In Western
music
Music is generally defined as the art of arranging sound to create some combination of form, harmony, melody, rhythm or otherwise expressive content. Exact definitions of music vary considerably around the world, though it is an aspect ...
, the term ''chroma feature'' or ''chromagram'' closely relates to twelve different
pitch classes
In music, a pitch class (p.c. or pc) is a set of all pitches that are a whole number of octaves apart; for example, the pitch class C consists of the Cs in all octaves. "The pitch class C stands for all possible Cs, in whatever octave positio ...
. Chroma-based features, which are also referred to as "
pitch class profiles", are a powerful tool for analyzing music whose pitches can be meaningfully categorized (often into twelve categories) and whose tuning approximates to the
equal-tempered scale. One main property of chroma features is that they capture harmonic and melodic characteristics of music, while being robust to changes in timbre and instrumentation.
Definition
The underlying observation is that humans perceive two musical pitches as similar in color if they differ by an octave. Based on this observation, a pitch can be separated into two components, which are referred to as ''tone height'' and ''chroma''.
[
] Assuming the
equal-tempered scale, one considers twelve chroma values represented by the set
:
that consists of the twelve pitch spelling attributes as used in Western music notation. Note that in the equal-tempered scale different pitch spellings such
C and D refer to the same chroma. Enumerating the chroma values, one can identify the set of chroma values with the set of integers , where 1 refers to chroma C, 2 to C, and so on. A
pitch class
In music, a pitch class (p.c. or pc) is a set of all pitches that are a whole number of octaves apart; for example, the pitch class C consists of the Cs in all octaves. "The pitch class C stands for all possible Cs, in whatever octave positio ...
is defined as the set of all pitches that share the same chroma. For example, using the
scientific pitch notation
Scientific pitch notation (SPN), also known as American standard pitch notation (ASPN) and international pitch notation (IPN), is a method of specifying musical Pitch (music), pitch by combining a musical Note (music), note name (with accidental ...
, the pitch class corresponding to the chroma C is the set
:
consisting of all pitches separated by an integer number of octaves. Given a music representation (e.g. a musical score or an audio recording), the main idea of chroma features is to aggregate for a given local time window (e.g. specified in beats or in seconds) all information that relates to a given chroma into a single coefficient. Shifting the time window across the music representation results in a sequence of chroma features each expressing how the representation's pitch content within the time window is spread over the twelve chroma bands. The resulting time-chroma representation is also referred to as
chromagram. The figure above shows chromagrams for a C-major scale, once obtained from a musical score and once from an audio recording. Because of the close relation between the terms chroma and pitch class, chroma features are also referred to as
pitch class profiles.
Applications
Identifying pitches that differ by an octave, chroma features show a high degree of robustness to variations in timbre and closely correlate to the musical aspect of harmony. This is the reason why chroma features are a well-established tool for processing and analyzing music data.
[
] For example, basically every chord recognition procedure relies on some kind of chroma representation.
[
][
][
][
] Also, chroma features have become the de facto standard for tasks such as
music alignment
Music can be described and represented in many different ways including sheet music, symbolic representations, and audio recordings. For each of these representations, there may exist different versions that correspond to the same musical work. Th ...
and synchronization
[
][
] as well as audio structure analysis.
[
] Finally, chroma features have turned out to be a powerful mid-level feature representation in content-based audio retrieval such as cover song
identification,
[
][
] audio matching
[
][
] or
audio hashing.
Computation of audio chromagrams
There are many ways for converting an audio recording into a chromagram. For example, the conversion of an audio recording into a chroma representation (or chromagram) may be performed either by using short-time Fourier transforms in combination with binning strategies
[
][
][
] or by employing suitable multirate filter banks.
[
Furthermore, the properties of chroma features can be significantly changed by
introducing suitable pre- and post-processing steps modifying spectral, temporal,
and dynamical aspects. This leads to a large number of chroma variants, which
may show a quite different behavior in the context of a specific music analysis scenario.][
]
See also
* Time-frequency analysis
* Time-frequency analysis for music signal
*Pitch (music)
Pitch is a perceptual property of sounds that allows their ordering on a frequency-related scale,
or more commonly, pitch is the quality that makes it possible to judge sounds as "higher" and "lower" in the sense associated with musical melodie ...
*Musical theory
Music theory is the study of the practices and possibilities of music. ''The Oxford Companion to Music'' describes three interrelated uses of the term "music theory". The first is the "Elements of music, rudiments", that are needed to understand ...
References
{{Reflist, 30em
External links
Chroma Toolbox
Free MATLAB implementations of various chroma types of pitch-based and chroma-based audio features
Harmonic Pitch Class Profile plugin
Music information retrieval
Music technology
Musicology
Time–frequency analysis