Programming Speech in WPF - Speech Synthesis

New Microsoft Speech API (SAPI) version 5.3, which is an integral part of Windows Vista, is a managed API that allows developers to write speech-enable applications in .NET Framework 3.0.  This speech functionality is defined in the System.Speech and its five sub namespaces. Physically, the speech API resides in System.Speech.Dll assembly.

Here is a list of five namespaces that defines Speech related functionality.

• System.Speech.Audioformat
• System.Speech.Recognition
• System.Speech.Recognition.SrgsGrammar
• System.Speech.Synthesis
• System.Speech.Synthesis.TtsEngine

To access Speech API in WPF, you must add System.Speech.Dll assembly reference to a project. Right click on the project name in Solution Explorer, select Add Reference and select System.Speech on the .NET Tab and select OK button as shown in Figure 1.

Figure 1.

This action will add System.Speech assembly reference and copy System.Speech.dll to the bin folder of your project.  Now you can import System.Speech related namespaces in your application.

Speech Synthesis

Speech Synthesis, known as text-to-speech in previous versions of SAPI, is a process of converting text to speech.

Windows Vista comes with a default voice called Microsoft Anna. Let's take a look at it. Go to Control Panel and click on Text to Speech. You will see Speech Properties dialog with two tabs - Text to Speech and Speech Recognition as you can see in Figure 2 and Figure 3.

Figure 2.

On Text to Speech dialog box, you will see Voice Selection dropdown showing Microsoft Anna. On this dialog, you may also test the voice and audio output. If you have more voices installed, you will see them in the dropdown list as well. You can install more voices when you install Microsoft Speech SDK. 5.1.

Figure 3.

Table 1 describes the classes available in System.Speech.Synthesis namespace.

In this article, our focus is on SpeechSynthesizer class and its methods and properties.

SpeechSynthesizer     

The SpeechSynthesizer generates text to speech.

The Speak method speaks the text synchronously. The following code creates a SpeechSynthesizer object and calls Speak method that says "Hello WPF.". By default, the SpeechSynthesizer uses Microsoft Mary voice.

SpeechSynthesizer ss = new SpeechSynthesizer();

ss.Speak("Hello WPF.");

SpeechSynthesizer Properties

The SpeechSynthesizer has four properties - Rate, State, Voice, and Volume that are used to get and set rate, state, voice, and volume of the speech. The value of rate is between -10 to 10 and value of Volume is between 0 and 100. The Voice is the VoiceInfo object and State is SynthesizerState object. I will discuss these properties in more details in my forthcoming articles.

Asynchronous Speech

The SpeakAsync method speaks asynchronously and takes a Prompt, PromptBuilder or string as input text.

SpeechSynthesizer ss = new SpeechSynthesizer();

ss.SpeakAsync("Hello WPF");

The Application

Based on above class, properties, and methods, I build an application that allows you to browse a text file, opens it in a RichTextBox control, set the volume and rate of the speech and speaks it for you.

The application UI looks like Figure 4.

Figure 4.

The XAML code for controls looks like following:

In this application, The Open a Text File button opens a file browser and you can open browse a text file. After that it reads the text file and opens it in the RichTextBox control. The code listing is shown below on the Open a Text File button click event handler.

private void OpenTextFileButton_Click(object sender, RoutedEventArgs e)

{

    OpenFileDialog dlg = new OpenFileDialog();

    dlg.InitialDirectory = "c:\\";

    dlg.Filter = "Text files (*.txt)|*.txt|All Files (*.*)|*.*";

    dlg.RestoreDirectory = true;

    if (dlg.ShowDialog() == System.Windows.Forms.DialogResult.OK)

    {

        LoadTextDocument(dlg.FileName);

        FileNameTextBox.Text = dlg.FileName;

    }

}

private void LoadTextDocument(string fileName)

{

    TextRange range;

    System.IO.FileStream fStream;

    if (System.IO.File.Exists(fileName))            {

        range = new TextRange(richTextBox1.Document.ContentStart, richTextBox1.Document.ContentEnd);

        fStream = new System.IO.FileStream(fileName, System.IO.FileMode.OpenOrCreate);

        range.Load(fStream, System.Windows.DataFormats.Text);

        fStream.Close();

    }

}

The Speak button click event handler sets the volume and rate of the speech after getting these values from the Volume and Rate ComboBoxes and sets the Rate and Volume property of SpeechSynthesizer and after that calls the Speak method.

The ConvertRichTextBoxContentsToString method below reads the contents of the RichTextBox and converts to a string.

Here is the Speak button click event handler.

private void TalkButton_Click(object sender, RoutedEventArgs e)

{

    ComboBoxItem volumeItem = (ComboBoxItem)VolumeList.Items[VolumeList.SelectedIndex];

    Int32 vol = Convert.ToInt32(volumeItem.Content.ToString());

    ComboBoxItem rateItem = (ComboBoxItem)RateList.Items[RateList.SelectedIndex];

    Int32 rate = Convert.ToInt32(rateItem.Content.ToString());

    talker.Volume = vol;

    talker.Rate = rate;

    talker.Speak(ConvertRichTextBoxContentsToString());

}

string ConvertRichTextBoxContentsToString()

{

    TextRange textRange = new TextRange(richTextBox1.Document.ContentStart,

        richTextBox1.Document.ContentEnd);

    return textRange.Text;

}

Summary

Speech API (SAPI) 5.3 is a managed API comes with Windows Vista. In this article, I discussed how we can use SAPI in a WPF application to build speech-enabled applications. This article covered the text-to-speech (TTS) or Speech Synthesis where we built an application that convert text to speech.  

In my next articles in this series, I will add more features to this application including word highligting and spell check. In the next part of this series, I will cover speech recognition in SAPI and WPF. 

I hope you enjoyed this article. All feedback and critics are most welcome. Feel free to post them at the bottom of this article.