{{Short description, Interlaced video signal variations
In video engineering, field dominance refers to the choice of which field of an
interlaced video
Interlaced video (also known as interlaced scan) is a technique for doubling the perceived frame rate of a video display without consuming extra Bandwidth (signal processing), bandwidth. The interlaced signal contains two field (video), fields ...
signal is chosen as 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 edit or switch occurs at the start of the even field (some equipment, such as vision mixers or switchers allow the field dominance to be set to 'none' which means the switch will occur on the next field boundary after the 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
480i is the video mode used for standard-definition digital video in the Caribbean, Japan, South Korea, Taiwan, Philippines, Myanmar, Western Sahara, and most of the Americas (with the exception of Argentina, Paraguay, and Uruguay). The ...
– often incorrectly referred to as
NTSC
NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170.
In 1953, a second ...
) or 1/50 of a second under the 625-line system (or
576i
576i is a standard-definition television, standard-definition digital video mode, originally used for Digitization, digitizing 625 lines, 625 line Analog television, analogue television in most countries of the world where the utility frequen ...
– often incorrectly referred to as
PAL
Phase Alternating Line (PAL) is a color encoding system for analog television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
). With interlaced systems there are an odd number of lines in each frame. This means that there is a half-line offset between the fields, therefore the lines in the second field will be positionally interleaved with the lines in the first field.
The lines are numbered in the order in which they are scanned (so it is incorrect to talk of 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 even field. In 625/50 systems, by convention, the first field in the frame is considered the odd field.
Selecting a consistent field dominance in vision switching 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, as it can lead to "flash fields" when old and new edits are made too close together.
The term field dominance is often incorrectly used to refer to field order, particularly when referring to a field order error such as can occur when converting between certain different video file formats. Analogue 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 analogue field sync. Digital formats use a single-bit in their 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
Progressive segmented Frame (PsF, sF, SF) is a scheme designed to acquire, store, modify, and distribute progressive scan video using interlaced equipment.
With PsF, a progressive frame is divided into two ''segments'', with the odd lines in one s ...
(PsF), which is a way of sending a progressively scanned picture over an interlaced system, each alternate field will contain the odd and even lines of the original progressive scan. But once it is being carried as an interlaced picture the lines are numbered in the order they are being sent as with any 'normal' interlaced system.