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Steven Ochmanski
Cisco Employee
Cisco Employee


Vector Sum Excited Linear Prediction (VSELP)

Complete Definition

Vector Sum Excited Linear Prediction (VSELP) is a speech coding method used  in the IS-54 standard. This codec was used in early TDMA cell phones in the  United States. It was also used in the first version of RealAudio for audio over  the Internet. The IS-54 VSELP standard was published by the Telecommunications  Industry Association in 1989.

IS-54 VSELP specifies an encoding of each 20 ms of speech into 159-bit  frames, thus achieving a raw data rate of 7.95 kbps. In an actual TDMA cell phone, the  vocoder output is packaged with error correction and signaling information,  resulting in an over-the-air data rate of 16.2 kbps. For internet audio, each 159-bit frame  is stored in 20 bytes, leaving 1 bit unused. The resulting file thus has a data  rate of exactly 8 kbps.

A major drawback of VSELP is its limited ability to encode non-speech sounds,  so that it performs poorly when encoding speech in the presence of background  noise. For this reason, use of VSELP has been gradually phased out in favor of  newer codecs.

VSELP is also the encoding method used in some early Motorola Type II  trunking systems. It was phased out in favor of the newer IMBE method.

Wikipedia CELP  Definition

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