# Audio Signal Glossary in librosa

** Published:**

Time Series

Typically an audio signal, denoted by y, and represented as a one-dimensional numpy.ndarray of floating point values. y[t] corresponds to

**amplitude**of the waveform at sample t.Samping rate

The (positive integer) number of samples per second of a time series. This is denoted by an integer variable

`sr`

.Frame

A short slice of time series used for analysis purposes. This usually corresponds to a single column of a spectrum matrix.

Window

A vector of function used to

**weight samples****within a frame**when computing a spectrogram.Frame length

The (positive integer) number of samples in an analysis window(or Frame). This is denoted by an integer variable n_fft

Hop length

The number of samples between successive frames, e.g., the columns of a spectrogram. This is denoted as a positive. This is denoted as a positive integer hop_length.

Window length

The length(width) of the window function(e.g., Hann window). Note that this can be smaller than the frame length used in a short-time Fourier transform. Typically denoted as a positive integer variable win_length.

Spectrogram

A matrix s where the rows index frequency bins, and the columns index frames(time). Spectrograms can be either real-valued or complex-valued. By convention, real-valued spectrograms are denoted as numpy.ndarrays S. while complex valued STFT matrices are denoted as D.

Onset(strength) envelope

An onset envelope onset_env[t] measures the strength of note onsets at frame t. Typically stored as a one-dimentional numpy.ndarray of floating-point values onset_envelope.

chroma

Also known as pitch class profile(PCP). Chroma representations measure

**the amount of relative energy in each pitch(音高) class**(e.g., the 12 notes(音符) in the chromatic scale) at given frame/time.