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30p vs. 60i - And Other Questions

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Peyton Todd
30p vs. 60i - And Other Questions
on Mar 28, 2009 at 12:52:26 am

Hello. I'm a linguist needing to record a video conversation in deaf sign language for the purpose of analyzing the head movements, eyebrow movements, eyeball movements, eye blinks, and eye squints accompanying their signs. There will be four signers conversing, seated on sofas opposite each other, two signers per sofa, and I will be recording them from the waist upward, indoors, with open windows off camera, hopefully on a sunlight day.

I want the best possible clarity of picture given my limited budget (say in the price range of a Canon VIXIA HV30). I'll need two cameras, of course (one for each pair of signers), and although this is likely to be a one-time event, I plan to purchase one camera for practice runs since I'm a relative newbie at video, and rent the other camera, although I'm debating buying two in case I screw up and need to do it again.

The other main consideration is the fact that the resulting videos will almost certainly never be shown on TV, but only via computer in four different situations: (1) in a video editor, for coding signers' facial expressions and extracting short clips (currently I have Vegas Movie Studio 9 Platinum for the PC); (2) shown to colleagues via files on CD or hard disk; (3) viewed by colleagues over the internet; (4) viewed by colleagues at conferences (projected from a laptop onto a large screen).

Here are my questions:

1. Should I plan to go with 30p or 60i? I don't want to miss an eye blink, so it would seem that 60i would be better, but of course each 60i frame would have only half as much information as would be contained in a 30p frame. Is it relevant to ask how many 'lines' out of the total 1080 an eyeball or eyebrow would take up, given that signers will be recorded only from the waist up plus a little more space over the head since some signs do occur above the head? (I assume I'm correct in assuming 1080 lines?)

2. What 'post' considerations would be relevant? Would 30p vs. 60i be best given that the end result will always be shown via a PC? (For example, would the conversion process force the result to one or the other?) And what about the fact that most people who view my clips will have a rather standard PC or Mac? In particular:

a. All the sign language video clips I've dealt with so far were in a 4:3 ratio, which is wide enough to show two signers side by side. Would standard video players on PCs and Macs adapt to the 16:9 ratio seamlessly, or would I have to convert to 4:3? And is it even possible to convert from 16:9 to 4:3? Would I lose clarity of the picture in doing so? (After all, it's only a slice of a larger picture, right?)

b. Will I need to worry about NTSC vs. PAL vs. SECAM or will it be the same for everyone all over the world as long as they have a PC or Mac?

c. Is there any consideration I've overlooked?

I apologize for the great length of this question, but I thought it best to provide you with all the relevant details.

Thanks in advance for your help!

Peyton Todd



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Gord Stephen
Re: 30p vs. 60i - And Other Questions
on Mar 29, 2009 at 8:50:38 pm

Hi Peyton,

Really interesting. I'm working on a project in ASL right now, so lately I've found myself thinking about similar things. My input in response to your questions:

1. 30p. The interlacing process means that with fast movements (such as hand movements), the field sets don't display fast enough to 'keep up' with the movements, meaning that one set of fields will show hands in one position and the other set will show them a little to the left or right. The result is a comb-like edge on moving objects - distracting and difficult to study. Also, 1/60th of a second is really fast, faster than an eye blink (how many times can you blink in one second?), so I wouldn't be that concerned about missing anything. That being said, your shutter speed is really important. Motion blurring is bad news in project like this. I'm shooting with the shutter speed locked at 1/100th of a second - but you might want to go even higher, I'm still getting a bit of blur in fast hand and arm movements (nothing that would impair the viewer's ability to understand what's being signed, but for analysis purposes it might be an issue).

2. Computer displays are progressive, and any video that you show on them will probably (or at least should) be deinterlaced anyways.

a) If you can, export your footage with a square pixel aspect ratio - then you won't have to worry about your footage appearing 'squished' to anyone. You can convert from 16:9 to 4:3, either by cropping the right and left sides of the video out or by 'letterboxing' the footage (resulting in black bars on the top and bottom of the screen). With the first option, you're not losing any picture quality, technically... But you are chopping off the sides of your image. With letterboxing, you're losing resolution, but keeping the entire image.

b) PAL/NTSC/SECAM are television broadcast standards - the video file you render (for example a .mov or a .wmv) is independent of that system. The only issue might be if someone didn't have the correct player or codec to play back the video (for example, if you emailed me a .mov file and I didn't have Quicktime).

c) Just remember to shoot with a high shutter speed. You could be shooting at a really high framerate, but with a low shutter speed, things would be a blurry mess. And to avoid the other kind of blur... make sure that your shots are always in focus. If your talent isn't moving around in the frame, manually focus in on them and leave the focus locked there. It's frustrating when you rely on your camera's autofocus only to discover that it decided for you that the plant in the background is the focus of the shot, leaving your actual subject fuzzy.

