librosa/core/spectrum.py
def phase_vocoder(D, rate, hop_length=None):
    """Phase vocoder.  Given an STFT matrix D, speed up by a factor of `rate`

    Based on the implementation provided by [1]_.

    .. [1] Ellis, D. P. W. "A phase vocoder in Matlab."
        Columbia University, 2002.
        http://www.ee.columbia.edu/~dpwe/resources/matlab/pvoc/

    Examples
    --------
    >>> # Play at double speed
    >>> y, sr   = librosa.load(librosa.util.example_audio_file())
    >>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
    >>> D_fast  = librosa.phase_vocoder(D, 2.0, hop_length=512)
    >>> y_fast  = librosa.istft(D_fast, hop_length=512)

    >>> # Or play at 1/3 speed
    >>> y, sr   = librosa.load(librosa.util.example_audio_file())
    >>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
    >>> D_slow  = librosa.phase_vocoder(D, 1./3, hop_length=512)
    >>> y_slow  = librosa.istft(D_slow, hop_length=512)

    Parameters
    ----------
    D : np.ndarray [shape=(d, t), dtype=complex]
        STFT matrix

    rate :  float > 0 [scalar]
        Speed-up factor: `rate > 1` is faster, `rate < 1` is slower.

    hop_length : int > 0 [scalar] or None
        The number of samples between successive columns of `D`.

        If None, defaults to `n_fft/4 = (D.shape[0]-1)/2`

    Returns
    -------
    D_stretched : np.ndarray [shape=(d, t / rate), dtype=complex]
        time-stretched STFT
    """

    n_fft = 2 * (D.shape[0] - 1)

    if hop_length is None:
        hop_length = int(n_fft // 4)

    time_steps = np.arange(0, D.shape[1], rate, dtype=np.float)

    # Create an empty output array
    d_stretch = np.zeros((D.shape[0], len(time_steps)), D.dtype, order='F')

    # Expected phase advance in each bin
    phi_advance = np.linspace(0, np.pi * hop_length, D.shape[0])

    # Phase accumulator; initialize to the first sample
    phase_acc = np.angle(D[:, 0])

    # Pad 0 columns to simplify boundary logic
    D = np.pad(D, [(0, 0), (0, 2)], mode='constant')

    for (t, step) in enumerate(time_steps):

        columns = D[:, int(step):int(step + 2)]

        # Weighting for linear magnitude interpolation
        alpha = np.mod(step, 1.0)
        mag = ((1.0 - alpha) * np.abs(columns[:, 0])
               + alpha * np.abs(columns[:, 1]))

        # Store to output array
        d_stretch[:, t] = mag * np.exp(1.j * phase_acc)

        # Compute phase advance
        dphase = (np.angle(columns[:, 1])
                  - np.angle(columns[:, 0])
                  - phi_advance)

        # Wrap to -pi:pi range
        dphase = dphase - 2.0 * np.pi * np.round(dphase / (2.0 * np.pi))

        # Accumulate phase
        phase_acc += phi_advance + dphase

    return d_stretch
def phase_vocoder(D, rate, hop_length=None):
    """Phase vocoder.  Given an STFT matrix D, speed up by a factor of `rate`

    Based on the implementation provided by [1]_.

    .. [1] Ellis, D. P. W. "A phase vocoder in Matlab."
        Columbia University, 2002.
        http://www.ee.columbia.edu/~dpwe/resources/matlab/pvoc/

    Examples
    --------
    >>> # Play at double speed
    >>> y, sr   = librosa.load(librosa.util.example_audio_file())
    >>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
    >>> D_fast  = librosa.phase_vocoder(D, 2.0, hop_length=512)
    >>> y_fast  = librosa.istft(D_fast, hop_length=512)

    >>> # Or play at 1/3 speed
    >>> y, sr   = librosa.load(librosa.util.example_audio_file())
    >>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
    >>> D_slow  = librosa.phase_vocoder(D, 1./3, hop_length=512)
    >>> y_slow  = librosa.istft(D_slow, hop_length=512)

    Parameters
    ----------
    D : np.ndarray [shape=(d, t), dtype=complex]
        STFT matrix

    rate :  float > 0 [scalar]
        Speed-up factor: `rate > 1` is faster, `rate < 1` is slower.

    hop_length : int > 0 [scalar] or None
        The number of samples between successive columns of `D`.

        If None, defaults to `n_fft/4 = (D.shape[0]-1)/2`

    Returns
    -------
    D_stretched : np.ndarray [shape=(d, t / rate), dtype=complex]
        time-stretched STFT
    """

    n_fft = 2 * (D.shape[0] - 1)

    if hop_length is None:
        hop_length = int(n_fft // 4)

    time_steps = np.arange(0, D.shape[1], rate, dtype=np.float)

    # Create an empty output array
    d_stretch = np.zeros((D.shape[0], len(time_steps)), D.dtype, order='F')

    # Expected phase advance in each bin
    phi_advance = np.linspace(0, np.pi * hop_length, D.shape[0])

    # Phase accumulator; initialize to the first sample
    phase_acc = np.angle(D[:, 0])

    # Pad 0 columns to simplify boundary logic
    D = np.pad(D, [(0, 0), (0, 2)], mode='constant')

    for (t, step) in enumerate(time_steps):

        columns = D[:, int(step):int(step + 2)]

        # Weighting for linear magnitude interpolation
        alpha = np.mod(step, 1.0)
        mag = ((1.0 - alpha) * np.abs(columns[:, 0])
               + alpha * np.abs(columns[:, 1]))

        # Store to output array
        d_stretch[:, t] = mag * np.exp(1.j * phase_acc)

        # Compute phase advance
        dphase = (np.angle(columns[:, 1])
                  - np.angle(columns[:, 0])
                  - phi_advance)

        # Wrap to -pi:pi range
        dphase = dphase - 2.0 * np.pi * np.round(dphase / (2.0 * np.pi))

        # Accumulate phase
        phase_acc += phi_advance + dphase

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

    Based on the implementation provided by [1]_.

    .. [1] Ellis, D. P. W. "A phase vocoder in Matlab."
        Columbia University, 2002.
        http://www.ee.columbia.edu/~dpwe/resources/matlab/pvoc/

    Examples
    --------
    >>> # Play at double speed
    >>> y, sr   = librosa.load(librosa.util.example_audio_file())
    >>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
    >>> D_fast  = librosa.phase_vocoder(D, 2.0, hop_length=512)
    >>> y_fast  = librosa.istft(D_fast, hop_length=512)

    >>> # Or play at 1/3 speed
    >>> y, sr   = librosa.load(librosa.util.example_audio_file())
    >>> D       = librosa.stft(y, n_fft=2048, hop_length=512)
    >>> D_slow  = librosa.phase_vocoder(D, 1./3, hop_length=512)
    >>> y_slow  = librosa.istft(D_slow, hop_length=512)

    Parameters
    ----------
    D : np.ndarray [shape=(d, t), dtype=complex]
        STFT matrix

    rate :  float > 0 [scalar]
        Speed-up factor: `rate > 1` is faster, `rate < 1` is slower.

    hop_length : int > 0 [scalar] or None
        The number of samples between successive columns of `D`.

        If None, defaults to `n_fft/4 = (D.shape[0]-1)/2`

    Returns
    -------
    D_stretched : np.ndarray [shape=(d, t / rate), dtype=complex]
        time-stretched STFT