Non-interlaced NTSC
 

Is there anyone that can explain how exactly non-interlaced NTSC works? I've heard a few reasons for it.

One is the last scanline is only half a scanline in interlaced modes, and in non interlaced modes, the last scanline is a full scanline.

But then I've heard others where the VSync has extra pulses to prevent the two fields from advancing basically.

So what exactly causes interlacing and no interlacing?

Interlace is inherent and automatic with an odd number of lines and correctly chosen H and V scan frequencies. A good way to explain it ito yourself is to draw the scan lines for a system with (say) 4.5 lines per field. Start at top left, then scan 4.5 lines, end at bottom centre. Fly back to top centre and continue, filling in the gaps as you go until you arrive at bottom right.

This page has useful info but there's nothing like trying it for yourself to get real understanding.
http://www.sparkysworld.co.uk/interlacing.htm

Non-interlaced NTSC can't really exist. If it's not interlaced, it's not NTSC. By definition. You can ghave a nasty kludge with 262 or 263 lines scanned 60 times per second, along with NTSC 3.58MHz subcarrier. The patterning caused by subcarrier in coloured areas will be truly horrible.

Subcarrier doesn't seem to be a problem with things like the PS1 or N64 game consoles. They appear completely fine when not interlacing in 240p mode though.

Early video games, like the Atari 2600, and various 80's era home computers did non-interlaced "NTSC". Interlace would have made ASCII characters on the TV screen hop up and down on the TV screen 30 times a second, and would be quite undesirable. Also TV sets of the time didn't care that it wasn't interlaced, as the vertical deflection only was coupled to the vertical sync pulses, and didn't care about the relationship to the horizontal sync. Later sets did have a horiz sync countdown to help time the vertical sync, but they also had an exception circuit to detect and allow non-interlaced vertical sync timing.

Yea, you can't see these scanlines and say it's interlacing.. It's clearly not.

http://www.youtube.com/watch?v=NFZ4DwaPF54