Old NTSC tricks: 240p?

John Foust jfoust at threedee.com
Mon Mar 16 09:20:07 CDT 2015

I'm trying to understand at a low level how some early computers 
and game consoles generated a non-standard form of NTSC.

The Wikipedia http://en.wikipedia.org/wiki/Low-definition_television says:

"Older video game consoles and home computers generated a nonstandard NTSC 
or PAL signal which sent a single field type which prevented fields from 
interlacing. This is equivalent to 240p and 288p respectively, and was 
used due to requiring less resources and producing a progressive 
and stable signal."

Another source says this was true for the "NTSC Atari 2600, Apple II 
family, Commodore 64, Nintendo Entertainment System, Sega Master 
System, and the vast majority of games for NTSC Genesis, Super NES, 
PlayStation, and Nintendo 64."

This page http://www.hdretrovision.com/240p/ calls it a "special timing 
signal" and gives examples of how contemporary flat-panel TVs can 
misinterpret the old signal.  The issue has spawned the creation of 
dozen of devices to give the retro look on new TVs.

- John

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