Decent inexpensive conversion between 720p and 1080i can be done, without downrezzing to SDTV by ending up with just 540 lines of vertical resolution.
The secret is to process the even 1080i fields differently from the odd 1080i fields, choosing different lines to blend or interpolate or double or drop. Sophisticated de-interlacing of 1080i to 1080p first will help but is not necessary.
If the processing does not de-interlace the 1080i first and also treats the odd and even fields the (exact) same way, the resolution will be cut to 540 lines vertically.
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Decent inexpensive conversion between 720p and 1080i can be done, without downrezzing to SDTV by ending up with just 540 lines of vertical resolution.
The secret is to process the even 1080i fields differently from the odd 1080i fields, choosing different lines to blend or interpolate or double or drop. Sophisticated de-interlacing of 1080i to 1080p first will help but is not necessary.
If the processing does not de-interlace the 1080i first and also treats the odd and even fields the (exact) same way, the resolution will be cut to 540 lines vertically.
More:
http://members.aol.com/ajaynejr/hdconv.htm