Speech coding: Difference between revisions

Revision as of 01:01, 20 March 2002

Revision as of 01:38, 20 March 2002

Line 1:

'''Speech coding''' is the [[audio compression|compression]] of speech (into a [[code]]) for [[telecommunication|transmission]] using [[audio signal processing]] and [[speech processing]] techniques.
The two most important applications using speech coding are [[mobile phone|mobile phones]] and [[internet phone|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 illegbility.
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 unneccesary in the speech coding context. In speech coding, the most important criterion is always preservation of intelligiblity of speech, with a constrained amount of transmitted data.
But it should be emphasised that intelligibility of speech includes, besides the actual litteralliteral content, also speaker identity, emotions, intonation, timbre etc. that are all important for perfect intelligibility.

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.

In addition to the actual speech coding of the signal, it is often neccessary 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.

In addition to the actual speech coding of the signal, it is often neccessary 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.

Major subfields:

* [[Wide-band speech coding]]

Major subfields:

** [[NMTGSM]]

* [[Wide-band speech coding]]

** [[GSMNMT]]

* [[Narrow-band speech coding]]

** [[NMT]]

* [[Narrow-band speech coding]]

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

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