Depending on how interlaced video is displayed, video artifacts can be less or more noticeable.
Interlaced source files sometimes appear with visible thicker lines in the Compressor 4.2.1 Preview window. Making this window 100%, 50% or 25% of source size can reduce the visibility of these video artifacts. If the Fit option or the Zoom to Fit option (Shift-Z) are selected, lines can be more noticeable.
The horizontal patterns visible when previewing interlaced video are similar to a moiré pattern, caused by the regular horizontal interlaced lines in the video. This effect is minimal when the video is viewed at 100% size. It's also less noticeable when when viewing the video at sizes that are exact powers of 2, like 25% or 50%
For other scaling sizes, the scaling algorithm used by the computer graphics system might exaggerate the horizontal interlace lines present in interlaced video. This can lead to obvious visual artifacts such as horizontal bands, sawtooth edges, steps, etc.
Video frame rates can also change the perception of these artifacts. The faster the video frame rate, the less time there is between each interlaced field. As a result, moving objects don't move as far between each interlaced field and the size of the sawtooth edges and other artifacts are reduced. Slower video frame rates produce more pronounced interlacing artifacts.
For example, since PAL video frame rates of 25 frames per second are slower than NTSC video frame rates of 29.97 frames per second, interlacing artifacts are more prominent in PAL interlaced video than in NTSC interlaced video.
Preview Display and Output
While the Compressor preview display might show interlacing artifacts to a greater or lesser degree depending on your scaling and frame rate, these artifacts don't effect the way Compressor processes your video.
If you maintain your video in interlaced format rather than applying a de-interlacing filter, the resulting output video can also be susceptible to the same artifacts when viewed on a progressive monitor. Depending on the scaling system used by your monitor or video player, you can see scaling artifacts similar to the ones you see in the Compressor Preview window when you resize the window.