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Speech Command Recognition Using Deep Learning

This example shows how to perform speech command recognition on streaming audio. The example uses a pretrained deep learning model. To learn how the deep learning model was trained, see Train Speech Command Recognition Model Using Deep Learning.

Load the pre-trained network. The network is trained to recognize the following speech commands: yes, no, up, down, left, right, on, off, stop, and go, and to otherwise classify audio as an unknown word or as background noise.

labels = 1×12 categorical
     down      go      left      no      off      on      right      stop      up      yes      unknown      background 

Load one of the following audio signals: noise, someone saying stop, or someone saying play. The word stop is recognized by the network as a command. The word play is an unknown word to the network. Listen to the signal.

audioData = {audioread("stopCommand.flac"),16e3,"stop"};

The pre-trained network takes auditory-based spectrograms as inputs. Use the supporting function extractAuditorySpectrogram to extract the spectrogram. Classify the audio based on its auditory spectrogram.

auditorySpectrogram = extractAuditorySpectrogram(audioData{1},audioData{2});
score = predict(net,auditorySpectrogram);
prediction = scores2label(score,labels,2);

Use the supporting function visualizeClassificationPipeline, to plot the audio signal, the auditory spectrogram, the network prediction, and a word cloud indicating the prediction scores.


Detect Commands from Streaming Audio

The model was trained to classify auditory spectrograms that correspond to 1-second chunks of audio data. It has no concept of memory between classifications. To adapt this model for streaming applications, you can add logic to build up decision confidence over time.

Create a 9-second long audio clip of the background noise, the unknown word, and the known command.

fs = 16e3;
audioPlay = audioread("playCommand.flac");
audioStop = audioread("stopCommand.flac");
audioBackground = 0.02*pinknoise(fs);
audioIn = repmat([audioBackground;audioPlay;audioStop],3,1);

Specify the classification rate in hertz. The classification rate is the number of classifications per second. Every classification requires 1 second of audio data.

classificationRate = 20; % Hz

Specify the time window for decisions. Decisions are made by considering all individual classifications in a decision time window.

decisionTimeWindow = 1.5; % seconds

Specify thresholds for the decision logic. The frameAgreementThreshold is the percent of frames within a decisionTimeWindow that must agree to recognize a word. The probabilityThreshold is the threshold that at least one of the classification probabilities in the decisionTimeWindow must pass.

frameAgreementThreshold = 50; % percent
probabilityThreshold = 0.7;

Use the supporting function, detectCommands, to simulate streaming command detection. The function uses your default audio device to play the streaming audio.

detectCommands( ...
    Input=audioIn, ...
    SampleRate=fs, ...
    Network=net, ...
    Labels=labels, ...
    ClassificationRate=classificationRate, ...
    DecisionTimeWindow=decisionTimeWindow, ...
    FrameAgreementThreshold=frameAgreementThreshold, ...

Detect Commands from Microphone Input

You can test the model by performing speech command recognition on data input from your microphone. In this case, audio is read from your default audio input device. The TimeLimit parameter controls the duration of the audio recording. You can end the recording early by closing the scopes.

The network is trained to recognize the following speech commands: yes, no, up, down, left, right, on, off, stop, and go, and to otherwise classify audio as an unknown word or as background noise.

detectCommands( ...
    SampleRate=fs, ...
    Network=net, ...
    Labels=labels, ...
    ClassificationRate=20, ...
    DecisionTimeWindow=1.5, ...
    FrameAgreementThreshold=50, ...
    ProbabilityThreshold=0.7, ...

Supporting Functions

Extract Auditory Spectrogram

function features = extractAuditorySpectrogram(x,fs)
%extractAuditorySpectrogram Compute auditory spectrogram
% features = extractAuditorySpectrogram(x,fs) computes an auditory (Bark)
% spectrogram in the same way as done in the Train Speech Command
% Recognition Model Using Deep Learning example. Specify the audio input,
% x, as a mono audio signal with a 1 second duration.

