Editing Mean Opinion Score (section)

== History and Development ==

=== Early Origins ===

The concept of Mean Opinion Score emerged in the telecommunications industry during the 1970s as telephone networks became more complex and digital transmission methods were introduced. Initially developed by the ITU, MOS provided a standardized way to assess voice transmission quality over telephone networks by aggregating human judgments of call quality.<ref>https://www.telecomtrainer.com/mos-mean-opinion-score/</ref>

The early methodology involved having listeners sit in controlled "quiet rooms" and score telephone call quality as they perceived it. This subjective testing approach had been in use in the telephony industry for decades before formal standardization, reflecting the industry's recognition that technical measurements alone could not capture the human experience of communication quality.<ref>https://en.wikipedia.org/wiki/Mean_opinion_score</ref>

=== Standardization ===

The methodology was formally standardized in ITU-T Recommendation P.800, "Methods for subjective determination of transmission quality," approved on August 30, 1996.<ref>https://www.itu.int/rec/T-REC-P.800-199608-I/en</ref> This recommendation established rigorous protocols for conducting subjective quality tests, including specific requirements for test environments and procedures.

The standardization specified that test subjects should be seated in quiet rooms with volumes between 30 and 120 cubic meters, reverberation times less than 500 milliseconds (preferably 200-300 ms), and room noise levels below 30 dBA with no dominant spectral peaks. These environmental controls ensured consistency and reliability in MOS evaluations across different testing facilities and organizations.

=== Evolution and Extensions ===

Following the success of P.800, the ITU-T developed additional recommendations to clarify and extend MOS methodology:

* '''ITU-T P.800.1''' (2003, updated 2016): Established terminology for different types of MOS scores, distinguishing between listening quality subjective (MOS-LQS), listening quality objective (MOS-LQO), and listening quality estimated (MOS-LQE) to avoid confusion about the source and nature of scores.<ref>https://studylib.net/doc/8277727/itu-t-rec.-p.800.1--03-2003--mean-opinion-score--mos--ter</ref>
* '''ITU-T P.800.2''': Prescribed how MOS values should be reported, emphasizing that MOS scores from separate experiments cannot be directly compared unless explicitly designed for comparison and statistically validated.
* '''ITU-T P.808''' (2021): Addressed crowdsourcing methods for conducting subjective evaluations, recognizing the need for scalable approaches to MOS testing in the digital age.<ref>https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-P.808-202106-I!!PDF-E&type=items</ref>