Stutter in iMovie 5 & 6 - causes and workarounds

Question

This topic discusses the problem of stutter in the playback of iMovie 5/6 projects — where the playback pauses repeatedly. The cause of the stutter is discussed here, with possible workarounds.
Over the last several weeks I've examined several stuttering projects, one provided by Marilyn Hudson and three by Benny Alford. Thanks to you both. They were very helpful.
I also examined several projects I created myself, after discovering how to force a project to stutter.
The tests I ran suggest that stutter is caused by the number of audio clips in the timeline of the project. When a large number of audio clips have been added to a project, the project begins to stutter. The stutter worsens as more audio clips are added, eventually causing the project to play very poorly, if it plays at all. The stutter improves as you remove audio clips, eventually allowing the project to play normally.
Frank Farmer, whose forum topic about stutter describes the problem nicely, is a good read:
http://discussions.apple.com/thread.jspa?threadID=495288&tstart=0
Benny's topic is also very helpful:
http://discussions.apple.com/thread.jspa?threadID=511773&tstart=100
BEFORE WE BEGIN, A CAVAET OR TWO...
Identifying software problems is never easy, especially when you don't have the code to read. So my conclusions may not be accurate in every respect. There may be "gaps" still to fill.
Certain features of the stuttering problem can vary from user to user and from project to project, so what I say may not apply to everyone or to every project. Likewise, your experience may not apply to others.
In light of that uncertainty, I hesitated a bit to even post this message, for some conclusions may be "wrong" for some users. Hopefully they will be "right" for most.
If your experience is hugely different than mine, please say. We will all benefit.
THE PROBLEM
Stuttering takes several forms. It may include pauses in video playback, pauses in audio playback, audio distortion, or some combination. I believe these all have the same cause, creating a variety of playback symptoms. (The behavior may vary because of a difference in video cards and other hardware features. The amount of RAM doesn't seem to matter. I didn't have lots of Macs to test.)
Stutter is not simply an inconvenience or nuisance. Stutter can make it impossible to edit a project. Playback becomes so poor you can't view the editing you've done.
Severe stutter can also make simple tasks impossible to do, like moving the playhead, starting/stopping playback, selecting clips, undoing changes and saving. Your editing grinds to a halt.
HOW THE PROJECTS WERE STUDIED
When I began I didn't know what I was looking for, of course. I had never seen stutter in any of my projects, but had read reports by others and had some guesses. I poked and prodded the projects looking for clues, aided by software I wrote to help evaluate the type and number of project clips. I tried to explore as many possible causes as I could think of, ruling out possible causes.
THE CAUSE
Every project I tested could be forced to stutter by adding a large number of audio clips. Conversely, every stuttering project could be made to stop by removing audio clips. The exact number of audio clips that caused stutter varied, but not the pattern.
Based on what I saw, the major contributing cause to stutter — perhaps the only cause — is the number of audio clips in the Timeline of the project. That will definitely cause the problem, not the length of the project, nor the project size, nor the iMovie version, nor the number of video clips.
I was unable to find any other contributing factor. You can't prove a negative, however, so I cannot be certain other causes don't exist. As we discuss this topic more perhaps other factors will emerge.
SEVERITY
The severity of the stutter varies by the number of audio clips in the timeline. Stutter begins with a few simple "popping" sounds, and gets worse as more audio clips are added. The popping is replaced by pauses and audio distortion. Stutter continues to worsen as you add more audio clips, eventually making the project hard to edit.
WHAT KIND OF AUDIO CAUSES STUTTER?
All kinds of audio clips cause stutter. The type of audio you add doesn't seem to matter. It's the total number of audio clips that matters, not the type.
The audio can be any of these types:
1) Songs imported to the project from iTunes or elsewhere;
2) Audio extracted from video clips; and
3) Voiceover clips — narrations you record in iMovie.
WHAT COUNTS?
It's the number of audio CLIPS in the Timeline that matters, not the number of audio files you import or create. Importing a song adds one clip to the timeline and one file to the project. Splitting that clip adds additional clips, increasing the likelihood of stutter. Splitting extracted audio clips adds to the clip count too.
Every audio split you do adds to the total, increasing the likelihood of stutter.
WHAT DOES NOT AFFECT STUTTER?
