[pvrusb2] PAL-Plus support

[Mike Isely]

On Fri, 7 Nov 2008, Carsten Meier wrote:

> Am Fri, 07 Nov 2008 16:55:44 +0100
> schrieb Bjorn Danielsson <pvr-usb2 at dax.nu>:
> 
> > Carsten Meier <cm at trexity.de> wrote:
> > >
> > > I think you misunderstood me a little bit. Just to be sure we're
> > > talking about the same thing: http://en.wikipedia.org/wiki/Palplus .
> > > I know, that my PVRUSB2 doesn't have any decoder-hardware.
> > >
> > > To assemble a true 16:9-picture of a PALPlus-broacast, only the
> > > lines hidden in the black letterbox-bars have to be reinserted into
> > > the picture. Can that be done with the PVRUSB2?
> > 
> > According to that Wikipedia article, PAL-plus is a very special
> > analog encoding that removes 25% of the visible scanlines and
> > hides their luminance information in the color subcarrier in
> > the top and bottom black sections.
> 
> Yes. The availability section of the wikipedia article is a little bit
> outdated, all major broadcasters in Germany send such a signal for most
> of its 16:9 content.
> 
> > In order to mix these lines
> > back into sequence again you would need either hardware support
> > or special software that converts the decoded video stream
> > (if possible) just before displaying the frames on the screen.
> > 
> 
> All that is needed is a framebuffer (which the PVRUSB2 has) and a
> little bit of logic to "unhide" the lines and to put them in sequence.
> (I'm not an expert on this, just my guess... ;) )
> Doing that in software on decoding of the MPEG stream is not possible
> because the additional information is already lost.

The framebuffer in this case is buried in the hardware.  Software on 
Linux doesn't get a chance to see the results until it is already in 
mpeg format.  That's kind of the whole point behind having a hardware 
mpeg encoder in the pipeline.

Having looked at the wikipedia article now, I see the part about the 
missing lines being hidden in the color subcarrier.  Extracting that 
back out is therefore not a simple transformation for existing hardware 
(forgetting for the moment that the hardware would ALSO have to do an 
out-of-order reassembly for the hidden lines).  This is something the 
cx25843 chip would have to do on its own, if it could be done at all.  
I'm not an expert on that chip; the pvrusb2 driver merely uses the 
cx25840 module in V4L to handle that part.  But a few google searches so 
far is not showing me any clues that might suggest palplus decoding 
support for that part.  I also note this comment at the bottom of the 
wikipedia article:

<quote>
The decoding of a 16:9 picture in PALplus is very costly and requires 
the use of the two fields of the full picture. Because of this, only 
expensive TVs such as 100 Hz TV sets can decode the 16:9 mode of 
PALplus.

For all this, the takeup of PALplus has been marginal, and the future of 
this standard does not look very bright, since broadcasters are 
switching to digital television broadcasting (DVB).
</quote>

That would seem to suggest low odds that the cx25843 chip could ever 
support this.

  -Mike


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Mike Isely
isely @ pobox (dot) com
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