Mutation:

Originally coined by David Rosenboom, the term mutation is applied by Larry Polansky to the process whereby some parameter (sign, magnitude or both) of one sound file (source) is 'mapped' onto that of another sound file (target). The mutation program uses the intervalic distance between FFT frames to gererate the contour or sign. A mutation function is applied to either uniformly or irregularly extrapolate sign, magnitude or both and paste it onto that of the target. For this reason the source and target must be identical in length (achieved in SoundHack by either an automatic truncation of the target or, if time scaling is selected, a time stretching/contracting of the target to match the length of the source). Once the mutation process is finished a file is created which contains the mutant soundfile which will maintain some characteristics of the source and the target depending upon specifications made in the mutation window.

Source: the first sound loaded into the mutation function is termed the source. During mutation, parameters (magnitude, sign or both) of the source sound are calculated and 'pasted' onto the target soundfile.

Target: the sound file in mutation onto which the parameters of the source file are pasted.

a) mutation index: allows for the selection of a number between 0 and 1, with 0 being the source sound and 1 being the target. In other words, if the mutation index number is closer to 1 the mutant file will result in characteristics closer to the target.

b) mutation function: the mutation function is also referred to as the "type" in the mutation program window. Here one may select from the five basic types of mutations or their two concatenations (mutations combining two types in succession). The function may also be edited to change dynamically over time by clicking on the "edit function" box which will allow one to either select a preset or draw (using the mouse) a custom dynamic mutation.

Mutation Types

There are five basic types of mutations and two concatenations. Each of them are of the same basic form and provide a resulting mutant which constitutes a spectral crossfade between source and target files.

The USIM (uniform signed interval mutation) is the most basic of all the mutation functions. The USIM operates on each spectral band interval for every FFT-frame, cross-fading amplitude differences between corresponding spectral bands (Polansky: '95).

The ISIM (irregular signed interval mutation) replaces the amplitude interval of the source by that of the target for each frame to create that of the mutation by adding the signed interval to the amplitude of the previous spectral frame.

The UUIM (uniform unsigned interval mutation) interpolates between the source and target interval, but retains the sign of the source interval. A completely mutated spectra by the UUIM mutation function has the magnitude intervals of the target but the signs of the source intervals.

The IUIM (irregular unsigned interval mutation) is similar to the UUIM, differing only in its use of stochastic pasting of magnitude intervals while retaining sign.

The LCM (linear contour mutation) does the opposite of the IUIM, pasting the signs of the target onto the magnitudes of the source, generally resulting in an unrecognizeable spectral transformation.

Concatenated mutations (mutations combining two functions into one) work by sending each frame the output of the first mutation to the input of the next. For the LCM/IUIM, each part of the concatenation mutates an independent set of stochastically chosen spectral bands. This and the other concatenation, the ICM/UUIM are generally less predictable and contain less of an image of the source and target soundfiles. Nevertheless, they make for some interesting results.

to hear examples of the five different mutation types and their two concatenations click on the types below.

Original sounds:

accordian

guitar

voice

creek

Mutants:

USIM accordian (source), guitar (target)	USIM creek (source), voice (target)
ISIM accordian (source), guitar (target)	ISIM creek (source), voice (target)
UUIM accordian (source), guitar (target)	UUIM creek (source), voice (target)
IUIM accordian (source), guitar (target)	IUIM creek (source), voice (target)
LCM accordian (source), guitar (target)	LCM creek (source), voice (target)

Concatenations:

LCM/UUIM accord (source), guitar (target)	LCM/UUIM creek (source), voice (target)
LCM/IUIM accordian (source), guitar (target)	LCM/IUIM creek (source), voice (target)

A rough idea of what a USIM mutation does to the spectrum of two sounds can be obtained from these spectral graphics. First, the spectrum of the source file (accordian), then of the target (guitar) and finally of the mutant (USIM).

Accordian	Guitar	Accordian .USIM. Guitar

The magnitude differences of the sign from the accordian is pasted onto the guitar. In effect, the amplitude differences between the intervals of the source and target are added to form the mutant. While the mutant maintains characteristics of both sounds, the accordian's sign is kept intact as can be seen in the spectrum of the mutation.