[Former Member 2410]

It’s a very interesting question though.

The main US networks are carried on cable in Canada, through stations near the border. However they are subject to Canada’s infamous simultaneous substitution rule, which basically says that if a local station is carrying the same episode of the same programme as an out of market station, the signal of the out of market station must be substituted by the cable provider for that of the local station for the duration. While the rule is written in broad enough terms it is squarely aimed at US stations and for the benefit of Canadian ones, and Canadian networks programme accordingly to maximise the benefit for themselves.

Presumably then, if no simultaneous substitution is in effect, the Canadian viewers of US stations would indeed see interruptions, but they might not if simultaneous substitution wasn’t in effect (although I would imagine the mere fact of the interruption would technically mean subsuming wasn’t required from that point).

Sim-sub was designed primary to protect the Canadian advertisers from losing out when shows are aired on both a U.S. and Canadian network at the same time. However, it's also been used to sub one Canadian station with another. Many Canadian networks air the same shows as their U.S. equivalents, at the same times, just to take advantage of sim-sub rules. This creates a situation in Newfoundland on NTV where shows are 'late' in order to meet Eastern time (Newfoundland has only one commercial channel - the independent NTV which shows a mix of CTV and Global shows).

As an aside, sim-sub can be quite complex, and it's the subject of much debate in Canada. There's been several court battles over it between, for example, Bell and the CRTC, especially over big events like the Super Bowl.

Not all providers treat sim-sub the same either, partly dependent on technology, and partly dependent on the channels they carry. For example Bell Satellite (formerly Bell ExpressVu) sim-sub's nationally. Shaw Direct (now owned by Rogers) sim-sub's by postal code and / or account. I've known areas that are in remote locations (including my own home) not be sim-subbed, for various reasons, or be sim-subbed on some channels but not others. I've had it where my TV wasn't sim-subbed, but a neighbours was. Equally, the channel that is subbed-in may be different between providers as they don't carry the same U.S. networks (Shaw carries Detroit and Rochester on the East Coast whereas Bell carries Boston IIRC), which is dependent on where their headend feeds come from.

In the same vain, if the sim-sub is late or early, even by a second or two, it can cause significant disruption to watching. Some stations (NTV comes to mind as it's my 'local') used to show very large opaque bugs (DOGs), covering the bug / DOG of the originating network, but these are now rare, if at all used.

IME, when breaking news airs on a U.S. station, it's not sim-subbed (as the Canadian networks aren't showing the same show). If it is subbed over, it's usually a mistake, or the Canadian network wants to keep to time / schedule. Occasionally, breaking Canadian news or some large event on the main networks has been broken in-to by a sim-sub, sometimes just by automation, sometimes by a deliberate choice (the latter being not very popular!)