Considered the first electrical speech synthesizer, VODER (Voice Operation DEmonstratoR) was developed by Homer Dudley at Bell Labs and demonstrated at both the 1939 New York World’s Fair and the 1939 Golden Gate International Exposition.
The VODER was operated by highly trained technicians who would manipulate a set of analog (continuous) controls that produced speech like sounds. It produced only two basic sounds: a tone generated by a radio valve to produce the vocal sounds and a hissing noise produced by a gas discharge tube to create the sibilants. These basic sounds were passed through a set of filters and an amplifier that mixed and modulated them to sound like speech.
Difficult to use and difficult to operate, VODER nonetheless paved the way for future machine-generated speech.