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Tuesday, July 24, 2001

Urban Legends of Video

A trio of common myths and misconceptions that arise when working with video.

Back to the computer. When video has been digitized, most applications present it to you a frame at a time. But those frames actually contain two interlaced fields. To process these correctly, you ideally want to separate them back out into individual fields. And to do that correctly, you need to know which field – the one that begins on the first line, or the one that begins on the line after that – came first in time. This has nothing to do with how you edited the video, or the electrical signals that numbered the fields on tape; it has to do with how those two fields were stored in a single frame on your disk drive.

How do we refer to this temporal field order inside a frame? Some use the term odd or even field first, based on one field starting at the first line and filling in the odd-numbered lines, and the other field starting on the second – or even-numbered – line. But unfortunately, not everyone calls that first line “one” (an odd number): Some refer to it as “zero” (an even number). Thus, the same file could be referred to in one program as even field first, and in another program as odd field first – and both mean the same thing. A better term to use is upper (the first line) and lower (the line below the first line).

Here’s where the confusion comes in: Most professional NTSC video systems write frames that are lower field first, meaning the first field in time is the one that starts on the second line from the top of the frame. They also captured that video off of tape first field dominant – in other words, “field 1” on the tape starts on the second line in the file, which would appear to be a contradiction (until you realize the two terms have nothing to do with each other).

This language barrier between the field order computer people are interested in, and the field dominance hardware people are interested in, has botched the final delivery of a project more than once. Also consider that some computer systems are indeed upper field first, and that some non-computer editing systems can be switched to second field dominant, and you can see just how much hair can be pulled out of one’s head on deadline – there’s almost no hope you can have a meaningful conversation on the subject with a post house engineer who’s not familiar with creating video on personal computers.

The short answer is: If you’re on a computer, you care about field order, not field dominance. When many say field dominance, they really mean field order. If they actually knew what they were talking about when they told you the field dominance of their system (such as a post house engineer), they’re telling you information you don’t need to know once you’re in the computer environment – your focus is on the field order inside a frame. And if you run across a manual or dialog box for a piece of software that uses the term “field dominance”, set the manufacturer straight, before they cause some real damage.

(In the rare cases where you need to reverse the field dominance of a videotape, see the sidebar on the last page of this article.)

Editing • Motion Graphics • Post Production •

(Page 2 of 5 pages for this article  <  1 2 3 4 >  Last »)

Staggering Mistakes: Reversing Field Order

Chris Meyer | 03/03- 12:23 PM

PMW-EX1 - use regen timecode

Adam Wilt | 02/21- 09:32 PM

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