The two primary technologies generating synthetic speech waveforms are concatenative synthesis and formant synthesis.

Concatenative synthesis is based on stringing together segments of recorded speech. It generally produces the most natural-sounding synthesized speech. However, differences between natural variations in speech and the nature of the automated techniques for segmenting the waveforms sometimes result in audible glitches in the output. Maximum naturalness typically requires speech databases to be very large, in some systems ranging into the gigabytes of recorded data, representing dozens of hours of speech.

Formant synthesis does not use human speech samples at runtime. Instead, the synthesized speech output is created using additive synthesis and an acoustic model.  Many systems based on formant synthesis technology generate artificial, robotic-sounding speech that would never be mistaken for human speech.  They are usually smaller programs than concatenative systems because they do not have a database of speech samples. They can therefore be used in embedded systems, where memory and microprocessor power are especially limited.