Because MOS so explicitly gets at the object of telecom service, MOS ratings are used and abused extensively. They're abused in the sense that over time engineers have observed that various objectively measured network parameters (both on TDM and IP networks) have very particular effects on MOS ratings. So rather than gather up 50 or so people and ask them to listen to voice samples every time there's a need to generate a MOS rating, these engineers calculate the likely MOS rating from observed conditions.
One way they do this is through a computational analysis of how known waveforms degrade when played across a voice link. The ITU-T set out to define these variations in the original MOS by way of Recommendation P.800.1. Among the definitions are the MOS Listening Quality Subjective (MOS-LQS) and MOS Listening Quality Objective (MOS-LQO) tests. P.800.1 doesn't actually explain how to run an objective test. Rather, it specifies that if you're running an objective test, you have to label it as such.
Objective measures of voice quality can either be passive or intrusive. If you aren't of a mind to run a subjective test, but you're still willing to grab a voice channel for testing, then consider using some computing horsepower and performing sound-file analysis. For intrusive tests, a known sound file gets placed onto a VoIP or TDM network at point A, then is extracted at point B. The two files are then compared, with the difference between them representing the deviation from the ideal. The ITU-T defines this as P.862, better known as the Perceptual Evaluation of Speech Quality (PESQ) test.
The latest changes to PESQ pay particular attention to the peculiarities of IP networks. For example, since IP networks may lose packets, a resultant speech file may end up slightly compressed compared to the source file. Its sound volume might also be higher or lower than the original file. PESQ calls for the resultant file to be normalized to account for these anomalies.
PESQ is fairly new and still evolving, with recent enhancements made to account for variations in noise cancellation and level matching. It's somewhat computationally intensive and primarily fit for probes or test instruments, though it's also implemented on carrier-side gateways. While PESQ is almost universally used over subjective MOS tests, MOS is still king, so the ITU-T has defined conversions from PESQ to MOS.
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