[mythtv-users] OT: HDTV TV's

Steven Adeff adeffs.mythtv at gmail.com
Thu May 11 14:58:12 EDT 2006

On 5/11/06, Ivan Kowalenko <ivan.kowalenko at gmail.com> wrote:
> > On 5/11/06, Daniel Kristjansson <danielk at cuymedia.net> wrote:
> >> On Thu, 2006-05-11 at 11:58 -0400, Michael T. Dean wrote:
> >>> On 05/10/2006 03:01 PM, Mike Frisch wrote:
> >>> Exactly.  And, technically, a 1920x1080 (1080p) display cannot fully
> >>> resolve a 1920x1080 input signal (whether 1080i or 1080p).
> >> Not true. A 1920x1080 display can fully resolve a 1080i/p image.
> That's always confused me. There are 1920x1080 pixels, right? And
> there are 1920x1082 pixels coming at you, right? Why CAN'T a display
> show that properly? I understand those two pixels are cropped. I
> mean, a 1024x768 display can fully resolve a 1024x768 signal, why
> can't a HDTV resolve its signal? It's all digital.

By standard it should, and all fixed pixel displays have no excuse.
The problem comes in non-fixed pixel displays (CRT, rear projection,
etc). Where issues like hourglass effect, and other edge issues come
in to play. Manufacturers decided to use overscan of the edges to make
those issues "dissapear". The proper method would be to use overscan
AND increase the output resolution so that the picture being displayed
still sits in the frame of view. But this would increase the costs of
the sets, which is not something they are willing to do yet. Once the
prices come down I bet the higher end models will start doing this.
Some manufactuers have a 1:1 mapping mode where they don't force
overscan but you end up with possible the possible edge problems since
its still only drawing the same number of pixels.

> >> But if it is not a CRT, a 1920x1080 display can not fully resolve
> >> a 720p image unless you severely letterbox that image. In order
> >> to fully resolve both 720p and 1080i in an edge to edge display
> >> you need 3840x2160 pixels.
> Can you explain that? As an HD n00b (I don't even have an HDTV to
> play with...) that seems a little confusing, the way it's phrased.
> I've seen LCDs display resolutions lower than their maximum before
> (1024x768 LCDs displaying signals as low as 640x480). And since 720p
> and 1080i/p have 16:9 aspect ratios, shouldn't they scale properly?
> Or are we talking about scaling up the resolution in such a way that
> we don't have "half pixels" (or where pixel A from the 720p signal,
> when scaled up fills 1.5 pixels, or something like that)?

this is where scaling comes in to play. 720p=1280x720,
1080i/p=1920x1080. So when you have a fixed pixel display, each pixel
is there, and you have to do something with it, so in order for a
1080p display to show a 720p signal it has to scale the image in such
a manner that it will use all the 1920x1080 pixels. The problem is
that this is not an evenly divisible scale, being that 1920/1280 does
not equal an even number. In order for a screen to be able to show
both 1080p and 720p resolution with out pixel mapping tradeoffs you
need their least common denominator. this still ends up using more
than the required number of pixels for "1:1" mapping, but its the
least evil.

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