QT calculation – Converting frames to milliseconds, coding question

[Ariane Fisher]

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. 🙂

[Steve Eisen]

You’re making my head spin. I gave up on math a long time ago. 🙂

Steve Eisen
Eisen Video Productions
Vice President
Chicago Final Cut Pro Users Group

[Michael Gissing]

Quicktime will report a strange frame rate because of incorrect metadata. When using quicktime conversion in FCP, I have found it essential to set the frame rate manually, not leaving it on the default “current”. In 25 fps land, it often says the file is 20.02 fps when it is actually 25fps.

Pain in the ring. I spent an afternoon looking for software to change the metadata to the correct frame rate.

So to add further pain Ariane, the metadata and the actual frame rate can differ which makes accurate length in millisecond calculation unreliable.

[Ariane Fisher]

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?

[Michael Gissing]

[Ariane Fisher] “So this has to be possible, or they wouldn’t offer that information, or am I just kidding myself?”

When software like Final Cut sets incorrect data, then that is a bug. If it makes it unreliable to java script based on the assumption of correct metadata, then I would guess yes. None of that stops Apple making claims that you can do it. It just boils down to reliability.

I am sure quicktime is in for major re-writing at the moment with an anticipated 64 bit version of FCP. Perhaps they will fix some of these issues at the same time, or introduce more bugs. I know none of this helps you at the moment.

[Ariane Fisher]

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.

[Steve Eisen]

I hope you’re wearing your hat!

Steve Eisen
Eisen Video Productions
Vice President
Chicago Final Cut Pro Users Group

[Doug Beal]

I think you need more tequila
earlier you posted
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

Now this is wrong there is no .72 frames
you have 20 seconds which is 480 frames at true 24 fps
then you have 5 frames that didn’t round into seconds for a total of 485 frames

switch the counter to frames in your sample quicktime and see what it counts for frames

Doug Beal
Editor / Engineer
Rock Creative Images
Nashville TN

[Bouke Vahl]

Second vote for working in frames rather than Msecs, as you probably always need an integer.
(My software mostly works in Msecs, as for syncing sound i sometimes want subframe accuracy. And sound has not a timebase, unless you call the sample rate timebase (some do).)
Video/film of course needs frames to be identified, and often also the specific field you’re on.

Now for the math, i normally work with (1000 / 1001.)

That means:
Pal is 1000 msecs / 25 frames = 40 Msecs duration per frame
NTSC (both drop AND non-drop!) = 1000 / 30 * (1000 / 1001)= 33.3
This is the same as 30 / 1.001, but explains it a bit better…

the real trouble lies in reverse-creating the bugs in QT / FCP to stay compatible…
(For 59.98, display in QT player is different from FCP…)
And, in FCP there is a lot of stuff going on ‘under the hood’ that is unclear. (Hence the tremendous amount of posts here where people go out of sync on imported sound files where there seems to be no logic at all…)

Bouke

https://www.videotoolshed.com/
smart tools for video pros

[Ariane Fisher]

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.