Fields, F1/F2, Interleave, Field Dominance And More

By Chris Pirazzi. Information provided by folks throughout the company.

Definition of F1 and F2

Interlaced video signals have a natural two-field periodicity. F1 and F2 are the names we give to each field in the sequence. If you look at the waveform of a video field on an oscilliscope, you can tell whether it is an F1 field or an F2 field by the shape of its sync pulses.

ANSI/SMPTE 170M-1994 defines Field 1, Field 2, Field 3, and Field 4 for NTSC (figure 7).

ANSI/SMPTE 125M-1992 defines the 525-line version of the bit-parallel digital CCIR-601 signal, using an NTSC waveform for reference. ANSI/SMPTE 259M-1993 defines the 525-line version of the bit-serial digital CCIR-601 signal in terms of the bit-parallel signal. 125M defines Field 1 and Field 2 for the digital signal.

CCIR Rep. 624-1-1978 defines Field 1 and Field 2 (figure 2) for 625-line PAL.

CCIR Recommendation 656 Describes a 625-line version of the bit-serial and bit-parallel CCIR-601 digital video signal. It defines Field 1 and Field 2 for that signal (table I).

We define F1 as an instance of Field 1 or Field 3.

We define F2 as an instance of Field 2 or Field 4.

Field Interlacing and Field Interleave

Generally the term 'interlacing' refers to a technique for signal encoding or display, and 'interleave' refers to a method of laying out the lines of video data in memory.

The term 'interleave' must be qualified when used in this way. It can also refer to how the samples of an image's different color basis vectors are arranged in memory, or how audio and video are arranged together in memory.

Field Interlacing

The way in which two fields should be interlaced together to produce a picture depends on:

which field is an F1 field and which field is an F2 field
whether the fields are from a 525- or 625-line signal.

It does NOT depend on:

what "order" the F1 and F2 fields are in
anything relating to field dominance

For 525-line analog signals, the picture should be produced in this manner: (F1 has 243 active lines, F2 has 243 active lines==486 active lines)

 field 1                          field 2  picture 0-based  picture 1-based
 *******                          *******  ***************  ***************
          (second half only)-----| l.283   0                1
  l.21 |-----------------------  |         1                2
       |  -----------------------|         2                3
       |-----------------------  |-- F2    3                4
  F1 --|  -----------------------|         4                5
       |-----------------------  |         ...              ...
       |  -----------------------|         ...              ...
       |-----------------------  |         483              484
       |  -----------------------| l.525   484              485
 l.263 |------(first half only)            485              486

For official 525-line digital signals, the picture should be produced in this manner: (F1 has 244 active lines, F2 has 243 active lines==487 active lines)

 
 field 1                          field 2  picture 0-based  picture 1-based
 *******                          *******  ***************  ***************
  l.20 |-----------------------            0                1
       |  -----------------------| l.283   1                2
  l.21 |-----------------------  |         2                3
       |  -----------------------|         3                4
       |-----------------------  |-- F2    4                5
  F1 --|  -----------------------|         5                6
       |-----------------------  |         ...              ...
       |  -----------------------|         ...              ...
       |-----------------------  |         483              486
       |  -----------------------| l.525   484              486
 l.263 |-----------------------            486              487

For practical 525-line digital signals, all current SGI VL hardware skips line 20 of the signal, and pretends that the signal has 486 active lines. As a result, we can think of the signal as having exactly the same interlacing charateristics and line numbers as the analog signal: (F1 has 243 active lines, F2 has 243 active lines==486 active lines)

 
 field 1                          field 2  picture 0-based  picture 1-based
 *******                          *******  ***************  ***************
          -----------------------| l.283   0                1
  l.21 |-----------------------  |         1                2
       |  -----------------------|         2                3
       |-----------------------  |-- F2    3                4
  F1 --|  -----------------------|         4                5
       |-----------------------  |         ...              ...
       |  -----------------------|         ...              ...
       |-----------------------  |         483              484
       |  -----------------------| l.525   484              485
 l.263 |-----------------------            485              486

