Axelrod Broadcast Solutions, LLC

Axelrod Broadcast Solutions, LLC

Framingham, MA, USA

SBE Certified technical consulting, solving audio, video, radio frequency,  and IT problems in cable, broadcast, commercial, and residential systems.

Deinterlacing archival video

December 02, 2013  /  Glenn Axelrod

I've been surprised by the poor quality of some digitized archival footage appearing in recent TV documentaries. You can see the problem where there are horizontal fringe effects in rapidly moving objects. Those artifacts are not in the original material and they are easily prevented. You need to deinterlace old TV recordings when converting them to a compressed MPEG format.

If you are using Handbrake on a Mac computer, the default setting seems to be no deinterlacing (at least that's what I found in my setup), but Handbrake actually contains some powerful tools. See the explanation at https://trac.handbrake.fr/wiki/DeinterlacingGuide

To make sure you have it working, select Window:Picture Settings, then Filters. Choose Decomb (Handbrake's improvement over generic Deinterlace) and select Decomb: Default.

That allowed me to make this:                                           into this:

s_interlacing3.png

Explanation: The images are closeups from two conversions of video showing a logo that was being both panned and zoomed, so there was a lot of motion in the recording. With no motion, the two images would be the same. Interlaced scanning in the North American system means the camera captures alternate lines of a frame 1/60 second (16ms) apart. That can be a long time when things are moving fast. Stacking both fields on top of each other to merge the alternating lines, instead of showing them in proper time sequence, looks bad. That's what the left side shows.

When a camera scans progressively, there are no "alternate" lines; every line in a frame is captured less than 1/40000 second (25 us) after its neighbor. This is why sports networks favor the higher temporal resolution of the 720p system over the higher spacial resolution of 1080i for high def. But older archival video is usually recorded in the analog version of 480i, and it must be deinterlaced for a decent digital conversion. Deinterlacing (or Handbrake's "decombing") uses knowledge of the recording process to reassemble the sequence of frames with moving elements in their proper places, as seen on the right.

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