[pvrusb2] Aspect ratio trouble

[Mike Isely]

On Mon, 31 Aug 2009, Bernhard Geier wrote:

> > Try some experiments with mplayer first.  I know this is not what you
> > want, but it will help to see what's happening to the stream.  Do a
> > 720x576 capture into a file with the video_aspect set to 4:3.  Now do
> > another capture into another file with the video aspect set to 16:9.
> > Play back each with mplayer.  You should see mplayer sizing the frame
> > correctly for the aspect ratio in each case.  I know this works - I've
> > done it (actually I've even done this mid-stream and mplayer still gets
> > it right).  But if it isn't working for you here with mplayer then we
> > need to investigate a problem with the driver or how you're using it.
> 
> Mplayer, Xine and MythTV show the right 4:3 or 16:9 frame, so this is working.
> 
> > If it *does* work, then we know that you're getting a properly formatted
> > stream.  So next I'd look at your playback app.  If the app isn't paying
> > attention to the embedded aspect ratio info, then let's hope there's a
> > way to manually set that within the app - perhaps there's another way to
> > tell the app that your pixels aren't actually square.  What app is this?
> > If you can't find a way to set the desired aspect ratio from within the
> > app, then the only option I think you have left is to try changing the
> > capture resolution so that the pixels are in fact square again.
> > (Actually if the app really is assuming square pixels, then 720x576
> > already isn't 4:3 - it's 5:4.)
> > 
> 
> The stream is technically perfect, if it's recorded in 4:3 all apps (and also
> my TV) show it in 4:3, it it's recorded in 16:9 in 16:9.
> The real problem are the black borders and the image stretching.
> Maybe I should not have mentioned the capure resolution but better that I'm
> living in a PAL country. As far as I know the pvrusb2 module does not know
> wide screen signalling (WSS), so I hoped to get rid of the black bars by
> setting pvrusb2 manually to 16:9.
> As long as I have set  pvrusb2's aspect ratio setting  to 4:3, everything
> looks fine: No picture deformation, and black bars at the top and the bottom
> if the content is in 16:9. If I set pvrusb2 to 16:9, the mpeg stream's aspect
> ratio is correctly set to 16:9, the 4:3 picture is taken (including the black
> bars at top and bottom) and, by the 16:9 flag in the stream, simply stretched
> horizontaly - instead of just recording the middle of the picture.

Wide Screen Signalling (WSS) is a specific type of VBI data.  As the 
pvrusb2 driver does not handle VBI, WSS won't work either.  So yes, your 
strategy here is what I would do as well (tell the driver to indicate 
16:9 content).

I think the problem you may be having is that the video content already 
has the black bars embedded in it, i.e. the broadcast station is 
actually sending you 16:9 content in a 4:3 frame and filling in the 
black bars themselves - also known as letterboxing.  What you really 
need actually is "anamorphic" video from the station where they 
horizontally squish the 16:9 content into a 4:3 frame and then send the 
WSS signal to tell the TV to stretch it back and locally generate the 
letterbox frame.  But it sounds like they aren't doing that.  
Anamorphic+WSS won't work properly on older TVs and TV stations may 
prefer to letterbox rather than use anamorphic with WSS if they are 
trying to stay compatible with such older TVs (but of course at the cost 
of a lot of wasted vertical resolution sending those black bars).

If that isn't the cause, I don't know what is.

  -Mike


-- 

Mike Isely
isely @ isely (dot) net
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