Online, 30fps looks best, even in PAL region (?)

fausseplanete wrote on 11/10/2011, 5:13 PM

A mathematical colleague pointed out that most laptops etc. have screen refresh rates of 60Hz which can't be changed (unlike for older CRT screens), and 27.97 is very-near divisible into 60, whereas 25 is not, the latter resulting in dropped frames and hence uneven motion.

So I put it to the test, and the argument seems to be borne out !

To check out the "30 fps is better than 25 fps" hypothesis, I got the camera out and recorded pans at 720p30 and 720p25, and edited and rendered each consistently with this. The respective results are at http://vimeo.com/31929495 (30 fps) and http://vimeo.com/31930183 (25 fps).

Here in the UK (25 fps PAL region), with my MacBook Pro running Windows 7 via Boot Camp, the 30 fps version looks far smoother to my eyes at least.

So to maximise the proportion of online audients enjoying smooth quality, maybe all online video should now be 30fps, regardless of PAL/NTSC region?

Comments

farss wrote on 11/10/2011, 6:01 PM

"So to maximise the proportion of online audients enjoying smooth quality, maybe all online video should now be 30fps, regardless of PAL/NTSC region? "

Agree, that's a fairly simple conclusion to reach as 30fps is more temporal resolution than 25fps plus to some extent what we perceive as resolution is a product of spatial and temporal resolution, perhaps because there is less motion blur.

There's a couple of bigger picture problems here though.

1) Shooting 30fps in Region50 countries can lead to all manner or problems with lighting.
2) If you need to deliver the content on both DVD and online it would still make more sense to shoot 25fps in Region50 and 30fps in Region60, assuming your clients are in the region you are shooting.

Bob.

Laurence wrote on 11/10/2011, 7:00 PM

Just a bit more on this. Incandescent lights don't flicker, and most new fluorescent lights flicker at very fast rates that don't cause problems with video rates. However, mercury vapor work lights and older fluorescent are still quite common so watch out if you are using 30p in a PAL area, like Bob said, watch out.

Most PAL players and TVs will play back 60i just fine, but they won't always do 24p without displaying the pulldown. Thus, 30p will often give you a nicer progressive look (since 60i display of 30p looks like 30p) than 24p (where it is likely to be viewed as 60i with added 3:2 pulldown).

As a general rule, you can read up on the rules of lighting for shooting 24p in an NTSC area, and use those rules when you shoot 30p in a PAL area.

Yeah I love 30p and use it for most everything these days.

fausseplanete wrote on 11/11/2011, 3:37 AM

Thanks for your comments, Bob & Laurence. I accept what you say.

But my main point (that I want to verify) was not about the greater temporal resolution or lighting issues (both granted) but instead about the fact that that most laptops etc. (I don't own an iPad but I bet they're the same) have a fixed screen refresh rate of 60Hz in any case, and on a lot of devices this cannot be changed. So its on display that dropped frames and hence uneven movement occur. Flickering is a further issue, I haven't looked into that yet - but raise some thoughts below.

I guess you (plural) were referring to flicker that happens on the recording side.

When recording in daylight of course, it's not an issue (apart from the risk of un-noticed (at the time) artificial lights in-shot. For my tests (linked in the initial post), 30 fps definitely best, both when played locally from a file and when played over Vimeo. Anyone agree/disagree? I hope I'm not just seeing what I expect to see !

However when shooting under controlled artificial lighting, I guess there's an argument for ensuring it avoids 50Hz. That impacts on kit-selection, e.g. suitable LED (all LEDs flicker, some at a problematic rate) or else tungsten etc. run from a 60Hz source, maybe from a 60Hz invertor. Sounds like fast fluorescents and LEDs will ultimately be the way to go.

But then there's the stuff that can't be controlled:

When computer screens are in-shot, I guess their own refresh rates (which might typically be 60Hz) matter only when what's displayed on their own screens is changing (e.g. moving). Modern screens are not like phosphor CRTs so I guess when their displayed image is not changing, virtually no flicker at all happens. But if they do have moving stuff, 60 fps might be best.

Ambient artificial lighting (at 50 Hz) is another matter of course. I wonder if, like the "new fluorescent lights" you mention, energy-saving bulbs flicker at high rates.

One further option I wonder about for mains-lit indoor scenes is shooting at 25 fps then converting (with motion-compensation etc. (e.g. by Twixtor or AviSynth as described by Nick Hope) to 30 fps for display. That shouldn't "invent" any flickering, because it's just an interpolation. Another "pain" to have to do in post of course, but at least it would be automatic. Conversely if shot outdoors at 30 fps then by same process could convert to 25 fps if needed for PAL DVDs etc.

Thanks Laurence for your suggestion that: "As a general rule, you can read up on the rules of lighting for shooting 24p in an NTSC area, and use those rules when you shoot 30p in a PAL area". I'll do just that.

Final thinks: I wonder is there some kind of video "notch filter" like there is for removing mains hum from audio? Maybe it would need to be a bit intelligent, something along the lines of a video denoiser that uses temporal information and frequency decomposition. Of course one can use a general denoiser (like Neat Video) to remove video flicker (I've done it for conferences where lighting is 50Hz but presentation screen is 60Hz) but a purpose-made tool would presumably be faster and produce better quality (akin to the use of de-hum as opposed to denoiser in an audio tool such as iZotope RX).

My aim here, apart from sharing-and-comparing the insight, is to establish an "ideal" new workflow, that might not only affect the shooting side but might also establish whether a client should be persuaded to relax their current requirement for on-line content to be at 25 fps.