[opendtv] Re: MPs back Ofcom stance on spectrum sale
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 25 May 2007 12:07:31 -0400
At 12:56 PM -0400 5/24/07, Tom Barry wrote:
I guess you guys just like to argue.
Bert specified the maximum amount of information in what I assumed
was a pure theoretical sense.
Perhaps, but he showed no indication of understanding the difference
between information content and sample rates.
You specified practical real worlds cases where images at increasing
resolution do NOT have flat MTF curves.
I described the issues that impact the actual information content and
gave some examples to explain this.
You are both right.
If you say so.
One of the images I play with is a huge panoramic still image of the
Valley of Fire that I've previously turned into a moving pan at
1080p/60 and posted on this list. Something like that can have a
huge amount of information due the opportunity for oversampling.
Oversampling cameras will eventually be able to do this to. And I
personally believe that reality has almost infinite detail if we
could actually capture it properly.
I think it would be accurate to say that there is almost infinite
detail in nature, however, when we sample reality into a 2D spatial
plane at a given sample rate, we place finite bounds on the amount of
information that can be captured. This is the basis for sampling
theory.
Of course right now the higher spatial frequencies of any video tend
to be quite limited due to the filtering of lenses, sensors, and
whatever that tend to average adjacent values. This also used to be
true for anything over 320x200 resolution too. But over the years
the almost flat part of the MTF curve is lengthening to the right
with new technology and demand. So we should not assume there will
always continue to be little high frequency information or that we
will be unable to compress it when there.
All true to some extent. But there are both theoretical and practical
limits to how much information we can throw away before we begin to
introduce distortions that can be seen by the human visual system.
Obviously, if there is no limit on the peak bitrate that can be used
to deliver a compressed image stream, we can keep these distortions
to a level that cannot be detected. Unfortunately we do not have that
luxury in most distribution systems, and ESPECIALLY in a mass market
DTV system.
And then there is another reality that must be considered. Eventually
we will reach the point where it is non-productive to deliver
additional image detail. This will primarily be driven by display
requirements. This is analogous to digital audio, where we are
already approaching the useful limits for mass market products, with
higher quality reserved for a small premium niche market.
You mentioned 320 x 200 resolution. This raises an important point.
As we sample at lower and lower frequencies, each sample becomes VERY
important, as it represents more information in the scene. Thus we
cannot compress at higher ratios lest we throw away information that
is critical to comprehension of the scene. As we increase the
sampling rate we begin to capture more details that can be distorted
slightly while still delivering a good quality picture. But we must
still remember that removing too much information form any source
leads to visible artifacts, thus we are better off resampling to a
lower resolution for emission so that we can maintain delivered image
quality.
Though I will agree that currently the delivery of higher frequency
video information in HDTV is fairly non-existent.
YUP! It would be fairly easy today to demonstrate that resampling to
1024 x 576 from any of the HDTV formats, then compressing properly
would deliver equal or better quality that what we see today.
This is where I believe Bert is having problems. He seem to think
that we are already delivering good quality HDTV with MPEG-2. The
truth is that even with H.264 we will still be lacking in bandwidth
to handle the peak bit rate requirements.
Regards
Craig
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