Table 2-1 Details of Five CODECs
CODEC Technique Bandwidth 20 ms Sample Size Quality
G.711 PCM 64 160 4.10
G.726 ADPCM 32, 24, 16 80, 40, 20 3.85
G.728 LDCELP 16 40 3.61
G.729 CS-ACELP 8 20 3.92
G.729A CS-ACELP 8 20 3.90
Voice quality is measured on a scale called Mean Opinion Score (MOS).
MOS has been scored by averaging judges’ scores: a MOS of 5 is perfect,
whereas 4 is toll quality, and anything less gets less and less acceptable.
Perceptual Speech Quality Measurement (PSQM) is a newer technique that
compares wave forms pre- and post-transmission and grades on a scale of 0
to 6.5. PSQM is repeatable and less arbitrary, but the non-traditional scale
made it hard to compare to MOS, so Perceptual Evaluation of Speech
Quality (PESQ) is a version of PSQM that uses an MOS scale.
All the ideas discussed in this section—sampling, quantization, encoding,
and compression—depend on specialized processors called Digital Signal
Processors (DSP). DSPs are also used for translating CODECs (transcoding)
and for conferencing.