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The two most important applications using speech coding are [[mobile phone]]s and [[internet phone]]s.
The techniques used in speech coding are similar to that in [[audio data 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 [[narrowband]] 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.
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In addition, most speech applications require low coding delay, as long coding delays interfere with speech interaction.
The [[A-law algorithm]] and the [[Mu-law algorithm]] are used in nearly all land-line long distance telephone communications. They can be seen as a kind of speech encoding, requiring only 8 bits per sample but giving effectively 12 bits of resolution.
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 prediction|linear predictive]] stage that models the spectral envelope and code-book based model of the residual of the linear predictive model.
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