Speech Command Recognition Using Deep Learning

This example shows how to train a deep learning model that detects the presence of speech commands in audio. The example uses the Speech Commands Dataset [1] to train a convolutional neural network to recognize a given set of commands.

To train a network from scratch, you must first download the data set. If you do not want to download the data set or train the network, then you can load a pretrained network provided with this example and execute the next two sections of the example: Recognize Commands with a Pre-Trained Network and Detect Commands Using Streaming Audio from Microphone.

Recognize Commands with a Pre-Trained Network

Before going into the training process in detail, you will use a pre-trained speech recognition network to identify speech commands.

Load the pre-trained network.

load('commandNet.mat')

The network is trained to recognize the following speech commands:

  • "yes"

  • "no"

  • "up"

  • "down"

  • "left"

  • "right"

  • "on"

  • "off"

  • "stop"

  • "go"

Load a short speech signal where a person says "stop".

 [x,fs] = audioread('stop_command.flac');

Listen to the command.

 sound(x,fs)

The pre-trained network takes auditory-based spectrograms as inputs. You will first convert the speech waveform to an auditory-based spectrogram.

Use the function extractAuditoryFeature to compute the auditory spectrogram. You will go through the details of feature extraction later in the example.

auditorySpect = helperExtractAuditoryFeatures(x,fs);

Classify the command based on its auditory spectrogram.

command = classify(trainedNet,auditorySpect)
command = 

  categorical

     stop 

The network is trained to classify words not belonging to this set as "unknown".

You will now classify a word ("play") that was not included in the list of command to identify.

Load the speech signal and listen to it.

x = audioread('play_command.flac');
sound(x,fs)

Compute the auditory spectrogram.

auditorySpect = helperExtractAuditoryFeatures(x,fs);

Classify the signal.

command = classify(trainedNet,auditorySpect)
command = 

  categorical

     unknown 

The network is trained to classify background noise as "background".

Create a one-second signal consisting of random noise.

x = 0.01 * randn(16e3,1);

Compute the auditory spectrogram.

auditorySpect = helperExtractAuditoryFeatures(x,fs);

Classify the background noise.

command = classify(trainedNet,auditorySpect)
command = 

  categorical

     background 

Detect Commands Using Streaming Audio from Microphone

Test your pre-trained command detection network on streaming audio from your microphone. Try saying one of the commands, for example, yes, no, or stop. Then, try saying one of the unknown words such as Marvin, Sheila, bed, house, cat, bird, or any number from zero to nine.

Specify the classification rate in Hz and create an audio device reader that can read audio from your microphone.

classificationRate = 20;
adr = audioDeviceReader('SampleRate',fs,'SamplesPerFrame',floor(fs/classificationRate));

Initialize a buffer for the audio. Extract the classification labels of the network. Initialize buffers of half a second for the labels and classification probabilities of the streaming audio. Use these buffers to compare the classification results over a longer period of time and by that build 'agreement' over when a command is detected. Specify thresholds for the decision logic.

audioBuffer = dsp.AsyncBuffer(fs);

labels = trainedNet.Layers(end).Classes;
YBuffer(1:classificationRate/2) = categorical("background");

probBuffer = zeros([numel(labels),classificationRate/2]);

countThreshold = ceil(classificationRate*0.2);
probThreshold = 0.7;

Create a figure and detect commands as long as the created figure exists. To run the loop indefinitely, set timeLimit to Inf. To stop the live detection, simply close the figure.

h = figure('Units','normalized','Position',[0.2 0.1 0.6 0.8]);

timeLimit = 20;

tic;
while ishandle(h) && toc < timeLimit

    % Extract audio samples from the audio device and add the samples to
    % the buffer.
    x = adr();
    write(audioBuffer,x);
    y = read(audioBuffer,fs,fs-adr.SamplesPerFrame);

    spec = helperExtractAuditoryFeatures(y,fs);

