[mythtv-users] Very slight jitter on high power machine

Brad Templeton brad+myth at templetons.com
Tue Dec 14 08:28:41 UTC 2004

On Mon, Dec 13, 2004 at 11:13:46PM -0800, Jarod Wilson wrote:
> 1080i just as many lines of resolution to the screen per frame as 540p. Both 
> send 540 lines, they are electronically identical. You're not throwing 
> anything away when you follow Doug's instructions. 1920x540p using bob deint 
> outputs essentially the same information to your screen as 1920x1080i, no 
> deint. For whatever reason, it works better for some folks. (Me, I output at 
> 1920x1080i with kernel deint).

If this is true I am misunderstanding something.

My understanding is that in an interlaced signal, such as 1080i, you have
a display with 1080 lines in it.  (We have fewer so there is some downscaling
in most current TVs but leave that out for now.)

One frame comes in 1/60th of a second.  It paints lines 0, 2, 4, 6 and
so on -- the even lines.   In the next 60th of a second, another frame
comes which paints lines 1, 3, 5 and so on.   While it is doing this,
the other lines are persisting from the prior frame.

Thus in 1/30th of a second, 1080 distinct lines of resolution have been
delivered.  Due to persistence you see them all. 

In 1920x540p, you would send the even frames and display them, and then the
odd frames would come and _erase the even frames_.  Or, with special
de-interlacing software, the frames would be a blend of the even and odd
line for best effect.

So I would maintain, unless this is wrong, that displaying 1080i at
540p throws away half the pixels, and thus is not something you want
to accept.

If you shot something at 60fps, and then display it interlaced, you
lose half of every frame.  When the image is still, you will get twice
the resolution from 1080i than 1920x540p.    With a still image, in fact
you get the same information as 1080p!

If the image is moving, however, 1080i is throwing away half of each frame
from the 60hz recording, while 1080p does not.

That's why sports are often done at 720p.  Half the pixels per frame but
twice as many frames per second -- good for fast motion.

Also important is that when things move in interlaced video, half the lines
move in advance of the next group and the eye is quick enough to see that.
You  need clever filtering to be rid of that.

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