Judder-free since 2008

I was minding my own business, listening to the Accidental Tech Podcast, when John Siracusa delivered a brand new thing to be neurotic and obsessive about: 24 frames-per-second content being delivered over 60 frames-per-second output.1

I’ve been casually streaming my films over my Apple TV like a chump, never stopping to think of the obvious question: “Is this 24 fps media2 actually getting to my TV at 24 fps?” Well, okay, it’s not very obvious. Anyhow, it turns out that a 60fps media player makes a choice when it gets this sort of content:

  1. Switch to 24 fps output
  2. Keep doing 60 fps, and stretch the 24 frames across 60 frames

One of the number 60’s many faults3 is that it isn’t divisible by 24, so the output will alternate between 3 and 2 frames per original frame. This post on Rtings.com explains it better, and has a special 24 fps video you can download to test your media setup. It’s just a 24-square grid which lights up each square once a second, sequentially. The test: take a picture with 1s exposure, and all the squares should be equally as bright if each frame is getting the same exposure.

While the Apple TV definitely outputs at 60 frames per second, there are some televisions that are clever enough to spot this sort of shit, and fix it for you. I tried it on my 8 year old telly, and was disappointed to see this (pardon the blurring):

But! It turned out someone (probably me) had maliciously switched my television to ‘Game’ entertainment mode. I flicked it across to ‘Cinema’, and tried again. This time I was a lot happier.

I’m not saying that this justifies the incredible amount I spent on an LCD just before they all became stupidly cheap. But it does make me feel better about it.

  1. He’s got form; see his previous discussion of native resolutions on TVs.

  2. Okay, so, while film is supposed to be 24 frames per second, technically a lot of media is more likely to be 23.976 frames per second, which is 24 × 1000 ÷ 1001. Here’s an explanation I found. You’re welcome.

  3. Other faults: ambiguous when rotated, upsettingly less than twice my current age.

Tom Charman