    % Classify the current spectrogram, save the label to the label buffer,
    % and save the predicted probabilities to the probability buffer.
    [YPredicted,probs] = classify(trainedNet,spec,'ExecutionEnvironment','cpu');
    YBuffer = [YBuffer(2:end),YPredicted];
    probBuffer = [probBuffer(:,2:end),probs(:)];

    % Plot the current waveform and spectrogram.
    subplot(2,1,1)
    plot(y)
    axis tight
    ylim([-1,1])

    subplot(2,1,2)
    pcolor(spec')
    caxis([-4 2.6445])
    shading flat

    % Now do the actual command detection by performing a very simple
    % thresholding operation. Declare a detection and display it in the
    % figure title if all of the following hold: 1) The most common label
    % is not background. 2) At least countThreshold of the latest frame
    % labels agree. 3) The maximum probability of the predicted label is at
    % least probThreshold. Otherwise, do not declare a detection.
    [YMode,count] = mode(YBuffer);

    maxProb = max(probBuffer(labels == YMode,:));
    subplot(2,1,1)
    if YMode == "background" || count < countThreshold || maxProb < probThreshold
        title(" ")
    else
        title(string(YMode),'FontSize',20)
    end

    drawnow
end

Load Speech Commands Data Set

Download and extract the data set [1].

url = 'https://storage.googleapis.com/download.tensorflow.org/data/speech_commands_v0.01.tar.gz';

downloadFolder = tempdir;
datasetFolder = fullfile(downloadFolder,'google_speech');

if ~exist(datasetFolder,'dir')
    disp('Downloading speech commands data set (1.5 GB)...')
    untar(url,datasetFolder)
end

Create an audioDatastore that points to the data set.

ads = audioDatastore(datasetFolder, ...
    'IncludeSubfolders',true, ...
    'FileExtensions','.wav', ...
    'LabelSource','foldernames')
ads = 

  audioDatastore with properties:

                       Files: {
                              ' ...\Local\Temp\google_speech\_background_noise_\doing_the_dishes.wav';
                              ' ...\AppData\Local\Temp\google_speech\_background_noise_\dude_miaowing.wav';
                              ' ...\AppData\Local\Temp\google_speech\_background_noise_\exercise_bike.wav'
                               ... and 64724 more
                              }
                     Folders: {
                              'C:\Users\bhemmat\AppData\Local\Temp\google_speech'
                              }
                      Labels: [_background_noise_; _background_noise_; _background_noise_ ... and 64724 more categorical]
    AlternateFileSystemRoots: {}
              OutputDataType: 'double'
      SupportedOutputFormats: ["wav"    "flac"    "ogg"    "mp4"    "m4a"]
         DefaultOutputFormat: "wav"

Choose Words to Recognize

Specify the words that you want your model to recognize as commands. Label all words that are not commands as unknown. Labeling words that are not commands as unknown creates a group of words that approximates the distribution of all words other than the commands. The network uses this group to learn the difference between commands and all other words.

To reduce the class imbalance between the known and unknown words and speed up processing, only include a fraction of the unknown words in the training set. Do not include the longer files with background noise in the training set. Background noise will be added in a separate step later.

Use subset to create a datastore that contains only the commands and the subset of unknown words. Count the number of examples belonging to each category.

commands = categorical(["yes","no","up","down","left","right","on","off","stop","go"]);

isCommand = ismember(ads.Labels,commands);
isUnknown = ~ismember(ads.Labels,[commands,"_background_noise_"]);

includeFraction = 0.2;
mask = rand(numel(ads.Labels),1) < includeFraction;
isUnknown = isUnknown & mask;
ads.Labels(isUnknown) = categorical("unknown");

adsSubset = subset(ads,isCommand|isUnknown);
countEachLabel(adsSubset)
ans =

  11×2 table

     Label     Count
    _______    _____

    down       2359 
    go         2372 
    left       2353 
    no         2375 
    off        2357 
    on         2367 
    right      2367 
    stop       2380 
    unknown    8186 
    up         2375 
    yes        2377 

Split Data into Training, Validation, and Test Sets

The data set folder contains text files, which list the audio files to be used as the validation and test sets. These predefined validation and test sets do not contain utterances of the same word by the same person, so it is better to use these predefined sets than to select a random subset of the whole data set.

