08 Nov What is the quality of audiovisual content

Measuring the perceived quality of an audiovisual content by end users has become a very important objective for broadcasters and content providers. The audiovisual content traffic through the Internet along with the new TV channels have grown significantly. It is essential for them to have knowledge about what they are providing to the final users and how well users perceive it.

When the contents are transformed into other formats and transmitted, either through the Internet or conventional TV (cable, terrestrial or satellite diffusion), more or less transmission errors will happen in the audio and/or in the video. Having mechanisms that detect these errors are necessary to determine the quality of the signal.

The success of an audiovisual content is related to the number of visualizations and the audience, and the quality of the content plays an important role in this. However, estimating the perceived quality is not easy. Mainly because determining how well a video looks, how well an audio sounds and how good is the interaction with the service or application is subjective and depends on the human perception.

The subjective metric used to assess the final perceived quality is known as QoE (Quality of Experience). Using the ITU’s (International Telecommunications Union) definition, the QoE is “the overall acceptability of an application or service, as perceived subjectively by the end user”. This metric takes into account the type of content, the degradations of the signal, the expectations, experiences and user perceptions (related to the Human Visual System and Human Auditory System), network conditions and device capabilities. QoE would be the degree of satisfaction or disapproval of a user with a specific service, taking into account all the previously mentioned factors.

These factors are classified in three different categories: Human Influencing Factors, System Influencing Factors and Context Influencing Factors.

Human Influencing Factors are related to the human users. In this group there are factors such as demographic and socio-economic status, emotional state, expectations, needs, and physical and mental constitutions. Aspects such as gender, age, visual and hearing acuity must also be considered to estimate the quality of experience.

System Influencing Factors determine, from a technical point of view, the quality of an application or service. This category is related to the capture (quality will depend on the imperfections in the optical system and on the sensor capabilities of the camera, environmental conditions and content characteristics), coding (essential to assign an adequate codec and bitrate to avoid the effect of blocks), transmission, storage and content playback. There is a classification in four subcategories: content, media, network and device.

Content factors correspond to temporal or spatial requirements, color, texture, 2D or 3D. Media factors depend on the characteristics of the content (resolution, frame rate, sampling rate, audio video synchronization) and the encoding characteristics. Network factors describe the transmission system in terms of packet loss, delays, freezes, jitter, network congestion and bitrate drops. Device factors take into account the capacity, performance and characteristics of the systems and devices with which the audiovisual contents are consumed. Features such as the device settings, screen size, screen resolution, viewing distance and viewing angle greatly influence the quality of experience.

Context Influencing Factors consider the characteristics related to the physical (space and location), temporal (time of the day), social (if the user is alone or is accompanied by more people), task (related to the purpose the user is consuming the audiovisual content for), technical (if user is using additional systems or devices to improve the experience) and economic (if the user has paid or not for the content).[/vc_column_text]

All these factors described above have to be considered when estimating the QoE. The estimation can be done with subjective assessments where users evaluate the quality of a set of degraded sequences following a specific methodology or with objective assessments, using mathematical algorithms to measure the audiovisual signal degradation and network conditions, taking into account the characteristics of the audiovisual content and the models of the human perception.

 

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