I've spent the last 2-3 months on researching TTS and ASR for FS for a project. Best TTS depends on what you consider important. Also, how do you plan on using it.
Here's some of the TTS engines I've run across with some pros/cons:
Festivate Lite (flite)
Pros:
- Free (comes with FS)
- simple to use
- 16K voice sounds decent
- Completely customizable
Cons:
- 8K voice sounds horrible over cell phone
NeoSpeech (VoiceWare) (around $300/port for 1 voice + $75 each addl voice)
Pros:
- My selection for best soundig voices
- Recently select by Stephen Hawkings for his voice (geek points!)
- Lots of Languages supported
- Free trial available
Cons:
- Custom C-Based API (FS interface coming soon)
- Large file size (Engine + SDK + 1 Voice = 900MB)
- Support is lacking (Company beed in Korean, time zone issues, etc)
Nuance ($500/port for 1 voice)
Pros:
- Wide Variety of Products
- Support MRCP
- Supports ASR as well (add'l fees)
- Excellent support
- Free trial
- Decent sounding voices at 8K and 16K
- Wide range of tuning parameters
Cons:
- Pricey
- Limited voice selection
- Limited support for 64-bit linux
AT&T (NaturalVoice) (no pricing info available)
Pros:
- Big company (solid in marketplace)
- Good suppport (user and developer)
- ASP model means no software to maintain
Cons:
- ASP model incurs delay
- Voices sound too digitized
- Limited support for 64-bit linux
Loquendo ($500/port for 1 voice + 15% addl voice)
Pros:
- Good sounding voices (almost as good as NeoSpeech)
- Wide variety of languages
- Excellent support
- Has free 30 day trial
- Supports MRCP
- Support ASR and Voice Recognition as well. (add'l fees)
- Small footprint (< 150MB)
Cons:
- Pricey
- Complicated install process
- Limited management/tuning capabilities
In the end, it was down to NeoSpeech or Loquendo for our application. We are currently running tests with NeoSpeech and assuming all goes well, we will select them. Though don't let that color your opinion too much after several "focus groups" we discovered the most important element in the equation is does your customer/boss like the sound of the voices, and that is a completely subjective decision.
-pete