Re: Grabbing 4:3 and 16:9

[Michel Bardiaux <mbardiaux@mediaxim.be>]

Hans Verkuil wrote:
> On Tuesday 26 February 2008 17:55:53 Michel Bardiaux wrote:
>> Daniel Glöckner wrote:
>>> On Tue, Feb 26, 2008 at 12:19:39PM +0100, Michel Bardiaux wrote:
>>>> Here in Belgium the broadcasts is sometimes 4:3, sometimes 16:9.
>>>> Currently, the card goes automatically in letterbox mode when it
>>>> receives 16:9, and our software captures the 4:3 frames at size
>>>> 704x576.
>>> The card does not go into letterbox mode. It's the broadcaster who
>>> squeezes the 16:9 picture into 432 lines surrounded by 144 black
>>> lines.
>> Let me rephrase to check I understood correctly. In analog TV, there
>> are no anamorphic broadcasts. When the WSS (accessible via /dev/vbi,
>> right?) states 16:9, then a 16:9 (sic) TV switches to a mode where it
>> crops 2x72 lines, then stretches the image both horizontally and
>> vertically to fill the whole 16:9 screen. Am I correct?
> 
> Yes, this is really true. Remember that the broadcast should still work 
> when received by an old 4:3 TV. The only way to ensure that it still 
> looks OK is to letterbox it. As mentioned before PALPlus allows the 
> broadcaster to encode additional information encoded in the black bars 
> to improve the image quality (never looked into that, though).
> 
> BTW, WSS does allow anamorphic broadcasts, although it is very rare. I 
> saw it once, but I've always suspected that someone made a 
> configuration error because anamorphic broadcasts look squashed on 
> normal 4:3 TVs.
> 
>> I must admit I have difficulty believing that. Could you give me the
>> URLs of sites explaining all that?
> 
> http://en.wikipedia.org/wiki/Widescreen_signaling
> 
> http://en.wikipedia.org/wiki/PALPlus

To summarize: I know now that PALPLUS is indeed widely used in Belgium 
for analog TV (via cable usually); that I can detect it from WSS using 
libzvbi.

But PALPLUS seems to be being deprecated; e.g. the wikipedia pages 
explain it was, but no longer is, used in the Netherlands. Just like 
analog-TV itself. So, unless I can find (any suggestions?) a ready-made 
open-source software decoder for PALPLUS, it is unlikely we will attempt 
to extract that info. Much better to work on digital-TV!

Thanks again to all who helped.

> 
> Regards,
> 
> 	Hans
> 
>>> Some fill the chroma part of the black lines with a PALPlus helper
>>> signal. Although the algorithms to decode PALPlus are well
>>> documented in ETS 300 731, I have never seen a software
>>> implementation.
>>>
>>>> 1. How do I sense from the software that the mode is currently
>>>> 16:9 or 4:3?
>>> Some broadcasters use WSS to signal 16:9.
>>> In Germany some signal 4:3 even on 16:9 shows.
>>> Read ETSI EN 300 294.
>>>
>>>> 2. How do I setup the bttv so that it does variable anamorphosis
>>>> instead of letterboxing? If that is at all possible of course...
>>> You can't. Bttv can't stretch vertically.
>>>
>>>   Daniel

-- 
Michel Bardiaux
R&D Director
T +32 [0] 2 790 29 41
F +32 [0] 2 790 29 02
E mailto:mbardiaux@mediaxim.be

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