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librosa.phase_vocoder

librosa.phase_vocoder(D, *, rate=None, t_out=None, kind='linear', hop_length=<DEPRECATED parameter>, n_fft=<DEPRECATED parameter>)[source]

Phase vocoder. Given an STFT matrix D, speed up by a factor of rate

Based on the implementation provided by [1].

This is a simplified implementation, intended primarily for reference and pedagogical purposes. It makes no attempt to handle transients, and is likely to produce many audible artifacts. For a higher quality implementation, we recommend the RubberBand library [2] and its Python wrapper pyrubberband.

Parameters:
Dnp.ndarray [shape=(…, n_bins, n_frames), dtype=complex]

STFT matrix

ratefloat > 0 [scalar]

Speed-up factor: rate > 1 is faster, rate < 1 is slower. Use this for constant-rate time-stretching.

t_outnp.ndarray [shape=(n_output,)], optional

Optional array of fractional input frame indices specifying the input time corresponding to each output frame. Values must lie in [0, n_frames). This can be used for variable-rate time-stretching. If given, rate must be None.

kindstr

Interpolation kind for magnitude interpolation, e.g., ‘linear’ or ‘nearest’. See scipy.interpolate.interp1d for a full list of supported options.

hop_lengthint > 0 [scalar] or None

The number of samples between successive columns of D.

If None, defaults to n_fft//4 = (D.shape[-2]-1)//2

Warning

This parameter is deprecated as of 1.0 and will be removed in 1.1. It is unused in the current implementation.

n_fftint > 0 or None

The number of samples per frame in D. By default (None), this will be inferred from the shape of D. However, if D was constructed using an odd-length window, the correct frame length can be specified here.

Warning

This parameter is deprecated as of 1.0 and will be removed in 1.1. It is unused in the current implementation.

Returns:
D_stretchednp.ndarray [shape=(…, n_bins, t / rate), dtype=complex]

time-stretched STFT. For variable-rate stretching, the output will have len(t_out) frames.

See also

pyrubberband

Examples

>>> # Play at double speed
>>> y, sr   = librosa.loadx('trumpet')
>>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
>>> D_fast  = librosa.phase_vocoder(D, rate=2.0, hop_length=512)
>>> y_fast  = librosa.istft(D_fast, hop_length=512)

>>> # Or play at 1/3 speed
>>> y, sr   = librosa.loadx('trumpet')
>>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
>>> D_slow  = librosa.phase_vocoder(D, rate=1./3, hop_length=512)
>>> y_slow  = librosa.istft(D_slow, hop_length=512)