I saw no evidence that these factors contribute to stutter:
1) The number of audio and video files (files stored in the project's Media folder).
2) Overlapping clips in an audio track.
3) The length of audio clips.
4) Which audio track the audio clips reside on.
5) The number of video clips.
6) The length of the project.
7) The size of the project (file size).
8) The size of the iMovie Trash.
HOW MANY AUDIO CLIPS WILL CAUSE STUTTER?
It varies a bit, so it's hard to say exactly. Projects I tested from one user began stuttering at 60 audio clips. Others began at 125. The number seems to vary from Mac to Mac. My guess is a typical maximum is 90-100 audio clips, but that's just a guess.
Although the number of clips may vary from user to user, my guess is the number may be relatively constant for all your projects. Your projects might tend to start stuttering at about 60 clips, 90 clips, 125 clips or whatever. But that's just a guess.
One of the reasons a precise number of clips is difficult to say is that the stutter itself varies from occasional popping to long pauses, affected by the number of audio clips. It's sometimes hard to know exactly when stuttering begins. As you remove audio clips from a stuttering project, for example, the stutter may appear to go away, but you may later hear occasional popping. Then when you remove five more audio clips you achieve perfect playback.
COUNTING AUDIO CLIPS
Note that iMovie 5 and 6 lets us count the number of audio clips in the Timeline. When you select clips with the mouse, iMovie displays the number of selected clips at the bottom of the iMovie window.
Unfortunately, the clip count is not displayed when the "Select All" menu command is used to select clips. You must physically drag across the audio clips to see the total.
HOW CAN I FIGURE OUT THE CLIP LIMIT FOR MY STUTTERING PROJECT?
It's not hard to figure out how many audio clips your Timeline can hold without stuttering. You can do a simple test where you remove audio clips until the stutter stops.
First Save the project. (Do NOT Save the project again during the test.) Then select ten audio clips or so, and hit the Delete key. Check for stutter. If the project stutters, remove more clips. Continue removing audio clips until the stutter stops, then count the remaining audio clips. To count the remaining clips, select all the audio clips with the mouse and check the total reported at the bottom of the iMovie window. Subtract a few — just to be safe — and that's your clip limit. (If you hear occasional popping sounds later, subtract another five clips.)
To restore the project to its previous state, choose File > Revert to Saved.
IS THIS JUST AN iMOVIE 6 PROBLEM? OR JUST AN iMOVIE 5 PROBLEM? OR WHAT?
Stuttering occurs in both iMovie 5 and iMovie 6. I saw the same stutter in both.
There were indications that stutter begins earlier in iMovie 6 — with 10% to 20% fewer audio clips. That may explain why some users report that stuttering began when they upgraded to iMovie 6. iMovie 5's tolerance for audio clips seems a bit higher, so projects that play okay in iMovie 5 might begin stuttering when upgraded to iMovie 6. They may have been "on the edge" in iMovie 5, but when upgrading to iMovie 6, they start stuttering.
Stuttering also occurs in iMovie 3 and 4. I could create stuttering projects in iMovie 3/4 too.
The only version of iMovie that refused to stutter was iMovie 2. (I stopped the test after adding 400 audio clips.) Note that iMovie 2 may use very different QuickTime routines than later versions of iMovie, which may explain its reluctance to stutter.
SO, WHERE'S THE PROBLEM?
In spite of what I just said about iMovie versions, I suspect this is not an iMovie problem. The problem may lie elsewhere. Here's why:
1. It's possible to create stuttering movies in QuickTime Player too, completely independent of iMovie. I used QuickTime Pro to add audio tracks to a movie, and found that a normal QuickTime movie can be made to stutter. It behaves just like iMovie. (QuickTime Pro's "Add" command lets you add multiple audio tracks to a movie. Each audio track is equivalent to an audio clip in iMovie.)
2. QuickTime Player also stutters when it plays the small QuickTime reference movie iMovie places inside the iMovie project. (iMovie 2 has no such movie, suggesting its reliance on QuickTime is different.)
The existence of stutter in both QuickTime Player and iMovie suggests the problem is in the QuickTime software itself, not iMovie. iMovie and QuickTime Player both use QuickTime system routines to edit/play our movies. I suspect that's where the problem lies. (I mean the QuickTime system software underlying both the QuickTime Player and iMovie, not the QuickTime Player application.)
