[mythtv-users] RE: nvidia new drivers hdtv/interlacing, any benefit for us non HDTV people?

Bruce Markey bjm at lvcm.com
Sun Jul 4 16:54:07 EDT 2004

Chris Vargas wrote:
> I'd like someone to confirm that interlacing modes
> truly have no effect on tv-out.

Truly. The fact that there is support for a variety of
interlaced modes available through the outputs for multi-
sync monitors has no impact on the output for the one
and only video vertical synchronization that is the NTSC

> If I'm understanding things correctly, interlaced TV
> Out (via composite or s-video) should be very useful,

It's not a matter of it might be "useful", it is a pre-

I have to assume that this statement presupposes that a
tv-out signal has a choice of being interlaced or not. If so, 
then no, you are not understanding things correctly =).

An NTSC compatible TV set only understands an input signal
that is ~59.97Hz interlaced (29.94Hz per field) with 525
lines of which up to 483 are intended to be visible. If an
input signal from, say, a computer graphics card tv-out was
progressive scan, or a very different vertical frequency or
number of scan lines in the raster, the TV set would show
nothing but garbage. Same for PAL but PAL uses more scan
lines at a lower frequency for about the same throughput.

An NTSC signal has always been, and will always be interlaced
else it would not be NTSC. The nVidia 4363 driver put out an
interlaced NTSC signal as does the 6106 as does the GATOS
"devel" branch as does the Matrox driver.

TV is overscanned so that the first and 483rd lines are
projected beyond the top and bottom of the screen so that
you don't see the rough edges and corners. However, you
do what to see all the way to the edges of an X11 desktop.
Therefore, tv-out cards underscan then scale and smooth
the X dimensions onto the available TV scan lines.

Say, X is configured for 800x600. The tv-out will use, maybe,
scan lines 10-470 so that the top and bottom will fit inside the
boundaries of the TV screen. Everything that is drawn in the
X buffers is 600 pixels high but when the tv-out card prepares
the output signal, it scales and smooths everything to fit
on the 460 lines for display. Same thing for 1024x768 (which
is too blurry to be useful for almost anything) and even
640x480 is scaled down to fit on 460 lines in this example.

If you playback a full screen video recording that is 480
pixels high at 1024x768, 800x600, or even 640x480, the output
is scaled on to 460 lines of the raster. Therefore, data from
both fields is seen in every frame. If the camera that recorded
the original image recorded each field at a different time
(not always the case) ~1/60sec apart, the edges of objects in
motion will be jagged.

Support for a variety of interlaced modes through the SVGA and
DVI outputs to multi-sync monitors has precisely zero impact on
the interlace artifacts seen on the scaled image output through
the NTSC compatible tv-out. The only similarity for these two
topics is that there is a key word that seems to contain the
same sequence of characters (i-n-t-e-r... ;-).

The issues of aligning the image or de-interlacing the image
if it cannot be aligned are exactly the same for tv-out with
the 4363 driver and tv-out for the 6105 driver.

--  bjm

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