In Canada, the Canadian Radio-television and Telecommunications Commission allows Canadian television broadcasters to require simultaneous substitution of their channel for U.S. channels when they choose to simulcast a show; consequently, they often do choose to do so. Although sometimes controversial, this practice is seen as as a way to protect the broadcast rights and commercial revenues of Canadian television broadcasters, by ensuring that the Canadian commercials are seen by nearly all viewers of the program.
For example, Global airs several popular American series such as The Simpsons. With simultaneous substitution, Global can ensure that advertisements purchased on their network are seen by nearly all Canadians watching the show at that particular time, rather than losing advertising revenue because some viewers were seeing the FOX affiliate's commercials instead.
This issue is at its most controversial during the Super Bowl, where the simsub rules usually block Canadian audiences from seeing the American commercials that they would actually want to view during the Super Bowl. The replacement ads are often the same ones seen on Canadian television at all other times of the year, although the amount of "original" creative content by Canadian advertisers during recent editions has increased significantly.[citation needed].
Simsubs can only occur when a local and a non-local station are airing identical programming. Using the Simpsons example above, Global can only simsub a FOX affiliate airing the same episode of The Simpsons in the same time slot (hence "simultaneous"). The network cannot simsub if FOX is airing a different episode, and it cannot simsub a FOX affiliate airing the show at a different time. Therefore Canadian viewers should not lose any content from other, non-simsubbed programs; however, this is only true if the cable, satellite, or MDS distributor performs the substitution accurately. If they mistake the start or end time, or even channel, of a broadcast or if the time is unexpectedly changed (for example, due to a long-running sportscast before it), viewers may lose out.
Simultaneous substitutions are performed by the cable, satellite or MDS distributor, from a list submitted by the station in advance. Simultaneous substitution applies only to those methods of distribution. U.S. terrestrial signals available in Canadian border markets cannot be simsubbed. Also, simsubs can only be applied by cable companies in areas where the local station is available terrestrially.