Where Does Field Dominance Originate?

By Chris Pirazzi. Information provided by folks throughout the company. Special thanks to Bruce Busby, Scott Pritchett, and Paul Spencer for review and historical perspective.

Field dominance is defined in Definitions: F1/F2, Interleave, Field Dominance, and More. Undoubtedly after reading that definition you have some questions.

What Has Field Dominance and When?

The cause-and-effect of field dominance is often confusing. Think of field dominance as a property that video material can acquire when it is edited with other material or grouped into frames in a computer. Once some video material has acquired a particular dominance, it must be manipulated with that dominance from then on.

Put another way,

Who Has to Worry About Field Dominance?

Once material has a field dominance, all subsequent devices which edit that material must use the same field dominance, so that their edits never produce half-frame edits as shown with material D above. This means that you need a way to set the dominance of VTRs, video switchers, VL devices, and Movie Library movies.

Modern VTRs and switchers offer this option.

Some SGI VL devices assume F1 dominance. The other SGI VL devices have a device-specific control to set their dominance. These controls include VL_SIR_FIELD_DOMINANCE (sirius), VL_EV1_DOMINANCE_FIELD (ev1), VL_MGV_DOMINANCE_FIELD (ev3), VL_MGC_DOMINANCE_FIELD (cosmo2), and VL_MVP_DOMINANT_FIELD (mvp). SGI created the new device-independent VL_FIELD_DOMINANCE control so that all devices could use the same control, but sadly only divo supports it in IRIX 6.5. Whether its dominance is fixed or settable, if a VL device is set to FA dominance (where A is 1 or 2 and B is the opposite):

You set the dominance of a Movie Library movie implicitly when you choose which fields to group into frames and when you set DM_IMAGE_INTERLACING while creating the movie's image track.

Equipment which does not perform edits, such as a video monitor, a waveform monitor, or a vectorscope, does not care about field dominance.

Why Is Field Dominance Selectable?

You might ask: "Why didn't the industry just choose and use some dominance (F1 or F2, doesn't matter) so that we never have to worry about it?" Well, welcome to video! Presented with an arbitrary choice, video engineers are incapable of making the same decision. Some engineers decided that F1 should be the dominant field because the number 1 comes before the number 2. As we saw in Definitions: F1/F2, Interleave, Field Dominance, and More, LTC and VITC timecodes are defined so that a new hh:mm:ss:ff occurs at an F1, so this meshes. Some engineers decided that the dominant field should be the field that includes the top line of a picture. For analog and practical digital 525 this is F2, for official digital 525 this is F1, for 625 this is F1.

Who was the first to make a dominance decision? The Ampex VR1000B 2-inch quad video deck from 1962, on which people did edits by "developing" the tape's magnetic control track into visible marks using a chemical and then splicing the tape at those marks with a razor blade, placed its control track marks every thirtieth of a second at---you guessed it---the beginnings of F2 fields. This greatly predates timecode formats like LTC and VITC. So in some sense F2 dominance is right because it was first.

You might also ask: "Why doesn't the industry just choose and use something now?" The original culprit decks from the sixties created a legacy, in the form of reels and reels of archival material, that was passed on to each new generation of VTR technology as studios transitioned to "the next" equipment. This legacy is still alive; all decks sold today have switchable field dominance, and studios still have material from "the last" equipment with edits on a certain field boundary.

Another, even more grotesque idiosyncrasy of analog video tends to dwarf the field dominance issue anyway: color framing. Edits on older VTRs which did not fall on a 2-frame (NTSC) or 4-frame (PAL) boundary relative to the analog signal's color subcarrier would generate unattractive pops and instabilities in the image at the edit point. Therefore, people were too busy worrying about which 2- or 4- frame boundary they had to edit on to worry about which field they had to edit on.

Modern component digital decks have a small chance of breaking the cycle: they have the ability to edit on arbitrary field boundaries, and they have no color framing idiosyncrasies. Studios might actually start editing on field boundaries, and the dominance issue will finally be dead. Editors will still have to maintain the alternation between field types though, since the fields are spatially distinct.

Field Dominance and 3:2 Pulldown

The issue of field dominance is further confused by 3:2 pulldown, a method of transferring 24 frame per second film images to the "60" field per second rate of 525-line video. 3:2 pulldown is described in Fields: Why Video Is Crucially Different from Graphics. Regardless of one's choice of field dominance, video generated through 3:2 pulldown can have video frames whose fields are from different film frames, possibly even different scenes of a film. So in a sense, the material has neither dominance, but instead has a repeating 10 field pattern. If 3:2 pulldown generated material is then edited without consideration of the 3:2 sequence, you get a sequence where scene changes occur at completely unpredictable field boundaries. This was a major issue for constant-angular-velocity consumer videodisks mastered from 24 frame per second footage, where customers wanted to achieve rock-solid F1/F2 still frames nomatter where they paused. CAV videodisks actually include a "white flag" marker in the vertical interval on all F1/F2 pairs that may contain scene changes, as a hint to the videodisk player not to pause there!