A voice call having quality comparable to that of an ordinary long distance call, originally placed over the analog circuit-switched public switched telephone network (PSTN). More recently, the quality measurement technique applied to evaluate various implementations of voice encoding techniques, such as pulse code modulation (PCM). In quantitative terms, a voice connection rated as having a mean opinion score (MOS) of 4.0 or better, on a scale of 1.0 (bad) to 5.0 (excellent).The MOS is a mean average of the subjective evaluations of a group of listeners. The ITU-T standardized the MOS evaluation process in the Conversation Opinion Test, which is documented in Annex A of Recommendation P.800. A companion, and more objective, standard is P.862, the Perceptual Evaluation of Speech Quality (PESQ), which addresses the effects of filters, jitter, and coding distortions. Both P.800 and P.862 can be implemented in hardware. See also distortion, filter, ITU-T, jitter, MOS, P.800, P.862, PCM, and PESQ.
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