Hi Dee-27
You are correct in saying that with an interlaced display, only half of the available lines that make up a full frame are displayed at a time. In your case of 720i the first field (field1) displays lines 1-3-5-7-9 etc to line 719 in a 50th of a second. The second field (field2) displays lines 2-4-6-8-10 etc to line 720 in the next 50th of a second. Then when the 2 fields are interlaced together (hence the term interlaced) on your display to form one frame you see 720 lines in a 25th of a second giving you flicker.