CAN I EXPORT PROJECTS STUTTERING PROJECTS TO THE CAMERA?
No, a stuttering projects cannot be successfully exported to the camera. The copy on the tape will stutter too. If iMovie can't play the project smoothly, it can't export smoothly to the camera.
CAN I EXPORT STUTTERING PROJECTS TO QUICKTIME?
The stutter does NOT affect exporting to a QuickTime movie. That export succeeds, I suspect, because the export to QuickTime is not a time-sensitive operation, unlike the export to the camera. The QuickTime export can take as long as required.
The ability to export successfully to QuickTime lets us use some of the workarounds described below.
CAN I BURN A DVD OF A STUTTERING PROJECT?
Yes, you can successfully burn a DVD of a stuttering project. The iDVD encoding acts a lot like exporting to QuickTime, apparently. iDVD takes whatever time it needs to encode the project; it's not a time-sensitive operation.
CAN STUTTERING PROJECTS BE REPAIRED?
Projects cannot be repaired so they will never stutter. The only permanent fix is for Apple to fix QuickTime, if that's where the problem lies.
Workarounds can remove the stutter, however, allowing you edit the project successfully.
IS THERE AN EASY WORKAROUND?
The workaround ADJUST PLAYBACK SETTINGS below is pretty easy to implement. Trouble is, it doesn't always work, and the improvement may only be temporary. But the workaround is easy to do, so it's worth a try.
This workaround adjusts your iMovie playback settings and Mac monitor settings to make it easier to play the movie. That reduces the playback burden on your Mac and may raise the number of audio clips the project can contain before stutter begins.
This workaround may also make it easier to edit your project, making it easier to apply OTHER workarounds. So it's a good first step before implementing (better) workarounds.
ARE THERE OTHER WORKAROUNDS?
Yes, there are several effective workarounds. They all eliminate the stutter in the iMovie project.
The workaround to choose depends on the amount of editing yet to do, and whether or not you want to preserve all the clips in the project as discrete clips. And, of course, the workaround must be convenient for you. Some require more "expertise" than others.
It goes without saying that the simplest workaround is to try to limit the number of audio clips you include in the Timeline. That's not an ideal solution for all projects, obviously, but when you can do it, that's the best solution.
The other workarounds are posted below, one workaround in each message. As you try them, please reply to that message when suggesting changes and reporting errors. Future readers should check for updated versions of workarounds posted later in this topic.
GENERAL OBSERVATIONS
• The type of stutter we see varies quite a lot. One project I tested, the large project authored by Marilyn, showed considerable VIDEO stutter when played on her iMac. The audio played fine. There was so much stutter the project could not be edited. When I moved Marilyn's project to my PowerBook 1.5 GHz G4, worse video stuttering occurred.
But when played on my desktop Mac (Dual 1 GHz PPC G4) the VIDEO played just fine but the AUDIO stuttered. The audio slowed down, as if playing in slow motion. A clip's audio could sometimes be heard well after the playhead moved beyond the end of the audio clip.
• It's easy to create a stuttering project. Here's how: Create a new project, record a few seconds of audio voiceover, Copy the voiceover clip and Paste it at the end of the project 9 times. (To move the playhead to the end before you Paste each time, press the End key on your keyboard.) Then select all 10 audio clips, Copy them, and Paste at the end of the project 9 times. The project now has 100 audio clips. Try playing the project. If it fails to stutter, Paste again at the end of the project. Repeat Pasting at the end until the stutter begins. Stutter will get progressively worse the more clips you Paste.
• I saw some strange window behavior when editing badly stuttering projects. The iMovie window began behaving strangely. Activating windows in OTHER applications windows stopped working normally. Clicking in the window of another application would activate the window — allow me type in it, for example — but the iMovie window refused to allow it to come forward. The iMovie window remained on top, although its three red, orange and green window titlebar buttons were disabled. It was as if the iMovie window had changed into a palette that refused to go to the background. Re-launching iMovie resolved the problem.
WHAT ABOUT iTUNES MUSIC?
There are reliable reports that (some types of?) iTunes music may affect stutter. Unfortunately, I wasn't able to reproduce that problem, no matter what music I tried. I can't suggest what type music to use — or avoid.
