[vdr] Re: Interlacing

[Tony Houghton <h@realh.co.uk>]

In <10693.1098879965@www41.gmx.net>, Jörg Knitter wrote:

> > Tony Houghton a écrit :
> > > Anyway, I presume the problem with these shows is that they're broadcast
> > > interlaced, and surely the reason for that is to match the interlacing
> > > on the TV, so deinterlacing shouldn't be necessary if the player could
> > > sync the interlacing of the stream to the interlacing of the TV output.
> > > This might be a bit tricky with the Voodoo, because the X modeline I
> > > have to use with it (see below) is not interlaced, so it must be doing
> > > the interlacing in hardware. I haven't worked out whether the software
> > > sees it as a 25Hz or 50Hz mode, but I think it's the latter, in which
> > > case I suppose it would have to provide the user with an option to force
> > > it to sync to even or odd frames IYSWIM.
> > > 
> > > Does anyone have any thoughts on this?
> 
> Hi,
> 
> sorry, I don´t see the real point in this discussion. Unfortunately, I can
> only tell the problems from the Windows side, so I would also like to know
> the situation on Linux.
> 
> First of all: If you output a video exactly with 720x576 pixels and use no
> deinterlacer or postprocessing, it is theoretically possible make it display
> 1:1 on the TV set (if the drivers output the image correctly resized i.e.
> with overscan area etc). The main problem (at least on Windows): It is not
> possible to determine if the upper or the lower field is displayed by the TV
> output of the graphics card. 

That's very much what I was thinking. On the Voodoo there's the added
complication that I think the mode as seen by the software is 50Hz which
is then interlaced in hardware, so a field order checkbox wouldn't solve
it. The software would see the same sync pulse for upper and lower
frames, so it would have a 50% chance of getting it right each time it
was started, so a user would need a sort of "jog button" to correct it.

> Additionally, if you play back a video on windows, the clock of the sound
> card is taken into reference. The disadvantage: There is stuttering from
> time to time on the monitor/tv-out. There are tools that change this
> behaviour: Reclock sets the sync on the video output and changes the sound
> output frequency on analogue output and inserts or deletes AC3/DTS frames if
> the A/V sync is not correct.
> 
> So the best solution would be if you had a TV output on your graphics card
> that is able to adjust the clock dynamically. ATI and nVidia are said to
> have a fixed TV output clock, the Matrox cards are said to have a variable
> one.

I think adjusting the sound to match the picture is a reasonable
compromise. It should be possible by adjusting the frequency rather than
skipping/adding samples, so it would be unnoticeable.

> And in fact: Matrox cards are the only cards I know that enable 1:1 output
> on Windows. There is a function called DVDMax that outputs the image
> correctly scaled and synced to the TV out. Unfortunately, this software also
> can´t determine the field order. This is why you get a field order checkbox
> in the drivers where you can say if the video played has to be output upper
> field first (e.g. DVB) or lower field first (e.g. DV videos, DV videos
> converted to MPEG-2).
> If you (still) have a windows partition and a Matrox card, just check it out
> by playing a DVD with deinterlacing set to 'weave'.

Lots of cards can output a 1:1 TV signal, but only via RGB. Matrox and
ATI are the only ones with composite sync, others need a simple external
circuit to combine the syncs. Voodoo cards can output such a signal on
their TV-out, but like I said, AFAICT it uses a 50Hz non-interlaced mode
and does the interlacing in hardware.

> I think the linux situation might be the same: If something like the DVDMax
> option exists in DFB, the Matrox cards might be the only ones that can
> output a video perfectly without any necessity of deinterlacing or
> post-processing.
> For all other solutions, you might have to do it like it is done with the
> Windows Media Center Edition: Deinterlace the material to output it to a TV
> set making it interlaced again. I like to compare this with compressing an
> WAV to MP3 to be able to play it back uncompressed - lot of people would not
> see (hear) the quality loss, but it is there and it would be unnecessary if
> graphics card design/drivers would be better. Nevertheless, a few years
> later, interlaced TV might be the past as we all will have flat screens that
> work progressive - so a deinterlacer is always necessary.

I think my only solution might be to ditch the Voodoo and get an NVidia
or ATI for decent hardware deinterlacing, but the reason I got the
Voodoo was because of the poor quality of my existing ATI card. I heard
NVidias are slightly better, so I'll try looking on eBay for an old slow
one with a good brand. But one disadvantage of NVidias over ATIs is that
they're more inclined to come with a nasty little fan.

-- 
TH * http://www.realh.co.uk