Voice Codecs2 groups of voice codecs:1. Waveform codecsExploit the redundant characteristics of the waveform itself. ex: Pulse Code Modulation PCM Adaptive Differential Pulse Code Modulation ADPCM 2. Source codecs Newer compression techniques that exploit knowledge of the source characteristics of speech generation ex: Linear Predictive Coding (PLC) Code Exited Linear Prediction (CELP) Multipulse, Multilevel Quantization (MP-MLQ) Pulse Code Modulation (PCM): The most common method of encoding an analog voice signal. It’s based on Nyquist theorem (if you sample at twice the highest frequency, you’ll achieve good-quality voice). Encode using 8 bits samples to encode the amplitude of speech. Will guarantee good quality voice up to 4000 Hz by sampling at 8000Hz PCM use 2x4000=8000 Hz sampling. (4000 Hz sufficient for voice quality) The transmission rate is 8000 x 8 bit = 64000 kbps. 2 basic PCM variations: u-law & a-law, u-law has a slight lower S/N compare to a-law. Adaptive Differential Pulse Code Modulation (ADPCM): Encodes using 4 bits samples. Transmission rate: 32 kbps. Unlike PCM, the 4 bits do not directly encode the amplitude of the speech, but do encode the differences in amplitude, as well as the rate of change of that amplitude. Voice Coding Standard:ITU-T standardizes PCM, ADPCM, CELP, MP-MLQ PCM coding schemes in its G-series recommendation:G.711: 64 Kbps PCM voice coding technique. used in the public phone network and through PBX G.726: ADPCM encoding at 40,32, 24 and 16 Kbps. Some public phone network and PBX support it. G.728: low delay variation of CELP voice compression at 16 Kbps G.729: CELP at 8 Kbps G.723.1 compression at low bit rate
LD-CELP: Low Delay-CELP CS-ACELP: Conjugated Structure Algebraic CELP MOS: mean opinion score |
$Date: Wed Jan 12 18:00:48 CET 2005 $ | © 2003-2005 Omar Gani |