Interlacing information

Introduction

Interlaced video is a technique of doubling the perceived frame rate of a video signal without consuming extra bandwidth. Since the interlaced signal contains the two fields of a video frame shot at two different times, it enhances motion perception to the viewer and reduces flicker by taking advantage of the persistence of vision effect. This results in an effective doubling of time resolution as compared with non-interlaced footage. However, interlaced signals require a display that is natively capable of showing the individual fields in a sequential order, and only traditional Cathode Ray Tube (CRT) based TV sets are capable of displaying interlaced signals. Interlacing only looks right on an old tube TV (CRT), if you have an LCD or a plasma, interlaced content looks horrible.

Interlacing is the use of two half-frames (fields) instead of one whole frame. The fields are shown: line 1,3,5... from the first field and line 2,4,6... from the second.

Most TV’s will show the picture this way, first field 1 then field 2. Computer monitors on the other hand will show both fields at once (screenshot), notice the strange horizontal lines? This is 'combing', or 'jaggies', anyone who's done a few video conversions will sooner or later come across these type of sources, especially when dealing with sources originally meant for TV broadcast.

These artefacts can be fixed through a process called de-interlacing. Only deinterlace when your source is truly interlaced, if you select the deinterlace method on a progressive source there will be degradation of fine detail within the video picture.

A good rule of thumb is anything made for TV broadcast will most probably be interlaced, it's quite easy to spot, play the movie on your PC, look for the combing, you’ll notice them when objects or people move. Most movies will be OK but remember deinterlacing, like all post processing, will add time to your encode.

Deinterlace or Decomb

Deinterlacing is a blunt weapon; it's applied to every frame without fail. video.NET does have another option, 'decomb'. This option only deinterlaces frames that are visibly interlaced. This saves time, and makes it safe. It won't destroy the detail in progressive sources the way deinterlace will.