3 Replies Latest reply on Mar 3, 2013 11:02 PM by Pierre Devereux

    Frame rate conversions

    Pierre Devereux Level 2

      Hi,

       

      And here, so far, I have been very happy, smugly reading the posts on frame rate issues and thinking to myself - "I am so glad I live in PAL country - Shoot 25FPS, playback 25FPS, no funny "So-called" Drop Frames or, how do you say it... "Pull Down" to worry about".

       

      Sigh

       

      We have signed up to a stock footage firm, and hope to use them to source a lot of our aerial and panoramic nature shots and...you guessed it... they come in either 23.976FPS or 29.97FPS flavour.

       

      I had a heart attack this morning, and almost blurted out my ignorance on the forums, because I imported some 4K footage (at 29.97FPS) into my 1080p 25FPS After Effects comp - the footage is of a fast pan accross some mountain passes. When I played it back, there was a terrible "Judder" and my heart sank. My first impression was that there was an issue with 4K footage in a 1080p project

       

      Thankfully I took some advice seen a few times on the firums from Dave - I sat back, took a deep breath, let it out.....and did it again.!

       

      Once I thought it through, it occured to me that the frame rates were different, and after a few tests using comps of different frame rates, I could prove that this was indeed the issue. I have now spent some time looking up frame rate conversions, and it looks to me that I am sitting at the wrong end of the Frame Rate Spectrum.

       

      Am I correct in saying that it is easier to convert from 25FPS to 29.97, rather than the other way around? So far, the best information I could glean from the sites I went to (and most of this is from comments posted by Dave) is that Twixtor should do a fine job at assisting in the conversions.

       

      I have recently bought Twixtor, so I am busy running a few tests to see whats what.

       

      I thought I would take this chance to have a chat about frame rate conversions, and see if there are any new improvements and suggestions on going about this. As can be seen from my comments above, to date, I have not had to deal with Drop Frames or 3:2 Pull downs, so I need to start from scratch learning about them. I will be doing some heavy reading in the next few days, but any other advice would be appreciated.

       

      Twixtor.....its you and me now........

       

       

      Pierre

        • 1. Re: Frame rate conversions
          Rick Gerard Adobe Community Professional & MVP

          If there's no audio then just change interpretation to 25fps. No conversion necessary because a frame is a frame. The aerial footage will just seem like the plane is flying a bit slower which is probably OK. If you have shots with people walking or some other activity that looks too slow then speed up the footage.

           

          If you still see juddering then you're seeing a critical panning speed stroboscopic effect in the pan. You have to change the speed of the pan by changing the speed of the clip.

           

          If your footage is interlaced then you should separate fields. The only reason you ever need to do frame rate conversions is to match the duration of your clips. Don't make things more complex than they need to be. Make all your judgements about clip speed and playback using the device the shot will be played back on. IOW look at it on a broadcast monitor if it's going to be broadcast, look at it projected if it's going to be projected, look at it on a computer display if the project is designed for distribution that way. It's the only way you can tell what's going on.

           

          The only time that it's critical to re-time footage is when your footage has sync sound or must be a specific length. I hope this helps.

          • 2. Re: Frame rate conversions
            Dave LaRonde Level 6

            Am I correct in saying that it is easier to convert from 25FPS to 29.97, rather than the other way around? ...Twixtor should do a fine job at assisting in the conversions.

             

            And here I thought you called Montreal home, where the frame rate is 29.97.  Silly me.

             

            From the viewpoint of the amount of image data available, 29.97 to 25 would probably be more effective at preserving image quality: for that conversion, you're eliminating frames, not adding them.  But it's still a messy job for software.   It's pretty easy to visualize the task:  in this example, I'll assume 29.97 is actually 30 to make it a bit easier.

             

            Make a 75-inch-long and narrow piece of paper.  If you substitute centimeters for inches, you'll save paper.  Turn it on its side.  It represents one second. 

            Divide the top into 25 equal 3-inch/cm parts; you can now see 25fps, the PAL frame rate. 

            Divide the bottom into 30 equal 2.5-inch/cm parts; you can now see 30 fps, just a hair's breadth off from the NTSC frame rate.  Observe how the divisions line up, then try to think about how a piece of software would have to work to change one into the other. 

             

            But there's more: this is a second of video, and video contains motion.  The camera moves, the subject in the shots move, and the motion typically isn't consistent.  When you're in the process of converting from one frame rate to another, you have to take the motion into account to create the images necessary to go from one rate to the other.

             

            Now, hardware like a Terranex box is the hands-down champion at frame rate conversion. It's fast and accurate, but hardware is expensive to use.  Software costs less, but the processing time is much longer, and the decisions the user makes in selecting the motion compensation algorithms come into play: choose the wrong one and the motion looks bad, creating poor images.

             

            Does this help any?

            • 3. Re: Frame rate conversions
              Pierre Devereux Level 2

              Hi,

               

              Thank you both for your responses. MUCH appreciated.

               

              Rick,

               

              I thought I was being clever by working out all the maths and differences etc, and here you go, pointing out that I can just interpret the footage! I seem to make a habit of over complicating things, NO PAIN NO GAIN!

              But seriously, thank you for this, I will try it out this morning and feedback to the forum.

               

              Dave,

               

              (the breathing technique does wonders for the blood pressure by the way) - Sunny South Africa is where I hang my hat.

               

              When you set the analogy out like that, it makes a lot of sense. I did get Twixtor to make quite a nice improvement on the clip, but I will give Rick's suggestion a shot first. As I have said on several occasions, once we sell the first Season, and the cashflow comes in, after buying a small tropical island, I want to invest in some new hardware, and when THAT day arrives, I will be chatting to you all again as to what to go/where to look.

               

              Thanks guys.

               

              Pierre