Re: [stella] Interlacing Success!

Subject: Re: [stella] Interlacing Success!
From: "Andrew Davie" <adavie@xxxxxxxxxxxxx>
Date: Sun, 18 Aug 2002 16:51:18 +1000
----- Original Message -----
From: "Clay Halliwell" <clay.h@xxxxxxxxxxxxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Sunday, August 18, 2002 2:35 PM
Subject: Re: [stella] Interlacing Success!

> Ummm. The usual 2600 scanline is equal to exactly one NTSC scanline.
> no way you're going to cut that height in half without rewiring the TV.

Nope.  A TV picture isn't 262 NTSC lines high (or 312 in PAL).  It is 525
and 625, respectively.

The '2600 programs to date have only been sending 262/312 lines to the TV,
but - and here's the crucial bit - the TV has been effectively displaying
the same data for two frames.  So lines (if you really look) are actually
two TV lines tall.  By jiggering the interlacing, we are able to actually
display different data on the odd and even frames of the TV interlaced
image, and so you are able to effectively DOUBLE your vertical resolution -
from 262 to 525, and from 312 to 625... give or take a scanline.

In conclusion, the usual 2600 scanline is equal to exactly TWO NTSC
scanlines.  And yes, you can cut that in half.  Your belief that a TV
displays only 262 lines (or 312 PAL) is probably the issue.  In any frame,
yes - there are roughly that many lines - but TVs draw pictures in
interlaced mode - where an actual image is composed of two frames - the odd
lines first, then the even lines on the next frame.  Each of those frames
being 262 lines (or 312 PAL).  Thus giving our actual TV resolution of 525
NTSC (625 PAL).

That's what this is all about :)


PS: Great work Billy - keep it up - you have a lot of keen readers following
your progress.  And Glenn I'm beginning to warm to the CC concept.

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