Re: [stella] Interlacing 101

Subject: Re: [stella] Interlacing 101
From: "Clay Halliwell" <clay.h@xxxxxxxxxxxxxxxx>
Date: Tue, 20 Aug 2002 21:00:30 -0500
----- Original Message -----
From: "Andrew Davie" <adavie@xxxxxxxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Tuesday, August 20, 2002 9:11 AM
Subject: Re: [stella] Interlacing Success!

> I still contend that on non-interlaced games, a '2600 'scanline' is
> 2 TV-lines tall.

Sigh... This is exactly like me arguing that I'm actually 12 feet tall, but
that the top 6 feet are invisible.

Let's back up and start at square one:

NTSC televisions run at 30 frames per second. Each frame is composed of two
interlaced fields. A new field is sent every 60th of a second. The odd and
even fields combine to form a single frame. Therefore, broadcast television
programs run at 30 FPS (approximately).

However, the onus of interlacing is entirely on the broadcaster. This allows
game consoles to, instead of generating odd and even fields, just generate
the odd (or even) field over and over, yielding a picture that updates at
60FPS. The downside is that you sacrifice half your vertical resolution
because the other field is never sent, and thus remains blank.

Okay, that's enough explaining of stuff you likely already knew. Let's get
back to this statement:

> I still contend that on non-interlaced games, a '2600 'scanline' is
> 2 TV-lines tall.

Okay, there's no such thing as a "2600 scanline" (and yes, I've been guilty
of using the term in this discussion). A scanline is nothing more or less
than the line produced by the electron gun beam crossing the face of the
picture tube. If by "2600 scanline" we mean a line of vertical single-line
resolution, then they're still equivalent, because the 2600 is only
generating half a frame's worth of data... those other scanlines are /never/
written, not even as black. They're skipped.

When you make the transition to an interlaced display, no scanlines are
getting "split". What's happening is that all those scanlines that were
laying dormant are suddenly active again, thanks to the second field being

> And I contend that we ARE doubling the vertical resolution, as I

I never disputed this. But all the muddled terminology surrounding this
achievement is driving me nuts.


BTW, has anybody considered hooking a scope to an old Astrocade to see how
it generates its interlaced display?

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