"HD ready" doesn't mean that 1080i is not a standardized HD format.  It is a term used to describe TVs capable of displaying 720p, 1080i, and 1080p, but which lack a built in tuner for receiving over-the-air broadcasts.

1080i was created as a standard to deal with over-the-air broadcasts (1080p exceeds the 3Gb/sec bandwidth limit of over the air HD signals).

Here is the EBU (European) standards doc for HD resolutions:

http://www.ebu.ch/CMSimages/en/tec_doc_t3299_tcm6-23327.pdf

The SMPTE standards doc for HD resolutions has to be purchased, and I'm not willing to do that for the sake of the argument.   However, here is a link to the corresponding ATSC document which refers to SMPTE 274M revision 7 which is the standard in question.

So... HD 1080i resolution is 1980 x 1080, with interlacing, and 720p resolution is 1280 x 720, progressive scan.  While a 720p image may be marginally more steady (I seriously doubt anyone can detect this on an HD TV), the 1080i image is definitely higher resolution.  Unless you're dealing with some of the LCD monitors that downscale 1080i to 1366 x 768.

you are right there, i was looking for something else and posted that in hast on my way out this morning, i apoligize for my lack of thought put into that post, i hope, when having more time tomorrow to try this again..... i kept thinking it was true hd while driving to and from today, but i know that has something to do with dolby, and not res

until then i salute you sir

1080i was not created as a low bandwidth broadcast standard. That is a common misconception. It is a standard made purely for CRT display technology, which back in '96 was pretty much the only consumer display technology on the market.

Unlike flat screens / pixel based display technologies, the bottleneck in CRT displays is not the number of pixels in the front of the glass tube, but rather the speed and accuracy of the electron beam that lights up these pixels. Higher resolutions = higher electron beam speeds = more expensive tubes.

To get around this limitation consumer CRT TV's draw each frame in two passes. First the electron beam draws every odd horizontal line (the even lines in between remain unlit/black), then it makes a second pass filling in the missing even horizontal lines. This is called interlacing, and this is why all older video standards (VHS, LaserDisc, SD broadcast signals, 1080i HD signals) are interlaced. They were designed for CRT displays.

The HD-Ready standard on the other hand, is designed purely for modern flat screen technology where interlacing is pointless. If you run into bandwidth problems, then it's a better solution to transmit 30 progressive frames per second, rather than 60 interlaced frames. To recieve HD-Ready certification, a TV must be able to display 720 progressive horizontal lines (or better) at 60 frames per second. Since 1080i CRT's are technically limited to 540 progressive horizontal lines at 60 frames per second, they don't qualify for HD-Ready certification despite offering a higher effective resolution per second than 720p displays.

The downside to interlacing, and why progressive is considered better, is mainly line shimmer. Because of the lightning fast response times of CRT, and the fact that the electron beam can only light up one pixel at a time, an ugly shimmering effect can be seen on very thin horizontal lines. This makes a Windows desktop unwatchable on a 1080i CRT. However with games and movies visible shimmer is extremely rare, and HD CRT's are generally still considered to be superior to flat screens as far as movie/game image quality goes.

Another problem with interlacing is that the frame rate is limited to 30 frames per second. This is not a problem with movies, as they are shot at 24 frames per second, but many games render at 60 frames per second. On a 1080i CRT these games will display odd lines from one frame, then fill in even lines from the next frame. This leads to motion artefacts. Typically there will be jagged edges on diagonal lines during motion. However this is a minor problem as in my experience most people will not be able to spot this even if you point it out to them. Motion artefacts on flat screens because of low response times is generally considered to be a bigger problem.

Sorry for the wall of text above.

But the difference between 720p and 1080i is difficult to explain in just a few lines. I already tried that and apparently failed.

no problem, you got some of what i was looking for i new a bit of 1080i history, did know it was mid 90s though aways thought it was late, knew about the refresh rate issues to... still missing the certification i was reading about, but maybe i halusinated it

Actually, that was a very informative wall of text.

You are right about 1080i being created for CRT HD displays, but then used for over-the-air after the fact.  I didn't state that correctly when I said it was created for over-the-air HD.

I still contend that 1080i looks better than 720p on modern HD TVs (not CRT based HD TVs), however.