Canada's broadcast regulator is making good on a promise to crack down on Canadian cable companies improperly cutting into U.S.-based programs after hearing from Canadians upset about not being able to see the final minutes of the finale of David Letterman's show.

The CRTC sent open letters to executives of Rogers and Shaw this week, making them aware of complaints they've received about the improper execution of what's known as simultaneous substitution or "simsub" which is when Canadian broadcasters take a feed of an event from a U.S. source and splice in their own commercials on the fly.

If it's done inelegantly or clumsily, what can often happen is that Canadian viewers see a feed that comes back from commercials late, or cuts out early.

Early goodbye to Dave

That's exactly what a handful of Canadians say happened to them on the finale of The Late Show with David Letterman last week.

Crammed with guests for the last show, the episode ran a few minutes past its scheduled end time of 1:35 a.m. "Right at 1:35 a.m. CTV cut in to my CBS feed and started showing a (presumably) canned episode of some stand up comic," viewer Calvin Daling told the commission. "After 33 years of being on TV, can he not go an extra few minutes? The simulcast was very frustrating as I missed a big moment in TV that I stayed up way late to watch."

"Since the final moments of series finales are some of the most poignant, I am tremendously upset by Shaw's actions," echoed another viewer, Aaron Klotz.

Marilee Sharp, another upset viewer who wrote to the commission, put it bluntly: "I want to know who's fault it is that the David Letterman final show on May 21 was cut off well before it ended."

The CRTC agrees with the viewers, asking the heads of networks to explain why that happened. Earlier this year, the CRTC changed the broadcast rules surrounding simsub so networks would have no excuses not to execute it properly and seamlessly.

"With such focus put on the execution of simultaneous substitutions in the last few months, it is surprising that these errors continue to happen — particularly during such a widely publicized television event," the commission wrote to Susan Wheeler, a vice-president at Rogers Media.

By next Tuesday, the broadcasters singled out by the CRTC must explain what happened and "explain what new measures will be taken to avoid similar situations in the future."

Under the letter of the law, the regulator has the right to punish a local broadcaster for simsub errors by taking away their right to do it for a period of time or for a type of programming. If it's a distributor, the regulator can force them to provide a compensatory rebate to its customers.

A spokesman for Rogers Communications says an "internal miscommunication" left customers who were watching its CBS feed in Ontario, New Brunswick and Newfoundland and Labrador miss the final minute of Letterman's show.

Shaw Communications says it is investigating what happened with the goal of preventing such errors in the future.