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If you mean the display elements have square shapes, then, yes, this implies a certain vertical and horizontal spatial frequency response, different from that with a Gaussian CRT spot. |
#2
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As oldtvnut correctly points out, in all sampling theory the idealised sample is infinitesimal in length. (Dirac delta function if anyone is that interested). In TV this is generalised to 2 dimensions rather than one. Practical pixels have a finite size and shape. For LCD displays and CCD cameras this ideally approaches a square having the same dimensions as the pixel spacing. This gives a zero order hold function and hence a loss of HF response on both axes which follows a sin(x)/x curve. The point I am trying to make is that the assumptions which underpin Kell Factor stem from the days when H scanning was a continuous function while vertical scan was sampled. These assumptions may well not apply when the picture is inherently sampled at the sensor on both axes. As a thought experiment consider a sensor and/or display where each pixel can be individually addressed. They can then be read or written in an arbitrary sequence*. I can conceive that this might affect motion protrayal (motion above a very slow rate is aliased in TV systems) but I cannot see how it might affect our perception of H and V resolution. Hence the Kell factor of a progressively scanned system using modern techniques should be unity. I may have overlooked something here. For example unless there is some kind of optical filter before the sensor there can be H and V aliasing. Or there may be performance problems of the sensor that affect the axes differently. *In doing this thought experiment I was influenced by BBC Research Report 1991/4 "Image Scanning using a Fractal Curve" by John Drewery. http://www.bbc.co.uk/rd/publications..._1991_04.shtml John Drewery had a superb understanding of scanning, sampling and spectra. Back in about 1975 I remember him demonstrating the 3 dimensional spectrum of TV signals (PAL in this case) using some wonderful models that he had the BBC Research Dept workshop make from pieces of coloured PTFE. Nowadays this would have been done by computer graphics. Last edited by ppppenguin; 08-03-2012 at 01:11 AM. |
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For a full explanation of 3-dimensional spectra resulting from scanning, I also recommend an out-of-print book by Pearson: http://www.amazon.com/Transmission-D...n+transmission |
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Always wondered why HDTV cameras 2k? Naive? |
Audiokarma |
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Image size is important. For cine camera replacement you want to be able to use your existing stock of 35mm prime lenses. Hence the sensor size needs to replicate 35mm film area. For TV the sensors are smaller. I haven't looked at what size sensors Super Hi-Vision uses but the fundamental resolution is about 8k x 4k. I saw a demonstration a few days ago at BBC Broadcasting House, some recordings from the Olympics. NHK and BBC have worked together to televise parts of the olympics on this new system. Only 3 cameras so a refreshing return to old fashioned production values, lots of lingering wide shots, minimal pans or zooms. You don't need closeups when you have that much resolution available. From my seat, about 30 feet from a 25 foot screen the pictures were perfectly detailed and flawless, even under difficult lighting conditions such as fireworks. The pictures were also being relayed to Bradford, Glasgow, Washington DC, Tokyo and Fukushima so some of you may have had a chance to see them. |
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and cinema camera innovations will drive this change. Have not noticed any Olympics motion artifacts with BBC?originated HD in 50HZ? but probably 60HZ for HiVision? The EBU needs to continue to push for 1080p for 7 & 8 mhz channels and dual 50/60hz standard |
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Some good points here. I recall John Watkinson wrote a paper in 1998 on Video Oversampling. He stated that because of the optical filtering ahead of the sensor, it is not necessary to use so many lines to deliver HD. If on the otherhand, the number of pixels on the sensor is substantially higher followed by the optical low pass filter, rescaling to fewer lines will not result in loss of spatial resolution. The only caveat is that oversampling only really works with non-interlaced video.
I believe we now underestimate the resolution of Image Orthicon video since resolution was limited by the structure of the target element and not by a digital imager's pixel array. Hence higher number of pixel imagers, rescaling and progressive scan is the future. Nevertheless, I would have liked to have seen what a 4" IO tube could yield in terms of spatial resolution. |
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