Because this example trains a single network, it only uses the validation set and not the test set to evaluate the trained model. If you train many networks and choose the network with the highest validation accuracy as your final network, then you can use the test set to evaluate the final network.

Read the list of validation files.

c = importdata(fullfile(datasetFolder,'validation_list.txt'));
filesValidation = string(c);

Read the list of test files.

c = importdata(fullfile(datasetFolder,'testing_list.txt'));
filesTest = string(c);

Determine which files in the datastore should go to validation set and which should go to test set.

files = adsSubset.Files;
sf = split(files,filesep);
isValidation = ismember(sf(:,end-1) + "/" + sf(:,end),filesValidation);
isTest = ismember(sf(:,end-1) + "/" + sf(:,end),filesTest);

adsValidation = subset(adsSubset,isValidation);
adsTrain = subset(adsSubset,~isValidation & ~isTest);

To train the network with the entire dataset and achieve the highest possible accuracy, set reduceDataset to false. To run this example quickly, set reduceDataset to true.

reduceDataset = false;
if reduceDataset
    numUniqueLabels = numel(unique(adsTrain.Labels));
    % Reduce the dataset by a factor of 20
    adsTrain = splitEachLabel(adsTrain,round(numel(adsTrain.Files) / numUniqueLabels / 20));
    adsValidation = splitEachLabel(adsValidation,round(numel(adsValidation.Files) / numUniqueLabels / 20));
end

Compute Auditory Spectrograms

To prepare the data for efficient training of a convolutional neural network, convert the speech waveforms to auditory-based spectrograms.

Define the parameters of the feature extraction. segmentDuration is the duration of each speech clip (in seconds). frameDuration is the duration of each frame for spectrum calculation. hopDuration is the time step between each spectrum. numBands is the number of filters in the auditory spectrogram.

Create an audioFeatureExtractor object to perform the feature extraction.

fs = 16e3; % Known sample rate of the data set.

segmentDuration = 1;
frameDuration = 0.025;
hopDuration = 0.010;

segmentSamples = round(segmentDuration*fs);
frameSamples = round(frameDuration*fs);
hopSamples = round(hopDuration*fs);
overlapSamples = frameSamples - hopSamples;

FFTLength = 512;
numBands = 50;

afe = audioFeatureExtractor( ...
    'SampleRate',fs, ...
    'FFTLength',FFTLength, ...
    'Window',hann(frameSamples,'periodic'), ...
    'OverlapLength',overlapSamples, ...
    'barkSpectrum',true);
setExtractorParams(afe,'barkSpectrum','NumBands',numBands);

Read a file from the dataset. Training a convolutional neural network requires input to be a consistent size. Some files in the data set are less than 1 second long. Apply zero-padding to the front and back of the audio signal so that it is of length segmentSamples.

x = read(adsTrain);

numSamples = size(x,1);

numToPadFront = floor( (segmentSamples - numSamples)/2 );
numToPadBack = ceil( (segmentSamples - numSamples)/2 );

xPadded = [zeros(numToPadFront,1,'like',x);x;zeros(numToPadBack,1,'like',x)];

To extract audio features, call extract. The output is a Bark spectrum with time across rows.

features = extract(afe,xPadded);
[numHops,numFeatures] = size(features)
numHops =

    98


numFeatures =

    50

The audioFeatureExtractor normalizes auditory spectrograms by the window power so that measurements are independent of the type of window and length of windowing. In this example, you post-process the auditory spectrogram by applying a logarithm. Taking a log of small numbers can lead to roundoff error. To avoid roundoff error, you will reverse the window normalization.

Determine the denormalization factor to apply.

unNorm = 2/(sum(afe.Window)^2);

To speed up processing, you can distribute the feature extraction across multiple workers using parfor.