There is a workaround that has worked successfully for some. That is to use iTunes to burn an Audio CD of the songs you want to use, then import the songs directly to iMovie from the Audio CD. See the workaround "BURN SONGS TO AUDIO CD" below.
THIS IS GETTING LONG
There's more I could say, but it won't add much value. We'll learn more as we gain more experience, I'm sure.
Good luck with your projects. Let's hope the underlying problem, whatever it is, is fixed soon. In the meantime, share your insights and discoveries.
Karl

Answer 1

WORKAROUND 3: Divide and Conquer.
BEST SUITED FOR
This method is best suited for projects where you still have quite a lot of editing to do, and the project is stuttering badly.
This is my favorite workaround, for it allows you to continue to edit the clips normally as discrete clips, but in a stutter-free environment.
SUMMARY
To reduce the number of audio clips in the project, this workaround divides your project into two smaller projects. Each project — each Part — contains a smaller number of audio clips so it plays smoothly. You can edit the clips normally, for the project plays smoothly.
Later, when you have finished editing the Parts, you re-assemble the Parts back into a single project. The method of reassembly we use will not cause stutter in that project.
As you add more audio clips to each Part, you may see a Part begin to stutter again, just like the original. You've hit your audio clip limit again. It's not a big deal. You simply divide that Part in half again to reduce the number of audio clips, and the new, even smaller Parts will play smoothly.
Usually it's necessary to divide a stuttering project just one time, but more may be needed.
ADVANTAGES
Each Part of the divided project contains half the clips of the original project. The clips are preserved as discrete clips, which allows you to edit clips normally. (Other methods join the video clips, audio clips, titles, and transitions into a single clip so they are no longer editable as discrete clips.)
DISADVANTAGES:
1) Because the project has been divided into two (or more) Parts, it's not possible to play your project from start to finish.That's not a big problem, but it's not what I would prefer. (If playing everything is important to you, see the MODIFIED DIVIDE AND CONQUER paragraph below.)
Of course, you'll be able to play all the Parts together when you re-join the Parts later.
2) This workaround can require significant amounts of disk space, especially if the clips imported from the camera are long-running clips. (An hour-long lecture, for example, with no scene breaks.)
If the clips are small, dividing a 10GB project in half will use an additional 10GB disk space (the total for the two Parts). As you split the Parts, it will temporarily use more.
If the clips in the original project were long-running clips, each Part may require the same disk space as the original project. If the original project was 10GB, each Part may use 10GB. This is because of iMovie's non-destructive editing feature, which copies the long Media file for the long clip in the original project to each Part.
Later, when joining the two Parts into one project, allow 13GB for each hour of total run time.
3) If the project contains many clips, it may be difficult to divide the project's video and audio tracks accurately. iMovie bugs may make it difficult to correctly select and delete clips. Care must be taken when selecting and removing clips. More details below.
TO DIVIDE THE PROJECT, FOLLOW THESE STEPS
• Open the stuttering project.
• If you haven't done it yet, adjust your Mac and iMovie settings as described in WORKAROUND 1: Adjust Playback Settings. This may make it easier for you to divide the stuttering project. (Return your settings to normal after dividing the project.)
• In iMovie preferences > General, turn ON the checkboxes "Snap to items in Timeline" and "Play sound effects when snapping." We will create bookmarks in a moment, and this makes them easier to use.
• From the iMovie menubar, choose View > Switch To Timeline Viewer. (Ignore if the Timeline already uses that view.)
• Locate a good place to divide the project in half. In a moment, you will move the playhead to that place and create a bookmark. Later, you will divide the project there.
(If the project already has many bookmarks, consider removing all the bookmarks to make it easier to find your new bookmark. Use the iMovie menu command Markers > Delete All Bookmarks.)
• If your audio clips are heavily grouped in one area of the project, split the project in the middle of that area, not the middle of the project. You goal is to divide the number of audio clips roughly in half.
• Look for a location like this, which is ideal:
- At the start (or end) of a video clip.
- Where that end of the video clip has no audio clip below it.
If you can't find a clip end that has no audio clip below it, move the playhead to the start/end of a video clip, select the audio clip below it, and split that audio clip.
If the video clip is so long you must split it, split the video clip where there is no audio clip directly below it.