For 625-line analog signals, the picture should be produced in this manner: (F1 has 288 active lines, F2 has 288 active lines)

 
 field 1                          field 2  picture 0-based  picture 1-based
 *******                          *******  ***************  ***************
  l.23 |--(second half only)---            0                1
       |  -----------------------| l.336   1                2
       |-----------------------  |         2                3
  F1 --|  -----------------------|         3                4
       |-----------------------  |-- F2    4                5
       |  -----------------------|         ...              ...
       |-----------------------  |         ...              ...
       |  -----------------------|         573              574
 l.310 |-----------------------  |         574              575
          ----(first half only)--| l.623   575              576

For 625-line digital signals, the picture should be produced in this manner: (F1 has 288 active lines, F2 has 288 active lines)

 
 field 1                          field 2  picture 0-based  picture, 1-based
 *******                          *******  ***************  ****************
  l.23 |-----------------------            0                1
       |  -----------------------| l.336   1                2
       |-----------------------  |         2                3
  F1 --|  -----------------------|         3                4
       |-----------------------  |-- F2    4                5
       |  -----------------------|         ...              ...
       |-----------------------  |         ...              ...
       |  -----------------------|         573              574
 l.310 |-----------------------  |         574              575
          -----------------------| l.623   575              576

All Field 1 and Field 2 line numbers match those in SMPTE 170M and CCIR 624. Both of the digital specs use identical line numberings to their analog counterparts. Warning: "Video Demystified" and many chip specs use nonstandard line numbers in some (not all) of their diagrams. Warning: 125M draws fictitious half-lines in figure 3 in very strange places that do not correspond to where the half-lines fall in the analog signal.

Picture line numbering scheme is shown both 0-based (like the movie library) and 1-based.

Field Interleave

Unless otherwise specified, this document assumes that the lines of image data in memory are arranged in top-to-bottom order.

The rules for interleaving two fields in memory depend on the same factors as those for interlacing:

which field is an F1 field and which field is an F2 field
whether the fields are from a 525- or 625-line signal.

The rules themselves are identical to those found under "PRACTICAL 525-line digital signals" and "625-line digital signals" above.

Field Dominance

Field dominance is relevant when transferring data in such a way that frame boundaries must be known and preserved. Examples:

GPI/VLAN/LTC triggered capture or playback of video data
edits on a VTR
interpretation of fields in a VLbuffer for the purposes of interlacing or de-interlacing.

Field dominance can be "F1 Dominant" or "F2 Dominant." It defines the meaning of a "frame."

For "F1 Dominant," a "frame" is an F1 field followed by an F2 field. This is the protocol recommended by all of the above specifications.

For "F2 Dominant," a "frame" is an F2 field followed by an F1 field. This is the protocol followed by several NY production houses for the 525-formats only.

Most older VTRs cannot make edits on any granularity finer than the frame. The latest generation of VTRs are able to make edits on arbitrary field boundaries, but can (and most often are) configured only to make edits on frame boundaries. Video capture or playback on a computer, when triggered, must begin on a frame boundary. Software must interleave two fields from the same frame to produce a picture. When software de-interleaves a picture, the two resulting fields are in the same frame.

Regardless of the field dominance, if there are two contiguous fields in a VLbuffer, the first field is always temporally earlier than the second one: under no circumstances should the temporal ordering of fields in memory be violated.

Thoroughly Ambiguous: Avoid The Terms "Even" and "Odd"

These terms are ambiguous and terribly overloaded. They must be avoided or carefully defined where used.

"Even and odd" could refer to whether a field's active lines end up as the even scanlines of a picture or the odd scanlines of a picture. In this case, one needs to additionally specify how the scanlines of the picture are numbered (zero-based or one-based), and one may need to also specify 525 vs. 625 depending on the context.

"Even and odd" could refer to the number 1 or 2 in F1 and F2, which is of course a totally different concept that only sometimes maps to the above. This definition seems somewhat more popular.

The VL unfortunately uses these terms in one place:

VL_CAPTURE_ODD_FIELDS captures F1 fields
VL_CAPTURE_EVEN_FIELDS captures F2 fields

Application to MSCs and DMediaInfo->sequence

The VL presents field numbers to a VL application in two contexts:

For video-to-memory or memory-to-video paths whose VL_CAP_TYPE is set to VL_CAPTURE_NONINTERLEAVED (fields separate, each in its own buffer), the functions vlGetFrontierMSC and vlGetUSTMSCPair return MSCs which count fields (in other VL_CAP_TYPEs, the returned MSCs do not count fields).
For any video-to-memory path, the user can use vlGetDMediaInfo to return the DMediaInfo structure contained in an entry in a VLBuffer. This structure contains a member called "sequence" which always counts fields (regardless of VL_CAP_TYPE).

