Speech synthesizers
Speech synthesis is the methods of generating artificial speech by mechanical means or by a computer algorithm. It it used when there is a need to communicate information acoustically, and nowadays is found in text-to-speech applications (screen text reading, assistance for the visually impaired) and virtual assistants (GPS navigation, mobile assistants such as Microsoft Cortana or Apple Siri), or in any other situation where the information usually available in text has to be passed acoustically. It often comes paired with voice recognition.
These applications require speech that is intelligible and natural-sounding. Today speech synthesis systems achieve great deal of naturalness compare to a real human voice. Yet they are still perceived as non-human because minor audible glitches still remain present in the outputted utterances. It may very well be that modern speech synthesis has reached the point in the Uncanny Valley where the closeness of the artificial speech is so near perfection that humans find it unnatural.