD] types of distortion

pix pix at test.at
Tue Jul 20 19:48:00 CEST 2004

linear remapping is not so hard if you can define your remapping
mathematically.  i'm a bit of a fan of this function (which i'll write
as an expr for handy use).

[expr~ 2/(1+pow($f2,-$f1))-1]

your input goes in the left, the signal on the right is a control
(sensible values >1).

in theory this maps input values between +/- infinity into +/- 1. but
because of the limited precision of floating point math, they eventually
just end up being exactly equal to +/- 1 at some range. for a control of
2 this is about +/- 80. for 1.01 this is about +/- 1500.

this function is normally called the sigmoid function and is used in
neural networks.. the control is normally 'e', which probably gives the
resulting curve a perfect quality that my primitive mathematical
understanding can not appreciate. 

also, foldover distortion is the effect you get when frequencies higher
than 0.5*samplerate (nyquist frequency) are produced. so-called because
they are 'folded' back in frequency space, so a frequency at
nyquist+100hz creates a kind of ghost frequency lurking around

the kind of distortion you are referring to is the kind of god-awful
clipping that old soundblaster cards (and surely other hardware) did.
where values where wrapped around the +/- boundaries instead of
clipping. you can recreate it (why!?) using wrap~ (after some massaging
of the signal) like this,

[+~ 1]
[/~ 2] //get the unclipped signal into [0,1]
[*~ 2]
[-~ 1] //shift it back to [-1,1]

(you could do the massaging using scale, but i'm never sure what library it's in)

... if anyone knows of a good description of all of the different
distortions that come into play when an FM or AM radio signal is poorly
recieved, please fill me in.


On Mon, Jul 19, 2004 at 09:22:34AM -0700, Lex Ein wrote:
> Add linear remapping, where the instantaneous signal is used as an index 
> to a table.
> Easy for 8,12,  and 16-bit audio.  Not so easy for 20-bit or 24-bit, heh.
> This can be used to create compression, expansion, clipping, dynamics 
> inversion, etc.
> L
> ba wrote:
> >what kinds of distortion are there other than clipping and foldover?  I 
> >figure fold over is where the signal is flipped back instead of clipping.  
> >what are some other common waveshaping effects?
> > 
> >
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