First, determine the number of partitions for the dataset. If you do not have Parallel Computing Toolbox™, use a single partition.

if ~isempty(ver('parallel')) && ~reduceDataset
    pool = gcp;
    numPar = numpartitions(adsTrain,pool);
else
    numPar = 1;
end

For each partition, read from the datastore, zero-pad the signal, and then extract the features.

parfor ii = 1:numPar
    subds = partition(adsTrain,numPar,ii);
    XTrain = zeros(numHops,numBands,1,numel(subds.Files));
    for idx = 1:numel(subds.Files)
        x = read(subds);
        xPadded = [zeros(floor((segmentSamples-size(x,1))/2),1);x;zeros(ceil((segmentSamples-size(x,1))/2),1)];
        XTrain(:,:,:,idx) = extract(afe,xPadded);
    end
    XTrainC{ii} = XTrain;
end

Convert the output to a 4-dimensional array with auditory spectrograms along the fourth dimension.

XTrain = cat(4,XTrainC{:});

[numHops,numBands,numChannels,numSpec] = size(XTrain)
numHops =

    98


numBands =

    50


numChannels =

     1


numSpec =

       25041

Scale the features by the window power and then take the log. To obtain data with a smoother distribution, take the logarithm of the spectrograms using a small offset.

XTrain = XTrain/unNorm;
epsil = 1e-6;
XTrain = log10(XTrain + epsil);

Perform the feature extraction steps described above to the validation set.

if ~isempty(ver('parallel'))
    pool = gcp;
    numPar = numpartitions(adsValidation,pool);
else
    numPar = 1;
end
parfor ii = 1:numPar
    subds = partition(adsValidation,numPar,ii);
    XValidation = zeros(numHops,numBands,1,numel(subds.Files));
    for idx = 1:numel(subds.Files)
        x = read(subds);
        xPadded = [zeros(floor((segmentSamples-size(x,1))/2),1);x;zeros(ceil((segmentSamples-size(x,1))/2),1)];
        XValidation(:,:,:,idx) = extract(afe,xPadded);
    end
    XValidationC{ii} = XValidation;
end
XValidation = cat(4,XValidationC{:});
XValidation = XValidation/unNorm;
XValidation = log10(XValidation + epsil);

Isolate the train and validation labels. Remove empty categories.

YTrain = removecats(adsTrain.Labels);
YValidation = removecats(adsValidation.Labels);

Visualize Data

Plot the waveforms and auditory spectrograms of a few training samples. Play the corresponding audio clips.

specMin = min(XTrain,[],'all');
specMax = max(XTrain,[],'all');
idx = randperm(numel(adsTrain.Files),3);
figure('Units','normalized','Position',[0.2 0.2 0.6 0.6]);
for i = 1:3
    [x,fs] = audioread(adsTrain.Files{idx(i)});
    subplot(2,3,i)
    plot(x)
    axis tight
    title(string(adsTrain.Labels(idx(i))))

    subplot(2,3,i+3)
    spect = (XTrain(:,:,1,idx(i))');
    pcolor(spect)
    caxis([specMin specMax])
    shading flat

    sound(x,fs)
    pause(2)
end

Add Background Noise Data

The network must be able not only to recognize different spoken words but also to detect if the input contains silence or background noise.

Use the audio files in the _background_noise_ folder to create samples of one-second clips of background noise. Create an equal number of background clips from each background noise file. You can also create your own recordings of background noise and add them to the _background_noise_ folder. Before calculating the spectrograms, the function rescales each audio clip with a factor sampled from a log-uniform distribution in the range given by volumeRange.

adsBkg = subset(ads,ads.Labels=="_background_noise_");
 numBkgClips = 4000;
if reduceDataset
    numBkgClips = numBkgClips/20;
end
volumeRange = log10([1e-4,1]);

numBkgFiles = numel(adsBkg.Files);
numClipsPerFile = histcounts(1:numBkgClips,linspace(1,numBkgClips,numBkgFiles+1));
Xbkg = zeros(size(XTrain,1),size(XTrain,2),1,numBkgClips,'single');
bkgAll = readall(adsBkg);
ind = 1;

for count = 1:numBkgFiles
    bkg = bkgAll{count};
    idxStart = randi(numel(bkg)-fs,numClipsPerFile(count),1);
    idxEnd = idxStart+fs-1;
    gain = 10.^((volumeRange(2)-volumeRange(1))*rand(numClipsPerFile(count),1) + volumeRange(1));
    for j = 1:numClipsPerFile(count)

        x = bkg(idxStart(j):idxEnd(j))*gain(j);

        x = max(min(x,1),-1);

