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Question about Blu-ray frame rates

Mentasm posted 2008 Aug 29 19:13

Hi, I hope this is the right place to ask about this. It seems to me that it's an authoring question, so here goes. I'm trying to get to the bottom of whether film-sourced material should be encoded at 23.976fps or exactly 24fps on Blu-ray. I was under the impression that BD players' 24Hz modes actually output at 23.976, and that encoding at exactly 24fps would cause problems when trying to play back in 24Hz mode. So far I have checked over 20 varied discs from all of the major studios/distributors (both UK and US releases) and they are all 23.976, save for one label - Optimum. Some of their recent and forthcoming titles are exactly 24fps and when I was watching them on my 120Hz set with 5-5 pulldown I noticed a number of glitches, almost like missing frames. They appeared quite regularly and I have not seen these errors on any of the 23.976 BDs. If I turn off the 24Hz output I don't see the glitches, so could it be the 24fps and th 24Hz modes don't get along? Now I'm by no means a video expert (which is why I'm posting here), but I did query this with the company and this is the answer they got from their production house in Germany: : I've talked to several interal and external technicians and none said that there is a problem with 24fps. Specially not in comparison with 23,976. Both, 24fps and 23,976fps are part of the Blu-ray specification and mandatory player support is given. Therefore we always use the native framerate from the HDCAM or HDCAM SR tape and encode that way. Otherwise we'd have to conform the audio and as there is no reason for not using either of both that's our standard procedure The majority of Blu-rays we're doing are either 23,976 or 24, however I'd have to investigate a bit further in order to give you any numbers. I'm also certain that there is a lot of titles on the market using 24fps. Nevertheless, this is the first time we hear about this issue. And also do all of the people from other authoring studios I have spoken to. One thing to bear in mind is that some screens might react differently to these framerates. But in order to exactly determine the technical details we'd have to get into contact with engineers from all hardware manufacturing companies around the world. I'm sure you understand that it is just not possible to cover all of them during qc. However, we're testing all of your titles on various setup and it appears that older screens tend to have issues handling movements on both, 23,976 and 24. But we don't see any differences between these 2 framerates. Would anyone care to comment on this? Everything I've seen and read indicates that 23.976 is the norm. Am I wrong, or are they simply fobbing me off? I always thought that it was 23.976 to maintain compatibility with NTSC frame rates? It seems to me that they're encoding at exactly 24fps because the UK is a 'PAL' country, but the HD spec mandates that all HD ready displays be able to handle 60HZ, doesn't it (and obviously 23.976 can go to 29.97fps with 2-3 pulldown)? I'll freely admit to being confused by this, mostly because I've come across a lot of conflicting info when surfing for answers. If anyone can enlighten me or - even better - point me in the direction of a few definitive articles, I'd be very grateful. Thanks!

rhegedus posted 2008 Sep 26 10:36

23.976 is what I'm used to - I think there may be some internal manipulation (1.001/1.000) by the player to get it to 24fps exactly. If you encode at 24fps and then this is manipulated by 1.001/1000 you get a frame rate of 24.024 which I suspect is causing your problems.

edDV posted 2008 Sep 26 11:16

Interesting question. I knew that both 24 and 23.976 fps were supported by the Blu-Ray standard but I don't know if the HDMI spec requires 24 fps support for "24p" capable HDTV sets. Over here everything is 23.976 for proper compatibility with 59.94 Hz and 119.88 Hz TV sets. Coming at it from the other direction, HD production cameras can be set to 23.976, 24 or 25 fps. I've always read that 24p is for film transfer where 23.976 and 25 are for HDTV. When 24 fps film is displayed on "NTSC" systems, playback is slowed to 23.976 fps. For "PAL" systems 24 fps film is sped up to 25 fps. I would assume Blu-Ray players do the same for 24 fps source but I'm not certain.

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