[173214] in North American Network Operators' Group
RE: Net Neutrality...
daemon@ATHENA.MIT.EDU (Keith Medcalf)
Sun Jul 20 12:10:04 2014
X-Original-To: nanog@nanog.org
Date: Sun, 20 Jul 2014 10:08:41 -0600
In-Reply-To: <CAP-guGXMBcxTyqStvDUuKzwid=0PoM1GQzXS3ydOYSom-t48LA@mail.gmail.com>
From: "Keith Medcalf" <kmedcalf@dessus.com>
To: "nanog@nanog.org" <nanog@nanog.org>
Cc: Rob Seastrom <rs@seastrom.com>
Errors-To: nanog-bounces@nanog.org
>An LED screen doesn't refresh the way a CRT does, right? The light
>doesn't flash and fade, it stays constant until the next change. So
>why would a 30 hz refresh rate make any difference at all for tasks
>which update the screen less often than 30 times a second? Mike did
>say he used it for doing software development.
You are absolutely correct Bill, however,
>Movies were shot at 24fps and TV shows at 30fps (60 interlaced), so
>I'm not sure where the harm would be there either.
In order to create a perception of movement, the images need to have "no im=
age" between them. 24 frame progressive (such as a movie theatre from real=
film) is usually projected as 48 frames using double shutters. It is the =
"blank/dark/no image" parts between the images that create the perception o=
f movement in the brain. For a scanning display (CRT) this is automatic --=
the persistence of each display frame is timed such that it only persists =
for one half a scan (which is why if you take a picture of a CRT displaying=
an image with a camera with a shutter speed faster than the refresh rate, =
you see a rolling black bar). The "blank/black" frames are created automat=
ically.
LCD displays, however, do not have these blank frames between the actual fr=
ames, which is why they do not create the appearance of motion correctly. =
Most LCD display devices, however, have a refresh rate of 60 Hz (some are h=
igher). When fed with a 30p signal, the display electronics should display=
blackness for every other 60p image. If you send a 60 Hz display a 60p si=
gnal, however, it will not have smooth motion.
LCD's designed to display moving pictures (ie, TVs) will run at even higher=
refresh rates (120 Hz for example) which allows a 60p display with proper =
blanking. In some cases the motion vectors are calculated and only the "mo=
ving bits" are blanked (or in some cases displayed as a complement image). =
Devices with even higher refresh rates do even more esoteric computations =
to determine the interstitial frames to create a proper perception of motio=
n by the brain.
Each manufacturer uses their own proprietary algorithms to determine what t=
o actually display -- some better some worse. Some even use a "scanning ba=
cklight" which makes the LCD display "emulate" the scanning behaviour of a =
CRT display allowing for a CRT-like creation of motion.
Now, back to regularly scheduled programming....
