[linux-dvb] Re: 768x576, 720x576, 704x576?! (was: black borders in xawtv)

[immo.benjes@philips.com]

On Thursday 28 Feb 2002 8:28 pm, you wrote:
> On Tue, 26 Feb 2002 jonathan.foster@philips.com wrote:
> > PAL is broadcast as 720x576 digital video.  But the analog signal is
> > only 702 pixels wide, so 9 pixels each side are left black.
> >wide, so 9 pixels each side are left black.
> >
> > Yep, crop the image before you display it.
>
> Thanks for the answer. Now I have some more questions. ;-)
>
> Analog PAL is 702x576 (really? not 704x576?). 
Analog PAL has no horizontal resolution. It is analog. And it has 575 lines 
(2 half lines). If you sample a PAL signal with 13.5 MHz you would end up 
with a resolution of 702x576 (there are no half lines in digital) with an 
active line of 52usec.

The 704 you often see comes from the MPEG coding as you require blocks of 8.

>The usual TV has an aspect
> ratio of 4:3, so the picture is somehow scaled to 768x576. So far, so
> good.
>
> "Digital PAL" is broadcast as 720x576 (why 720 and not 702?). 
Good question. Note that the active digital line acording to ITU 601 is 
53,3333 usec instead of 52 usec. To be compatible with ITU-470 (PAL) the 
first and the last 9 samples are set to the blanking level.
The aspect ratio of 4:3 is defined for the analog PAL signal (52 usec with 
575 active lines).


>However, as
> the source is the analog PAL 702x576 picture, 
Not necessarily. If you use old PAL content yes, but in the studio they are 
working with ITU 601 (720x576). New content will be in that format.

>the 9 pixel borders on each
> side are encoded into the MPEG2 stream. Again, the usual TV has an aspect
> ratio of 4:3. When scaling from 720 to 768 i see two problems:
> - The black borders are still there as they get scaled as part of
>   the picture
> - The picture is NOT the same as the analog one: The analog one gets
>   scaled horizontally by the factor 768/702, the digital one gets scaled
>   horitontally by the factor 768/720. So the analog picture is slightly
>   more "stretched" horizontally?!
See above: the definition of 4:3. Streching the 702 to 768 is more accurate. 

>
> What I want is simply to get a good picture output to my TV (without
> using the DVB board video out, but by using the TV out of my graphics
> board). 
Than make sure you have RGB output :-)

?Just cropping the picture is not possible, as the TV is fixed to
> 4:3 and i want to use the whole screen!
> So, as far as i see, there are two possible solutions:
> - Take the 768x576 picture with the black borders and scale it
>   horizontally AND vertically. 
The black boards should disapear anyway when your graphic cards generates a 
correct timing. If you see black bars the timing of your card isn't correct.
Play around with your X modelines. 

The black borders at the left and right
>   side will dissapear, but parts of the picture at the top and the
>   bottom edge will also not be visible.
> - Take the 768x576 picture with the black borders and scale it
>   horizontally ONLY. The black borders at the left and right side will
>   dissapear, but the picture will be stretched horizontally some more.
Adjust this with your modeline.


>
> What's the way that is used on the video out of the DVB card? I can't see
> any black boarders this way?!

They have a correct timing.

Regards

Immo

>
> Thanks in advance, Jonny <jonny@1409.org>



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