Field dominance

In video engineering , field dominance refers to the choice of which field of interlaced video signal is chosen to the point at which video edits or switches occur.

There are two main choices for field dominance: odd or even. With odd field dominance the edit or switch occurs at the start of the odd field. With even field dominance the field of the field or the field of change switch has been pressed).

Interlacing divides the frame into two fields, each containing half the number of lines. Each field is scanned in 1/60 second under the 525-line system (or 480i – often incorrectly referred to as NTSC ) or 1/50 of a second under the 625-line system (or 576i – often incorrectly referred to as the PAL ). With interlaced systems there are a number of lines in each frame. This means that there is a half line between the fields, therefore the lines in the second field will be positively intertwined with the lines in the first field.

The lines are numbered in the order they are scanned (so it is incorrect to talk about the ‘odd numbered lines’ and the ‘even numbered lines’ when referring to interlaced video – but see PsF Line Numbers In 525/60 systems By convention, the first field in the frame is considered the same field. In 625/50 systems, by convention, the first field in the frame is considered the odd field.

Selecting a dominant field of vision and linear editing systems will maintain color framing synchronization. Re-editing old video material already edited with a different field dominance convention can be problematic, it can lead to “flash fields” when old and new edits are made too close together.

The term field dominance is often incorrectly used when referring to a particular field of view. Analogous 525/60 systems field one, line one starts when the falling edge of the first equalizing pulse is coincident with the start of a line. In 625/50 systems field one line one starts with line sync being coincident with the falling edge of the first broad pulse in the analog field sync. Digital formats use a single-bit Timing Reference Sequence as the “Field” flag. Field flag bit F = 0 marks the first field of each frame. F = 1 Marks the second field.

PsF Line Numbers

In Progressive Segmented Frame (PsF), which is a way of sending a progressive scanned picture to an interlaced system, each alternate field will contain the odd and even lines of the original progressive scan. But they are as usual in the process of being in line with each other.

See also

  • Color framing

External links