Video Signal Processing : Using deinterlacing filters

You should have an understanding about why you'd want to do some video signal processing. Even if you've done a great job producing and capturing your video, there are still fundamental differences between television and computer monitor displays that should be compensated for. To do this, you need to de-interlace your video and adjust your color for RGB monitors.

Using de-interlacing filters
Most editing platforms have de-interlacing filters built into them. As we saw in Figure 10.1, the problem is dealing with the artifacts that arise when two fields of interlaced video are combined to make a single frame of progressive video. Three methods are commonly used to deal with interlaced video:

Blending: This approach combines the two fields, but it's vulnerable to interlacing artifacts, as shown in Figure 10.1.

Interpolation: This approach attempts to shift parts of one field left or right to compensate for the artifacts. This is very computationally complex, because only parts of the field should be interpolated. For example, in Figure 10.1, we want to interpolate the parts of the frame that include the moving minivan, but not those that contain static elements such as the trees in the background.

Discarding: This approach discards one field and uses a single field twice in a single frame of progressive video. The resulting frame therefore has half the vertical resolution of the original frame, but without the interlacing artifacts.

Editing and encoding platforms distinguish themselves by how they deal with interlacing artifacts. De-interlacing video on two different platforms generally yields different quality results. Where you choose to do your de-interlacing depends on where you can get the best quality. If you're staying in the broadcast world for your editing phase, it makes more sense to de-interlace during the encoding phase. This is demonstrated for you in the next section.

However, we have to come clean about de-interlacing. For the most part, it isn't necessary for most podcasts. If you're encoding your podcasts for viewing on a video iPod (or other portable media device), chances are good that you're targeting a resolution of 320×240. At this resolution, most encoding software drops the second field by default! If you've got only 320 lines of resolution, it doesn't make sense to process the second field, so you don't have any interlacing artifacts to deal with. This is a very good reason to target 320×240 for your podcasts: The de-interlacing problem goes away.

If, however, you're targeting browser-based playback for your podcast and decide to use a resolution larger than 320×240 — such as 400×300, 480×360, or 640×480 — you need to de-interlace your video during the encoding phase. So, for you mavericks, the next section shows where to find the de-interlacing filter in a number of software applications.

Where to find de-interlacing filters
If you're hoping to de-interlace your video (assuming that your final video podcast resolution is larger than 320×240), you need to make sure your encoding application has de-interlacing filters. Most, but not all, do. If you're targeting the QuickTime format, use an encoding application such as Sorenson Squeeze, because QuickTime Pro doesn't include a de-interlacing filter.

Sorenson Squeeze includes a de-interlacing filter in the filter settings window, shown in Figure 1. Double-click any filter to open the filter settings window. The de-interlacing filter is on by default in the preset filters.

Figure 1: Sorenson Squeeze offers de-interlacing in the filter settings window.


If you're targeting the Windows Media Format, you can use the de-interlacing filter included in the Windows Media Encoder. The de-interlacing filter is on the Processing tab of the Session Properties window, shown in Figure 2.


Figure 2: The Windows Media Encoder offers a de-interlacing filter in the processing settings.

If your encoding application doesn't have a de-interlacing filter, chances are good that your editing platform will. Vegas includes the de-interlace setting in the Project Properties window, shown in Figure 3. Select Project Properties from the File menu or type Alt+Enter, and then select the deinterlacing method you want from the drop-down menu.


Figure 3: Vegas offers a de-interlacing filter in the project properties window.