Hyperbolic tangent distortion. Provides normalized (i.e. in range (-1,+1))
or non-normalized processing (i.e. (-0.7616, 0.7616) for input (-1,+1)).
Normalized processing is the default but is more expensive to use.
Saturation defaults to 1.0; larger values will increase the distortion
quality.

Modified hyperbolic tangent distortion. Provides separate shaping of
positive and negative parts of signal (i.e., asymetric waveshaping). 
distort1 using the following formula:

       exp(asig * (shape1 + pregain)) - exp(asig * (shape2 - pregain))
aout = ---------------------------------------------------------------
       exp(asig * pregain)            + exp(-asig * pregain)

ARGS 

pre-gain - determines the amount of gain applied to the signal before 
  waveshaping. A value of 1 gives slight distortion.

post-gain - determines the amount of gain applied to the signal after 
  waveshaping.

shape1 - determines the shape of the positive part of the curve. A value of 0 
  gives a flat clip, small positive values give sloped shaping.

shape2 - determines the shape of the negative part of the curve.

(The above quoted from the Csound manual entry for distort1; see link below.)

Based on Csound's distort1 opcode by Hans Mikelson. For further information,
see:

http://csound.github.io/docs/manual/distort1.html
http://www.csoundjournal.com/ezine/winter1999/processing/
http://folk.ntnu.no/oyvinbra/gdsp/Lesson4Modtanh.html