Old NTSC tricks: 240p?

Ryan K. Brooks ryan at hack.net
Mon Mar 16 12:31:04 CDT 2015

Great description, thanks for posting this.


> On Mar 16, 2015, at 10:07 AM, Adam Sampson <ats at offog.org> wrote:
> John Foust <jfoust at threedee.com> writes:
>> I'm trying to understand at a low level how some early computers 
>> and game consoles generated a non-standard form of NTSC.
> There's a pretty good description here, with diagrams of the video
> waveforms involved for both PAL and NTSC (both use the same idea):
>  http://martin.hinner.info/vga/pal.html
> In brief: a video signal consists of a series of fields (about 60 per
> second for NTSC). Each video field starts with a series of "vertical
> sync" pulses that returns the electron beam to the top of the screen
> (there's also a "horizontal sync" pulse at the end of each line, which
> moves the beam back to the left of the screen). There are different
> vertical sync pulse sequences for odd and even fields, so the monitor
> knows to offset the even field by half a line. A normal video signal
> alternates between odd and even fields; in the kind of non-interlaced
> signal you're talking about, every frame starts with the odd field
> vertical sync, so the monitor always pulls the electron beam back to the
> same place.
> Cheers,
> -- 
> Adam Sampson <ats at offog.org>                         <http://offog.org/>

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