Speech processing is the study of speech signals and the processing methods of these signals.
The signals are usually processed in a digital representation, so speech processing can be regarded as a special case of digital signal processing, applied to speech signal.
It is also closely tied to natural language processing (NLP), as its input can come from / output can go to NLP applications. E.g. text-to-speech synthesis may use a syntactic parser on its input text and speech recognition's output may be used by e.g. information extraction techniques.
Speech processing can be divided into the following categories:
Speech recognition, which deals with analysis of the linguistic content of a speech signal.
Speaker recognition, where the aim is to recognize the identity of the speaker.
Enhancement of speech signals, e.g. audio noise reduction.
Speech coding, a specialized form of data compression, is important in the telecommunication area.
Voice analysis for medical purposes, such as analysis of vocal loading and dysfunction of the vocal cords.
Speech synthesis: the artificial synthesis of speech, which usually means computer-generated speech.
Speech enhancement: enhancing the perceptual quality of a speech signal by removing the destructive effects of noise, limited capacity recording equipment, impairments, etc.