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.
- 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||...||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:
Now say we try and edit in material D to replace material B following F2
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:
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:
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
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):
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.
- 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.
Equipment which does not perform edits, such as a video monitor, a
waveform monitor, or a vectorscope, does not care about field
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
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
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!