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Ariane Fisher
March 19, 2011 at 1:04 am in reply to: QT calculation – Converting frames to milliseconds, coding question

I’ll probably take this discussion onto the developer’s forum. Do you guys want me to post my findings here? Or does no one care about the advanced mathematical vagaries of Quicktime? On an up note, I may have figured out the mismatched sync of audio and video when importing xmls. Or is that old news?
Ariane Fisher
March 18, 2011 at 7:46 pm in reply to: QT calculation – Converting frames to milliseconds, coding question

Aha! Thank you so much. Even though I’ve spent hours on the Developer site, I somehow missed this page. Easy to do when you’re frustrated. I believe I should be able to find my answers there. Well, hopefully.

Are there forums for developers as well? I don’t want to be the newbie on there asking the dumb questions. Hopefully mine are technically challenging enough to merit posting there. 🙂
Ariane Fisher
March 18, 2011 at 6:51 pm in reply to: QT calculation – Converting frames to milliseconds, coding question

If there’s an app, plug-in or anything for it… well that would be awesome. I feel no need to re-invent the wheel, especially with the impending possibility of a complete QT rewrite and this all getting thrown out. It’s also possible that QT is not the right software to use for this. No desire to touch flash.

I know there are platforms like Kaltura out there that manipulate video online, but they don’t meet my needs for several reasons. I’m ok with doing the java manipulations, it’s finding the correct frame number in the first place, that is my issue. I can’t figure out how QT knows where it’s at within a movie.

From reading the java scripting for QT manual, I know this “must” be possible. Also, from seeing QT accurately reinterpret the duration in ms to frames, I know there must be some consistent calculation involved. I think I’m getting close, but I need to understand the underlying mathematics behind QT. What a geek I am!
Ariane Fisher
March 18, 2011 at 5:57 pm in reply to: QT calculation – Converting frames to milliseconds, coding question

I agree, there is no such thing as .72 frames. That was a step in the calculation. I’m an engineer by training and like to thoroughly label all my units, so I can easily trace any errors in my assumptions.

So, now I’ll ask questions line by line.

[Doug Beal] “you have 20 seconds which is 480 frames at true 24 fps”

When you say true 24fps, do you mean 24 frames/1.001seconds, or do you mean 24.0000 frames per second? This media is 24/1.001 out of camera.

[Doug Beal] “switch the counter to frames in your sample quicktime and see what it counts for frames”

Thank you so much! Didn’t know I could do that. I had originally run the clip through Compressor and overlaid timecode for diagnosis purposes. It’s reading the frames correctly, but I still need to understand the mathematical conversion from frames to milliseconds, as QT inherently reads it this way. I’m using the online QT metadata (which is in milliseconds) to manipulate the movie with javascript. Until I can understand the conversion, I can’t accurately manipulate it. Then, there’s also the issue that the online QT metadata is showing a different time in ms than what the same file shows on the desktop. Fun stuff.
Ariane Fisher
March 18, 2011 at 1:25 am in reply to: QT calculation – Converting frames to milliseconds, coding question

I’m not actually looking at the metadata out of FCP. Straight out of the camera, QT says the clip duration is 20.27 seconds. The clip is 20 seconds, 5 frames long. Where is the .27 from? It’s not from 5/(24/1.001), that is .21. To make it even better, looking at the same clip online, the QT metadata then says 20.288. Even if there is an error, surely it must be a consistent error for which I can account? Bangs head into wall immediately followed by shot of tequila.
Ariane Fisher
March 18, 2011 at 12:59 am in reply to: QT calculation – Converting frames to milliseconds, coding question

Yes, I keep hearing that, but… Apple has an entire manual devoted to java scripting of Quicktime on their developers site. In order to script QT, you must have reliable numbers on which to script. So this has to be possible, or they wouldn’t offer that information, or am I just kidding myself?
Ariane Fisher
March 17, 2011 at 10:44 pm in reply to: QT calculation – Converting frames to milliseconds, coding question

Don’t say that. I’m already feeling like ramming my head into a wall. Several times. And then several more. And the next kid who whines about “when am I ever going to use math in the real world” is going to get smacked really hard by me. 🙂
Ariane Fisher
March 17, 2011 at 10:19 pm in reply to: QT calculation – Converting frames to milliseconds, coding question

I know some guys at AJA, maybe I can try there. I emailed Chris Pirazzi. Maybe beer will help. 🙂
Ariane Fisher
March 17, 2011 at 10:04 pm in reply to: QT calculation – Converting frames to milliseconds, coding question

Luckily I have an engineering degree. 🙂

So, if I have 20 seconds, 5 frames of video at 24fps NDF, the calculation should be:
(20seconds*(24frames/1.001second)) + (5frames/(24/1.001))=479.72 frames

Which I need to convert back into seconds for QT. So I multiply:
479.72*(1.001/24), which should equal QT’s number of 20.27 seconds – but it doesn’t. I’ll try to contact Chris Perazzi as well. Thank you.
Ariane Fisher
March 17, 2011 at 8:20 pm in reply to: QT calculation – Converting frames to milliseconds, coding question

I read through that post and retried the math. Still doesn’t work (or I’m doing it wrong). And it doesn’t explain the discrepancy between the QT calculations on the desktop and those in mediainfo. This media was 24fps NDF, a 20 second clip.

Any math/QT geek want to explain this to me? According to that site, at 24fps NDF, a 20 second, 5 frame clip should be:
20 seconds + 5*(1.001/24) or 20.208seconds, not 20.27 which is what QT says.

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