Ringing

The ringing artifact occurs when the quantization of individual DCT coefficients due to irregularities in high frequencies in the reconstruction of macroblocks. The effect is related to Gibb's phenomenon. It is more visible in high contrast edges, in areas with smooth textures. It is easily visible on next example, around the edges of the objects, apparing like duplications of their contours.

Fig. 4. Example of high ringing artifact

Noise

Noise is defined as an uncontrolled or unpredicted pattern of intensity fluctuations that affects to the perceived quality of an image.

There are multiple types of noise impairments commonly produced by compression algorithms. Two common impairments are mosquito noise and quantization noise.


Other artifacts related are flickering, in images with high texture content, or aliasing, when the content of the scene is above the Nyquist rate, either spatially or temporally

Video Quality Assessment 135

must not exceed 30º. Conditioning of the room.

Materials / Test sequences.

the observer.

Sound.

Presentation of results

their scores.

Observers

The screen must be arranged in contrast and brightness and the angle of observation

The room must be conditioned in lighting, with seat comfort and no reflections on screen. Because the viewers will spend a considerable time and this must not affect to

The minimum number of participants in the study, for the sample to be enough is 15, but it is recommended to find as many observers as possible, 60 or more. The viewers are preferred to be non-professional, possessing no trained-eyes. Professional viewers

There must be a representative variety of age and sex in viewers. The observers must

There must be a selection of sufficient test material, including different type of contents emitted by a conventional TV channel (sports, movies, news, documentary, etc.), and with different settings of spatial (more or less level of detail and high frequencies) and

Each sequence must have a duration of between 10 and 20 seconds due to human memory and perception, to assure a correct viewing, neither too short than the viewers do not have time to observe the image in detail, nor too long to avoid causing fatigue in

A number of organizations have developed test still pictures and sequences, whose use

The results must be presented in detail. All information is necessary to validate the study and verify its good performance. Data given must include: details of the test configuration and materials, type of source and displays, number of subjects or observers that participated, reference system used and its specific variations, scores and

Logistic curve-fitting and logarithmic axis will allow a straight line representation,

Audio quality assessment is preferably developed independently of the video assessment. In fact, it is recommended not to use sound or audio in video studies, in

Furthermore, in case of using it, selection of the accompanying audio material should be considered at the same level of importance as the selection of video material.

order to avoid distractions in the observer, modifying their opinion, off target.

tend to search for the impairment and their opinion is not always so impartial.

make a previous training to understand the objective of the test.

temporal (faster or slower contents) complexity.

mean scores adjusted to 95% confidence interval.

which is the preferred form of presentation, as legible as possible.

is recommended for the assessment.
