Simple Audio Classification with Keras

Introduction

In this tutorial we will build a deep learning model to classify words. We will use tfdatasets to handle data IO and pre-processing, and Keras to build and train the model.

We will use the Speech Commands dataset which consists of 65,000 one-second audio files of people saying 30 different words. Each file contains a single spoken English word. The dataset was released by Google under CC License.

Our model is a Keras port of the TensorFlow tutorial on Simple Audio Recognition which in turn was inspired by Convolutional Neural Networks for Small-footprint Keyword Spotting. There are other approaches to the speech recognition task, like recurrent neural networks, dilated (atrous) convolutions or Learning from Between-class Examples for Deep Sound Recognition.

The model we will implement here is not the state of the art for audio recognition systems, which are way more complex, but is relatively simple and fast to train. Plus, we show how to efficiently use tfdatasets to preprocess and serve data.

Audio representation

Many deep learning models are end-to-end, i.e. we let the model learn useful representations directly from the raw data. However, audio data grows very fast – 16,000 samples per second with a very rich structure at many time-scales. In order to avoid having to deal with raw wave sound data, researchers usually use some kind of feature engineering.

Every sound wave can be represented by its spectrum, and digitally it can be computed using the Fast Fourier Transform (FFT).

A common way to represent audio data is to break it into small chunks, which usually overlap. For each chunk we use the FFT to calculate the magnitude of the frequency spectrum. The spectra are then combined, side by side, to form what we call a spectrogram.

It’s also common for speech recognition systems to further transform the spectrum and compute the Mel-Frequency Cepstral Coefficients. This transformation takes into account that the human ear can’t discern the difference between two closely spaced frequencies and smartly creates bins on the frequency axis. A great tutorial on MFCCs can be found here.

After this procedure, we have an image for each audio sample and we can use convolutional neural networks, the standard architecture type in image recognition models.

Downloading

First, let’s download data to a directory in our project. You can either download from this link (~1GB) or from R with:

dir.create("data")

download.file(
  url = "http://download.tensorflow.org/data/speech_commands_v0.01.tar.gz", 
  destfile = "data/speech_commands_v0.01.tar.gz"
)

untar("data/speech_commands_v0.01.tar.gz", exdir = "data/speech_commands_v0.01")

Inside the data directory we will have a folder called speech_commands_v0.01. The WAV audio files inside this directory are organised in sub-folders with the label names. For example, all one-second audio files of people speaking the word “bed” are inside the bed directory. There are 30 of them and a special one called _background_noise_ which contains various patterns that could be mixed in to simulate background noise.

Importing

In this step we will list all audio .wav files into a tibble with 3 columns:

• fname: the file name;
• class: the label for each audio file;
• class_id: a unique integer number starting from zero for each class – used to one-hot encode the classes.

This will be useful to the next step when we will create a generator using the tfdatasets package.

Generator

We will now create our Dataset, which in the context of tfdatasets, adds operations to the TensorFlow graph in order to read and pre-process data. Since they are TensorFlow ops, they are executed in C++ and in parallel with model training.

The generator we will create will be responsible for reading the audio files from disk, creating the spectrogram for each one and batching the outputs.

Let’s start by creating the dataset from slices of the data.frame with audio file names and classes we just created.

Now, let’s define the parameters for spectrogram creation. We need to define window_size_ms which is the size in milliseconds of each chunk we will break the audio wave into, and window_stride_ms, the distance between the centers of adjacent chunks:

window_size_ms <- 30
window_stride_ms <- 10

Now we will convert the window size and stride from milliseconds to samples. We are considering that our audio files have 16,000 samples per second (1000 ms).

window_size <- as.integer(16000*window_size_ms/1000)
stride <- as.integer(16000*window_stride_ms/1000)

We will obtain other quantities that will be useful for spectrogram creation, like the number of chunks and the FFT size, i.e., the number of bins on the frequency axis. The function we are going to use to compute the spectrogram doesn’t allow us to change the FFT size and instead by default uses the first power of 2 greater than the window size.

