“I know this question is outside the realm of audio, but I bought my Mac and Final Cut Pro software from Sweetwater and have a question about it. I have been generating freeze frames from moving video as part of a project. They look fine on my computer and on the LCD screen of my camera. However, when I play the project on a TV some of those still images flicker or shake. Any idea what is causing this?”
Yes. But first I’ll address your “outside the realm of audio” comment. Audio and video are increasingly being used together in creative ways. Us audio guys have lamented for years the fact that people in video don’t pay enough attention to audio. Now the tables are turning somewhat as many of us old audio guys are realizing that we need to be competent in video in order to get certain kinds of audio work, particularly DVD, multimedia, and Internet oriented stuff. Further, as more and more audio and video people work together, it is incumbent upon both to learn about the important concerns of the other so we can better communicate and work together. In light of that we are happy to help answer basic video questions.
The problem you are experiencing is the result of video interlacing in your still frame(s). Normal NTSC video uses two fields for every frame of video. These alternately paint the odd and even lines of video on your TV screen. Many cameras also record the video this way (though it can be turned off on some). They capture one, and then rescan the image to capture the other. Of course all of this happens for each frame. If your image is moving on the screen it is quite possible the information between the two video fields is different due to the slight time difference between when they are captured or scanned. When you later freeze one frame of this video to make a still frame you still have those two fields present, and if the information between them is different enough (which depends on your shutter speed and how fast the image was moving) the image will appear to shake or vibrate as the two slightly different images are painted on your TV screen. The problem doesn’t normally show up on computer monitors because they have a different method (as well as scan frequency) of delivering the image to the screen.
The solution to this problem is to de-interlace your still picture frames. The process involves deciding whether you want to display the odd or even field. Then it simply removes the other and copies the one you chose so it gets painted as both the odd and even. This function is available under the Video Effects menu in your Final Cut software. It’s also a plug-in for Photoshop, in case you happen to be working with the images there.