[opendtv] Re: Aspect ratio (Was: 20060807 Mark's Monday Memo)
- From: Jeroen Stessen <jeroen.stessen@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 15 Aug 2006 08:56:06 +0200
Hello,
Tom Barry wrote:
> The couple of BBC HD shows I've seen have been encoded at 1440x1080i
> AVC, with non-square pixels.
Sometimes you need non-square pixels to achieve a square resolution...
Especially in the days of the (interlaced) CRT, horizontal reconstruction
of the sampled signal was much more perfect than vertical reconstruction.
Horizontally you can use your DAC, a high-order anti-imaging filter, and
then the analog gamma correction is done by the tube. This distortion
creates some extra higher harmonics which add to the sharpness impression.
Vertically you only have the overlapping lines for reconstruction, and
this is compromised by the interlaced scanning grid. The line structure
will remain somewhat visible, and the only real remedy is to start with
a higher number of lines than would be necessary according to Nyquist.
Same goes for a camera: if you have an oversampled camera and you can
downsample electronically then you can deliver the same information on a
coarser sampling grid than when you must output an original sampling grid.
That's because an electronic anti-aliasing filter can be much more complex
and perfect than a mere optical anti-aliasing filter. There is often more
opportunity for horizontal processing than vertical, hence the asymmetry.
It has been said before, and it is not untrue, that 1080i is more or less
equivalent to 720p. If it weren't for different compression efficiency
then you could have it for the price of 540p. But let's not start another
war on that.
My point is that the aspect ratio of a sample grid should be judged in
combination with the perfection of sampling and reconstruction in the two
directions. Nowadays with CCD cameras and progressive matrix displays and
a progressive sampling grid you can argue that a square aspect ratio makes
most sense for the grid. But this wasn't always so.
More interesting would be to discuss whether an image representation in
"bandlets" even still has a sampling grid. A sampling grid is just an
artificial addition to an image for the convenience of digital signal
processing. It does not belong to the image, it is just an annoyance.
If someone knows a way of getting rid of it, then please do...
Greetings,
-- Jeroen
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