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app_festival is an application that allows one to send text-to-speech commands
to a background festival server, and to obtain the resulting waveform which
gets sent down to the respective channel. app_festival also employs a waveform 
cache, so invariant text-to-speech strings ("Please press 1 for instructions") 
do not need to be dynamically generated all the time. 

You need : 

1) festival, patched to produce 8khz waveforms on output. Patch for Festival
1.4.2 RELEASE are included. The patch adds a new command to festival 
(asterisk_tts). 

2) My patches to asterisk that provide variable substitution and quoting to 
the Asterisk Extension Logic. This is not really a requirement, but without
this, app_festival is mostly useless (you could very well use prerecorded
voices for static information). 

3) Before running asterisk, you have to run festival-server with a command 
like : 

/usr/local/festival/bin/festival --server > /dev/null 2>&1 &