We will now use dataset_map which allows us to specify a pre-processing function for each observation (line) of our dataset. It’s in this step that we read the raw audio file from disk and create its spectrogram and the one-hot encoded response vector.

# shortcuts to used TensorFlow modules.
audio_ops <- tf$contrib$framework$python$ops$audio_ops

ds <- ds %>%
  dataset_map(function(obs) {
    
    # a good way to debug when building tfdatsets pipelines is to use a print
    # statement like this:
    # print(str(obs))
    
    # decoding wav files
    audio_binary <- tf$read_file(tf$reshape(obs$fname, shape = list()))
    wav <- audio_ops$decode_wav(audio_binary, desired_channels = 1)
    
    # create the spectrogram
    spectrogram <- audio_ops$audio_spectrogram(
      wav$audio, 
      window_size = window_size, 
      stride = stride,
      magnitude_squared = TRUE
    )
    
    # normalization
    spectrogram <- tf$log(tf$abs(spectrogram) + 0.01)
    
    # moving channels to last dim
    spectrogram <- tf$transpose(spectrogram, perm = c(1L, 2L, 0L))
    
    # transform the class_id into a one-hot encoded vector
    response <- tf$one_hot(obs$class_id, 30L)
    
    list(spectrogram, response)
  }) 

Now, we will specify how we want batch observations from the dataset. We’re using dataset_shuffle since we want to shuffle observations from the dataset, otherwise it would follow the order of the df object. Then we use dataset_repeat in order to tell TensorFlow that we want to keep taking observations from the dataset even if all observations have already been used. And most importantly here, we use dataset_padded_batch to specify that we want batches of size 32, but they should be padded, ie. if some observation has a different size we pad it with zeroes. The padded shape is passed to dataset_padded_batch via the padded_shapes argument and we use NULL to state that this dimension doesn’t need to be padded.

ds <- ds %>% 
  dataset_shuffle(buffer_size = 100) %>%
  dataset_repeat() %>%
  dataset_padded_batch(
    batch_size = 32, 
    padded_shapes = list(
      shape(n_chunks, fft_size, NULL), 
      shape(NULL)
    )
  )

This is our dataset specification, but we would need to rewrite all the code for the validation data, so it’s good practice to wrap this into a function of the data and other important parameters like window_size_ms and window_stride_ms. Below, we will define a function called data_generator that will create the generator depending on those inputs.

data_generator <- function(df, batch_size, shuffle = TRUE, 
                           window_size_ms = 30, window_stride_ms = 10) {
  
  window_size <- as.integer(16000*window_size_ms/1000)
  stride <- as.integer(16000*window_stride_ms/1000)
  fft_size <- as.integer(2^trunc(log(window_size, 2)) + 1)
  n_chunks <- length(seq(window_size/2, 16000 - window_size/2, stride))
  
  ds <- tensor_slices_dataset(df)
  
  if (shuffle) 
    ds <- ds %>% dataset_shuffle(buffer_size = 100)  
  
  ds <- ds %>%
    dataset_map(function(obs) {
      
      # decoding wav files
      audio_binary <- tf$read_file(tf$reshape(obs$fname, shape = list()))
      wav <- audio_ops$decode_wav(audio_binary, desired_channels = 1)
      
      # create the spectrogram
      spectrogram <- audio_ops$audio_spectrogram(
        wav$audio, 
        window_size = window_size, 
        stride = stride,
        magnitude_squared = TRUE
      )
      
      spectrogram <- tf$log(tf$abs(spectrogram) + 0.01)
      spectrogram <- tf$transpose(spectrogram, perm = c(1L, 2L, 0L))
      
