Hypermedia today is used as a synonymous of hypertext. Nevertheless when Nelson coined the terms [NELSON 1965] 'hypermedia' referred to different media types, such as films, sounds and videos, arranged in a non-linear fashion and 'hypertext' was limited to the non-linear arrangement of text only. The first hypertext systems started focusing mainly in text [ENGELBART 1985] probably for technical reasons. As new systems were developed functionality for including graphics was progressively added. This led to the association of the terms hypertext and hypervideo as referring to the same concept, the non-linear arrangement of linear information. As the technology evolved new media capabilities were added to the systems, audio and video among others.

There are two perspectives of how to look at time-based hypermedia. The first is to observe the use of time-based media in a hypermedia (or hypertext) environment. This time-based media presents intrinsic characteristics that differentiate it from other static media, such as text and graphics. Basically time-based media represent a dynamic from of media were the tempo is determined by the author [LIESTěL 1994]. This time dependency leads us to a different rhetoric. In static media the time concept was left to the user who decided when to read and the speed of reading. In dynamic media, these concepts are intrinsically predetermined by the information nature. While a text node can be presented all at once in the screen a video node (or an audio node) needs to be played in order to convey any sense. Different static representations such as buttons or micons may be used, but it is the images (or sounds) change over time what provides meaning to the video. This implies that functionality such as link representation and navigation should be synchronized considering the time factor of the dynamic nodes. Therefore, the inclusion of time-dependent media sucha as video impose the concept of time for the whole system in order to provide acceptable aesthetics and a proper rhetoric. This is true, even for situations when non time-dependent media is linked to time-dependent media. This leads to consider the presentation of the whole hyperdocument.

Another perspective on time-based hypermedia is the consideration of the whole user experience when accessing a hyperdocument regardless of the kind of contentents. This perspective is concerned with the presentation over time of different nodes and with the coordination and synchronization of concurrent nodes or sub-presentation threads. The next section presents a brief description of some of the research done in both perspectives of time-based hypermedia.

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