[keithwinstein]

My understanding is that jitter refers to essentially stochastic variation of an ideally isochronous/periodic process or signal. You might say "the event is supposed to occur exactly every 1 second; in practice we observed jitter of +/- 50 milliseconds," referring to the RMS (1-standard-deviation estimate) of a sample of inter-event durations. Wikipedia has a nice article: https://en.wikipedia.org/wiki/Jitter

In the context of video, I think the most common use of "judder" is about the deterministic and periodic variation in timing that occurs when content of one frame rate is shown on a different-rate display. The most common situation is a "3:2 pulldown," where 24 frame-per-second film content is adapted to a 60 field-per-second or frame-per-second video signal. This is done by repeating one film frame for 3 video fields/frames (so 3/60 seconds), and then the next film frame for 2 video fields/frames, then the next one for 3, then 2, then 3, then 2, etc. (2/5 = 24/60 so it works out on average.) That repeating variation in frame duration or number of repetitions is seen as "judder." With a cinema projector or recent TV, you don't have this; the frames are shown 1/24 s apart and with an equal number of flashes each. But on an older 60 Hz TV, you'll have judder.

(I've also seen "judder" occasionally used to refer to the stuttery motion that comes from showing 24 fps content on a sample-and-hold display, like an OLED [without black-frame insertion] or a "good" LCD that just shows each frame with constant brightness for the entire 1/24 s and then almost-instantly switches to the next frame. But I don't think this is the correct usage.)

[toast0]

Adding on, jitter is something you can address with better precision (various mechanical and later electronic means were devised to ensure a precise time between frames during recording and projection to reduce the jitter between frames in a film) or in modern times, larger buffers (if you're streaming video and packet delays vary by 50 ms, a small buffer will allow for even playback). Jitter buffers improve perceived quality at the expense of delay and memory use, but for recorded video and audio, delay isn't a big deal.

On the other hand, judder is often due to a limitation of the system. Fixed frequency displays can't display off frequency content without compromises. And TV broadcasting was tied to fixed frequency displays. NTSC had no hope of 24fps support, sending two fields per frame at 48Hz or three fields per frame at 72Hz would both be way too far from the spec frequency of 60 Hz. I wonder if PAL@48Hz could have worked though --- 4% off spec is a lot, but also not that much.

And bringing it back to old time film that's not at 24 fps, that's definitely more hopeless.

ATSC and I presume DVB can broadcast content as 24fps, and the decoder will do the 3:2 pulldown as needed, but variable rate displays can show 24 fps (it's also usually not too hard to detect and invert 3:2 pulldown, at the cost of adding a few frames of delay). I don't think modern broadcasting does better for a 22fps old-timey film than analog broadcasting did though; it's still not something the system was built for, and you're still going to have off speed playback or a pattern of some frames shown more times than others (which is my essential definition of judder)