G.728
G.728 , also LD-CELP , designates a codec described by ITU-T for the compression of speech for use in telephony and IP telephony . The specialty of G.728 is its extremely low latency of five samples . At a sampling rate of 8 kHz, this corresponds to a time offset of 0.625 ms. The algorithm is therefore also known as LD-CELP , for Low Delay Code Excited Linear Prediction , and is based on the CELP compression method.
For compression in the encoder, the PCM voice signal is converted at 64 kbit / s into a serial bit stream with 16 kbit / s. The decoder generates a PCM voice signal from the coded data stream. The compression factor of four is fixed in the procedure. As is customary in the telephony sector, the PCM voice signal can be present with a non-linear sampling characteristic according to the μ-law or A-law method .
The linear prediction as a basis function is achieved by a 50th order LPC filter . For the implementation of the entire process, which in floating-point arithmetic or fixed-point arithmetic is available, a is a digital signal processor with a processing power of around 30 MIPS per channel is necessary.
Individual evidence
- ↑ ITU-T G.728 specification (in English)
- ↑ LD-CELP source code in C