Signal Properties
Signal Types
Signals in music and audio DSP can be categorized based on their properties. The list of categories below is not exhaustive but contains the most frequent types of sinals.
Stochastic Signals
Stochastic - or random - signals are those signals whose instantaneous (the value of the signal at a given point in time) value can not be predicted. They do not have a fundamental frequency or pitch. Such random signals can, however, be modeled as stochastic processes. This means a specific behavior and porperties can be expected - the frequency content and value range.
Some examples for stochastic signals:
Noise (white, pink, brown)
Breath/Noise in wind instruments
White Noise Example:
Deterministic Signals
A deterministic signal is fully predictable and can thus be expressed in a mathematical function. For such signals the instantaneous value can be calculated for any point in time.
Periodic Signals
Periodic signals are the most common type of deterministic signal. All periodic signals have a fundamental frequency - $f_0$ - the rate of repetition. The period $T$ of a periodic signal is the inverse of the frequency:
$$ T = \frac{1}{f}$$
For musical instruments and speech, the fundamental frequency is in the range of our hearing. It results in pitch, the perceptual concept related to fundamental frequency.
Some examples for periodic signals:
sine wave
square wave
saw tooth
Sine Example:
Quasi-Periodic Signals
Music and speech consist of a mix of both deterministic and stochastic signal components. Even pitched instruments and the human voice are never fully predictable.
Transients
In audio DSP, transients are short signal segments, characterized by a rapid change of signal properties. There is no exact definition, but in general they have a length between $ 1 - 100 \mathrm{ms} $. Most transients in speech and music signals are part of the attack - the beginning of a sound. They contain noise and rapidly changing sinusoidal components, like the struck of a guitar string, the hammer hitting a piano string or the early breath noise in wind and brass instruments. Transients are very important for recognizing instruments and defining their individual qualities.
Some examples for transients are:
Handclap
Snare drum
Attack segments
Noise bursts
Textures
Sound textures are created by a random sequence of similar events. Similar to their visual counterpart, they create a repeating surface.
Some examples for textures:
Rain
Fire crackling
Vinyl crackling
Frying bacon in a pan