Training and making predictions for a sound classifier model is a three stage process:
- Signal preprocessing
- A pretrained neural network is used to extract deep features
- A custom neural network is used to make the predictions
Details below about each stage.
Several audio transformations happen during this stage. For someone without sound domain knowledge, this is probably the most complicated part. Nothing about this stage is updated based on the input data (i.e. nothing is learned in this stage).
At a high level, the preprocessing pipeline does the following:
- The raw pulse code modulation data from the wav file is converted to floats on a [-1.0, +1.0] scale.
- If there are two channels, the elements are averaged to produce one channel.
- The data is resampled to only 16,000 samples per second.
- The data is broken up into several overlapping windows.
- A Hamming Window is applied to each windows.
- The Power Spectrum is calculated, using a Fast Fourier Transformation.
- Frequencies above and below certain thresholds are dropped.
- Mel Frequency Filter Banks are applied.
- Finally the natural logarithm is taken of all values.
The preprocessing pipeline takes 975ms worth of audio as input (exact input length depends on sample rate) and produces an array of shape (96, 64).
VGGish is a pretrained Convolutional Neural Network from Google, see their paper and their GitHub page for more details. As the name suggests, the architecture of this network is inspired by the famous VGG networks used for image classification. The network consists of a series of convolution and activation layers, optionally followed by a max pooling layer. This network contains 17 layers in total.
This network is kept static during model training. We have removed the last three layers of the original VGGish model. We use the widest layer, from the original network, as our input data for the final stage. This modified VGGish model outputs a double vector of length 12,288. On non-Linux systems, the model has also been eight bit quantized to reduce its size.
This is the only stage which is updated based on the input data. During training of a sound classifier this model is trained using the features from VGGish and the input labels. During prediction, only a forward pass is done using this network. This custom neural network is a simple three layer neural network. The first two layers are dense layers, with 100 units each. These layers use RELU activation). The final layer is a softmax. The number of units in this layer is equal to the number of labels.