      # transform the class_id into a one-hot encoded vector
      response <- tf$one_hot(obs$class_id, 30L)
      
      list(spectrogram, response)
    }) %>%
    dataset_repeat()
  
  ds <- ds %>% 
    dataset_padded_batch(batch_size, list(shape(n_chunks, fft_size, NULL), shape(NULL)))
  
  ds
}

Now, we can define training and validation data generators. It’s worth noting that executing this won’t actually compute any spectrogram or read any file. It will only define in the TensorFlow graph how it should read and pre-process data.

set.seed(6)
id_train <- sample(nrow(df), size = 0.7*nrow(df))

ds_train <- data_generator(
  df[id_train,], 
  batch_size = 32, 
  window_size_ms = 30, 
  window_stride_ms = 10
)
ds_validation <- data_generator(
  df[-id_train,], 
  batch_size = 32, 
  shuffle = FALSE, 
  window_size_ms = 30, 
  window_stride_ms = 10
)

To actually get a batch from the generator we could create a TensorFlow session and ask it to run the generator. For example:

sess <- tf$Session()
batch <- next_batch(ds_train)
str(sess$run(batch))

List of 2
 $ : num [1:32, 1:98, 1:257, 1] -4.6 -4.6 -4.61 -4.6 -4.6 ...
 $ : num [1:32, 1:30] 0 0 0 0 0 0 0 0 0 0 ...

Each time you run sess$run(batch) you should see a different batch of observations.

Model definition

Now that we know how we will feed our data we can focus on the model definition. The spectrogram can be treated like an image, so architectures that are commonly used in image recognition tasks should work well with the spectrograms too.

We will build a convolutional neural network similar to what we have built here for the MNIST dataset.

The input size is defined by the number of chunks and the FFT size. Like we explained earlier, they can be obtained from the window_size_ms and window_stride_ms used to generate the spectrogram.

We will now define our model using the Keras sequential API:

model <- keras_model_sequential()
model %>%  
  layer_conv_2d(input_shape = c(n_chunks, fft_size, 1), 
                filters = 32, kernel_size = c(3,3), activation = 'relu') %>% 
  layer_max_pooling_2d(pool_size = c(2, 2)) %>% 
  layer_conv_2d(filters = 64, kernel_size = c(3,3), activation = 'relu') %>% 
  layer_max_pooling_2d(pool_size = c(2, 2)) %>% 
  layer_conv_2d(filters = 128, kernel_size = c(3,3), activation = 'relu') %>% 
  layer_max_pooling_2d(pool_size = c(2, 2)) %>% 
  layer_conv_2d(filters = 256, kernel_size = c(3,3), activation = 'relu') %>% 
  layer_max_pooling_2d(pool_size = c(2, 2)) %>% 
  layer_dropout(rate = 0.25) %>% 
  layer_flatten() %>% 
  layer_dense(units = 128, activation = 'relu') %>% 
  layer_dropout(rate = 0.5) %>% 
  layer_dense(units = 30, activation = 'softmax')

We used 4 layers of convolutions combined with max pooling layers to extract features from the spectrogram images and 2 dense layers at the top. Our network is comparatively simple when compared to more advanced architectures like ResNet or DenseNet that perform very well on image recognition tasks.

Now let’s compile our model. We will use categorical cross entropy as the loss function and use the Adadelta optimizer. It’s also here that we define that we will look at the accuracy metric during training.

model %>% compile(
  loss = loss_categorical_crossentropy,
  optimizer = optimizer_adadelta(),
  metrics = c('accuracy')
)

Model fitting

Now, we will fit our model. In Keras we can use TensorFlow Datasets as inputs to the fit_generator function and we will do it here.

model %>% fit_generator(
  generator = ds_train,
  steps_per_epoch = 0.7*nrow(df)/32,
  epochs = 10, 
  validation_data = ds_validation, 
  validation_steps = 0.3*nrow(df)/32
)

