Simple audio recognition: Recognizing keywords

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This tutorial demonstrates how to preprocess audio files in the WAV format and build and train a basic automatic speech recognition (ASR) model for recognizing ten different words. You will use a portion of the Speech Commands dataset (Warden, 2018), which contains short (one-second or less) audio clips of commands, such as "down", "go", "left", "no", "right", "stop", "up" and "yes".

Real-world speech and audio recognition systems are complex. But, like image classification with the MNIST dataset, this tutorial should give you a basic understanding of the techniques involved.

Setup

Import necessary modules and dependencies. You'll be using tf.keras.utils.audio_dataset_from_directory (introduced in TensorFlow 2.10), which helps generate audio classification datasets from directories of .wav files. You'll also need seaborn for visualization in this tutorial.

pip install -U -q tensorflow tensorflow_datasets

import os
import pathlib

import matplotlib.pyplot as plt
import numpy as np
import seaborn as sns
import tensorflow as tf

from tensorflow.keras import layers
from tensorflow.keras import models
from IPython import display

# Set the seed value for experiment reproducibility.
seed = 42
tf.random.set_seed(seed)
np.random.seed(seed)

2023-10-27 05:56:12.403728: E external/local_xla/xla/stream_executor/cuda/cuda_dnn.cc:9261] Unable to register cuDNN factory: Attempting to register factory for plugin cuDNN when one has already been registered
2023-10-27 05:56:12.403772: E external/local_xla/xla/stream_executor/cuda/cuda_fft.cc:607] Unable to register cuFFT factory: Attempting to register factory for plugin cuFFT when one has already been registered
2023-10-27 05:56:12.405347: E external/local_xla/xla/stream_executor/cuda/cuda_blas.cc:1515] Unable to register cuBLAS factory: Attempting to register factory for plugin cuBLAS when one has already been registered

Import the mini Speech Commands dataset

To save time with data loading, you will be working with a smaller version of the Speech Commands dataset. The original dataset consists of over 105,000 audio files in the WAV (Waveform) audio file format of people saying 35 different words. This data was collected by Google and released under a CC BY license.

Download and extract the mini_speech_commands.zip file containing the smaller Speech Commands datasets with tf.keras.utils.get_file:

DATASET_PATH = 'data/mini_speech_commands'

data_dir = pathlib.Path(DATASET_PATH)
if not data_dir.exists():
  tf.keras.utils.get_file(
      'mini_speech_commands.zip',
      origin="http://storage.googleapis.com/download.tensorflow.org/data/mini_speech_commands.zip",
      extract=True,
      cache_dir='.', cache_subdir='data')

Downloading data from http://storage.googleapis.com/download.tensorflow.org/data/mini_speech_commands.zip
182082353/182082353 [==============================] - 1s 0us/step

The dataset's audio clips are stored in eight folders corresponding to each speech command: no, yes, down, go, left, up, right, and stop:

commands = np.array(tf.io.gfile.listdir(str(data_dir)))
commands = commands[(commands != 'README.md') & (commands != '.DS_Store')]
print('Commands:', commands)

Commands: ['stop' 'up' 'yes' 'down' 'no' 'left' 'right' 'go']

Divided into directories this way, you can easily load the data using keras.utils.audio_dataset_from_directory.

The audio clips are 1 second or less at 16kHz. The output_sequence_length=16000 pads the short ones to exactly 1 second (and would trim longer ones) so that they can be easily batched.

train_ds, val_ds = tf.keras.utils.audio_dataset_from_directory(
    directory=data_dir,
    batch_size=64,
    validation_split=0.2,
    seed=0,
    output_sequence_length=16000,
    subset='both')

label_names = np.array(train_ds.class_names)
print()
print("label names:", label_names)

Found 8000 files belonging to 8 classes.
Using 6400 files for training.
Using 1600 files for validation.

label names: ['down' 'go' 'left' 'no' 'right' 'stop' 'up' 'yes']

The dataset now contains batches of audio clips and integer labels. The audio clips have a shape of (batch, samples, channels). 

train_ds.element_spec

(TensorSpec(shape=(None, 16000, None), dtype=tf.float32, name=None),
 TensorSpec(shape=(None,), dtype=tf.int32, name=None))

