USB connection on the TV set may not be capable of transferring large sums of data such as 1080p video stored on an USB flash disk to be played on TV set. If it were a PC, you could use USB 3.0 for the purpose. USB 3.0 can transfer data ten times denser than USB 2.0 that you can find on TV sets.
For practical purposes and for your information only, we can say that 1080i video carries only half the data of 1080p video. 720p video carries a little under 1080i video data. Therefore, USB on the TV set may be able to transfer both 1080i and 720p video data equally well to be played on the TV set. By the way, you mention 768p video. I do not know about 768p video.
Conversely, you can try a bluray player, a bluray movie disk and an hdmi connection to check if your TV set is capable of full HD, if that's just what you want to see.