Speech Synthesis, otherwise known as Text to Speech (TTS), is a technology that quickly synthesizes a human voice using text as input. Speech synthesis  is the default behavior for voice calls on the Tropo platform. The Tropo ‘say‘ verb is the one that provides the TTS capability, by taking a string of text and speaking it back. It is of course possible for this verb to take a URL to a ‘wav’ or ‘mp3′ file for pre-recorded audio to be played as well.

When it comes to teaching your application to speak we follow the Perl ethos of making “the simple things easy and difficult things possible”. So your application may speak very well with the simplicity of our APIs, or it may be as sophisticated and emotional as you like through Tropo exposing powerful capabilities for giving your voices character.

For our first example we will simply say:

say 'I like squirrels!'

Which then renders this audio.

Next, we may choose from a voice that speaks any number of languages supported by Tropo (US/UK English, Castilian/Mexican Spanish, French, German, Italian & Dutch). Lets give French a try for our next example:

say "J'aime les écureuils!", :voice => 'florence'

Which then renders this audio.

Now, those were the simple examples that anyone may use to add a little speech to their applications. But, remember, we also make the difficult possible for those who want to really make their characters speak. As sometimes simply customizing the voice is not enough. There are cases when you’d also like control over pitch, volume and intonation. Tropo natively supports a standard called the Synthesized Speech Markup Language (SSML).

The Speech Synthesis Markup Language (SSML) is a W3C standard for controlling the pace, tone, pitch and all around sound of computer generated voices. Here’s a Ruby script that repeats the same sentence four times; each at a gradually lower speed:

answer 
say "<speak> I like squirrels!. 
I <prosody rate='-10%'>like squirrels!</prosody> 
I <prosody rate='-30%'>like squirrels!</prosody>  
I <prosody rate='-50%'>like squirrels!</prosody> 
</speak>"  
hangup

Which renders this audio. The previous example made use of the rate property of the SSML prosody element to control the playback speed. There are many other elements and attributes you may use, including: emphasis, phoneme, etc. To learn more about SSML and related technologies check out the W3C site at http://www.w3.org/TR/speech-synthesis/.

If you would like to call in and listen to these examples live, you may do so by dialing +990009369991429940 on Skype (free) or calling +1.408.940.5920 from any phone. What are you waiting for? Get started by signing up for an always free developer account @ Tropo.com.