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librosa.core.cqt

librosa.core.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='reflect', res_type=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

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.core.stft and np.pad.

res_typestring [optional]

The resampling mode for recursive downsampling.

By default, cqt will adaptively select a resampling mode which trades off accuracy at high frequencies for efficiency at low frequencies.

You can override this by specifying a resampling mode as supported by librosa.core.resample. For example, res_type=’fft’ will use a high-quality, but potentially slow FFT-based down-sampling, while res_type=’polyphase’ will use a fast, but potentially inaccurate down-sampling.

Returns
CQTnp.ndarray [shape=(n_bins, t), dtype=np.complex or np.float]

Constant-Q value each frequency at each time.

Raises
ParameterError

If hop_length is not an integer multiple of 2**(n_bins / bins_per_octave)

Or if y is too short to support the frequency range of the CQT.

See also

librosa.core.resample
librosa.util.normalize

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.util.example_audio_file())
>>> C = np.abs(librosa.cqt(y, sr=sr))
>>> librosa.display.specshow(librosa.amplitude_to_db(C, ref=np.max),
...                          sr=sr, x_axis='time', y_axis='cqt_note')
>>> plt.colorbar(format='%+2.0f dB')
>>> plt.title('Constant-Q power spectrum')
>>> plt.tight_layout()
>>> plt.show()
../_images/librosa-core-cqt-1_00_00.png

Limit the frequency range

>>> C = np.abs(librosa.cqt(y, sr=sr, fmin=librosa.note_to_hz('C2'),
...                 n_bins=60))
>>> C
array([[  8.827e-04,   9.293e-04, ...,   3.133e-07,   2.942e-07],
       [  1.076e-03,   1.068e-03, ...,   1.153e-06,   1.148e-06],
       ...,
       [  1.042e-07,   4.087e-07, ...,   1.612e-07,   1.928e-07],
       [  2.363e-07,   5.329e-07, ...,   1.294e-07,   1.611e-07]])

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([[  1.536e-05,   5.848e-05, ...,   3.241e-07,   2.453e-07],
       [  1.856e-03,   1.854e-03, ...,   2.397e-08,   3.549e-08],
       ...,
       [  2.034e-07,   4.245e-07, ...,   6.213e-08,   1.463e-07],
       [  4.896e-08,   5.407e-07, ...,   9.176e-08,   1.051e-07]])