        Xbkg(:,:,:,ind) = extract(afe,x);

        if mod(ind,1000)==0
            disp("Processed " + string(ind) + " background clips out of " + string(numBkgClips))
        end
        ind = ind + 1;
    end
end
Xbkg = Xbkg/unNorm;
Xbkg = log10(Xbkg + epsil);
Processed 1000 background clips out of 4000
Processed 2000 background clips out of 4000
Processed 3000 background clips out of 4000
Processed 4000 background clips out of 4000

Split the spectrograms of background noise between the training, validation, and test sets. Because the _background_noise_ folder contains only about five and a half minutes of background noise, the background samples in the different data sets are highly correlated. To increase the variation in the background noise, you can create your own background files and add them to the folder. To increase the robustness of the network to noise, you can also try mixing background noise into the speech files.

numTrainBkg = floor(0.85*numBkgClips);
numValidationBkg = floor(0.15*numBkgClips);

XTrain(:,:,:,end+1:end+numTrainBkg) = Xbkg(:,:,:,1:numTrainBkg);
YTrain(end+1:end+numTrainBkg) = "background";

XValidation(:,:,:,end+1:end+numValidationBkg) = Xbkg(:,:,:,numTrainBkg+1:end);
YValidation(end+1:end+numValidationBkg) = "background";

Plot the distribution of the different class labels in the training and validation sets.

figure('Units','normalized','Position',[0.2 0.2 0.5 0.5])

subplot(2,1,1)
histogram(YTrain)
title("Training Label Distribution")

subplot(2,1,2)
histogram(YValidation)
title("Validation Label Distribution")

Define Neural Network Architecture

Create a simple network architecture as an array of layers. Use convolutional and batch normalization layers, and downsample the feature maps "spatially" (that is, in time and frequency) using max pooling layers. Add a final max pooling layer that pools the input feature map globally over time. This enforces (approximate) time-translation invariance in the input spectrograms, allowing the network to perform the same classification independent of the exact position of the speech in time. Global pooling also significantly reduces the number of parameters in the final fully connected layer. To reduce the possibility of the network memorizing specific features of the training data, add a small amount of dropout to the input to the last fully connected layer.

The network is small, as it has only five convolutional layers with few filters. numF controls the number of filters in the convolutional layers. To increase the accuracy of the network, try increasing the network depth by adding identical blocks of convolutional, batch normalization, and ReLU layers. You can also try increasing the number of convolutional filters by increasing numF.

Use a weighted cross entropy classification loss. weightedClassificationLayer(classWeights) creates a custom classification layer that calculates the cross entropy loss with observations weighted by classWeights. Specify the class weights in the same order as the classes appear in categories(YTrain). To give each class equal total weight in the loss, use class weights that are inversely proportional to the number of training examples in each class. When using the Adam optimizer to train the network, the training algorithm is independent of the overall normalization of the class weights.

classWeights = 1./countcats(YTrain);
classWeights = classWeights'/mean(classWeights);
numClasses = numel(categories(YTrain));

timePoolSize = ceil(numHops/8);

dropoutProb = 0.2;
numF = 12;
layers = [
    imageInputLayer([numHops numBands])

    convolution2dLayer(3,numF,'Padding','same')
    batchNormalizationLayer
    reluLayer

    maxPooling2dLayer(3,'Stride',2,'Padding','same')

    convolution2dLayer(3,2*numF,'Padding','same')
    batchNormalizationLayer
    reluLayer

    maxPooling2dLayer(3,'Stride',2,'Padding','same')

    convolution2dLayer(3,4*numF,'Padding','same')
    batchNormalizationLayer
    reluLayer

    maxPooling2dLayer(3,'Stride',2,'Padding','same')

    convolution2dLayer(3,4*numF,'Padding','same')
    batchNormalizationLayer
    reluLayer
    convolution2dLayer(3,4*numF,'Padding','same')
    batchNormalizationLayer
    reluLayer

    maxPooling2dLayer([timePoolSize,1])

    dropoutLayer(dropoutProb)
    fullyConnectedLayer(numClasses)
    softmaxLayer
    weightedClassificationLayer(classWeights)];

