Simultaneous substitution

Simultaneous substitution (also known as simsubbing or signal substitution) is a practice mandated by the Canadian Radio-television and Telecommunications Commission (CRTC) requiring cable television, direct broadcast satellite (DBS), IPTV and MMDS television distribution companies in Canada to distribute the signal of a local or regional over-the-air station in place of the signal of a foreign or non-local television station, when the two stations are broadcasting identical programming simultaneously.

The practice has become controversial because its implementation will often pre-empt the signals of American networks available through Canadian cable and satellite providers such as those of ABC, CBS, NBC and Fox. Simsubbing usually receives nationwide attention in the days leading up to the annual broadcast of the Super Bowl, where the famed high-profile Super Bowl commercials are virtually blocked from viewing on Canadian television. The Canadian network that broadcasts the National Football League championship game is eligible to request that the American broadcaster's signal be replaced in Canada with its own signal, so long as both broadcasts are telecast simultaneously.

The CRTC first commissioned simsubbing in 1972, and it is sometimes erroneously called "simulcasting", the name of a practice different from simultaneous substitution in that there is no signal replacement. According to the CRTC, the practice of simultaneous substitution is necessary "to protect the rights of broadcasters, to enable television stations to draw enough advertising revenue and to keep advertising money in the Canadian market". Canadian broadcast television networks, which must request each and every substitution on an individual basis, have been criticized for exploiting the regulation and not investing enough money into Canadian content.

...
Wikipedia