Once lists of index references are available then time-based indexes can be build. These have an entry every n seconds, with the references to all of the media and the sources for those elements. If there were, say, 12 sources then each new index element would have a list of 12 index references. Each of the 12 references would be for a time closest to the second boundary in the original indexes.
In figure 9.13 there is an example showing a time-based index referencing three other indexes. There are 3 index references for each element of the time-based index. The time-based index has an element for each second of elapsed time, and the references associated with each element point to the index element whose time is closest to the number of elapsed seconds.