On Thu, Jan 06, 2005 at 09:49:42PM -0700, David Farber wrote: > Sorry for the off-topic post, but I expect that someone on this list has the > technical knowledge to answer this question. > > How is the aspect ratio of a video signal negotiated between two video > devices? For example, let's say I have a DVD player capable of 480p output. > Am I correct is assuming that I would get a larger, higher resolution picture > if I plugged this dvd player into a 17-inch 16:9 480p-capable tv/monitor than > if I plugged the same dvd player into a 17-inh 4:3 4800-capable tv-monitor? > > Can the dvd player detect the presence of a 16:9 tv versus a 4:3 tv? Or is > the 16:9 tv set scaling the 4:3 picture?
Analog TV transmission -- via Component video, VGA, SVideo or composite video -- has no concept of the width of the screen, ie the length of the horizontal lines. It does know how many lines there are but nothing really tells it how many pixels wide the lines are. (In theory, one could imagine trying to guess it, but since you are allowed to change how many pixels you fit in a line and the aspect ratio is supposed to stay the same this would not work.) So that means any device that feeds a TV analog signals needs to know the aspect ratio of the TV it is feeding (so it can insert bars in the right place when displaying content of a different ratio) or the TV has to have a setting to change what aspect ratio it appears to be (ie. it inserts the bars, or zooms or whatever.) Digital TV signals (ATSC etc.) contain information about what aspect ratio they are. Thus a cable box can, knowing the aspect ratio of the TV it is feeding, figure out the right thing to do (put bars on a 4:3 signal going to a 16:9 TV, or letterbox a widescreen signal on a 4:3 etc.) DVDs of course also contain info about what aspect ratio they should be. Many DVDs are released in anamorphic widescreen, and the player knows to display them fully on 16:9 TVs and letterbox on 4:3. In theory, DVI might be able to have the TV know what it's getting but I think even there you are allowed to send different pixel counts but I am less familiar with this. This is one of the big pains early widescreen consumers have.
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