Speech coding is the compression of speech (into a code) for transmission using audio signal processing and speech processing techniques.

The two most important applications using speech coding are mobile phones and internet phones.

The techniques used in speech coding are similar to that in audio compression and audio coding where knowledge in psychoacoustics is used to transmit only data that is relevant to the human auditory system. For example, in narrow-band speech coding, only information in the frequency band 400Hz to 3500Hz is transmitted but the reconstructed signal is still adequate for intelligibility.

However, speech coding differs from audio coding in that there is a lot more statistical information available about the properties of speech. In addition, some auditory information which is relevant in audio coding can be unnecessary in the speech coding context. In speech coding, the most important criterion is always preservation of intelligibility of speech, with a constrained amount of transmitted data.

It should be emphasised that the intelligibility of speech includes, besides the actual literal content, also speaker identity, emotions, intonation, timbre etc. that are all important for perfect intelligibility.

In addition, most speech applications require low coding delay, as long coding delays interfere with speech interaction.

The most common speech coding scheme is Code-Excited Linear Predictive (CELP) coding, which is used for example in the GSM standard. In CELP, the modelling is divided in two stages, a linear predictive stage that models the spectral envelope and code-book based model of the residual of the linear predictive model.

In addition to the actual speech coding of the signal, it is often necessary to use channel coding for transmission, to avoid losses due to transmission errors. Usually, speech coding and channel coding methods have to be chosen in pairs in order to get the best overal coding results.

The Speex project is an attempt to create a free software speech coder, unemcumbered by patent restrictions.

Major subfields:

See also: Digital signal processing, Speech processing, Audio signal processing, Data compression, Telecommunication, Mobile phone, Psychoacoustic model, Vector quantization.