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QT calculation – Converting frames to milliseconds, coding question
I need to understand the conversion in QT from fps to frames in milliseconds. I have a file that is 00:00:20:05 at 24fps. When I open it in QT it says 20.27 seconds. I would have thought it would be 5frames/23.976fps or 20.206s. I’m unsure where the 270 ms comes from. When I upload it to my server and read the mediainfo file, it says 20.288 seconds.
Obviously mediainfo is using some other factor than how QT calculates duration on the desktop. If this is the wrong forum to list this question, please direct me to the correct one. Thanks much.
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45 Replies
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Hi Ariane,
You are right. QT Player here makes a rough calculation.
A milliseconds precision is not necessary on a movie player.
For accuracy fallow the TC, and better if you check it in FC than QT.
rafael -
Thanks Rafael,
I agree, the world would be a much better place if all my calculations could be based on pure timecode. I’m doing java scripting based on these numbers. The java reads the mediainfo file, which QT provides in milliseconds, not timecode.
I searched in the MTEditing.pdf doc from the Apple Developers page as well as the manual for java scripting QT. I spent about 3 hours scouring the internet last night to find this calculation.
This is before the java portion though. It’s a fundamental QT issue, for which I have no answer.
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Hi Ariane,
I can’t help you about scripting and so, but some guys around – Andy Mees or Bouke Vahl -know well.
rafael -
Am I in the right forum to reach them? Or can I message them on here?
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You need this website: https://lurkertech.com/lg/timecode/
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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. -
[Ariane Fisher] “Am I in the right forum to reach them? Or can I message them on here?
“ You can contact Bouke through his web site. Andy will shows up soon.
rafael -
I’m not a code banger (nor am I lawyer, in other forums) 🙂
That guy wrote the QT standard, so he’s a go-to source. There’s a problem to solve with regard to dropframe or non-dropframe.
try the bigger page perhaps … https://lurkertech.com/lg/video-systems/
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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 framesWhich 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. -
I know some guys at AJA, maybe I can try there. I emailed Chris Pirazzi. Maybe beer will help. 🙂
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