librosa.cqt

librosa.cqt(y, *, sr=22050, hop_length=512, fmin=None, n_bins=84, bins_per_octave=12, tuning=0.0, filter_scale=1, norm=1, sparsity=0.01, window='hann', scale=True, pad_mode='constant', res_type='soxr_hq', dtype=None)[source]

Compute the constant-Q transform of an audio signal.

This implementation is based on the recursive sub-sampling method described by 1.

1

Schoerkhuber, Christian, and Anssi Klapuri. “Constant-Q transform toolbox for music processing.” 7th Sound and Music Computing Conference, Barcelona, Spain. 2010.

Parameters
ynp.ndarray [shape=(…, n)]

audio time series. Multi-channel is supported.

srnumber > 0 [scalar]

sampling rate of y

hop_lengthint > 0 [scalar]

number of samples between successive CQT columns.

fminfloat > 0 [scalar]

Minimum frequency. Defaults to C1 ~= 32.70 Hz

n_binsint > 0 [scalar]

Number of frequency bins, starting at fmin

bins_per_octaveint > 0 [scalar]

Number of bins per octave

tuningNone or float

Tuning offset in fractions of a bin.

If None, tuning will be automatically estimated from the signal.

The minimum frequency of the resulting CQT will be modified to fmin * 2**(tuning / bins_per_octave).

filter_scalefloat > 0

Filter scale factor. Small values (<1) use shorter windows for improved time resolution.

norm{inf, -inf, 0, float > 0}

Type of norm to use for basis function normalization. See librosa.util.normalize.

sparsityfloat in [0, 1)

Sparsify the CQT basis by discarding up to sparsity fraction of the energy in each basis.

Set sparsity=0 to disable sparsification.

windowstr, tuple, number, or function

Window specification for the basis filters. See filters.get_window for details.

scalebool

If True, scale the CQT response by square-root the length of each channel’s filter. This is analogous to norm='ortho' in FFT.

If False, do not scale the CQT. This is analogous to norm=None in FFT.

pad_modestring

Padding mode for centered frame analysis.

See also: librosa.stft and numpy.pad.

res_typestring

The resampling mode for recursive downsampling.

dtypenp.dtype

The (complex) data type of the output array. By default, this is inferred to match the numerical precision of the input signal.

Returns
CQTnp.ndarray [shape=(…, n_bins, t)]

Constant-Q value each frequency at each time.

Notes

This function caches at level 20.

Examples

Generate and plot a constant-Q power spectrum

>>> import matplotlib.pyplot as plt
>>> y, sr = librosa.load(librosa.ex('trumpet'))
>>> C = np.abs(librosa.cqt(y, sr=sr))
>>> fig, ax = plt.subplots()
>>> img = librosa.display.specshow(librosa.amplitude_to_db(C, ref=np.max),
...                                sr=sr, x_axis='time', y_axis='cqt_note', ax=ax)
>>> ax.set_title('Constant-Q power spectrum')
>>> fig.colorbar(img, ax=ax, format="%+2.0f dB")

Limit the frequency range

>>> C = np.abs(librosa.cqt(y, sr=sr, fmin=librosa.note_to_hz('C2'),
...                 n_bins=60))
>>> C
array([[6.830e-04, 6.361e-04, ..., 7.362e-09, 9.102e-09],
       [5.366e-04, 4.818e-04, ..., 8.953e-09, 1.067e-08],
       ...,
       [4.288e-02, 4.580e-01, ..., 1.529e-05, 5.572e-06],
       [2.965e-03, 1.508e-01, ..., 8.965e-06, 1.455e-05]])

Using a higher frequency resolution

>>> C = np.abs(librosa.cqt(y, sr=sr, fmin=librosa.note_to_hz('C2'),
...                 n_bins=60 * 2, bins_per_octave=12 * 2))
>>> C
array([[5.468e-04, 5.382e-04, ..., 5.911e-09, 6.105e-09],
       [4.118e-04, 4.014e-04, ..., 7.788e-09, 8.160e-09],
       ...,
       [2.780e-03, 1.424e-01, ..., 4.225e-06, 2.388e-05],
       [5.147e-02, 6.959e-02, ..., 1.694e-05, 5.811e-06]])
../_images/librosa-cqt-1.png