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Does anyone know for sure (read: give sources) why there isn't any 720p CRT HDTVS?
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Well, the source is my own opinion (for what it's worth).
The answer is, why would the CRT manufacturers limit their horizontal resolution when it's not required.
Digital displays are fixed pixel devices and as such are limited by the physical display elements that can be manufactured on the screen. The 720P vertical resolution (720 vertical elements) has approximately 1280 horizontal elements (horizontal resolution) for a 16:9 (1.77) aspect ratio.
A CRT (that is far easier to manufacture) uses phosphor dots and doesn't use discretely addressed pixels that constrain it to a native resolution. The 1080i display will have a theoretical horizontal resolution of 1920 to maintain the 16:9 aspect ratio.
Up to now it has been quite difficult to manufacture a digital display with 1920 fixed pixel horizontal elements. This meant the digital display had to step down to 1280 horizontal elements with its associated 720 element vertical limitation. - not that many people can tell the difference between 1080i and 720P when done well.
brucek