Interlacing Overview

Briefly, for higher resolution (640x480 and up) video, the scan lines from two low-res fields are merged into one (taller) frame of video. The normal NTSC 60 fields per second are "interlaced" into one 30 fps stream. If this is news to you, there's a nice overview (with pictures!) at the Puffin Designs, Inc web page.

Somtimes, when the Buz codec goes to read or produce interlaced video, it seems to sometimes get the odd/even field order wrong. The result is that pairs of horizontal scanlines are reversed. (This also means the two fields are in the wrong chronological order.) This can appear as "jitter", excessively blurry horizontal edges, flickering, or or other annoying artifacts. It's not subtle.

"When does it happen?"

First, it only happens at vertical resolutions greater than 240 -- those that use both interlaced fields. When Buz captures or plays video with 240 lines or less, there is no field order problem.

My own experience is that VidCap32 always shows jitter when I record at high resolutions and play it right back. VideoWave does things differently. If you record and play back, it's fine. The problem then appears after rendering a produced video. This table shows my experience with Buz and the interlacing problem. If your experience has been different, do tell.

Capture	Render	Playback	Result
VidCap32.exe		VideoWave	OK
VidCap32.exe		ActiveMovie	Bad
VideoWave		VideoWave	OK
VideoWave		ActiveMovie	Bad
VideoWave	VideoWave	VideoWave	Bad
VideoWave	VideoWave	ActiveMovie	VERY Bad

Basically, you take one hit for rendering with VideoWave, and one more for playback with ActiveMovie.

"I don't see it.. how do I make it happen?"

The easiest way to see it is simply to use VidCap32 to capture a video segment at 720x480, and play it back using the standard Windows ActiveMove or "mplayer.exe". The analog output and on-screen display will have visible interlacing problems. Or it can be done with just the included Iomega tools. Using the Buz MGI VideoWave, capture a segment of video at 720x480. Use as little compression (highest bandwidth) as possible, so we don't confuse compression artifacts with the interlacing problem.

Switch to VideoWave edit mode, and you should end up with a fine snippit of video in the preview window. Hook up an NTSC monitor to the Buz video out, and play the clip. It looks great! If your video card supports it, the video preview window on your computer screen should also show good video. This video does not have the "interlacing problem".

Drag that one video clip to the production film strip, and "Produce" the video. Be sure to use an output setting of "AVI Compressed", MJPEG format, at 720x480 resolution and 100k/frame. (MGI, in it's infinite wisdom, will render the whole video frame-by-frame, so be patient.) Note that the default render output is always 352x240, so be sure to change it.

Play the produced video and look at it closely. Wow! Doesn't look as good as the original, does it? See how horizontal edges seem to have "extra" lines, or maybe just jump around a lot? There it is.

If you use Premiere, it only re-renders that part of the video that has been edited. If you use effects, text overlay, or anything that causes Premiere to render video, you will see the same jitter effect on that segment.

But wait, it gets worse. Try playing the VideoWave produced video using any of the common Windows AVI playback tools, like mplayer. When using the default Windows ActiveMovie AVI playback system, the interlacing problem is doubled and the video is really bad. It seems the fields get jumbled an additional time by ActiveMovie.

Here, let me show you.

Here are some examples of the jitter problem. This is a single frame of captured video, from a Tektronix Pathfinder NTSC (composite) pattern generator. It's about as clean a video signal as the Buz could ask for.

They're still frames, but show the effect in perfect detail. I want all my video to look like the "Good" example. Let's hope Iomega hurries up and publishes the fix for this. In fact, why not call them today to request it?