Contents

Voice Over IP (VOIP)

This example shows how to use the UDP Send and UDP Receive System objects to transmit audio data over a network.

Introduction

User Datagram Protocol (UDP) is part of the Internet Protocol (IP) suite. UDP provides efficient transmission of data, but does not guarantee reliability, data order, or data integrity. These characteristics make UDP suitable for streaming audio and video data, but not for binary files and similar situations where data loss is unacceptable. The following block diagram shows the operations involved in the this example:

You can run this example entirely on one system. However, to use UDP on a network, it is best to run the example on two networked computers. If the second computer has MATLAB® installed, you can run the example on it directly. Otherwise, use MATLAB® Coder™ to generate a stand-alone executable that can run on the second computer.

To start, run the following script on your computer. While the script is running, the UDPSender object transmits audio sine wave data to the UDPReceiver object via the localhost port, 127.0.0.1, and then outputs the signal on your audio speakers. If you have a microphone, you can change the value of useMicrophone to 'true' in order to transmit live audio instead of the sine wave.

Having run the script on a single computer, copy the script to the second computer. In both scripts, replace the IP address, 127.0.0.1, with the IP address of the opposite computer. When you run both scripts, the UDP objects transmit audio signals to each other across the network.

Initialization

% Initialize several configuration parameters.
useMicrophone   = false;
IP_address      = '127.0.0.1';
IP_port         = 30000;
sampleRate      = 44100;
nChannels       = 1;
freq            = [100 99];
samplesPerFrame = 512;

% Create System objects to send local information to a remote client.
if useMicrophone
    hLocalSource = dsp.AudioRecorder('SampleRate', sampleRate,...
        'NumChannels', nChannels); %#ok<UNRCH>
else
    hLocalSource = dsp.SineWave('SampleRate', sampleRate,...
        'Frequency', freq(1:nChannels),...
        'SamplesPerFrame', samplesPerFrame);
end
hRemoteSink = dsp.UDPSender('RemoteIPAddress', IP_address, ...
    'RemoteIPPort', IP_port);

% Create System objects to listen to data produced by the remote client.
hRemoteSource = dsp.UDPReceiver('LocalIPPort', IP_port,...
    'MaximumMessageLength', samplesPerFrame,...
    'MessageDataType', 'double');
hLocalSink  = dsp.AudioPlayer('SampleRate', sampleRate);

Stream Processing Loop

fiveSeconds = 5*sampleRate;
for i=1:(fiveSeconds/samplesPerFrame)
    % Connect the local source to the remote sink.
    % In other words, transmit audio data.
    localData = step(hLocalSource);
    step(hRemoteSink, localData(:));

    % Connect the remote source to the local sink
    % In other words, receive audio data.
    remoteData = step(hRemoteSource);
    if ~isempty(remoteData)
        step(hLocalSink, remoteData);
    end
end

Release

Here you call the release method on the System objects to close any open files and devices.

release(hLocalSource);
release(hLocalSink);
release(hRemoteSource);
release(hRemoteSink);

Debugging your Network Connection

If you are unable to transmit the signal over the network, check the following items:

  • That your firewall is not blocking the IP port numbers you are using. If needed, consult your system administrator or operating system documentation.

  • That you are using a free IP port number. One not being in use by another application.

  • That the operating system is not restricting the port number to a privileged user. For example, Linux typically restricts ports below 1024 for use by root user.

If needed, please consult your system administrator or operating system for more information.

Summary

This example shows how to stream bidirectional audio over a UDP connection.

References

The Wikipedia entry on UDP

Postel, J., User Datagram Protocol, http://tools.ietf.org/html/rfc768

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