Epoch 1/10
1415/1415 [==============================] - 87s 62ms/step - loss: 2.0225 - acc: 0.4184 - val_loss: 0.7855 - val_acc: 0.7907
Epoch 2/10
1415/1415 [==============================] - 75s 53ms/step - loss: 0.8781 - acc: 0.7432 - val_loss: 0.4522 - val_acc: 0.8704
Epoch 3/10
1415/1415 [==============================] - 75s 53ms/step - loss: 0.6196 - acc: 0.8190 - val_loss: 0.3513 - val_acc: 0.9006
Epoch 4/10
1415/1415 [==============================] - 75s 53ms/step - loss: 0.4958 - acc: 0.8543 - val_loss: 0.3130 - val_acc: 0.9117
Epoch 5/10
1415/1415 [==============================] - 75s 53ms/step - loss: 0.4282 - acc: 0.8754 - val_loss: 0.2866 - val_acc: 0.9213
Epoch 6/10
1415/1415 [==============================] - 76s 53ms/step - loss: 0.3852 - acc: 0.8885 - val_loss: 0.2732 - val_acc: 0.9252
Epoch 7/10
1415/1415 [==============================] - 75s 53ms/step - loss: 0.3566 - acc: 0.8991 - val_loss: 0.2700 - val_acc: 0.9269
Epoch 8/10
1415/1415 [==============================] - 76s 54ms/step - loss: 0.3364 - acc: 0.9045 - val_loss: 0.2573 - val_acc: 0.9284
Epoch 9/10
1415/1415 [==============================] - 76s 53ms/step - loss: 0.3220 - acc: 0.9087 - val_loss: 0.2537 - val_acc: 0.9323
Epoch 10/10
1415/1415 [==============================] - 76s 54ms/step - loss: 0.2997 - acc: 0.9150 - val_loss: 0.2582 - val_acc: 0.9323

The model’s accuracy is 93.23%. Let’s learn how to make predictions and take a look at the confusion matrix.

Making predictions

We can use thepredict_generator function to make predictions on a new dataset. Let’s make predictions for our validation dataset.
The predict_generator function needs a step argument which is the number of times the generator will be called.

We can calculate the number of steps by knowing the batch size, and the size of the validation dataset.

df_validation <- df[-id_train,]
n_steps <- nrow(df_validation)/32 + 1

We can then use the predict_generator function:

predictions <- predict_generator(
  model, 
  ds_validation, 
  steps = n_steps
  )
str(predictions)

num [1:19424, 1:30] 1.22e-13 7.30e-19 5.29e-10 6.66e-22 1.12e-17 ...

This will output a matrix with 30 columns – one for each word and n_steps*batch_size number of rows. Note that it starts repeating the dataset at the end to create a full batch.

We can compute the predicted class by taking the column with the highest probability, for example.

classes <- apply(predictions, 1, which.max) - 1

A nice visualization of the confusion matrix is to create an alluvial diagram:

library(dplyr)
library(alluvial)
x <- df_validation %>%
  mutate(pred_class_id = head(classes, nrow(df_validation))) %>%
  left_join(
    df_validation %>% distinct(class_id, class) %>% rename(pred_class = class),
    by = c("pred_class_id" = "class_id")
  ) %>%
  mutate(correct = pred_class == class) %>%
  count(pred_class, class, correct)

alluvial(
  x %>% select(class, pred_class),
  freq = x$n,
  col = ifelse(x$correct, "lightblue", "red"),
  border = ifelse(x$correct, "lightblue", "red"),
  alpha = 0.6,
  hide = x$n < 20
)

We can see from the diagram that the most relevant mistake our model makes is to classify “tree” as “three”. There are other common errors like classifying “go” as “no”, “up” as “off”. At 93% accuracy for 30 classes, and considering the errors we can say that this model is pretty reasonable.

The saved model occupies 25Mb of disk space, which is reasonable for a desktop but may not be on small devices. We could train a smaller model, with fewer layers, and see how much the performance decreases.

In speech recognition tasks its also common to do some kind of data augmentation by mixing a background noise to the spoken audio, making it more useful for real applications where it’s common to have other irrelevant sounds happening in the environment.

The full code to reproduce this tutorial is available here.