[mythtv-users] Driving Australian TVs

[William Uther]

Hi,
   I've been building a MythTV box.  I have a VisionPlus digital DVB-T 
capture card (giving me a PAL, MPEG2, 16:9, 50Hz, interlaced video 
stream).  I'm trying to find the best way to get a picture on my TV (a 
4:3 aspect ratio, 100 Mhz Grundig with vga, scart, svideo and composite 
interfaces).  I'd like to be able to watch the 16:9 signal both 
letterboxed (see the whole thing with top and bottom black stripes) and 
zoomed (just grab the middle 4:3 section out of the 16:9 signal - lose 
the left and right edges of the video stream).

   I've tried two methods so far, but neither is entirely satisfactory.  
Can anyone suggest a good approach?

   Here is what I've tried:

   i) Use VGA out on the myth box at 640x480x60Hz into the vga in port 
on the TV.

   This works ok, but I get a horizontal combing effect during video 
motion - I assume this is an interlacing issue.  I've tried a few of 
the myth de-interlacing algorithms, but I don't really like them - the 
resulting picture quality isn't as good as a digital set top box going 
straight into the TV (which I assume keeps the interlacing - although 
I'm not sure how this would work at both 16:9 letterbox and 4:3 Zoom).
   There is also a noticeable jitter from the 50-60Hz conversion.

   ii) Use a home-brew VGA -> SCART cable.  The majority of the circuit 
I got from:

   http://www.nexusuk.org/projects/vga2scart/index.php

   although there seem to be a few problems with that design.  The main 
one being that I also needed to provide a few volts between pins 16 and 
18 to tell the TV this was an RGB input.  (And not all of the TV's 
three SCART inputs support RGB, which took me a while to figure out too 
:)

   I then tried the modeline on that website, and the one posted by 
Stephen Williams on this list:

   Modeline "768x576pali" 14.76  768 789 858 944  576 580 583 625 -hsync 
-vsync interlace
   ModeLine "720x576pali" 13.875 720 744 808 888 576 581 586 625 -hsync 
-vsync interlace

   Both of these generate an image that is too large for the TV screen.  
More than that, changing the mode line by small amounts doesn't seem to 
help - I'm beginning to wonder if the TV has some auto-centering 
circuitry that is throwing things off.  sigh.

   The other problem is that the 'interlace' part of the modeline seems 
to turn off Xv support (Savage drivers).  This in turn switches off the 
ability to zoom in Myth - I only get the 16:9 image squished into 4:3.  
I don't like the stretching.  Again, sigh.

Anyway... what are other people here doing to get PAL 16:9 interlaced 
MPEG2 into a PAL 4:3 interlaced TV?

   I'd be willing to buy a new video card with TV out (currently I'm 
using a cheap motherboard with built-in vga), but I'd want to be sure 
that I'm not going to have more problems with interlacing etc.

Thanks for any help,

Will        :-}

--
Dr William Uther                            National ICT Australia
Phone: +61 2 9385 6357             School of Computer Science and 
Engineering
Email: willu at cse.unsw.edu.au             University of New South Wales
Web: http://www.cse.unsw.edu.au/~willu/        Sydney, Australia