What is Interlaced Output?
NOTE: This is explained MUCH more clearly in pages on the Internet.
Before the days of computers and HD, all TV signals used an interlaced signal. In an interlaced signal each FRAME is broadcast in two parts called FIELDs. (1 FRAME= 2 FIELDs)
The signal broadcast contains all the odd lines in a picture 1,3,5etc (the first field) Followed by all the even lines in a picture 2,4,6,etc (the second field). Too make up the whole picture you first paint the lines of the first field then paint the lines of the second field interleaved with the first.
This is in contrast to a progressive signal where all the lines are broadcast consecutively i.e. 1,2,3,4,5,6etc.
Why? Well basically it saves bandwidth, you can get nearly twice the resolution picture for less than twice the bandwidth. In the early days of TV this was very important. With the new hight resolution pictures, it still remains important hence the use of 1080i.
So with PAL you get: 25 Frames per second consisting of 50 Fields per second.
or with NTSC : 30 (approx) Frames per second consisting of 60 Fields per second.