Mixing 24p 25p 30p framerates

Today you might easily find yourself in a situation where a client asks you to produce content in multiple formats such as PAL or NTSC. Or simply, the client has their own footage in one format but the camera you own is another. So the question really becomes how do we work with so many different formats, and what single format, if any format, is the best to use for distribution to all formats?

This article helps to answer some of that based on my personal experience as a digital cinema filmmaker since the early 90′s who prior to the advent of 24P capable cameras was getting motion-like film look using 50i long before it existed with the switch of a button on a camera. I will discuss some of the tools and methods I used which for a long time were my best kept secrets. (You read that correctly, 24p from 50i – yes!)

Let’s say your camera can shoot either format, PAL or NTSC . Which should you choose to shoot your content in? Traditionally people would tell you to “always chose the format of final distribution format”. Today, in 2011 that is becoming more and more meaningless. These days digital media delivery takes place in a variety of methods from computers, phones, media-set-top boxes, and a variety of other types of devices – all of which are format-proof since they can play practically any digital format and often the only limiting factor is the codec.

Prior to High Definition, Standard Definition PAL had a slightly higher resolution which offered some benefit if you were maybe looking at a potential film-out. In the digital world however since the aspect ratios between the two formats are different, adaptive pixel interpolation was needed to convert between the two which could result in some loss of image sharpness and could introduce ugly jaggies on edges. In the world of HD however, the resolution of PAL and NTSC are exactly the same making the only difference the frame rate.

How to interchange between PAL and NTSC? When you shoot 24p or 25p the speed is only 4% off from one other. Either one can ean be imported to the alternate timeline which effectively only alters the length of your project by 4%. Audio can then be squeezed or stretched by 4% easily without adjusting the pitch and you can use Adobe Audition, Pro Tools, or many audio editing programs will do this. For example if the video of your 5 minute production changes by 4%, the net effect is 12 seconds of time either way depending which way you convert. Most people will not notice any change in length of time since it would require them to know what the original time was to begin with. For example a band might notice on their music video, “Hey, what happened to 12 seconds of our song?”

What about 30p? Stay away from it unless you are absolutely sure you want it and you know why you want it. 30p is 20% off in speed from 24p or 25p and any attempt to interpolate footage to try and yield 24p or 25p from it is an absolute mess! If you think you want 30p for a less “juddery” filmlook but just can’t decide, shoot 60i since you can create 30p from it using field interpolation. You can even create good looking 24p from 60i using post production plugins. Should you decide you want 25p, shoot 60i anyway and use the 4% trick I mentioned to turn your 60i -> to 24p -> 25p. This still seems like a mess, but for the absolutely uncertain 60i is safe and also gives you a lot of options. (50i also can easily go to 25p and then 24p if needed.)

The bottom line would seem to indicate that the interlaced formats are the most flexible of all formats providing due to its ability of going to 30p, 24p and even 25p. (Remember that 50i can also be converted to 24p or 25p quite easily but will not go to 30p as well.) But if you’re going to shoot for a film motion-like look then stick with 24p or 25p and avoid 30p like the plague.

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