Today I was thinkering with the "pyramid" LD capture, and, when I was looking for the files, I encountered an old test script for upscaling, abandoned two years ago. Idea was good, so I managed to improve it - as now I have more knowledge than at that time - and results are quite good.
The idea is simple: take two captures of the same content, one PAL, one NTSC, then merge them to "squeeze" every bit of details from it... everyone knows that a PAL capture has 576 horizontal lines, while NTSC has 480... now, when a movie was transferred to video, usually original resolution was higher than that (not all the times, but often); so, the PAL and NTSC "received" different lines of image... see the next image (obviously intentionally exagerated...):
As you can see, the red, green and blue lines have different thickness in PAL and NTSC; the aim of this technique is to combine those different lines to recreate an image closer to the original one.
To test my theory, I took some high definition images, resized each dimension to 1/3 to simulate NTSC, and to 2/5 to simulate PAL, then I wrote a script to mix and upscale the simulated PAL and NTSC images. Here you are the close-ups of the results; PAL and NTSC are the simulated images upscaled with pointresize, PALup and NTSCup are upscaled using a bicubicresize, to simulate a simple upscaler, PaNup is my script (oh, how much I love acronyms - PaNup=PAL and NTSC upscaled) - no noise reduction is used:
(you can download original, simulated PAL and NTSC, and PaNup images here - 14MB)
Of course, the validity of this tests are questionable, as the PAL and NTSC images are simulated; nevertheless, these give an idea of what could be achieved using "simple and poor" low definition media, like VHS or better laserdisc, and DVD too! Not HDTV, OK, but still quite a good result.
The problem is, in real life, it's close to impossible to find a movie which has a PAL and NTSC version that match each other 100% - usually they use different masters, so cropping and color grading are different... but, in those almost-impossilbe cases where a PAL and a NTSC laserdisc (and VHS) are virtually the same, the ideal condition is to capture the PAL at 768x576 and the NTSC at 640x480 (actual letterbox images at 2.35:1 will be 768x326 and 640x272, at 1.85 will be 768x416 and 640x346); if the capture card could capture at 720x576/480, only the Y axis will benefit - this is also the case of DVD.
Real life test could be found in this thread.