In both of these cases, there should be the following correlation:

these values should be 0%2 if they represent an F1 field
these values should be 1%2 if they represent an F2 field

This is a relatively new convention and is not yet implemented on all devices.

Related Concepts in the SGI Movie Library

The movie library has an interlacing concept as well. It is different than the video one because a split-field image in the movie library does not always represent a complete video frame (it could be clipped, or not derived from video), and because both top-to-bottom and bottom-to-top ordering of video lines in images is supported.

This discussion will first assume top-to-bottom orientation of video lines in images.

The movie library has no parameter relating to field dominance in any way whatsoever. However, a choice of field dominance must be made when creating a movie file, because a movie file encodes pairs of fields into what it calls "frames," and all data transfers are on frame boundaries.

A movie library "frame," or "image," is defined by two parameters:

DM_IMAGE_LAYOUT: describes the current interleave of the image in memory. DM_LAYOUT_FULL_FRAME or DM_LAYOUT_SPLIT_FIELDS
DM_IMAGE_INTERLACING: describes the original interlacing characteristics of the signal that produced this image (or lack of interlacing characteristics).

The MV uses a zero-based line numbering scheme for DM_LAYOUT_FULL_FRAME images (line numbers 0,1,2,3,...).

An image which is DM_IMAGE_NONINTERLACED must be DM_LAYOUT_FULL_FRAME.

An image which is DM_IMAGE_INTERLACED_* can be full-frame or split field. The presence of the DM_IMAGE_INTERLACING parameter allows one to go between split-field and full-frame format at will.

A DM_IMAGE_INTERLACED_ODD image is an image such that the scanlines of the first field in the DM_LAYOUT_SPLIT_FIELD representation are meant to occupy the lines 1/3/5/7/... of the DM_LAYOUT_FULL_FRAME representation.

A DM_IMAGE_INTERLACED_EVEN image is an image such that the scanlines of the first field in the DM_LAYOUT_SPLIT_FIELD representation are meant to occupy the lines 0/2/4/8/... of the DM_LAYOUT_FULL_FRAME representation.

By "first field," we mean the image that is temporally first AND first in memory.

If an MV image is marked as bottom-to-top instead of top-to-bottom, then all temporal ordering and memory ordering rules become exactly reversed. A picture:

         top-to-   bottom-to
         bottom    top
 
DM_LAYOUT_FULL_FRAME:

         0         .
         1         5
         2         4
         3         3
         4         2
         5         1
         .         0

DM_LAYOUT_SPLIT_FIELD and DM_IMAGE_INTERLACED_EVEN

         0         .
         2         5
         4         3
         .         1
        ---       ---
         1         .
         3         4
         5         2
         .         0

DM_LAYOUT_SPLIT_FIELD and DM_IMAGE_INTERLACED_ODD

         1         .
         3         4
         5         2
         .         0
        ---       ---
         0         .
         2         5
         4         3
         .         1

Relation Between Movie Library and Video Concepts

Using the movie library and video definitions above, it is possible to come up with a movie-library-style description for any buffer containing an interleaved frame or two abutted fields of video data. Below, we give an example derivation of the movie-library-style definitions for a top-to-bottom buffer containing unclipped video data (a buffer containing all the video found on the charts above for analog 525, practical digital 525, analog 625, and digital 625-line signals).

The buffer's DM_IMAGE_LAYOUT can be determined simply by seeing whether the lines are split up into fields or interleaved together.

The buffer's DM_IMAGE_INTERLACING depends on many factors. For a signal with F1 dominanance, a frame consists of an F1 field followed by an F2 field (temporally and in memory). The DM_IMAGE_INTERLACING parameter tells us on which picture lines to place the first field's data. So we have:

for an analog or practical digital 525-line image, DM_IMAGE_INTERLACED_ODD
for an analog or digital 625-line image, DM_IMAGE_INTERLACED_EVEN

However, if the signal has F2 dominance, where a frame consists of F2 followed by F1, the first field is now an F2 field so we have:

for an analog or practical digital 525-line image, DM_IMAGE_INTERLACED_EVEN
for an analog or digital 625-line image, DM_IMAGE_INTERLACED_ODD