27-09-2017, 07:26 PM
The optimal aperture shape for more than 2 lines is a matter of debate. It's always a compromise, if only because there may be vertical aliasing on the input picture. A sharp transition from white to black going down the screen is aliased. Interlace makes things worse as you can't readily interpolate between spatially adjacent lines. If you do, then moving pictures are truly horrible. The Pineapple converter had an option to do this (easy with a framestore) and the results were great on still images, otherwise useless.
The interpolation aperture is theoretically infinitely wide and truncated to 2, 3 or more lines in practice. Truncating an aperture gives spurious effects which is why windowing is used. Hamming, Hanning and other windows are common. Look it up and prepare to be confused. However this doesn't help much here, the visual impression is what matters so there is no theoretically optimal aperture. Make the aperture too sharp, with excessive negative coefficients, and the effect will be unpleasantly edgy and nasty. Make it too soft, perhaps with all positive coefficients, and vertical resolution will suffer.
The BBC research reports (cited in one my articles, see my website, and all downloadable from the BBC R&D website) show their conclusions that led to the CO6/509 converter with its 4 line aperture. They reckoned that 3 lines was a perceptible improvement on 2 while 4 was just about visible over 3 on the most critical material. I think some later work suggested going up to 6, 9 or even more lines but I don't think this gave any improvement for ordiary standards conversion.
Don't forget that as you add more lines to the aperture the picture will move down the screen. In theory the lines before the centre of the aperture are in negative time and the centre stays still. In practice the centre of the aperture moves downwards. Just a matter of tweaking the output vertical position slightly.
The interpolation aperture is theoretically infinitely wide and truncated to 2, 3 or more lines in practice. Truncating an aperture gives spurious effects which is why windowing is used. Hamming, Hanning and other windows are common. Look it up and prepare to be confused. However this doesn't help much here, the visual impression is what matters so there is no theoretically optimal aperture. Make the aperture too sharp, with excessive negative coefficients, and the effect will be unpleasantly edgy and nasty. Make it too soft, perhaps with all positive coefficients, and vertical resolution will suffer.
The BBC research reports (cited in one my articles, see my website, and all downloadable from the BBC R&D website) show their conclusions that led to the CO6/509 converter with its 4 line aperture. They reckoned that 3 lines was a perceptible improvement on 2 while 4 was just about visible over 3 on the most critical material. I think some later work suggested going up to 6, 9 or even more lines but I don't think this gave any improvement for ordiary standards conversion.
Don't forget that as you add more lines to the aperture the picture will move down the screen. In theory the lines before the centre of the aperture are in negative time and the centre stays still. In practice the centre of the aperture moves downwards. Just a matter of tweaking the output vertical position slightly.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
