MOS Mean Opinion Score

The Mean Opinion Score (MOS) is a subjective quality metric used to assess the quality of audio or video transmission. MOS is widely used in the telecommunications industry to measure the perceived quality of voice and video calls, as well as in other industries where quality of experience (QoE) is important, such as streaming media.

In this article, we will explore what MOS is, how it is calculated, how it is used in the telecommunications industry, and some of the limitations of the metric.

What is MOS?

MOS is a numerical scale ranging from 1 to 5 that represents the perceived quality of a transmitted signal. MOS is based on the subjective assessments of a group of human listeners who evaluate the quality of the signal using a standardized test. The listeners rate the quality of the signal on a scale from 1 to 5, with 1 being the worst possible quality and 5 being the best possible quality.

MOS was originally developed by the International Telecommunication Union (ITU) in the 1970s as a way to assess the quality of voice transmissions over telephone networks. The MOS scale was later adapted to evaluate the quality of video and audio transmissions in a variety of applications, including videoconferencing, streaming media, and internet protocol (IP) telephony.

The MOS score is a powerful tool for measuring the subjective quality of a transmission because it takes into account the perceptions of human listeners, who are the ultimate judges of quality. Unlike objective measures such as signal-to-noise ratio (SNR), MOS reflects the listener's experience of the transmission, including factors such as noise, distortion, delay, and other impairments that can affect the quality of the transmission.

How is MOS calculated?

MOS is calculated by taking the average of the individual scores assigned by a group of listeners who have evaluated the quality of a transmission. The listeners are typically selected from a representative sample of the target audience for the transmission, and they evaluate the transmission using a standardized test.

The MOS test typically involves presenting a series of audio or video clips to the listeners, who are asked to rate the quality of each clip on a scale from 1 to 5. The clips are carefully selected to represent a range of different conditions that can affect the quality of the transmission, such as background noise, distortion, delay, and other impairments.

Once the listeners have rated the clips, their individual scores are averaged to produce an overall MOS score for the transmission. The MOS score provides a quantitative measure of the perceived quality of the transmission that can be used to compare different transmission technologies, assess the impact of different impairments on quality, and optimize the performance of transmission systems.

How is MOS used in the telecommunications industry?

MOS is widely used in the telecommunications industry to assess the quality of voice and video calls. In the case of voice calls, MOS is used to evaluate the quality of the audio signal, including factors such as background noise, echo, and other distortions that can affect the intelligibility of the conversation. In the case of video calls, MOS is used to evaluate the quality of the video signal, including factors such as resolution, frame rate, and compression artifacts that can affect the clarity and smoothness of the video.

MOS is also used in the testing and development of transmission systems to ensure that they meet the requirements for quality of service (QoS) and quality of experience (QoE). For example, MOS tests can be used to compare different transmission technologies, such as 3G, 4G, and 5G, and to assess the impact of different impairments, such as noise, delay, and packet loss, on the quality of the transmission.

In addition to telecommunications, MOS is used in other industries where QoE is important, such as streaming media services. Streaming platforms often rely on MOS to assess the quality of video and audio content delivered to users. By monitoring MOS scores, streaming providers can identify potential issues in the delivery pipeline, such as buffering, latency, or compression artifacts, and take corrective measures to improve the user experience.

Furthermore, MOS is used in research and development of new transmission technologies and algorithms. Engineers and researchers use MOS tests to evaluate the performance of innovative techniques designed to enhance audio and video quality. By comparing MOS scores before and after implementing a new technology, they can quantify the improvements achieved and make informed decisions about the effectiveness of their solutions.

The MOS score is also valuable in service-level agreements (SLAs) and contracts between service providers and customers. It helps establish quality benchmarks and guarantees for voice and video services. For instance, a telecommunications provider might include a clause in their contract specifying a minimum MOS score that must be met to ensure satisfactory service. If the MOS score falls below the agreed threshold, the provider may be required to take remedial actions or compensate the customer.

Additionally, MOS scores can be used for benchmarking and competitive analysis. Telecommunications companies can compare their services against those of their competitors by conducting MOS tests and evaluating the resulting scores. This allows them to identify areas where they excel or lag behind and make strategic decisions to improve their market position.

It's worth mentioning that different MOS testing methods exist, such as ITU-T P.800 for voice quality or ITU-T P.910 for video quality. These methods provide guidelines for conducting subjective tests and ensure standardization in the MOS evaluation process. Adhering to standardized procedures helps ensure consistency and comparability of MOS scores across different testing scenarios.

Limitations of MOS

While MOS is a widely used and valuable metric for evaluating quality, it has certain limitations that need to be considered:

1. Subjectivity: MOS is based on subjective opinions of human listeners, which introduces a degree of variability and subjectivity into the scores. Different listeners may have different perceptions and preferences, leading to some level of inconsistency in the scores.
2. Limited scope: MOS primarily measures perceived quality based on impairments introduced during the transmission process. It does not take into account other factors that can influence the overall user experience, such as content relevance, user interface, or personal preferences.
3. Simplified rating scale: The 1 to 5 rating scale used in MOS tests provides a relatively coarse granularity for capturing quality perceptions. This simplicity may not fully capture the nuances and subtleties of the listeners' opinions.
4. Context dependency: MOS scores can be influenced by the context in which the tests are conducted. Factors like listener fatigue, environmental conditions, or the specific testing setup can impact the scores and their comparability.
5. Lack of real-time assessment: MOS tests are typically conducted after the transmission has occurred, which means they do not provide real-time feedback on the quality of ongoing calls or streams. Real-time assessment requires alternative metrics and techniques.

Despite these limitations, MOS remains a widely accepted and valuable tool for assessing the perceived quality of audio and video transmissions. It provides a standardized and quantifiable measure that can guide decision-making in various domains, including telecommunications, streaming media, and research and development.

In conclusion, the Mean Opinion Score (MOS) is a subjective quality metric that quantifies the perceived quality of audio and video transmissions. It is calculated by averaging the ratings provided by a group of human listeners who evaluate the quality of a transmission. MOS is extensively used in the telecommunications industry, streaming media services, and research and development to assess quality, compare technologies, and improve user experiences. While MOS has its limitations, it remains a valuable tool for evaluating transmission quality and guiding decision-making processes.