I had a brainwave about frame-rates and TV refresh rates and experimented changing the output to the TV to 60hz (NTSC) from 50hz (PAL), on the assumption that Walking Dead is an American TV show and probably NTSC (and therefore about 30fps rather than 25fps, displaying better in 60hz).
Playback is better when the output is set to 60hz. Coincidentally (or not?) the laptop screen itself (which tends to display video better, less choppy) is also 60hz.
However, my reasoning is wrong, because I inspected the iTunes TV file this morning and it is actually film rate (ie. about 24fps), not 30fps as I thought.
It probably just plays better at 60hz because it has more frames to play with.
As an experiment I tried setting the TV output to 24hz, trying to match exactly what the video was, but that looked rubbish. I'm not actually sure what iTunes plays the video back at. It's a technical how-computers-work issue I'm not capable of dealing with without falling asleep.
I'm pretty amused that frame-rates in regards to PAL/NTSC are still a problem in the 21st centurty.
I was pretty impressed by the huge range of frame-rates my TV will display though, clearly setup to suppose devices like Blue-Ray which can output at the frame-rate originally recorded (ie. film rate or 24fps rather than doing horrible things to the picture to fit it in PAL (25fps, video is sped up) or NTSC (30fps, voodoo).
Some quick Googling tells me that this isn't an uncommon problem (playing 24fps video smoothly on a 50hz/60hz TV). I suspect this is the first time I've seen the problem because this is the first 24fps video I've seen. I think the graphics card is responsible for smoothing things out?
Meanwhile, setting to 60hz has improved it.
Clearly I'm more obsessed by this than I need to be. I notice this sort of panning choppy playing in store bought NTSC DVDs, caused (I suspect) by the shows being filmed in 24fps then converted to 30fps, which involves some dodginess. You don't see it in PAL because they just speed it up to 25fps, not skipping any frames.
All of which is stupid.