• Place the playhead where you want to divide the project, precisely at the start (or end) of a video clip. Create your bookmark.
• Write down the location (the playhead position) of the bookmark you created. If you've moved the playhead away from the bookmark, first re-locate the playhead exactly over the bookmark. Use the menu command Markers > Next Bookmark (or Previous Bookmark) to go there.
The playhead position is displayed just under iMovie's video Monitor. The time shown will be something like "33:52:10". Write it down.
• Save the project. Quit iMovie.
• In the Finder, click on the project and choose File > Duplicate. The Finder will require several minutes to duplicate the project. The Finder will name it "<Project Name> copy.iMovieProject", where <Project Name> is the original name of your project.
• When the Finder is done duplicating it, rename the duplicate project "Part 1 <Project Name>.iMovieProject". (We'll refer to this part now as "Part 1".)
• Open Part 1 in iMovie.
• In a moment, you will remove all the video and audio clips AFTER the bookmark, leaving the video clips and audio clips BEFORE the bookmark intact. They will comprise Part 1. iMovie sometimes makes it hard to select the clips you want to remove, so go slowly and carefully here.
• Read this paragraph and the three after it before you continue. Starting at the end of the project and moving towards the bookmark, select groups of audio clips in the bottom Timeline track and delete them. (Select the clips, then hit the Delete key.) After removing a group of audio clips from the bottom track, remove (any) audio clips above them in the middle track, then remove the video clips above those. Removing the video clips makes the timeline shorter, easier to work with.
• As you approach the bookmark, be careful. When you select an audio clip that's located close to the bookmark, iMovie sometimes (inappropriately) selects an audio clip or two BEFORE the bookmark, even though you don't drag over it. (I suspect those are audio clips where Direct Trimming has been used, and iMovie gets a bit confused.) It's important you not remove any selected audio clip located before the bookmark. You must unselect those before hitting the Delete key. To do that, hold down the Command key — the Apple key — and click on any selected clip located before the bookmark. Make sure only the audio clips you want to delete are selected, THEN hit the Delete key.
• If you mess up, don't worry. Use iMovie's Undo command. Or to undo all your changes, choose File > Revert to Saved, which reverts Part 1 back to when you opened it.
• Sometimes iMovie fails to delete all the video clips you've selected. (Another bug.) As you approach the bookmark, zoom in on the Timeline to view the bookmark area more closely. After pressing the Delete key to delete the video clips you've selected just beyond the bookmark, make sure all the video and audio clips after the bookmark were removed. If not, press the Delete key again.
• How go back and do what you just read.
• When all the audio and video clips after the bookmark have been deleted, save Part 1.
• Play Part 1. If it doesn't play smoothly, it still contains too many audio clips. That's unusual, but it happens. Later, you will have to divide it again, but don't worry about that now. You can divide it again after finishing Part 2.
• To discard the clips no longer needed in Part 1, empty the iMovie trash. Emptying the trash will save the project again, automatically. (It may be necessary to also empty the Finder trash to recover the disk space.)
• We now move on to Part 2. It's a bit different, so don't skip reading these instructions.
• Quit iMovie, closing Part 1.
• Duplicate the original project again. Name this copy "Part 2 <Project Name>.iMovieProject".
• Open Part 2 in iMovie.
• IMPORTANT: For Part 2, it's crucial that you lock the audio at the second half of the project before removing video clips in the first half, which you are about to do. Locking prevents the audio from sliding over when you remove the video clips in the front half, losing sync with the video.
To lock the clips , move the playhead to the start of the first audio clip you'll keep, then select all the audio clips and choose Advanced > Lock Audio Clip at Playhead.
• Starting at the bookmark and moving back towards the start of the project, select audio clips in the BOTTOM audio track and delete them.
• Repeat for the audio clips in the TOP audio track (the middle timeline track.)
• You are about to remove the video clips before the bookmark. There's a couple of things you need to know first.
1) Removing the video clips will dislodge the bookmark. Ignore its new position. The bookmark will no longer be located next to the dividing clip. That's okay. You won't be using the bookmark again.
2) iMovie may not actually remove all the selected video clips when you hit the Delete key. A few video clips may remain, still selected, at the (new) beginning of the project. Do not click the mouse for clicking it will unselect those clips. Before clicking the mouse, use the zoom slider below the timeline to zoom in on the (new) start of the project. If you see selected clips there, hit the Delete key AGAIN to remove them.
