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Date:      Thu, 18 Nov 1999 18:05:12 +1200
From:      "Craig Harding" <crh@outpost.co.nz>
To:        Terry Lambert <tlambert@primenet.com>
Cc:        chat@freebsd.org
Subject:   Re: Judge: "Gates Was Main Culprit"
Message-ID:  <19991118050528.7618214C0D@hub.freebsd.org>
In-Reply-To: <199911180228.TAA25734@usr08.primenet.com>
References:  <000001bf30b5$b82c5470$021d85d1@youwant.to> from "David Schwartz" at Nov 16, 99 08:39:22 pm

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Terry Lambert wrote:

> Even ignoring this, the rotating record head is away from the
> tape media for longer than the vertical blanking interval, and
> that means that you get a 1.5 reduction in frame rate.  This
> reduction in frame rate is even more noticible because of
> horizontal retrace in PAL vs. NTSC.
> 
> The result is that the 525 lines of vertical resolution are reduced
> to 200 for VHS, 400 for SuperVHS

Erm, actually I think you're getting a couple of things confused here 
Terry.

Firstly, I don't know what you're trying to say about frame rate, as 
far as I'm aware PAL video runs at 50 fields (25 frames) per second 
and NTSC runs at 60 fields (30 frames) per second [1].

As for resolution, you've been confused by the terminology. PAL
always displays (approximately) 625 scanlines on the screen, and
NTSC always shows 525. That's actual horizontal scans by the
electron gun across the width of the screen, half in each interlaced
field.

When we talk about the resolution of a tape format (or a camera or
monitor) in video circles, we're referring to the horizontal
resolution, which is loosely equivalent to the frequency response of
the intensity signal of the electron beam as it sweeps across the
display.

The resolution is measured by the only handy unit available in 
pre-computer display times - screen lines. So we're talking about 
resolution in terms of roughly square notional pixels, which means we 
use the size of vertical screen lines to talk about horizontal 
resolution.

Some camera test charts actually have a resolution grid on them, with 
horizontal and vertical lines drawn in an increasingly-finer 
gradient. When you can no longer distinguish individual lines, you've 
hit the resolution limit of the camera (or monitor, or whatever).

VHS, as Terry mentioned, has about 200-240 lines resolution. Super 
VHS has theoretically close to 500, where as the broadcast 
format BetaSP is only 450. This leads some people to claim that Super 
VHS is superior to BetaSP because they are ignoring SVHS's terrible 
colour resolution.

Modern broadcast video cameras have a horizontal resolution of about 
850 lines.
-- 
Craig Harding                crh@outpost.co.nz    "I don't know about God, I
Outpost Digital Media Ltd    crh@inspire.net.nz    just think we're handmade"
http://www.outpost.co.nz     ICQ# 26701833                 - Polly


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