This dataset only contains single channel audio, so use the tf.squeeze function to drop the extra axis:

def squeeze(audio, labels):
  audio = tf.squeeze(audio, axis=-1)
  return audio, labels

train_ds = train_ds.map(squeeze, tf.data.AUTOTUNE)
val_ds = val_ds.map(squeeze, tf.data.AUTOTUNE)

The utils.audio_dataset_from_directory function only returns up to two splits. It's a good idea to keep a test set separate from your validation set. Ideally you'd keep it in a separate directory, but in this case you can use Dataset.shard to split the validation set into two halves. Note that iterating over any shard will load all the data, and only keep its fraction. 

test_ds = val_ds.shard(num_shards=2, index=0)
val_ds = val_ds.shard(num_shards=2, index=1)

for example_audio, example_labels in train_ds.take(1):  
  print(example_audio.shape)
  print(example_labels.shape)

(64, 16000)
(64,)

Let's plot a few audio waveforms:

label_names[[1,1,3,0]]

array(['go', 'go', 'no', 'down'], dtype='<U5')

plt.figure(figsize=(16, 10))
rows = 3
cols = 3
n = rows * cols
for i in range(n):
  plt.subplot(rows, cols, i+1)
  audio_signal = example_audio[i]
  plt.plot(audio_signal)
  plt.title(label_names[example_labels[i]])
  plt.yticks(np.arange(-1.2, 1.2, 0.2))
  plt.ylim([-1.1, 1.1])

png

Convert waveforms to spectrograms

The waveforms in the dataset are represented in the time domain. Next, you'll transform the waveforms from the time-domain signals into the time-frequency-domain signals by computing the short-time Fourier transform (STFT) to convert the waveforms to as spectrograms, which show frequency changes over time and can be represented as 2D images. You will feed the spectrogram images into your neural network to train the model.

A Fourier transform (tf.signal.fft) converts a signal to its component frequencies, but loses all time information. In comparison, STFT (tf.signal.stft) splits the signal into windows of time and runs a Fourier transform on each window, preserving some time information, and returning a 2D tensor that you can run standard convolutions on.

Create a utility function for converting waveforms to spectrograms:

  • The waveforms need to be of the same length, so that when you convert them to spectrograms, the results have similar dimensions. This can be done by simply zero-padding the audio clips that are shorter than one second (using tf.zeros).
  • When calling tf.signal.stft, choose the frame_length and frame_step parameters such that the generated spectrogram "image" is almost square. For more information on the STFT parameters choice, refer to this Coursera video on audio signal processing and STFT.
  • The STFT produces an array of complex numbers representing magnitude and phase. However, in this tutorial you'll only use the magnitude, which you can derive by applying tf.abs on the output of tf.signal.stft.
def get_spectrogram(waveform):
  # Convert the waveform to a spectrogram via a STFT.
  spectrogram = tf.signal.stft(
      waveform, frame_length=255, frame_step=128)
  # Obtain the magnitude of the STFT.
  spectrogram = tf.abs(spectrogram)
  # Add a `channels` dimension, so that the spectrogram can be used
  # as image-like input data with convolution layers (which expect
  # shape (`batch_size`, `height`, `width`, `channels`).
  spectrogram = spectrogram[..., tf.newaxis]
  return spectrogram

Next, start exploring the data. Print the shapes of one example's tensorized waveform and the corresponding spectrogram, and play the original audio:

for i in range(3):
  label = label_names[example_labels[i]]
  waveform = example_audio[i]
  spectrogram = get_spectrogram(waveform)

  print('Label:', label)
  print('Waveform shape:', waveform.shape)
  print('Spectrogram shape:', spectrogram.shape)
  print('Audio playback')
  display.display(display.Audio(waveform, rate=16000))

Label: go
Waveform shape: (16000,)
Spectrogram shape: (124, 129, 1)
Audio playback


Label: no
Waveform shape: (16000,)
Spectrogram shape: (124, 129, 1)
Audio playback


Label: left
Waveform shape: (16000,)
Spectrogram shape: (124, 129, 1)
Audio playback

Now, define a function for displaying a spectrogram:

def plot_spectrogram(spectrogram, ax):
  if len(spectrogram.shape) > 2:
    assert len(spectrogram.shape) == 3
    spectrogram = np.squeeze(spectrogram, axis=-1)
  # Convert the frequencies to log scale and transpose, so that the time is
  # represented on the x-axis (columns).
  # Add an epsilon to avoid taking a log of zero.
  log_spec = np.log(spectrogram.T + np.finfo(float).eps)
  height = log_spec.shape[0]
  width = log_spec.shape[1]
  X = np.linspace(0, np.size(spectrogram), num=width, dtype=int)
  Y = range(height)
  ax.pcolormesh(X, Y, log_spec)

