Comment by toast0 | Hacker News Reader

Comment by toast0 | Hacker News Reader

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> I never understand why playing at wrong FPS for these old-time footage is a thing. It was so obviously wrong.

Old-time footage isn't always at 24 fps, but once the film industry standardized hard on 24 fps, not standard film becomes very hard to use correctly, even in a projection environment --- projectors were engineered around 24 fps all the time, and likely can't be adjusted.

Tv of course was fixed at 60 fields per second (or close enough) or 50 in Europe. Even today, a US market tv is going to do terrible things with a 50 fps signal (although I gather EU market tvs work at 50 and 60 and probably 24). If your old-time film was shot at 22.3 fps, you don't have good options to show it, except on a computer with a VRR display, and even then, I don't know if the whole chain will actually be fine with it. Options are: pretend it is 24 fps and it plays a bit fast and you can use your 24fps pipeline; show some frames twice and have nasty judder; interpolate 24 fps from the data you have which is kind of terrible and only recently a real option.

Modern TVs in any market will take 24 (23.976) hz, 50 hz, and 60 (59.94) hz. It's been pretty common to support all common refresh rates since displays went all-digital.

The vast majority of midrange and better TVs can also spot 24 fps content being sent in a 60 hz signal and render it properly.

Most TVs however will not look good with 50 fps content in a 60 hz signal and you will want to set your output device to adjust its refresh rate based on the frame rate of the content being played. Set top streaming boxes actually worth using have this option, but it is usually disabled by default.

The way movies used to be shown on TV was the movie was projected on a screen and the TV camera was pointed at the screen. The "interpolation" was simply the analog aspect of the TV camera tube.

Low tech, simple, and effective.

Some of my old family movies were converted to VHS that way.

Btw, the industry term for transferring film to video is telecine:

https://en.wikipedia.org/wiki/Telecine

And going the other way, before the introduction of videotape, the only way to store TV programs was to use a kinescope to produce a film recording of the broadcast:

https://en.wikipedia.org/wiki/Kinescope

Wow, the tech was really intricate! ¹

It could invert negative film to positive, and it could color correct scene by scene by looking at correction codes or counting frame numbers, then applying the correct color correction.

Later, broadcast TV started using flying spot scanners instead of just a video camera in front of a film projector. ²

1: https://en.wikipedia.org/wiki/Film_chain

2: https://en.wikipedia.org/wiki/Flying-spot_scanner

Off topic, but what's the difference between the term jitter and judder?

I've tried to google it multiple times, but every time, I can only find various forum posts which are all over the place (if not straight up contradict to each other).

From context I would guess that judder is a global effect (e.g. when the film pauses and then suddenly jerks forward playing), and that jitter is more local (e.g. to do with the movements of the actual characters on screen moving with sudden velocity)

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.)

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)

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