[opendtv] Re: Math of oversampling - a simple comparison
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Thu, 28 Apr 2005 08:45:02 -0400
At 7:36 AM -0400 4/28/05, Tom Barry wrote:
>Your filter really is quite impressive. But I tend to visualize
>the sudden loss of detail just by visualizing a possible MTF
>curve. If there was only mild detail in the original above 544
>then filtering that amount would not be noticeable. But possibly
>when you filter at 480 then the cut off point starts climbing the
>steeper shoulder of the MTF curve, suppressing frequencies that
>really exist in the image to some degree. And the actual
>perceived sharpness is the square of this ratio, so it is more
>like (480/544)^2 = .778.
Tom is on the right track here. The difference between 480P and 544P
is indeed visible to the average viewer. This is my take on why, and
why this is very important.
As I suggested (hopefully correctly), the ability to represent high
contrast details in an image is related to the cut-off frequency of
the required filters to meet the requirements of the sampling
theorem. As we increase the density of the sampling grid, we can
represent higher frequency details with increased contrast, up to the
point where the samples accurately reproduce the intensity of the
information "seen" by the camera (or created via a synthetic test
signal).
What I believe we are dealing with here is the relationship between
the visual acuity of the viewer and the ability of any sampling grid
to equal or surpass that level of acuity. There are many metrics at
work here. The number of vertical lines in an image determines the
level of vertical detail that can be passed (the use of interlace and
the required filtering also influences this). Likewise the number of
samples per line determines the level of horizontal detail that can
be represented. When the world agreed to standardize on a common
sampling rate for SDTV - i.e. ITU-R.BT601 - we made the horizontal
resolution the same for everyone, but we left the rather large
difference between 480 and 576 lines in place. 576 line systems
traded off temporal resolution for increased vertical spatial
resolution - to me these images have ALWAYS appeared to be sharper.
Based on many tests that I have performed over the years, it appears
that both PAL and NTSC are not capable of saturating the acuity of
the average viewer at the designed viewing distance. If we eliminate
interlace and compare high resolution images that are downsampled
respectively to 480P and 576P there is a very noticeable difference
in vertical detail. This example is a bit more extreme than the
difference between 480P and 544P that Jeroen noted, but it relates to
the same phenomenon.
Somewhere in the continuum from 480P to 544/576P to 720P we begin to
saturate the human visual system, at least in terms of the highest
level of detail that the average viewer can perceive. Clearly things
keep improving along this continuum, but once we get passed the stuff
that everyone can see easily, the addition of more detail become less
critical or noticeable. I think Tom is correct about "the square of
the ratio." As we raise the area under the MTF curve, we reach the
point where everyone perceives a sharp image - as we keep going each
incremental step becomes less obvious.
This is one of the reasons that I have long believed that we should
raise the floor on image quality to 544/576P as a baseline. 480P cuts
off well within the average acuity of viewers. With 576P we see very
significant improvements. But we also need to increase the level of
horizontal detail to match. With 601 the limit of 720 samples per
line becomes a significant factor. There is a huge difference in
perceived detail when we increase from 720 x 576 to 1024 x 576. &20
samples per line was adequate for an interlaced system since the
vertical detail is so "low," but when we remove the interlace
footprint we need to bring the horizontal resolution up to match.
If Tom and Jeroen have time to play, I would like to see a down/up
sample pass through 1024 x 576P. My educated guess is that it will be
nearly impossible to see the difference from the original.
Regards
Craig
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