Plot the example's waveform over time and the corresponding spectrogram (frequencies over time):

fig, axes = plt.subplots(2, figsize=(12, 8))
timescale = np.arange(waveform.shape[0])
axes[0].plot(timescale, waveform.numpy())
axes[0].set_title('Waveform')
axes[0].set_xlim([0, 16000])

plot_spectrogram(spectrogram.numpy(), axes[1])
axes[1].set_title('Spectrogram')
plt.suptitle(label.title())
plt.show()

png

Now, create spectrogram datasets from the audio datasets:

def make_spec_ds(ds):
  return ds.map(
      map_func=lambda audio,label: (get_spectrogram(audio), label),
      num_parallel_calls=tf.data.AUTOTUNE)

train_spectrogram_ds = make_spec_ds(train_ds)
val_spectrogram_ds = make_spec_ds(val_ds)
test_spectrogram_ds = make_spec_ds(test_ds)

Examine the spectrograms for different examples of the dataset:

for example_spectrograms, example_spect_labels in train_spectrogram_ds.take(1):
  break

rows = 3
cols = 3
n = rows*cols
fig, axes = plt.subplots(rows, cols, figsize=(16, 9))

for i in range(n):
    r = i // cols
    c = i % cols
    ax = axes[r][c]
    plot_spectrogram(example_spectrograms[i].numpy(), ax)
    ax.set_title(label_names[example_spect_labels[i].numpy()])

plt.show()

png

Build and train the model

Add Dataset.cache and Dataset.prefetch operations to reduce read latency while training the model:

train_spectrogram_ds = train_spectrogram_ds.cache().shuffle(10000).prefetch(tf.data.AUTOTUNE)
val_spectrogram_ds = val_spectrogram_ds.cache().prefetch(tf.data.AUTOTUNE)
test_spectrogram_ds = test_spectrogram_ds.cache().prefetch(tf.data.AUTOTUNE)

For the model, you'll use a simple convolutional neural network (CNN), since you have transformed the audio files into spectrogram images.

Your tf.keras.Sequential model will use the following Keras preprocessing layers:

For the Normalization layer, its adapt method would first need to be called on the training data in order to compute aggregate statistics (that is, the mean and the standard deviation).

input_shape = example_spectrograms.shape[1:]
print('Input shape:', input_shape)
num_labels = len(label_names)

# Instantiate the `tf.keras.layers.Normalization` layer.
norm_layer = layers.Normalization()
# Fit the state of the layer to the spectrograms
# with `Normalization.adapt`.
norm_layer.adapt(data=train_spectrogram_ds.map(map_func=lambda spec, label: spec))

model = models.Sequential([
    layers.Input(shape=input_shape),
    # Downsample the input.
    layers.Resizing(32, 32),
    # Normalize.
    norm_layer,
    layers.Conv2D(32, 3, activation='relu'),
    layers.Conv2D(64, 3, activation='relu'),
    layers.MaxPooling2D(),
    layers.Dropout(0.25),
    layers.Flatten(),
    layers.Dense(128, activation='relu'),
    layers.Dropout(0.5),
    layers.Dense(num_labels),
])

model.summary()

Input shape: (124, 129, 1)
Model: "sequential"
_________________________________________________________________
 Layer (type)                Output Shape              Param #   
=================================================================
 resizing (Resizing)         (None, 32, 32, 1)         0         
                                                                 
 normalization (Normalizati  (None, 32, 32, 1)         3         
 on)                                                             
                                                                 
 conv2d (Conv2D)             (None, 30, 30, 32)        320       
                                                                 
 conv2d_1 (Conv2D)           (None, 28, 28, 64)        18496     
                                                                 
 max_pooling2d (MaxPooling2  (None, 14, 14, 64)        0         
 D)                                                              
                                                                 
 dropout (Dropout)           (None, 14, 14, 64)        0         
                                                                 
 flatten (Flatten)           (None, 12544)             0         
                                                                 
 dense (Dense)               (None, 128)               1605760   
                                                                 
 dropout_1 (Dropout)         (None, 128)               0         
                                                                 
 dense_1 (Dense)             (None, 8)                 1032      
                                                                 
=================================================================
Total params: 1625611 (6.20 MB)
Trainable params: 1625608 (6.20 MB)
Non-trainable params: 3 (16.00 Byte)
_________________________________________________________________

