720i isn't a broadcast standard, but apparently, it is a common format for upconverting pre HD formats.
Quote:
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Originally Posted by Wikipedia
Wikipedia: Older (pre-HDTV) recordings on video tape such as Betacam SP are often either in the form 480i60 or 576i50. These may be upconverted to a higher resolution format (720i), but removing the interlace to match the common 720p format may distort the picture or require filtering which actually reduces the resolution of the final output.
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Probably what a lot of the sets do with composite inputs. especially the CRT based models, but I am guessing.