Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting modeled parameters in a compact bitstream.
[1] Common applications of speech coding are mobile telephony and voice over IP (VoIP).
For example, in voiceband speech coding, only information in the frequency band 400 to 3500 Hz is transmitted but the reconstructed signal retains adequate intelligibility.
At the time of their design, their 33% bandwidth reduction for a very low complexity made an excellent engineering compromise.
Their audio performance remains acceptable, and there was no need to replace them in the stationary phone network.
In CELP, linear prediction coefficients (LPC) are computed and quantized, usually as line spectral pairs (LSPs).
The modified discrete cosine transform (MDCT) is used in the LD-MDCT technique used by the AAC-LD format introduced in 1999.