Configure the Keras model with the Adam optimizer and the cross-entropy loss:

model.compile(
    optimizer=tf.keras.optimizers.Adam(),
    loss=tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True),
    metrics=['accuracy'],
)

Train the model over 10 epochs for demonstration purposes:

EPOCHS = 10
history = model.fit(
    train_spectrogram_ds,
    validation_data=val_spectrogram_ds,
    epochs=EPOCHS,
    callbacks=tf.keras.callbacks.EarlyStopping(verbose=1, patience=2),
)

Epoch 1/10
2023-10-27 05:56:28.451655: E tensorflow/core/grappler/optimizers/meta_optimizer.cc:961] layout failed: INVALID_ARGUMENT: Size of values 0 does not match size of permutation 4 @ fanin shape insequential/dropout/dropout/SelectV2-2-TransposeNHWCToNCHW-LayoutOptimizer
WARNING: All log messages before absl::InitializeLog() is called are written to STDERR
I0000 00:00:1698386190.386122  482694 device_compiler.h:186] Compiled cluster using XLA!  This line is logged at most once for the lifetime of the process.
100/100 [==============================] - 5s 12ms/step - loss: 1.7434 - accuracy: 0.3752 - val_loss: 1.2626 - val_accuracy: 0.6159
Epoch 2/10
100/100 [==============================] - 1s 8ms/step - loss: 1.1574 - accuracy: 0.5969 - val_loss: 0.9141 - val_accuracy: 0.7305
Epoch 3/10
100/100 [==============================] - 1s 8ms/step - loss: 0.8926 - accuracy: 0.6880 - val_loss: 0.7522 - val_accuracy: 0.7643
Epoch 4/10
100/100 [==============================] - 1s 8ms/step - loss: 0.7327 - accuracy: 0.7359 - val_loss: 0.6947 - val_accuracy: 0.7812
Epoch 5/10
100/100 [==============================] - 1s 8ms/step - loss: 0.6291 - accuracy: 0.7756 - val_loss: 0.6382 - val_accuracy: 0.8034
Epoch 6/10
100/100 [==============================] - 1s 8ms/step - loss: 0.5495 - accuracy: 0.8056 - val_loss: 0.5947 - val_accuracy: 0.7982
Epoch 7/10
100/100 [==============================] - 1s 8ms/step - loss: 0.5055 - accuracy: 0.8213 - val_loss: 0.5552 - val_accuracy: 0.8281
Epoch 8/10
100/100 [==============================] - 1s 8ms/step - loss: 0.4423 - accuracy: 0.8461 - val_loss: 0.5224 - val_accuracy: 0.8411
Epoch 9/10
100/100 [==============================] - 1s 8ms/step - loss: 0.4088 - accuracy: 0.8572 - val_loss: 0.4882 - val_accuracy: 0.8464
Epoch 10/10
100/100 [==============================] - 1s 8ms/step - loss: 0.3496 - accuracy: 0.8786 - val_loss: 0.4704 - val_accuracy: 0.8529

Let's plot the training and validation loss curves to check how your model has improved during training:

metrics = history.history
plt.figure(figsize=(16,6))
plt.subplot(1,2,1)
plt.plot(history.epoch, metrics['loss'], metrics['val_loss'])
plt.legend(['loss', 'val_loss'])
plt.ylim([0, max(plt.ylim())])
plt.xlabel('Epoch')
plt.ylabel('Loss [CrossEntropy]')

plt.subplot(1,2,2)
plt.plot(history.epoch, 100*np.array(metrics['accuracy']), 100*np.array(metrics['val_accuracy']))
plt.legend(['accuracy', 'val_accuracy'])
plt.ylim([0, 100])
plt.xlabel('Epoch')
plt.ylabel('Accuracy [%]')

Text(0, 0.5, 'Accuracy [%]')

png

Evaluate the model performance

Run the model on the test set and check the model's performance:

model.evaluate(test_spectrogram_ds, return_dict=True)

13/13 [==============================] - 0s 5ms/step - loss: 0.5219 - accuracy: 0.8293
{'loss': 0.5219233632087708, 'accuracy': 0.8293269276618958}

