What causes the aspect ratio to be different?

Old movies were filmed in the 4:3 ratio. That gives you blank areas at the left and right on your widescreen TV. Not-so-old movies were filmed in some variation of a widescreen format, but they were still standard-definition, which gives you the "letterbox" look--blank areas on all 4 sides. Modern HD video fills your 16:9 screen from corner to corner.

So it's not the channel, but what the channel is broadcasting at the moment.

As you've noted, some commercials, promotions, brief announcements, etc. are presented in 4:3 or letterbox because the producers didn't pay the extra money to record them in HD.

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Not all of the channels and tv shows have changed to the HD settings yet. so not all are going to be fitting your whole screen.

Also look for if your set has an auto aspect feature in the menu.

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