2 groups of voice codecs:1. Waveform codecs
Exploit the redundant characteristics of the waveform itself.
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
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:
64 Kbps PCM voice coding technique.
used in the public phone network and through PBX
ADPCM encoding at 40,32, 24 and 16 Kbps.
Some public phone network and PBX support it.
low delay variation of CELP voice compression at 16 Kbps
CELP at 8 Kbps
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|