Hope that helps!

Gord


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Todd Terry
Re: 30p vs. 60i - And Other Questions
on Mar 29, 2009 at 9:32:45 pm

Well, I may be giving some conflicting advice regarding the shutter speed, depending on your end usage.

You say its "purpose of analyzing the head movements, eyebrow movements, eyeball movements, eye blinks, and eye squints accompanying their signs."

If you plan to study individual frames, say toggle through the video very slowly, then yes, you might want to shoot with a higher than "normal" shutter speed (providing you have plenty of light, since exposure will start dropping like a rock when you crank up the shutter).

But... if you plan on primary usage to be viewing this video at "normal" speed, then I would never shoot at a higher-than-normal shutter speed ("normal" shutter being 1/60th of a second for 60i and 30p video, or 1/48th of a second for 24p video). If you shoot with a higher-than-normal shutter, then you motion is going to be very choppy and stuttery when viewed at normal speed. For something with a lot of movement such as sign language, I can imagine the end result could range anywhere from annoying to darn near unwatchable. Your brain will interpret the motion blur as "normal," but a higher-than usually shutter would be very stroby and juddery. Higher shutter speeds should be reserved for specific applications... such as studying individual still frames, creating a specific look (i.e., "Gladiator" or "Saving Private Ryan"), or when shooting video that you intend to slow-mo later.

Just my opinion, of course... but I think one of the most annoying things you'll often see is normal-speed video that was shot with a high-speed shutter... unless it was done for specific effect.


T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com






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Peyton Todd
Re: 30p vs. 60i - And Other Questions
on Mar 30, 2009 at 3:43:31 pm

Many thanks, guys. The really important piece of information up front is the decision to go with 30p, since that determines which camcorder to buy, which now seems definitely to be the Canon Vixia HV30. Thanks, too, for alerting me to the shutter speed and focus issues, which I would otherwise have been ignorant of. Fortunately the shutter speed question is one I should be able to experiment with by recording my own signing before I record my native signers.

But are still two issues I'm wondering about:

1. Do I need to buy more sophisticated video editing software to convert from 16:9 to 4:3 without letterboxing? I just rendered an approximaely square-shaped crop from one of my 4:3 videos, both on my ancient Vegas Video 3.0 and on the Vegas Movie Studio 9.0 Platinum which I just purchased, hoping to get off cheap. Unfortunately, in both cases the result came out letterboxed, and I can find no option in the latter (including in its skimpy manual) for telling it not to letterbox.

2. For embedding clips in PDFs and sharing them over the internet, I will usually want to convert them to small files in a 4:3 ratio using codecs most people with have - such as mpg or mov (although suggestions are welcome). It will not be much of a problem that these are lossy, since I can always ship the original HD versions to anyone who doubts my coding decisions. But is there anything relevant to that issue that I need to know now, before I buy the camcorder?

Thanks again,
Peyton


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Gord Stephen
Re: 30p vs. 60i - And Other Questions
on Mar 31, 2009 at 2:38:22 am

A higher shutter speed does create an unnatural effect and can seem quite 'video-y' - but hey, it *is* video, and for a technical product such as you're describing there's no need to hide that. Generally I would stick to 1/48 or 1/60 as well, but for my application, 1/100 makes myself and my clients happy. I'll admit that going much over 1/100 would probably be a little extreme, though. That all being said, your experiments will be your most valuable feedback.

Are you absolutely sure that you want/need to convert to 4:3 for final output? I'm not familiar with Vegas, or its output options, but if it were me I would just stick with 16:9 clips as my final output and not worry about cropping, letterboxing, or anything else. No need to sacrifice resolution or parts of the frame. It *should* be possible to crop the sides if that's what you need to do. Maybe try starting a 4:3 project and importing the 16:9 footage into it? (But could you get a 4:3 project in HD?) I'm just speculating now, I really don't know.

All that's for later down the road though, and that stuff shouldn't have an impact on your choice of camera. It is good to think about now though. If you're going to crop the sides of your footage for 4:3 output, just make sure that you're conscious of that while shooting and you keep your subjects (and their extremeties) well within the centre of the frame... but without some sort of guide on the viewfinder to help with that framing, it'll be tricky and risky... You know your project best - if it needs to be output in 4:3, then it needs to be output in 4:3. But why not just use the full 16:9 to your advantage? There shouldn't be any issue playing it back - widescreen video is supported by common video players and codecs.

Hope that helps.

Gord


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Peyton Todd
Re: 30p vs. 60i - And Other Questions
on Apr 1, 2009 at 2:14:24 pm

Hi - This is just a quick note of thanks to both of you. Without your help, I might have made some serious mistakes.

Peyton


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