iMovie: size of exported movies - 10 times larger than input

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iMovie: size of exported movies - 10 times larger than input

iMovie has been nothing but an exercise in frustration.

Last one: I need to edit some movies (conference recording, mostly slide shows with audio). It is simple enough. Clear fluff from the front and the end as people fiddle with the presentation, insert a clean slide title at the front and a closing slide at the end. I managed to get that done.

Now I want to export the result back as an .mp4 file. The trouble is that the output is much MUCH larger than the input. The input is some 49MB. The output is 590MB.

I tried reducing quality and resolution. Even at 540p and low quality, the output is still 230MB.

Why is that ? What other options do I have to make the result at least comparable to the input file in size ?

BTW: the input sound is mono. But the output is stereo. Not sure if this explains the effect on size ...

And the saving is incredibly slow too - it takes over 4 minutes to export one movie. Surely there must be more efficient ways of doing this ? Or I do miss some settings / options ?

Albert

MacBook Pro 13", macOS 10.13

Posted on Mar 26, 2020 7:27 AM

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Answer 1

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Rich839

Level 8

47,646 points

Mar 26, 2020 9:21 AM in response to Rich839

Regarding what options you have to make the file a smaller size, probably the easiest is to share it to Email instead of File, and under the Resolution options you will have options to make the video Small, Medium, or Large. You can really compress it down. I tried it and compressed a 47.8 mb video down to 1.7 mb using the small option. The Medium or small options would give you a larger file, of course.

Another way to reduce the size of your video would be to share it out to File and then re-share it from QuickTimePlayer at 480p.

Another way would be to use the free download, Handbrake, to re-encode your shared out video as H. 264, Mp4/AAC (even if it is already that) and select the Web Optimized option from the Summary menu. I tried that with a 1.1 GB video and reduced it to 687.9 MB with Web Optimized.

Be aware that the more you compress the file the less quality.

-- Rich

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Answer 2

Rich839

Level 8

47,646 points

Apr 6, 2020 9:38 AM in response to agodfrin

Great. You've got it solved. Just to add to the info, there is a slider in the "Custom" setting under "Quality" in the share out box that lets you reduce or increase the Mbps of a clip.

Have fun!

-- Rich

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Answer 3

Rich839

Level 8

47,646 points

Mar 26, 2020 8:03 AM in response to agodfrin

The likely answer is that your input video was very highly compressed and iMovie unpacked it for editing and rendered a less compressed version. Hence bigger size. Less compressed is better for quality and editing, and is desirable unless one intends to upload to the internet, or email it somewhere, or has storage space limitations.

-- Rich

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Answer 4

agodfrin Author

Level 1

38 points

Mar 26, 2020 11:07 AM in response to Rich839

I found the actual reason why the files are so strikingly different in size. It's the bitrate. The original recording has a bitrate of 105kbps. The output from iMovie is 1974 kbps!

I don't know why the input bitrate is so low. I assume it was used by the recording system: it is essentially the recording of slides shown on computer and projection screen, with the audio of the presenter. So the video is not exactly very dynamic: the same picture stays up for 10 seconds at the time with only the audio changing. For that a very low bitrate is quite acceptable.

But iMovie does not know that: it assumes I'm actually editing a real movie where the scene changes constantly, so it applies a reasonable bitrate, that essentially matches the 360p/480p internet movies.

I'm trying Handbrake now. It looks like it can do the job on what iMovie produces ...

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Answer 5

agodfrin Author

Level 1

38 points

Apr 6, 2020 8:23 AM in response to Rich839

To close the subject. I finally used ffmpeg (http://ffmpeg.org). I also tried handbrake, but ffmpeg is fully command-line, so it was easy to put together a small shell script to process all my files, like this:

$ for f in `find . -name *.mp4|sort`; do time ffmpeg -i $f -c:v libx264 -maxrate 160k -bufsize 10M ../Compressed/$f; done

This has the effect to convert the mp4 files to a lower bitrate (160kbps). The resulting size is just a little higher that the originals, and there is no degradation in quality (keeping in mind that the movies are just slide shows with and audio track).

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Answer 6

Rich839

Level 8

47,646 points

Mar 26, 2020 9:22 AM in response to Rich839

With the email option, you would drag the shared email video from the email to the desktop, where you could use the video for whatever purpose you want.

-- Rich

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Answer 7

Rich839

Level 8

47,646 points

Mar 26, 2020 11:23 AM in response to agodfrin

Low bitrate means more compression. High bitrate means less compression. So, as you have found, you are exporting to a much higher bitrate, less compressed video, than you started with.

-- Rich

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iMovie: size of exported movies - 10 times larger than input

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