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Subject: Re: [AE] OT resolution trivia
From: Chris Zwar <chris@chriszwar.com>
In-Reply-To: <list-4636500@media-motion.tv>
Date: Sat, 25 Feb 2012 10:22:32 +1100
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References: <list-4636500@media-motion.tv>
To: "After Effects Mail List" <AE-List@media-motion.tv>

On 25/02/2012, at 3:59 AM, Chris Meyer wrote:
> I suspect it's an analog hangover.

Yes, this is it.  It's all a historical artefact.

Prior to digital and HD, TV standards around the world could be =
categorised as either 525 or 625 lines, usually (but not always) =
corresponding to NTSC and PAL.  However the horizontal resolution would =
vary depending on the analogue bandwidth that was allocated in each =
country - usually between 5 and 8 mhz.  In the truest sense, analogue TV =
did not have a horizontal resolution in pixels, it had a bandwidth.

If you look at character generators and TV video cards from that period, =
they didn't actually specify a "resolution" in pixels that we would =
recognise today- they would specify a timing resolution in nanoseconds.  =
So a 6ns character generator would have a higher resolution than an 8ns =
character generator (because 6ns is faster).  But they'd all work at =
either 525 or 625 lines.

It's a bit like looking at the difference between VHS and Beta SP.  =
Decks in the same country will record the same signal with the same =
number of lines - they're either 525 or 625 line decks.  But VHS has =
appalling bandwidth compared with Beta SP, so the image quality is much =
lower.  But you can't measure the difference between them in pixels, as =
they don't have pixels... just bandwidth.  But they record either PAL or =
NTSC, regardless of how well they do it.

While everyone on this list is more likely to recognise terms like ITU-R =
601, REC 601, D1 rather than something like '6ns', the significance of =
those letters is often lost.  Basically they represent an agreed =
industry standard on how to take a 'squirt' of analogue bandwidth and =
convert it into a digital format.  The reason we got stuck with 720 =
horizontal pixels and rectangular pixel aspect ratios lies in the maths =
behind that standard.  But it doesn't mean that analogue PAL and NTSC =
actually have 720 horizontal pixels, only that 720 is the agreed =
standard.  Early non-linear systems such as the Media 100 digitised =
video with square pixels - 640x480 - and I remember reading an article =
that argued it provided a higher quality image than 720x480, as the same =
amount of bandwidth was distributed amongst fewer pixels, so the image =
suffered from less compression (hence higher quality).

The point is that the number of lines was always constant, regardless of =
the image quality and analogue bandwidth of a particular device, so =
that's how they're described.

FWIW I found an old copy of 'broadcast engineer' from the late 90s which =
looked at the proposed standards for ATSC/HD.  One of the engineers =
interviewed said "I hope they choose 1035 lines.  Anything else would be =
stupid..." It made me smile.


-Chris
 =20=