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Ill use CBC as an example,.
CBC is broadcast on SD ch 3 or HD on Ch 602
CBC only broadcasts in High definition...
consequently if your box in in HD mode, (and lets face it, why wouldnt it be!!) than CBC fills the screen in HD but in SD ch3 it adds the bars to the top and bottom,
if your watching a SD show on a HD channel this effects appears on the sides. Its purely how the signal is broadcast!