Re: Grabbing 4:3 and 16:9
- Date: Thu, 28 Feb 2008 15:12:17 +0100
- From: Michel Bardiaux <mbardiaux@xxxxxxxxxxx>
- Subject: Re: Grabbing 4:3 and 16:9
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@xxxxxxxxxxx
Mediaxim NV/SA
Vorstlaan 191 Boulevard du Souverain
Brussel 1160 Bruxelles
http://www.mediaxim.com/
--
video4linux-list mailing list
Unsubscribe mailto:video4linux-list-request@xxxxxxxxxx?subject=unsubscribe
https://www.redhat.com/mailman/listinfo/video4linux-list