• If you don't understand what you just read, read it again. Then select all the video clips before the bookmark and delete them. Zoom in to check for any selected clips that were not deleted, and press the Delete key again, if necessary.
• Save Part 2 and empty the iMovie trash.
• If everything worked as it should, the Part 2 audio and video should still be in sync and the project should play smoothly. If it doesn't play smoothly, Part 2 still contains too many audio clips. You'll have to divide Part 2 again later.
• If either Part 1 or Part 2 needs to be divided again, use the same technique we just used, but instead of duplicating and dividing the original project, duplicate and divide Part 1 or Part 2. First read WHAT IF THE PARTS BEGIN STUTTERING LATER? just below for a naming suggestion.
WHAT IF A PART BEGINS TO STUTTER LATER?
As you add add more new audio clips to Part 1 or Part 2, these projects may eventually start to stutter, just like the original project did. That's okay, for now you know how to eliminate stutter by dividing the project. When dividing Part 1 or Part 2 use new names like "Part 1A" and "Part 1B", or the names that work for you.
HOW TO RE-JOIN THE COMPLETED PARTS
When you are done editing all the Parts, you will likely want to join the Parts back together in a single project that you send to iDVD, export to QuickTime, or export to the camera. (See below when exporting to the camera.)
Don't worry, the method we use to join the Parts won't increase the number of audio clips, so the joined project won't stutter.
FOLLOW THESE STEPS TO RE-JOIN THE PARTS
1. Create a new iMovie project. Name it "Joined <Project Name>.iMovieProject" or something. Save the project.
2. One by one, locate the movie "Timeline Movie.mov" that's inside each of your Part projects and drag that movie into the Joined project timeline. (Dropping the movie on the Timeline imports Part 1 to the Joined project.) First import Part 1, then Part 2, etcetera.
To locate the Timeline Movie inside the Part project you must first open the project package to see what's inside. In the Finder, Control-click on the project icon and choose "Show Package Contents" from the popUp menu. A Finder window will open, showing the contents of the project package.
Double-click on the Cache folder. Inside you'll find the QuickTime movie "Timeline Movie.mov". (It's a special kind of QuickTime movie, called a reference movie, that has no audio or video of its own, just pointers to files in the project's Media folder.)
Drag the Timeline Movie.mov into the Joined project timeline. That will import the audio and video of that Part as a single clip to the Joined movie. Because all the audio clips are "flattened" as part of one video clip, the Joined project has no audio clips, only one video clip for each Part. So the Joined project won't stutter.
When all the Parts have been imported, add your Chapter Markers for iDVD to the project, if desired, and send the project to iDVD.
EXPORTING TO THE CAMERA
When exporting the divided project to the camera you may prefer to export the individual Parts projects instead of the Joined project. The reason is that the Parts projects contain discrete clips, just like the original project, while the joined project contains only a few clips, one for each Part.
When exporting a project to the camera iMovie preserves the clips as clips. Later, if you re-import that tape back to a new iMovie project, iMovie returns the exported clips as clips.
If you want your tape to contain all the clips of all the Parts, export the Parts to the camera, not the Joined project. The clips will return as discrete clips when the tape is re-imported. Clips like titles and transitions will no longer be editable, but they will return as discrete clips.
(Some clips in the Parts don't return as discrete clips. Any adjacent clips iMovie has edited are merged into one clip when exported to the camera. So two adjoining title clips will return as a single clip.)
MODIFIED DIVIDE AND CONQUER
If you want to able to play Part 1 as you edit Part 2 — instead of playing one Part at a time — you can modify the Divide and Conquer method to include Part 1 in your Part 2 project. This requires more disk space, but that may not matter.
When you done editing Part 1 and are ready to start editing Part 2, drag the Timeline Movie.mov from Part 1 to the beginning of the timeline in Part 2 (after locking the audio clips in Part 2). That delivers all of Part 1 as a single clip in Part 2. That lets you play Part 1 as you edit Part 2. (When joining the Parts, skip importing Part 1 to the Joined project. It will arrive with Part 2.)
FINALLY
This is the most complicated workaround, but for some projects, arguably the best. If I've missed something, please say so we can avoid problems.
Karl