why scan line of interlace is form middle point, and not horizontal?

Question

I am new to video format. I searched about interlacing scan mode.and fount

I want to know why the scan line is start at the middle of the line and not horizontal. Is LED display and CRT display same as the picture is showing?

Answer 1

I think your figure is not accurate: The first line of odd and even field start at the same vertical position with different horizontal positions.

For LED display, everything is controlled by digital circuit. The content of a LED display is progressive, and it is flushed/updated line by line.

For CRT display, the electron beam which light up the screen is controlled by analog circuit. There are two plates that control the horizontal and vertical shift of the beam. So the scan line is a slash line.

• If, We start a field at the end of last line.

  It means that voltage of vertical plate should reset, and with one more line shift for even field.

• If, We start the field at the middle of last line.

  It means that voltage of vertical plate only need to do reset.

As mankind's eye does not care about the starting position of field, the latter solution goes with a very simple analog circuit.