Signal Processing (contrib)

tf.contrib.signal is a module for signal processing primitives. All operations have GPU support and are differentiable. This module is especially helpful for building TensorFlow models that process or generate audio, though the techniques are useful in many domains.

Framing variable length sequences

When dealing with variable length signals (e.g. audio) it is common to "frame" them into multiple fixed length windows. These windows can overlap if the 'step' of the frame is less than the frame length. tf.contrib.signal.frame does exactly this. For example:

.html# A batch of float32 time-domain signals in the range [-1, 1] with shape
.html# [batch_size, signal_length]. Both batch_size and signal_length may be unknown.
signals = tf.placeholder(tf.float32, [None, None])

.html# Compute a [batch_size, ?, 128] tensor of fixed length, overlapping windows
.html# where each window overlaps the previous by 75% (frame_length - frame_step
.html# samples of overlap).
frames = tf.contrib.signal.frame(signals, frame_length=128, frame_step=32)

The axis parameter to tf.contrib.signal.frame allows you to frame tensors with inner structure (e.g. a spectrogram):

.html# `magnitude_spectrograms` is a [batch_size, ?, 129] tensor of spectrograms. We
.html# would like to produce overlapping fixed-size spectrogram patches; for example,
.html# for use in a situation where a fixed size input is needed.
magnitude_spectrograms = tf.abs(tf.contrib.signal.stft(
    signals, frame_length=256, frame_step=64, fft_length=256))

.html# `spectrogram_patches` is a [batch_size, ?, 64, 129] tensor containing a
.html# variable number of [64, 129] spectrogram patches per batch item.
spectrogram_patches = tf.contrib.signal.frame(
    magnitude_spectrograms, frame_length=64, frame_step=16, axis=1)

Reconstructing framed sequences and applying a tapering window

tf.contrib.signal.overlap_and_add can be used to reconstruct a signal from a framed representation. For example, the following code reconstructs the signal produced in the preceding example:

.html# Reconstructs `signals` from `frames` produced in the above example. However,
.html# the magnitude of `reconstructed_signals` will be greater than `signals`.
reconstructed_signals = tf.contrib.signal.overlap_and_add(frames, frame_step=32)

Note that because frame_step is 25% of frame_length in the above example, the resulting reconstruction will have a greater magnitude than the original signals. To compensate for this, we can use a tapering window function. If the window function satisfies the Constant Overlap-Add (COLA) property for the given frame step, then it will recover the original signals.

tf.contrib.signal.hamming_window and tf.contrib.signal.hann_window both satisfy the COLA property for a 75% overlap.

frame_length = 128
frame_step = 32
windowed_frames = frames * tf.contrib.signal.hann_window(frame_length)
reconstructed_signals = tf.contrib.signal.overlap_and_add(
    windowed_frames, frame_step)

Computing spectrograms

A spectrogram is a time-frequency decomposition of a signal that indicates its frequency content over time. The most common approach to computing spectrograms is to take the magnitude of the Short-time Fourier Transform (STFT), which tf.contrib.signal.stft can compute as follows:

.html# A batch of float32 time-domain signals in the range [-1, 1] with shape
.html# [batch_size, signal_length]. Both batch_size and signal_length may be unknown.
signals = tf.placeholder(tf.float32, [None, None])

.html# `stfts` is a complex64 Tensor representing the Short-time Fourier Transform of
.html# each signal in `signals`. Its shape is [batch_size, ?, fft_unique_bins]
.html# where fft_unique_bins = fft_length // 2 + 1 = 513.
stfts = tf.contrib.signal.stft(signals, frame_length=1024, frame_step=512,

.html# A power spectrogram is the squared magnitude of the complex-valued STFT.
.html# A float32 Tensor of shape [batch_size, ?, 513].
power_spectrograms = tf.real(stfts * tf.conj(stfts))

.html# An energy spectrogram is the magnitude of the complex-valued STFT.
.html# A float32 Tensor of shape [batch_size, ?, 513].
magnitude_spectrograms = tf.abs(stfts)

You may use a power spectrogram or a magnitude spectrogram; each has its advantages. Note that if you apply logarithmic compression, the power spectrogram and magnitude spectrogram will differ by a factor of 2.

Logarithmic compression

It is common practice to apply a compressive nonlinearity such as a logarithm or power-law compression to spectrograms. This helps to balance the importance of detail in low and high energy regions of the spectrum, which more closely matches human auditory sensitivity.

When compressing with a logarithm, it's a good idea to use a stabilizing offset to avoid high dynamic ranges caused by the singularity at zero.

log_offset = 1e-6
log_magnitude_spectrograms = tf.log(magnitude_spectrograms + log_offset)

Computing log-mel spectrograms

When working with spectral representations of audio, the mel scale is a common reweighting of the frequency dimension, which results in a lower-dimensional and more perceptually-relevant representation of the audio.

tf.contrib.signal.linear_to_mel_weight_matrix produces a matrix you can use to convert a spectrogram to the mel scale.

.html# Warp the linear-scale, magnitude spectrograms into the mel-scale.
num_spectrogram_bins = magnitude_spectrograms.shape[-1].value
lower_edge_hertz, upper_edge_hertz, num_mel_bins = 80.0, 7600.0, 64
linear_to_mel_weight_matrix = tf.contrib.signal.linear_to_mel_weight_matrix(
  num_mel_bins, num_spectrogram_bins, sample_rate, lower_edge_hertz,
mel_spectrograms = tf.tensordot(
  magnitude_spectrograms, linear_to_mel_weight_matrix, 1)
.html# Note: Shape inference for <a href="../../api_docs/python/tf/tensordot"><code>tf.tensordot</code></a> does not currently handle this case.

If desired, compress the mel spectrogram magnitudes. For example, you may use logarithmic compression (as discussed in the previous section).

Order matters! Compressing the spectrogram magnitudes after reweighting the frequencies is different from reweighting the compressed spectrogram magnitudes. According to the perceptual justification of the mel scale, conversion from linear scale entails summing intensity or energy among adjacent bands, i.e. it should be applied before logarithmic compression. Taking the weighted sum of log-compressed values amounts to multiplying the pre-logarithm values, which rarely, if ever, makes sense.

log_offset = 1e-6
log_mel_spectrograms = tf.log(mel_spectrograms + log_offset)

Computing Mel-Frequency Cepstral Coefficients (MFCCs)

Call tf.contrib.signal.mfccs_from_log_mel_spectrograms to compute MFCCs from log-magnitude, mel-scale spectrograms (as computed in the preceding example):

num_mfccs = 13
.html# Keep the first `num_mfccs` MFCCs.
mfccs = tf.contrib.signal.mfccs_from_log_mel_spectrograms(
    log_mel_spectrograms)[..., :num_mfccs]