Mac81
Level 1
(4 points)
Q: QT Decreases File Size by 90% After Minor Trim
As a test, I used QT 10 to open a 60 second gopro .mov video, and trimmed a couple seconds.
After saving the new file to 1080 (same as the source file), I saw that QT had decreased the file size by 90%.
I assume that means that QT also threw away pixels, and decreased the quality of the image.
Why does QT do this?
Is there an editor that does not?
Thanks.
Mac
iMac (Retina 5K, 27-inch, Late 2015), OS X El Capitan (10.11.1)
Jon Walker
Level 6
(18,603 points)
As a test, I used QT 10 to open a 60 second gopro .mov video, and trimmed a couple seconds. After saving the new file to 1080 (same as the source file), I saw that QT had decreased the file size by 90%. I assume that means that QT also threw away pixels, and decreased the quality of the image.
I suspect it may be doing a number of different things here. Compare the "Inspector" window for both the original and the merged files. Has the frame rate changed? (I.e., you failed to mention if the source file was 1080p30, 1080p60, 1080p120, or 1080p240.) What is the combined average data rate? (I.e. you failed to mention the data rate, Profile, Level, or Key Frame Rate of your source file.) Basically, to create a final file that is merged, flattened, and compatible with most third-party players, your file was re-encoded. I suspect your workflow may have limited output to 30 fps, as well as, applying the built-in context adaptive compression settings that are optimized for transcoding speed, visual quality, and file size output. (I.e., QT X does not offer user settings that control data rate or distribution across frames.) Thus, by definition, your file was compressed by "throwing away" data that may or may not affect the visual quality of your output depending on how you view it.
Why does QT do this?
This is what QT X was programmed to do by Apple.
Is there an editor that does not?
That depends on what you really want for your output.
QT 7 Pro can trim and merge segments without re-encoding the content but does so by creating "edits" within the file itself. Unfortunately, these "edits" only play correctly in player apps programmed to understand them. in addition, this workflow usually places the merged A/V data on separate tracks. The only way to ensure the edits play correctly in all apps AND flatten the content to a single audio and a single video track is to export/re-encode the edited content to a new file container which is what the QT X player is doing. The only real difference between QT X and third-party editors would likely be the ability to employ user options to manually control output settings.
The real question here (as someone already mentioned) is whether or not you need original file "quality" and/or original file frame rates for whatever you plan to do with the merged content. Normally I would use the free GoPro Studio app to combine the various steps (trimming, playback speed control, merging, adding transitions, adding titles, and export for a specific targeted use). While it may not be as intuitive as other apps, it is available free from the GoPro web site.
