[mythtv-users] OT: 1080i question

Doug Larrick doug at ties.org
Tue Dec 28 13:38:29 UTC 2004

Michael J. Lynch wrote:
> If anyone can shed light on this it would be appreciated.  As I understand
> it, resolution for 16:9 with a 1080i monitor is 1920x1080 where 1920 can
> be calculated by:
> 1920 = (1080 / 9) * 16
> Does this hold true for 4:3 also.  E.G.  Is horizontal resolution for 4:3
> aspect ratio with 1080 lines of vertical resolution 1440?
> 1440 = (1080 / 3) * 4
> If both of these are true, is it possible, and is there a video card 
> that will
> produce 1920x1080i and/or 1440x1080i?  If so, does anyone have
> modelines for each these?
> Are the h and v sync timing, pulse widths, porches, etc the same for both
> or are they different?
> I have a 16:9 aspect HD monitor and when I watch a 4:3 aspect program
> on a digital channel, I get vertical black bars to the right and left of 
> the
> active video.  E.G.  The picture is centered in the screen.  Does anyone
> know if this is done by adjustment of the porch widths or is it done by
> filling the extra 480 pixels with whatever (black seems to be the choice)?

A couple things...

1. It's not necessary (or even desirable, sometimes) for pixels to be 
square.  You can scale the dotclock (and other parameters) and add 
pixels horizontally, so long as the horizontal refresh rate remains 
constant.  I use 960x540p and 1920x540p modes that seem the same to the 
  TV (H and V refresh rates the same); it's just that one has twice the 
dot clock -- and twice the pixels -- as the other.  But you really 
shouldn't play these tricks with anything but a CRT -- if the display 
device has native pixels (LCD, DLP, plasma), use that resolution, even 
if its pixels are not square.

2. 4:3 content displayed by Myth on a 16:9 set has black bars, produced 
by one of three methods:
   a. part of the frame, inserted by the broadcaster,
   b. black area outside the active video area, inserted by MythTV, or
   c. MythTV outputting a 4:3 mode; inserted by the TV.

You would use MythTV's zooming controls (w key) to manage a or b, and 
the set's own zooming controls to manage c.  Myth will only do c if 
you've set it up that way.

So for your immediate question, there's no reason to generate a 
1440x1080i mode; MythTV will generate the black bars itself.

nVidia cards are capable of 1080i output (or various relatives of 1080i, 
such as 540p).  I think there's a Via card that can do it too (somebody 
help me out here).

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