I used pore words and verbiage to keep an incredibly complex subject simple, which was obviously a mistake. It takes a great deal of time and effort to explain it in a way that others can understand, which is why I did not want to get into this. I do not claim to be an expert.
Seeing how I’ve dug a hole for myself with poor / incorrect verbiage, I’ll attempt to dig the hole even deeper.
In short, using the word eliminate/delete was a very poor choice of words, and as John pointed out, just wrong. Native information was also a poor descriptor. In my mind, ‘native’ means that a frame is in the original format as shot without any rearranging of pixel content in any way, shape or form. 1080P/24 is the only ‘native’ film format currently available for home entertainment that I am aware of. 1080P/30 and 1080P/25 are two ‘native’ video formats that I am familiar with. Why I chose the word eliminate is harder to describe and I’ll probability just create more confusion trying to explain my rational. It has always been a mind set of mine, and one I need to reconsider.
Per my understand………(maybe John can educate me differently)
All data that was converted from 1080p/24 is there in an interlace 1080i video stream. The data is there, but it is rearranged and also duplicated for further rearrangement during the 3:2 telecine pull down process to convert 24 fps to 25 Hz and 30 Hz video, which is further processed to get 60 Hz for NTSC and 50 Hz PAL displays (and also 72 and 120 Hz displays). Frames in an interlaced video stream are composed of two separate fields each, one field representing the odd number of horizontal lines of information, the other field representing the even number of horizontal rows of information. Some frames contain the odd and even rows from the same frame, and some frames contain fields from two separate frames. For every four frames of film, telecine conversion from film to video creates 5 frames, consisting of 10 fields for 30 Hz video. Instead of simply doubling two fields and tripling the other two fields to get the ten video fields for five frames [(A-A-A, B-B, C-C-C, D-D), which would make the video stream appear uneven (pause and hold affect)], the fields within each video frame are staggered in a sequentially reoccurring order for smoother playback (A1-A2, A1-B2, B1-C2, C1-C2, D1-C2). Legacy CRT displays would light up the fields in sequence, but not the whole frame. New digital displays light up the entire frame. When an interlaced stream is converted to a progressive stream, these fields are re-woven in an attempt to arrange them back to the original film based frame sequence. The process of reverse telecine to put all the fields back together in the proper sequence still baffles me, so I will not even try to describe it.
Digging my hole deeper, there are industry standards for the different 1080P formats which also specify different timing for each frame rate. Timing relates to the number of pixel clocks in the horizontal lines of information. [Vertical timings are standard to all 1080P formats.] In each 1080 horizontal line there are 1920 pixel clocks and also a blanking period that is composed of a front porch, a sync pulse and a back porch. Blanking is there for CRT displays to make sure each horizontal line (or field) is placed properly as it retraces across the screen from right to left. (the beam is turned off during the retracing period, which is where the term blanking came from). Pixel clocks for the sync pulse and back porch remain the same for all formats, but the front porch clocks vary from format to format. The front porch clock variation difference is for 48, 50 and 60 Hz to ensure proper sequential line placement. The sync pulse is what triggers the retrace and is towards the middle of the blanking period and the back porch is at the end of the blanking period. The front porch is at the front of the blanking period and varies in length. The back porch keeps the electron beam turned off long enough for the entire passage of the beam from right to left (maintaining proper sequential line placement timing). This blanking period no longer applies to modern digital displays, but the legacy signal / timings remain to standardize and maintain proper pixel clock timings of all the different types of display formats (1080P48/50/60/72/120 Hz).
While you’re probably thinking, “so what, how does this apply to the 1080i / 720 – 1080P comparison?”, it has to do with what you see. Timing to maintain proper horizontal interlaced line placement frequently gets out of sync. It also applies to the process converting an interlaced video stream to a progressive video stream. This is why I maintain the position that, in MY words, NATIVE 1080p/24 or any down conversion of that format will always yield a better picture than any other format, especially any interlaced video stream. Only the very best video processors can take a, in my words ‘non native’ video stream and convert it back to an original 24 fps or doubled/tripled 24 fps format, or apply progressive scanning for 60 Hz output.
DVD, HD/DVD, BR, broadcast HD, broadcast SD, video, gaming and film all vary to some degree in how these processing parameters interact and how well or poor the video processor will interpret each feed and lock onto it for proper conversion and output.
I'm putting my shovel away and warming up the back hoe......