Train Network

Specify the training options. Use the Adam optimizer with a mini-batch size of 128. Train for 25 epochs and reduce the learning rate by a factor of 10 after 20 epochs.

miniBatchSize = 128;
validationFrequency = floor(numel(YTrain)/miniBatchSize);
options = trainingOptions('adam', ...
    'InitialLearnRate',3e-4, ...
    'MaxEpochs',25, ...
    'MiniBatchSize',miniBatchSize, ...
    'Shuffle','every-epoch', ...
    'Plots','training-progress', ...
    'Verbose',false, ...
    'ValidationData',{XValidation,YValidation}, ...
    'ValidationFrequency',validationFrequency, ...
    'LearnRateSchedule','piecewise', ...
    'LearnRateDropFactor',0.1, ...
    'LearnRateDropPeriod',20);

Train the network. If you do not have a GPU, then training the network can take time.

trainedNet = trainNetwork(XTrain,YTrain,layers,options);

Evaluate Trained Network

Calculate the final accuracy of the network on the training set (without data augmentation) and validation set. The network is very accurate on this data set. However, the training, validation, and test data all have similar distributions that do not necessarily reflect real-world environments. This limitation particularly applies to the unknown category, which contains utterances of only a small number of words.

if reduceDataset
    load('commandNet.mat','trainedNet');
end
YValPred = classify(trainedNet,XValidation);
validationError = mean(YValPred ~= YValidation);
YTrainPred = classify(trainedNet,XTrain);
trainError = mean(YTrainPred ~= YTrain);
disp("Training error: " + trainError*100 + "%")
disp("Validation error: " + validationError*100 + "%")
Training error: 1.526%
Validation error: 5.1539%

Plot the confusion matrix. Display the precision and recall for each class by using column and row summaries. Sort the classes of the confusion matrix. The largest confusion is between unknown words and commands, up and off, down and no, and go and no.

figure('Units','normalized','Position',[0.2 0.2 0.5 0.5]);
cm = confusionchart(YValidation,YValPred);
cm.Title = 'Confusion Matrix for Validation Data';
cm.ColumnSummary = 'column-normalized';
cm.RowSummary = 'row-normalized';
sortClasses(cm, [commands,"unknown","background"])

When working on applications with constrained hardware resources such as mobile applications, consider the limitations on available memory and computational resources. Compute the total size of the network in kilobytes and test its prediction speed when using a CPU. The prediction time is the time for classifying a single input image. If you input multiple images to the network, these can be classified simultaneously, leading to shorter prediction times per image. When classifying streaming audio, however, the single-image prediction time is the most relevant.

info = whos('trainedNet');
disp("Network size: " + info.bytes/1024 + " kB")

for i = 1:100
    x = randn([numHops,numBands]);
    tic
    [YPredicted,probs] = classify(trainedNet,x,"ExecutionEnvironment",'cpu');
    time(i) = toc;
end
disp("Single-image prediction time on CPU: " + mean(time(11:end))*1000 + " ms")
Network size: 286.7314 kB
Single-image prediction time on CPU: 3.1647 ms

References

[1] Warden P. "Speech Commands: A public dataset for single-word speech recognition", 2017. Available from https://storage.googleapis.com/download.tensorflow.org/data/speech_commands_v0.01.tar.gz. Copyright Google 2017. The Speech Commands Dataset is licensed under the Creative Commons Attribution 4.0 license, available here: https://creativecommons.org/licenses/by/4.0/legalcode.