This equipment has been at the World Exhibition in New York in the year 1939 and presented at the Golden Gate International Exposition. Pretty amazing for the time, or? So much know the Wikipedia to VODER (Voice Operation DEmonstratoR) Bell Labs:

Mit dem Voder (»Voice Operation Demonstrator«) Homer Dudley, it was possible for the first time, with an electrical device to synthesize speech. The Voder had but still be operated manually. In order to master its operation, it required an approximately one-year training. He was first introduced 1939 at the World Exhibition in New York.

By means of a wrist switch could determine, whether the original signal by a sine wave generator for voiced sounds (the frequency could be varied by a pedal) or a noise generator for unvoiced sounds generated.

This signal has now been using several, controlled by the user electric filter, insofar changed, that whole sentences revealed.

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The Voder - Homer Dudley (Bell Labs) 1939