kmedcalf at dessus.com
Sun Jul 20 16:08:41 UTC 2014
>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 image" 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 of 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 automatically.
LCD displays, however, do not have these blank frames between the actual frames, 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 higher). 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 signal, 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 "moving 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 motion by the brain.
Each manufacturer uses their own proprietary algorithms to determine what to actually display -- some better some worse. Some even use a "scanning backlight" 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....
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