A Curious Interlacing Issue

[Mahoney]

Hi all,

Just beginning to get my head round this :-). Wondered if anyone is interested / feels like chipping in...

Here's the setup. I am in the UK. I have a Sky digibox capable of kicking out PAL in either 4:3 or 16:9 format - so always 576 horizontal lines @50 interlaced. Not sure whether it's 756 or 720 vertical lines? And I presume it's the same number of vertical lines whether in 4:3 or 16:9 mode? Or am I wrong in that assumption? Doesn't really matter - the capture card copes either way and it's the capture card that determines the format of the MPEG-2 file. I have set it to output 16:9.

I am capturing that analogue signal using a Hauppauge PVR 500, sold in the UK so it's a PAL one. So the resulting MPEG-2 file is 720*576@50 interlaced.

I am using an RGB-> SCART cable to display VGA on my TV. To achieve this I set Powerstrip to output a 768*576 resolution (the TV didn't like 720*576).

I am then playing my resulting file using SageTV and the latest nVidia PureVideo decoder, set to do bob deinterlacing as that is the only way to get it to kick out 50 frames a second rather than 25. When outputing in source format that's fine - no interlacing issues so long as its the same number of horizontal lines. See the attached Excel file to understand why.

Problem comes when I tell Sage to play it in 16:9 format. So I've got a 720*576i MPEG2 file that Sage needs to resize to 16:9 and output in 768*576i. I *guess* that what Sage is doing is:

1) Changing the actual picture to 768*432 by dumping 144 horizontal lines evenly spaced, so 1 in every 4, and upsizing the vertical resolution by 48 pixels
2) Sticking 72 black lines on top and bottom

The result has interlacing issues - basically combing.

Here's my guess at the issue; I think the nVidia decoder is doing deinterlacing *before* SageTV resizes the image. So in memory my computer has a progressive 720*576 image. If I output in "source" then that's OK, because when the graphics card then displays that frame interlaced it has exactly the same number of horizontal lines so it always shows the new lines. However, if you remove one line in every four you get some really ugly combing in the interlaced image, whether it was bob or weave deinterlacing you did. See the attached file for a demonstration of this.

OK, so at one level the resolution is pretty simple - set the Sky box to output 4:3, record 4:3, view 4:3 and there's no problem at all! However, I would quite like the extra view of the pitch (I basically watch sport) in 16:9.

What's confusing me is that if I use the Sage decoder, it can do it - 16:9 looks fine.. So use the SageTV decoder I hear you cry - unfortunately it doesn't use the hardware MPEG decoding ability of my Radeon card, and whilst my measly 1Ghz Athlon processor can *just* cope, it can't give Girder any CPU time at all, resulting in 30-40 second response times for pressing a button on the remote! The Intervideo non-CSS and Power DVD decoders can't avoid the combing, either.

Any bright spark know why the Sage decoder can avoid the combing, and how to get that effect using a decoder that utilises hardware decoding?

Thanks,
Rob

[nielm]

Originally Posted by Mahoney

Not sure whether it's 756 or 720 vertical lines?

for analogue video, there is no real 'vertical' resolution -- ie no horizontal 'pixels', the signal is truly analogue (even if the original source is digital)...

Problem comes when I tell Sage to play it in 16:9 format. So I've got a 720*576i MPEG2 file that Sage needs to resize to 16:9 and output in 768*576i. I *guess* that what Sage is doing is:

1) Changing the actual picture to 768*432 by dumping 144 horizontal lines evenly spaced, so 1 in every 4, and upsizing the vertical resolution by 48 pixels
2) Sticking 72 black lines on top and bottom

The result has interlacing issues - basically combing.

Not surprising

The order is:
Mpeg2-decode (giving 2 interlaced fields per frame)
Deinterlace (giving 1 frame at source resolution)
Scale to required display resolution (not just drop lines, but merge them)
Display on desktiop

Because of the scaling + repositioning of the video, your scanlines no linger match the displayed scanlines, giving you nasty interlacing artifacts.

Why does sagedecoder work? Unknown -- it may be doing a different deinterlacing method (blend or more likely discard).

I am guessing you have a 4x3TV ( you would not need to letterbox the video otherwise).

If you did have a 16x9 TV, the 'correct' solution would be to switch the TV's display Aspect ratio between 4x3/16x9... (which is what the digibox would do), that way your Sage box will always output 768x576i and the TV will scale it horizontally.


I am afraid that I don't think you will be able to achieve what you want...

PS: www.100fps.com if you didnt know it already

[Mahoney]

I am guessing you have a 4x3TV ( you would not need to letterbox the video otherwise).

If you did have a 16x9 TV, the 'correct' solution would be to switch the TV's display Aspect ratio between 4x3/16x9... (which is what the digibox would do), that way your Sage box will always output 768x576i and the TV will scale it horizontally.

Yup, it's a 4:3 TV. Oh well, no biggie - I'm more interested in trying to understand the problem than solving it . To which end, thanks for the explanation and the link - still lots to learn and understand there.

Rob