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Note: Updated version availableYou are reading the old 1990s SGI version of this document. A more modern, updated version is available here.
Field dominance is defined in Definitions: F1/F2, Interleave, Field Dominance, and More. Undoubtedly after reading that definition you have some questions.
Put another way,
... | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | ... |
pre-existing tape material | ... | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | ... |
---|---|---|---|---|---|---|---|---|---|---|---|
new material: F1 dominant edit | F1 | F2 | F1 | F2 | F1 | F2 | |||||
new material: F2 dominant edit | F2 | F1 | F2 | F1 | F2 | F1 |
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:
F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 |
A | B | C |
Now say we try and edit in material D to replace material B following F2 dominance:
F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | |
before | A | B | C | |||||||||||
material D | D | |||||||||||||
after | A | B | D | C |
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.
video signal | ... | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | ... |
---|---|---|---|---|---|---|---|---|---|---|---|
VL frames | ... | frame | frame | frame | frame | ... |
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 | ... | F2 | F1 | F2 | F1 | F2 | F1 | F2 | F1 | F2 | ... |
---|---|---|---|---|---|---|---|---|---|---|---|
VL frames | ... | frame | frame | frame | frame | ... |
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.
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):
Equipment which does not perform edits, such as a video monitor, a waveform monitor, or a vectorscope, does not care about field dominance.
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.
Support This Site | I work on this site in my off hours. Please help me to push aside my day job and work on it more by supporting the site in one of these ways: |
Use your credit card or PayPal to donate in support of the site. | |
Use this link to Amazon—you pay the same, I get 4%. | |
Learn Thai with my Talking Thai-English-Thai Dictionary app: iOS, Android, Windows. | |
Experience Thailand richly with my Talking Thai-English-Thai Phrasebook app. | |
Visit China easily with my Talking Chinese-English-Chinese Phrasebook app. | |
I co-authored this bilingual cultural guide to Thai-Western romantic relationships. | |
Copyright | All text and images copyright 1999-2023 Chris Pirazzi unless otherwise indicated. |