[ivtv-users] [ivtv-devel] NTSC & WSS (was: ivtv TODO list)

Hans Verkuil hverkuil at xs4all.nl
Mon Jan 18 20:42:23 CET 2010

On Monday 18 January 2010 20:20:53 Jay R. Ashworth wrote:
> ----- "Hans Verkuil" <hverkuil at xs4all.nl> wrote:
> > On Monday 18 January 2010 19:42:45 Devin Heitmueller wrote:
> > > On Mon, Jan 18, 2010 at 1:35 PM, Hans Verkuil <hverkuil at xs4all.nl> wrote:
> > > > - NTSC and WSS. I still do not know how NTSC determines whether the source is
> > > >  4x3 or 16x9. PAL uses the WideScreen Signal (WSS). A similar feature exists
> > > >  for NTSC, but it is unclear whether it is actually used by broadcasters.
> > > >  Nobody seems to know.
> > > 
> > > The reality is that for NTSC there is no standard.  There is an NTSC
> > > variant of WSS discussed in the WSS specification, as well as a way of
> > > representing the info in EIA-608.  As far as I have been able to
> > > gather though, neither have actually ever been used in production. If
> > > someone wants to offer some evidence to the contrary, I would be happy
> > > to add the support to tvtime and test it with some of my tuner boards
> > > (and fix any bugs that in the driver I find).
> > 
> > Does that mean that you have to adjust your TV every time the format
> > changes!?
> > 
> > Usually when I am in the US and watch TV in my hotel room the picture is
> > always distorted. I really hope that is not normal behavior for NTSC and
> > widescreen TVs.
> The *actual* reality is that an NTSC signal has an inherent *signal* aspect
> of 4:3.  If you're looking at it off a composite cable, or a cable/OTA tuner
> you can force 4:3.  The *image* aspect may be different: wider aspects 
> are generally accomplished by letterboxing, though some consumer camcorders
> will record a 16:9 image as a 4:3 signal by doing an anamorphic squeeze of the
> video image.

I only know the situation from a PAL/SECAM perspective: here the use of WSS is
standard. If missing, then the source is 4x3, if it is present then it will tell
you exactly what aspect ratio it is. Usually 4x3 or 16x9 (i.e. letterboxed,
possibly with extra PALplus encoding although that isn't used much anymore),
but I have also seen anamorphic encoding (pretty rare, though).

In practice it works perfectly for PAL and SECAM. I'm frankly amazed that
there is no such equivalent for NTSC in use. I always assumed it was encoded
in the XDS or with some proprietary encoding that I didn't know about.



> If it came in from a digital source, like a DTV tuner, things are murkier.
> I *assume* there's an aspect flag, and I assume tuners will set it, but 
> both of these issues should be orthogonal to IVTV, I think, cause we're
> only concerned with composite NTSC and OTA RF NTSC-M, and you can safely
> force 4:3 on both.
> This is my technical understanding based on 20 years of making and editing 
> the stuff; if anyone has a counterexample concerning broadcast or composite
> transmission that I haven't already noted, bring citations.  :-)
> Cheers,
> -- jra

Hans Verkuil - video4linux developer - sponsored by TANDBERG

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