% Design audioFeatureExtractor object
persistent afe segmentSamples
if isempty(afe)
    designFs = 16e3;
    segmentDuration = 1;
    frameDuration = 0.025;
    hopDuration = 0.01;

    numBands = 50;
    FFTLength = 512;

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

    afe = audioFeatureExtractor( ...
        SampleRate=designFs, ...
        FFTLength=FFTLength, ...
        Window=hann(frameSamples,"periodic"), ...
        OverlapLength=overlapSamples, ...

% Resample to 16 kHz if necessary
if double(fs)~=16e3
    x = cast(resample(double(x),16e3,double(fs)),like=x);

% Ensure the input is equal to 1 second of data at 16 kHz.
x = resize(x,segmentSamples,Side="both");

% Extract features
features = extract(afe,x);

% Apply logarithm
features = log10(features + 1e-6);


Visualize Classification Pipeline

function visualizeClassificationPipeline(audioData,net,labels)
%visualizeClassificationPipeline Visualize classification pipeline
% visualizeClassificationPipeline(audioData,net,labels) creates a tiled
% layout of the audio data, the extracted auditory spectrogram, and a word
% cloud indicating the relative prediction probability of each class.

% Unpack audio data
audio = audioData{1};
fs = audioData{2};
knownlabel = audioData{3};

% Create tiled layout

% Plot audio signal in first tile
title("Known Class = "+knownlabel)

% Plot auditory spectrogram in second tile
auditorySpectrogram = extractAuditorySpectrogram(audio,fs);

% Plot network predictions as word cloud in third tile
scores = predict(net,auditorySpectrogram);
prediction = scores2label(scores,labels,2);
title("Predicted Class = "+string(prediction))

    function plotAuditorySpectrogram(auditorySpectrogram)
        %plotAuditorySpectrogram Plot auditory spectrogram

        % extractAuditorySpectrogram uses 25 ms windows with 10 ms hops.
        % Create a time vector with instants corresponding to the center of
        % the windows
        t = 0.0125:0.01:(1-0.0125);

        bins = 1:size(auditorySpectrogram,2);

        shading flat
        xlabel("Time (s)")
        ylabel("Bark (bins)")

    function plotAudio(audioIn,fs)
        %plotAudio Plot audio

        t = (0:size(audioIn,1)-1)/fs;
        xlabel("Time (s)")
        grid on
        axis tight


Plot Streaming Features

function detectCommands(options)
%detectCommand Detect commands
% detectCommand(SampleRate=fs,Network=net,Labels=lbls,ClassificationRate=cr, ...
%   DecisionTimeWindow=dtw,FrameAgreementThreshold=fat,ProbabilityThreshold=pt, ...
%   Input=audioIn)
% opens a timescope to visualize streaming audio and a dsp.MatrixViewer to
% visualize auditory spectrograms extracted from a simulation of streaming
% audioIn. The scopes display the detected speech command after it has been
% processed by the streaming algorithm. The streaming audio is played to
% your default audio output device.
% detectCommand(SampleRate=fs,Network=net,Labels=lbls,ClassificationRate=cr, ...
%   DecisionTimeWindow=dtw,FrameAgreementThreshold=fat,ProbabilityThreshold=pt, ...
%   TimeLimit=tl)
% opens a timescope to visualize streaming audio and a dsp.MatrixViewer to
% visualize auditory spectrograms extracted from audio streaming from your
% default audio input device. The scopes display the detected speech
% command after it has been processed by the streaming algorithm.

    options.Input = []
    options.TimeLimit = inf;

% Isolate the labels
labels = options.Labels;

if isempty(options.Input)
    % Create an audioDeviceReader to read audio from your microphone.
    adr = audioDeviceReader(SampleRate=options.SampleRate,SamplesPerFrame=floor(options.SampleRate/options.ClassificationRate));

