Many systems can be modelled by functions from the time domain signal into some other domain, i.e. by transformations rather than operations. The best-known example of this kind of system is the spectrum analyser, mo delled by the Fourier transformation. However, in principle, any function which yields a representation of the signal which is not a function of time can be seen as a transformation. For example, the extent to which two signals are si milar, correlation, can also be seen as a transformation.
Transformations can often be `cascaded' or `composed' in order to yield a comple x operation. For instance, multiplication of the Fourier transforms of two signa ls and application of the inverse Fourier transform results in a function of tim e, namely a filtering operation.