If you use a frame rate that is too high when you export iMovie for use on YouTube, the picture will look muddied and pixelated. What’s too high? 30 frames per second.
When you ‘Share’ or export an iMovie, choose Expert Settings. You will get a quick message that the iMovie is compressing the video, but then a settings window is displayed.
YouTube preparation work well with the following settings:
Frame rate: 15 fps
Frame reordering: yes
Encoding mode: multipass
Dimensions: 320×240 (also using 640×480)
Sampling rate: 48.000 kHz
Channels: Stereo (L R)
Bit rate: 160 kbps
Perpare for Internet Streaming: Fast Start
Frame rates at 15 fps show a little jump, but these settings keep the file size smaller and prevent muddy pictures with pixellations. Keep in mind that scenes with lots of color and motion contain a lot more information in each frame and are more likely to be pixellated or muddy from frame to frame — sometimes even at 15 fps.