    % Create a dsp.AsyncBuffer to buffer the audio streaming from your
    % microphone into overlapping segments.
    audioBuffer = dsp.AsyncBuffer(options.SampleRate);
    % Create a dsp.AsyncBuffer object. Write the audio to the buffer so that
    % you can read from it in a streaming fashion.
    audioBuffer = dsp.AsyncBuffer(size(options.Input,1));

    % Create an audioDeviceWriter object to write the audio to your default
    % speakers in a streaming loop.
    adw = audioDeviceWriter(SampleRate=options.SampleRate);

newSamplesPerUpdate = floor(options.SampleRate/options.ClassificationRate);

% Convert the requested decision time window to the number of analysis frames.
numAnalysisFrame = round((options.DecisionTimeWindow-1)*(options.ClassificationRate) + 1);

% Convert the requested frame agreement threshold in percent to the number of frames that must agree.
countThreshold = round(options.FrameAgreementThreshold/100*numAnalysisFrame);

% Initialize buffers for the classification decisions and scores of the streaming audio.
YBuffer = repmat(categorical("background"),numAnalysisFrame,1);
scoreBuffer = zeros(numel(labels),numAnalysisFrame,"single");

% Create a timescope object to visualize the audio processed in the
% streaming loop. Create a dsp.MatrixViewer object to visualize the
% auditory spectrogram used to make predictions.
wavePlotter = timescope( ...
    SampleRate=options.SampleRate, ...
    Title="...", ...
    TimeSpanSource="property", ...
    TimeSpan=1, ...
    YLimits=[-1,1], ...
    Position=[600,640,800,340], ...
    TimeAxisLabels="none", ...
specPlotter = dsp.MatrixViewer( ...
    XDataMode="Custom", ...
    AxisOrigin="Lower left corner", ...
    Position=[600,220,800,380], ...
    ShowGrid=false, ...
    Title="...", ...
    XLabel="Time (s)", ...
    YLabel="Bark (bin)");

% Initialize variables for plotting
currentTime = 0;
colorLimits = [-1,1];

% Run the streaming loop.
loopTimer = tic;
while whileCriteria(loopTimer,options.TimeLimit,wavePlotter,specPlotter,options.Input,audioBuffer)

    if isempty(options.Input)
        % Extract audio samples from the audio device and add the samples to
        % the buffer.
        audioIn = adr();

    % Read samples from the buffer
    y = read(audioBuffer,options.SampleRate,options.SampleRate - newSamplesPerUpdate);
    % Extract an auditory spectrogram from the audio
    spec = extractAuditorySpectrogram(y,options.SampleRate);
    % Classify the current spectrogram, save the label to the label buffer,
    % and save the predicted probabilities to the probability buffer.
    score = predict(options.Network,spec);
    YPredicted = scores2label(score,labels,2);
    YBuffer = [YBuffer(2:end);YPredicted];
    scoreBuffer = [scoreBuffer(:,2:end),score(:)];
    % Plot the current waveform and spectrogram.
    ynew = y(end-newSamplesPerUpdate+1:end);

    % Declare a detection and display it in the figure if 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(scoreBuffer(labels == YMode,:));
    if YMode == "background" || count < countThreshold || maxProb < options.ProbabilityThreshold
        wavePlotter.Title = "...";
        specPlotter.Title = "...";
        wavePlotter.Title = string(YMode);
        specPlotter.Title = string(YMode);
    % Update variables for plotting
    currentTime = currentTime + newSamplesPerUpdate/options.SampleRate;
    colorLimits = [min([colorLimits(1),min(spec,[],"all")]),max([colorLimits(2),max(spec,[],"all")])];
    specPlotter.CustomXData = [currentTime-1,currentTime];
    specPlotter.ColorLimits = colorLimits;

    if ~isempty(options.Input)
        % Write the new audio to your audio output device.

    function tf = whileCriteria(loopTimer,timeLimit,wavePlotter,specPlotter,Input,audioBuffer)
        if isempty(Input)
            tf = toc(loopTimer)<timeLimit && isVisible(wavePlotter) && isVisible(specPlotter);
            tf = audioBuffer.NumUnreadSamples > 0;

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