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HD 1080p - but at what frame rate??


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#1 of 5 OFFLINE   Michael Osadciw

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Posted April 15 2006 - 03:58 AM

Bare with me here, this post is loaded and may not be entirely clear with how I'm saying it: Some of the things I've been thinking about regarding the new HD formats is in relation to the frame rate the movie will be delivered in. 24fps - if films are transferred at 24 frames per second, that means there will be no doubling up every other frame to get to 30fps as we've been messing around with on NTSC. But I've seen writings that are talking 1080/30i output on players (Toshiba). So the question is are the discs encoded with 1080/24p and the player is adding a few extra frames to bring it up to 30fps? It doesn't stop there. What about the audio? If an HD film is transferred at native 24fps that means movies will have a slightly shorter runtime than what we are used to on NTSC DVD. That also means we'll hear the audio at it's original speed matching 24fps because it's slowed down ever so slightly for NTSC to match with 30fps. I guess that would have to mean that if an HD-disc is encoded in 24fps and the user sets the player to output for example 1080/30i, the player will have to slow the audio down to match the additional frames that I'm assuming the player will create. I'd assume that having a film transferred at its native 24 frames per second is ideal for HD disc because adding additional frames on the disc before user-selectable options to change resolutions and frame rates to match a different scan rate is NOT the film as intended. This way, if I have a display capable of accepting 1080/24p I should be able to have that viewing option from the disc. Many upcoming HD-disc releases could potentially have different frame rates on the one disc. A film could be 24fps and special features could be 480/30i (video camera "on location" making of special features) and 1080/30i (interviews with an HD cam). With different formats, could an HD-player have a "native" output option that sends all video out as originally encoded (leaving it to the display to sync to) or maybe the player will upconvert it all to a single output? If there is only a single-output option, I can't imagine that working if the selection is 1080/24p). Do you understand were I'm getting at? Am I correct on these assumptions? There has been a lot of talk about HD and resolution, but I thought I'd throw out the frame rate question for some clarification if anyone has any input. Mike

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#2 of 5 OFFLINE   ChristopherDAC

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Posted April 15 2006 - 07:02 AM

Currently, I'm not aware of any display that actually accepts a 24 Hz input. The output of the player will be 30Hz or 60Hz, probably not 48 or 72, and the extra scans will be produced in the usual way [just as with properly-flagged film-source DVDs] by alternately rendering each frame, in the player, as 2 and 3 fields. There is no change of the running time involved [unlike with PAL in which 24 fps is sped up to 25 fps], because the 30 or 60 video frames occur in the same second which the 24 film fames occupied. Likewise there is no change in the audio. Audio is slightly, imperceptibly altered for NTSC video, because the film and the accompanying sound are slowed down by a factor of 1001/1000 to accomodate the screwy framerate of NTSC colour TV ; this change will no longer occur with "true" 30 or 60 fps video, which is the HiVision standard.

#3 of 5 OFFLINE   GregK

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Posted April 15 2006 - 02:45 PM

I'm guessing we will probably continue on with the slight varients (23.976, 29.97, 59.97 etc) as it makes cross conversion to the other legacy NTSC formats easier as well as cleaner. Ditto for display devices. While the North American ATSC standard allows for both standards, as in 24 or 23.97, 60 or 59.97 and so on, it appears we will stick with the slight offsets. ABC is using 720p HDTV @ 59.97 progressive frames per second. NBC / CBS / (and on occasion PBS) use 1080 HDTV interlaced at 59.97 fields per second. From a purist approach I do wish it were easier to ditch the slight offsets, but as Christopher noted, the end result is very slight and really is imperceptible.

#4 of 5 OFFLINE   Michael Osadciw

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Posted April 18 2006 - 09:20 AM

Toshiba has told me that 1080/24p is stored on the disc with flags on specific frames that need to be doubled to get to 30fps for 1080/60i. When players are available with 1080/24p output, the player will ignore those flags and output the original 1080/24p. That's good news. I didn't ask about syncing the audio with the additional frames by slowing it down. I'm assuming that is a given. Mike

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#5 of 5 OFFLINE   DaViD Boulet

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Posted April 28 2006 - 06:52 AM

Michael, You're right that (almost all) film-based 1080 content on both HD DVD and BD will be 1080 24P encoded. How this gets sent to the display is entirely player-dependent. To start HD DVD only does 1080I at 60. To start BD players will do 1080p at 60 and some will also do 1080P at 24. Yes...there *are* displays (some projectors for instance) that can already accept a native 1080 24P signal. In the not-too-distant future expect more and more HD DVD and BD players to give videopiles fuller control so that a native "24 rate" can be obtained to avoid all added judder from the 3-2 repitition we see today for the 60Hz market. THERE IS NO FLICKER with "on"-technology diplays (like DLP, LCD, Plasma etc.) so 24 frame playback would look just fine and appear no different that 48 or 72 frame playback if the source was 24 frame to begin with. Lots of talk over at AVS about this. The projector manufacturers know we want to say goodbye to stupid 3-2 60Hz with new technologies and several manufacturers are working hard to get us there.
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