Pd (Pure Data) is a graphic computer music language for real-time applications and was written by Miller S. Puckette. Pd can also be read and understood as public domain.

There has been a desire for composition machines ever since mathematics was used as the basis for composition (approx. 16th–17th century). Leibnitz and Marin Mersenne (1588–1646) used “combining” and “composing” as equivalent terms.

The composition computer was first developed in the time of musique concrète and serial music. One of its first applications was the use of random generators, which were controlled by paper tape readers.

In order to create music by means of a computer, first a mathematical model representing the music must be found. The simplest representation is a list of discrete values (samples), the audio signal. This means that the entire information of the music is combined in one dimension and is plotted over time (in the time domain). Further representations are multidimensional vectors, such as the spectrogram, the sonogram in the frequency-time domain, or wavelet analysis in the wavelet domain. All these representations use an uninterrupted signal as a mathematical model.