**4.1 Introduction**

The user's opinion in video technologies is important. For that reason, the evaluation of a video compression codec must be developed with its help, in order to know the level of acceptance of it.

Several studies have been developed to measure the quality of new technology systems every period. In first instance, it was used to analyze the impact of digital television in the transition from analogue to digital. And in the transition from standard definition to highdefinition TV, including the studies of main encoders (MPEG-2, H.264/AVC) in the adaptation to different formats and storage, last time assessment was developed for blu-ray codecs. Also for the Definition of settings or quality parameters: bitrate, resolution (e.g. in HD: 720p or 1080i) or features for a TV channel, for example.

Next generation of encoding codecs HVC (High-efficiency Video Codec) which represents the evolution of H.264 is being analyzed by subjective assessment, to measure the impact on users and the necessity of its establishment.

So, it is clear that subjective quality evaluation is still important and it will always be together with the objective one. Methods and technologies change, but the purpose of video quality is still the same.

In this section, the most frequent techniques to develop a subjective quality study, in order to evaluate a determined system. There is a great variety of subjective testing methods, depending on the way of presenting the video sequences to the observers.

Then, there are single or double stimulus experiments, depending on if the original image is presented or only the degraded one.

The problem with this kind of studies is the requirement for a large number of observers viewing a limited amount of video contents, because if the time of contents viewed is extended for two long, the tiredness and fatigue of the observers will affect to the wished measurement. But it is still very used nowadays and will be necessary to introduce the objective assessment to explain in next section.
