Where Does Field Dominance Originate? - Lurker's Guide - lurkertech.com
lurkertech.com Lurker's Guide Where Does Field Dominance Originate?

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Note: Updated version available

You are reading the old 1990s SGI version of this document. A more modern, updated version is available here.

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,

  • Video material is born when video fields come out of a video camera, a video signal generator, or a field-based computer graphics algorithm. At this point, there is no need to impose any frame boundaries on the material: it has no field dominance.

    ...F2F1F2F1F2F1F2F1F2...

  • Say at some point you cut out part of this material and edit or record it onto a video tape that has different, pre-existing material. Say that pre-existing material has no field dominance (for example, say it's all camera black). Assuming your VTR works in frames as almost all VTRs do, you have to make a choice: do you cut the new material into the pre-existing material on the tape at an F1 field or an F2 field?

    pre-existing tape material...F2F1F2F1F2F1F2F1F2...
    new material: F1 dominant editF1F2F1F2F1F2
    new material: F2 dominant editF2F1F2F1F2F1

    Once you make that choice, then the material on the tape acquires a field dominance. All subsequent edits to that material need to begin on the same field type.

    To see why this is so, assume we have edited together material A, B, and C following F1 dominance:

    F1F2F1F2F1F2F1F2F1F2F1F2F1F2
    ABC

    Now say we try and edit in material D to replace material B following F2 dominance:

    F1F2F1F2F1F2F1F2F1F2F1F2F1F2
    beforeABC
    material DD
    afterABDC

    We have created an unpleasant edit where one field of material B still pops up at the edit point. Field dominance is the protocol which video engineers invented to prevent this problem.

  • Say you switch between two pieces of material that have no field dominance using a video switcher. The switcher has to decide on a field boundary to make its switch; it has assigned a field dominance to its output material.

  • Say at some point you bring material that does not have a field dominance into a computer in such a way that pairs of fields are grouped into a frame. For example, say you are using VL_CAPTURE_INTERLEAVED in the VL. You have to make a choice: do you set your VL device to F1 dominance so that it will group an F1 field followed by an F2 field into a frame:

    video signal...F2F1F2F1F2F1F2F1F2...
    VL frames...frameframeframeframe...

    Or do you set your VL device to F2 dominance so that it will group an F2 field followed by an F1 field into a frame:

    video signal...F2F1F2F1F2F1F2F1F2...
    VL frames...frameframeframeframe...

    Once you have made this choice, you have determined the boundaries on which the material may be edited. You have given the material a field dominance.

    Even VL_CAPTURE_NONINTERLEAVED's behavior is affected by the field dominance. See Hints for Vid-to-Mem Applications for more information about this.

  • Say you have video fields from somewhere other than the VL. You will still have to make a choice of field dominance if you want to store the fields into a Movie Library movie. This is because the Movie Library groups your fields into frames and its editing commands work on frame boundaries.

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):

  • each VL_CAPTURE_INTERLEAVED frame will consist of a FA field temporally followed by an FB field.
  • the UST stamp on each VL_CAPTURE_INTERLEAVED frame, and the USTs returned by vlGetUSTMSCPair(), will be that of an FA field.
  • the first field you get in VL_CAPTURE_NONINTERLEAVED mode, whether free-running or GPI triggered, will be an FA field.
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!

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CopyrightAll text and images copyright 1999-2017 Chris Pirazzi unless otherwise indicated.
Support This Site

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Copyright

All text and images copyright 1999-2017 Chris Pirazzi unless otherwise indicated.