[Rudy81]

I have tried to understand the issues with 24fps and 30fps video but still cannot put it all together in my thick brain. I have a Sharp Z12000, an LG combo HD-DVD/BD player, Nvidia GT8600 video card and the latest nvidia drivers, plus Windows XP Pro. On many HD titles there is no problem. However, on some titles that have slow camera pans, such as Roy Orbison's Black and White Night, the picture at times gets "jerky", or what I believe is called Judder. Is that the correct term?


I have tried refresh rates of 48Hz and 60 Hz at a 720p settings. Amazingly, the 60 Hz refresh rate is better. I am using the OEM Power DVD Ultra to play HD content. CPU usage never goes above 50% on any movie I have watched.


Can anyone explain what the problem is in layman's terms and suggest how to best run my resolution settings and refresh rates?


Thanks in advance for your help.

 

[IanD]

Whilst movies are generally encoded in VC-1 at 24fps (and would thus be best displayed at multiples of that refresh rate), the video stream contains pulldown flags which are hints to the player on how to repeat fields to generate a 60i or 30fps output. Because fields are repeated in the 24fps->60i conversion in an uneven 3:2 pattern, some frames remain on screen for longer than others and this gives rise to a juddery effect in motion pans. It is not as noticeable on more stationery scenes.


I think PowerDVD automatically adds pulldown when playing VC-1 material, thus generating 30fps with that juddery effect. It has to cater for the lowest common denominator out of 24fps, 30fps and 60i material that can be encoded on disc and this is 60i (which can be readily converted to 30fps via various de-interlacing techniques).


If you look at VC-1 titles with IME, the overlayed IME is generally 480p30, so PowerDVD has to output everything at 30fps.


Since PowerDVD doesn't have an option to perform IVTC (to remove the repeated fields and return the video to 24fps), my guess is that it always outputs 30fps with pulldown if required.


I could be mistaken on this issue as I have no definitive way of demonstrating what PowerDVD actually does.


So, if you attempt to play 30fps pulldown material on a 60Hz display, you might notice the pulldown judder effect on motion pans. If you attempt to play 30fps material at 24/48/72Hz (or conversely 24fps at 60Hz), you will notice additional drop frame issues due to the mismatch between input and output refresh rates.


Since you get the best results at 60Hz, the source is probably 30fps and probably the best you will get without some additional processing.


If you can find a way to perform IVTC (sometimes drivers support this with graphics cards, some displays will do this on inputs and outboard video processors can also do this) and display at a multiple of 24fps, you might be able to remove all judder type effects.


If you want to struggle with remuxing your titles, some have had success in creating .mkv containers for the audio/video and playing back with other players which can ignore the pulldown flags and thus output the original 24fps. PowerDVD will not play these alternative container formats.


It's not an easy task to obtain 24fps output to display at multiples of 24Hz, yet.

 

[Rudy81]

Thank you very much for the explanation. It puts it all together. I am slowly beginning to understand the basic principles of video and its limitations when playing film material. Fortunately, this problem is only evident on a few titles.