why convert 1080i to 720p?

[IndieRockSteve]

not really a newbie, more of a lurker. Anyway, I've been trying to figure out why everyone seems to convert 1080i to 720p?

I'm looking to begin converting my 1080i hdtv ts's to something to save space, especially now that x264 is quite mature and ready to handle this, and I've built a computer for playback with the processing power required. I notice everyone does conversion to 720p with a few random (Band of Brothers) 1080p conversions.

I'd like to encode my 1080i to 1080p and obviously keep the 720p sources at 720p as I plan to buy a 1080p tv in the next 6months, but is there a reason for the 720p conversion everyone seems to do?

thanks!

[Revgen]

Because most people hate interlacing. Plus 720p looks better since it's 720 full lines of resolution, while 1080i is just 540 lines that have been interlaced and look ugly.

[IndieRockSteve]

ah, so 1080i is the same amount of actual picture information as 540p? (excuse my ignorance in this area)

so if I wanted to keep all the original picture data from the 1080i source and use the least bits to store it I would want to do an encode at 1920x540p? (regardless of playback issues, on a theoretical level)

but aren't the alternating lines of an interlaced video still holding different picture information?

[scharfis_brain]

as long as your 1080i streams contain 3:2 or 2:2 pulldowned contents (usually film) your can bring 1080i back lossless to true 1080p using a fieldmatcher or an IVTC.

you'll only get the 'halved' vertical resulution, if true interlaced contents are being transmitted. Like sports or news for example.
But if you use a sophisticated deinterlacer on that stuff, it will almost look like true 1080p.

[IndieRockSteve]

ah ok. so if its "filmed" in 1080i it will really be 540p, but true 1080p sources converted to 1080i can be IVTC'd(etc) back to a reasonably close to original 1080p stream?

Now, for things like HDTV shows and HDTV broadcast movies, which of these scenarios matches? Are they filmed in 1080p and broadcast in 1080i or filmed and broadcast in 1080i?

thanks!

[scharfis_brain]

uhm, no.

if the base framerate is 24p, 30p or 25p the telecined 1080i60 (24p and 30p) or 1080i50 (25p) will be FULLY revertable! not only close to.

if the base framerate is 60p or 50p the resulting 1080i will be true interlaced meaning that we loose detail (eg. resolution) with it.
But sinc we got vertically shifted fields, true 1080i is NOT 540p. the 30 or 25 hz vertical jitter will suggest a much higher resolution to your eyes.
And if you are using a motion adaptive deinterlacer, you'll get static areas being fully reverted (lossless again!) to 1080i, while moving areas remain at 'halved' resolution, but this can be improved with intelligent interpolation algorithims like EDI or ELA to eliminate stairstepping and flicker.

And if you want to go further you can make usage of a motion compensated deinterlacer, which even will return moving areas to full 1080p resolution and detail avoiding loss of detail, flicker or stairstepping.

And please! Do never ever compare 1080i with 540p.
If one would do so, you would throw away a LOT of image information and detail. Also you'd receive a horrible vertical jitter.

[IndieRockSteve]

ok, thanks for that much detailed answer. It jives with my understanding of how interlacing works and my personal findings learning to use IVTC, etc.

Is there a reason then that so many convert 1080i to 720p?

[IndieRockSteve]

oh, and what options are there for a motion adaptive deinterlacer?

thanks!

[Teegedeck]

Try TIVTC (http://forum.doom9.org/showthread.php?t=82264) for telecined material; very sophisticated, easily handled. TDeint (same thread) is a good choice for truely interlaced content.

Is there a reason then that so many convert 1080i to 720p?
I guess many people don't have the hardware to play back 1080p without stutter and/or don't have a 1920x1080 display... I know I don't.

[theAlchemist]

I get the impression that some of you really live inside HDTV :) Maybe one of you can answer this question that bothers me for some time now.
What is the technological reason for using any interlaced video format with HDTV?
As far as I understand the interlacing technique was invented to get around the fluorescent's short persistence phenomenon, in order to reduce flickering on Cathode Ray Tube displays (CRTs). With the invention of the plasma screens and since HDTV targets mostly those screens, this technique seems to be obsolete and not necessary anymore (plasma screens do not suffer from the short persistence like CRTs). I expected any new format to abandon the interlacing technology completely and concentrate on compression using the modern technology (Spatial / Temporal redundancy reduction using Motion Compensation and DCT).
Transmitting 60 fields per second or 30 full frames per second consumes the same bandwidth however if the video is interlaced the receiver must de-interlace it, which consumes serious amount of CPU power and result in inferior quality.
I do give the credit to the expert who decided to make the most advanced HDTV format interlaced. Therefore I assume there must be a reason but I just can't figure it out. Can you help?

[Lil' Jer]

What is the technological reason for using any interlaced video format with HDTV?

Bandwidth savings. 1080i takes less bandwidth to broadcast then 1080p from what I've heard.