For the last couple of hours, I’ve been watching Scott Pelley provide sober, restrained, extraordinarily smooth coverage of the bomb blasts that occurred at today’s Boston Marathon.

Yet the CBS News coverage has also highlighted the unavoidable challenges, and pitfalls, associated with extended coverage of this sort of event, where anchors have more time to fill than new information, and are forced to rely on the same handful of images, replayed on what amounts to a continuous loop.

Pelley, to his credit, kept repeating — over and over again — what was known, and what wasn’t. There was no wild speculation, no jumping to conclusions, nothing to instill panic. It took some time before he would officially label the incident terrorism, later calling it “an act of terrorism, by whom we do not know.”

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Everything about him projected a sense of calm and authority. Local anchors, who often make all those mistakes, should be forced to watch as homework for the next time a similar situation arises in their market.

Nevertheless, those measured tones have been juxtaposed with the same image — again and again — of the two bombs exploding near the finish line. For those joining the coverage in progress, this is a service. For those watching hoping for some new crumb of information, it’s the numbing effect of seeing the carnage replayed because, well, it’s TV, and they have to show something.

This has been the issue from time immemorial — TV’s great strength (immediacy) and glaring weakness (images without context). TV news operates in these situations without a net, without an editor.

As stated, Pelley (chosen somewhat arbitrarily, in this case) has distinguished himself throughout the day as a worthy heir to the anchors he clearly seeks to emulate — including the trio (Peter Jennings, Dan Rather, Tom Brokaw) who reigned and defined the position for more than 20 years, through any number of tragedies.

But watching the coverage throughout the afternoon offered another reminder that these first drafts of history are messy. And for all the gadgetry at our disposal, that’s a problem there’s really no way to fix — other than having the good sense to turn off the coverage at the point where you realize there’s nothing more new to say.