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Audio Features

Audio Features Preprocessing

Ludwig supports reads in audio files using Python's library SoundFile therefore supporting WAV, FLAC, OGG and MAT files.

  • audio_file_length_limit_in_s: (default 7.5): float value that defines the maximum limit of the audio file in seconds. All files longer than this limit are cut off. All files shorter than this limit are padded with padding_value
  • missing_value_strategy (default: backfill): what strategy to follow when there's a missing value in a binary column. The value should be one of fill_with_const (replaces the missing value with a specific value specified with the fill_value parameter), fill_with_mode (replaces the missing values with the most frequent value in the column), fill_with_mean (replaces the missing values with the mean of the values in the column), backfill (replaces the missing values with the next valid value).
  • in_memory (default true): defines whether image dataset will reside in memory during the training process or will be dynamically fetched from disk (useful for large datasets). In the latter case a training batch of input images will be fetched from disk each training iteration. At the moment only in_memory = true is supported.
  • padding_value: (default 0): float value that is used for padding.
  • norm: (default null) the normalization method that can be used for the input data. Supported methods: null (data is not normalized), per_file (z-norm is applied on a “per file” level)
  • audio_feature: (default { type: raw }) dictionary that takes as input the audio feature type as well as additional parameters if type != raw. The following parameters can/should be defined in the dictionary:
  • type (default raw): defines the type of audio features to be used. Supported types at the moment are raw, stft, stft_phase, group_delay. For more detail, check Audio Input Features and Encoders.
  • window_length_in_s: defines the window length used for the short time Fourier transformation (only needed if type != raw).
  • window_shift_in_s: defines the window shift used for the short time Fourier transformation (also called hop_length) (only needed if type != raw).
  • num_fft_points: (default window_length_in_s * sample_rate of audio file) defines the number of fft points used for the short time Fourier transformation. If num_fft_points > window_length_in_s * sample_rate, then the signal is zero-padded at the end. num_fft_points has to be >= window_length_in_s * sample_rate (only needed if type != raw).
  • window_type: (default hamming): defines the type window the signal is weighted before the short time Fourier transformation. All windows provided by scipy’s window function can be used (only needed if type != raw).
  • num_filter_bands: defines the number of filters used in the filterbank (only needed if type == fbank).

Example of a preprocessing specification (assuming the audio files have a sample rate of 16000):

name: audio_path
type: audio
  audio_file_length_limit_in_s: 7.5
    type: stft
    window_length_in_s: 0.04
    window_shift_in_s: 0.02
    num_fft_points: 800
    window_type: boxcar

Audio Input Features and Encoders

Audio files are transformed into one of the following types according to type in audio_feature in preprocessing.

  • raw: audio file is transformed into a float valued tensor of size N x L x W (where N is the size of the dataset and L corresponds to audio_file_length_limit_in_s * sample_rate and W = 1).
  • stft: audio is transformed to the stft magnitude. Audio file is transformed into a float valued tensor of size N x L x W (where N is the size of the dataset, L corresponds to ceil(audio_file_length_limit_in_s * sample_rate - window_length_in_s * sample_rate + 1/ window_shift_in_s * sample_rate) + 1 and W corresponds to num_fft_points / 2).
  • fbank: audio file is transformed to FBANK features (also called log Mel-filter bank values). FBANK features are implemented according to their definition in the HTK Book: Raw Signal -> Preemphasis -> DC mean removal -> stft magnitude -> Power spectrum: stft^2 -> mel-filter bank values: triangular filters equally spaced on a Mel-scale are applied -> log-compression: log(). Overall the audio file is transformed into a float valued tensor of size N x L x W with N,L being equal to the ones in stft and W being equal to num_filter_bands.
  • stft_phase: the phase information for each stft bin is appended to the stft magnitude so that the audio file is transformed into a float valued tensor of size N x L x 2W with N,L,W being equal to the ones in stft.
  • group_delay: audio is transformed to group delay features according to Equation (23) in this paper. Group_delay features has the same tensor size as stft.

The encoders are the same used for the Sequence Features. The only difference is that time series features don't have an embedding layer at the beginning, so the b x s placeholders (where b is the batch size and s is the sequence length) are directly mapped to a b x s x w (where w is W as described above) tensor and then passed to the different sequential encoders.

Audio Output Features and Decoders

There are no audio decoders at the moment (WIP), so audio cannot be used as output features.

Audio Features Measures

As no audio decoders are available at the moment, there are also no audio measures.