Loss of quality when trimming and outputting in original resolution

Nothing to do but troubleshoot.

What is the exact format of the video clip? Open it in QuickTimePlayer and click on Window/Show Movie Inspector and check the format in the box that appears.

Try these diagnostic procedures and see if they resolve the issue:

Deleting Preferences

Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears.

iMovie will open in a new library. Reopen your old library to get back to your projects.  Deleting preferences is a safe

procedure that will not cause data loss or disruption to your project.

New Project

Another thing you can try is to copy your media into a newly created project and see if that cures the problem. To do

that, do an Edit/Select All, Edit/Copy, of the media in your project and then Edit/Paste it into the newly created project.

Safe Mode

You could also try booting up in Safe Mode (see Finder menu for instructions) and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly.

Updating iMovie

To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the

trash. Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash and return it to the Applications folder.

Resetting the System Management Controller

Try resetting the System Management Controller (but no longer available for the new M1 computers):

https://support.apple.com/en-us/HT201295

Resetting the NVRAM or PRAM

Try resetting the NVRAM or PRAM:

https://support.apple.com/en-us/HT204063

Contact Apple Support

If none of the above resolve your issue, contact Apple Support.  The techs there have the capability of remotely accessing your computer that we cannot do here.  To contact apple support click on the Get Support item in the upper right hand corner of this forum’s screen.  Navigate to a place where you can enter your phone number.

-- Rich

The format information looks fine.

You might try running your clip through Handbrake and see if converting to Mp4/AAC makes any difference when you import the converted clip into iMovie and then export it. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering

often cures issues. Handbrake will do it by default.

You can get Handbrake here:

https://handbrake.fr/

A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.

Also, when you click on the Settings button above and to the right of the timeline, what is the resolution shown for the project?

If still no luck with any of the troubleshooting, then a call to Apple Support might help.

-- Rich

There shouldn't be any change in quality, but it depends on your settings. Try these settings:

Also, you can choose the Custom setting under Quality and move the slider all the way to the right and see if that helps.

-- Rich

Also, if you export in Best Quality (pro res) you might technically get a bit better quality, although 4x file size.

The difference might not be discernible to the human eye, however. The setting is intended for professional quality editing more than anything else.

-- Rich