Summary: The easy workaround to get iMovie work with unsupported MP4 files is to convert MP4 to Apple InterMediate Codec(AIC) MOV which ensures the best result when using in iMovie 08/09/11.
MP4, as the most popular video format, is widely used for saving media datum. And it can be found in lots of places, such as camcorder(GoPro/Sony or other camcorders), websites(YouTube/Fox or other Video websites). It is a common video format for users to share. However, the users met the problem while importing MP4 videos to iMovie for further editing.
"For some reason iMovie '11 won't let me import videos. Specifically, I'm trying to add MP4 files that I copied to my hard drive from my GoPro HD 960. For GoPro users, I used the R2 (720p) format. They open properly in QuickTime and VLC but can't be imported into iMovie (or iPhoto)".
"I downloaded many MP4 movies from Internet, and want to import them into iMovie for editing to create a new movie.But the iMovie won’t accept the mp4 videos. Can you help me?"
Does iMovie accept MP4 videos? According to the Apple official website, it does support MP4 files encoded with MPEG4/AAC or H.264/AAC data. That is to say, if a MP4 file contains an incompatible video codec with iMovie, it will be rejected by iMovie.
So if you are unable to locate your MP4 files into iMovie, then you need to find another way to perform the task. As we all know, the most compatible video codec for iMovie is AIC, then transocoding MP4 files to AIC with a third party program is an easy way to edit MP4 videos in iMovie.
To fulfill your task, you can just get Brorsoft's Video Converter for Mac, a powerful video converter, editor and player. It offers an optimized preset Apple Intermediate Codec .mov for iMovie for easy access. Besides transcoding MP4 video to iMovie supported videos with optimized settings while keeping the video quality, the all-in-one and easy-to-use toolkit can convert all other popular video formats like AVI, MXF, Canon/Nikon DLSR MOV, WMV, MKV, FLV, ASF, M2TS, VOB, etc. to iMovie natively supported video formats on Mac OS X (Mavericks and Mountain Lion included) with fast encoding speed. See how below about converting MP4 files to iMovie format.
Free Download MP4 to iMovie Converter:
(Note: Click here for OS X 10.5)
How to edit MP4 videos in iMovie with AIC vodec
Step 1: Load MP4 video files to the program by clicking “Add” button, or drag the MP4 videos to the software directly.
Step 2: Click "format" bar to select "iMovie and Final Cut Express--> Apple Intermediate Codec(AIC)(*.mov)" as the output format for Mac iMovie.
Tip: Before conversion, you can click the "Settings" icon to customize the video/audio settings for iMovie: If you want to get a smaller ones, please decrease the video size from 1920*1080 to 1440*1080 or 1280*720.
Step 3: Click the Arrow icon under the preview window, and the program will start transcode MP4 to AIC MOV for iMovie 08/09/11.
After the MP4 to iMovie Mac conversion finished, you can freely import your transcoded MP4 files in iMovie '08, iMovie '09, iMovie '11, iMovie HD etc for editing.
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Source:MP4 Converter Download For iMovie editing mp4 file