Display a confusion matrix

Use a confusion matrix to check how well the model did classifying each of the commands in the test set:

y_pred = model.predict(test_spectrogram_ds)

13/13 [==============================] - 0s 3ms/step

y_pred = tf.argmax(y_pred, axis=1)

y_true = tf.concat(list(test_spectrogram_ds.map(lambda s,lab: lab)), axis=0)

confusion_mtx = tf.math.confusion_matrix(y_true, y_pred)
plt.figure(figsize=(10, 8))
sns.heatmap(confusion_mtx,
            xticklabels=label_names,
            yticklabels=label_names,
            annot=True, fmt='g')
plt.xlabel('Prediction')
plt.ylabel('Label')
plt.show()

png

Run inference on an audio file

Finally, verify the model's prediction output using an input audio file of someone saying "no". How well does your model perform?

x = data_dir/'no/01bb6a2a_nohash_0.wav'
x = tf.io.read_file(str(x))
x, sample_rate = tf.audio.decode_wav(x, desired_channels=1, desired_samples=16000,)
x = tf.squeeze(x, axis=-1)
waveform = x
x = get_spectrogram(x)
x = x[tf.newaxis,...]

prediction = model(x)
x_labels = ['no', 'yes', 'down', 'go', 'left', 'up', 'right', 'stop']
plt.bar(x_labels, tf.nn.softmax(prediction[0]))
plt.title('No')
plt.show()

display.display(display.Audio(waveform, rate=16000))

png

As the output suggests, your model should have recognized the audio command as "no".

Export the model with preprocessing

The model's not very easy to use if you have to apply those preprocessing steps before passing data to the model for inference. So build an end-to-end version:

class ExportModel(tf.Module):
  def __init__(self, model):
    self.model = model

    # Accept either a string-filename or a batch of waveforms.
    # YOu could add additional signatures for a single wave, or a ragged-batch. 
    self.__call__.get_concrete_function(
        x=tf.TensorSpec(shape=(), dtype=tf.string))
    self.__call__.get_concrete_function(
       x=tf.TensorSpec(shape=[None, 16000], dtype=tf.float32))


  @tf.function
  def __call__(self, x):
    # If they pass a string, load the file and decode it. 
    if x.dtype == tf.string:
      x = tf.io.read_file(x)
      x, _ = tf.audio.decode_wav(x, desired_channels=1, desired_samples=16000,)
      x = tf.squeeze(x, axis=-1)
      x = x[tf.newaxis, :]

    x = get_spectrogram(x)  
    result = self.model(x, training=False)

    class_ids = tf.argmax(result, axis=-1)
    class_names = tf.gather(label_names, class_ids)
    return {'predictions':result,
            'class_ids': class_ids,
            'class_names': class_names}

Test run the "export" model:

export = ExportModel(model)
export(tf.constant(str(data_dir/'no/01bb6a2a_nohash_0.wav')))

{'predictions': <tf.Tensor: shape=(1, 8), dtype=float32, numpy=
 array([[ 0.134065 ,  1.5670671, -1.9424224,  3.8415747, -5.531281 ,
         -1.9281846, -2.909515 , -2.7880256]], dtype=float32)>,
 'class_ids': <tf.Tensor: shape=(1,), dtype=int64, numpy=array([3])>,
 'class_names': <tf.Tensor: shape=(1,), dtype=string, numpy=array([b'no'], dtype=object)>}

Save and reload the model, the reloaded model gives identical output:

tf.saved_model.save(export, "saved")
imported = tf.saved_model.load("saved")
imported(waveform[tf.newaxis, :])

INFO:tensorflow:Assets written to: saved/assets
INFO:tensorflow:Assets written to: saved/assets
{'class_ids': <tf.Tensor: shape=(1,), dtype=int64, numpy=array([3])>,
 'predictions': <tf.Tensor: shape=(1, 8), dtype=float32, numpy=
 array([[ 0.134065 ,  1.5670671, -1.9424224,  3.8415747, -5.531281 ,
         -1.9281846, -2.909515 , -2.7880256]], dtype=float32)>,
 'class_names': <tf.Tensor: shape=(1,), dtype=string, numpy=array([b'no'], dtype=object)>}

Next steps

This tutorial demonstrated how to carry out simple audio classification/automatic speech recognition using a convolutional neural network with TensorFlow and Python. To learn more, consider the following resources: