30

fFR = 1.001 ≈29.97Hz

It is confusing to refer to fields as odd and even. Use first field and second field instead.

This chapter details the scanning, timing, sync structure, and picture structure of 480i29.97 (525/59.94/2:1) video. The scanning and timing information in this chapter applies to all variants of 480i video, both analog and digital. The sync information relates to component analog, composite analog, and composite digital systems.

Frame rate

480i video represents stationary or moving two-dimen- sional images sampled temporally at a constant rate of 30⁄1.001 frames per second. For studio video, the tolerance on frame rate is normally ±10 ppm. In practice the tolerance applies to a master clock at a high frequency, but for purposes of computation and standards writing, it is convenient to reference the tolerance to the frame rate.

Interlace

A frame is conveyed as a sequence of 525 horizontal raster lines of equal duration, uniformly scanned top to bottom and left to right. Scanning has 2:1 interlace to form a first field and a second field; scan lines in the second field are displaced vertically by half the vertical sampling pitch, and delayed temporally by half the frame time, from scanning lines in the first field. In MPEG-2 terms, the first field is the bottom field.

Lines are numbered consecutively throughout the frame, starting at 1.

Table 39.1 480i line assignment

EQ Equalization pulse

BR Broad pulse

CC Closed caption

[n] Line number relative to start of second field (deprecated)

§V=0 in RP 125-1992, and in the 480i version of BT.601-4 (1994); in later standards, V=1 for these lines.

◊The thick vertical bar at the right indicates lines carried in 480i or 480p MPEG-2 according to SMPTE RP 202. (The vertical center of the picture is located midway between lines 404 and 142.) Unfortunately, 480i DV systems digitize a range one image row up from this.

‡In analog terminology, lines 1 through 3 are considered part of the first field; lines 264 and 265 are considered part of the second field.

Concerning closed captions, see

ANSI/EIA/CEA-608-B, Line 21

Data Services.

For details concerning VITC line assignment, see SMPTE RP 164,

Location of Vertical Interval Timecode.

Table 39.1 opposite shows the vertical structure of a frame in 480i video, and indicates the assignment of line numbers and their content.

In legacy equipment, the picture may start as early as line 20 or line 283. However, video on lines 21 and 284 is liable to be replaced by line 21 closed caption data upon NTSC transmission, so it is pointless to provide more than 483 picture lines in the studio. With the wide use of 480i DV and MPEG-2 systems, I argue that it is pointless to provide more than 480 lines; however, 483 lines were broadcast in analog NTSC.

Lines 10 through 21 and 273 through 284 may carry ancillary (“vertical interval”) signals either related or unrelated to the picture. If vertical interval timecode (VITC) is used, it should be located on line 14 (277); a second, redundant copy can be placed on line 16 (279). Failing line 14, line 18 (281) is suggested. See

Vertical interval timecode (VITC), on page 402.

Line sync

Horizontal events are referenced to an instant in time denoted 0H. In the analog domain, 0H is defined by the 50%-point of the leading (negative-going) edge of each line sync pulse. In a component digital interface, the correspondence between sync and the digital information is determined by a timing reference signal (TRS) conveyed across the interface. (See SDI and HD-SDI sync, TRS, and ancillary data, on page 433.)

In an analog interface, every line commences at 0H with the negative-going edge of a sync pulse. With the exception of vertical sync lines, which I will describe in a moment, each line commences with a normal sync pulse, to be described. Each line that commences with normal sync may contain video information. Every line that commences with a sync pulse other than normal sync maintains blanking level, except for the intervals occupied by sync pulses.

Field/frame sync

To define vertical sync, the frame is divided into intervals of halfline duration. Each halfline either contains no sync information or commences with the assertion of a sync pulse having one of three durations, each having a tolerance of ±0.100 µs:

EIA and FCC standards in the United States rounded the equalization pulse duration to two digits, to

2.3 µs, slightly less than the theoretical value of 2.35 µs. Equipment is usually designed to the letter of the regulation, rather than its intent.

Line 263 commences with a normal sync pulse and has an equalization pulse halfway through the line. Line 272 commences with an equalization pulse and remains at blanking with no sync pulse halfway through the line. For analog details, see Chapter 2, Analog SD sync, genlock, and interface, of Composite NTSC and PAL: Legacy Video Systems.

SMPTE RP 168, Definition of Vertical Interval Switching Point for Synchronous Video Switching.

•A normal sync pulse having a duration of 4.7 µs

•An equalization pulse having half the duration of a normal sync pulse

•A broad pulse, having a duration of half the line time less the duration of a normal sync pulse

Each set of 525 halflines in the field commences with a vertical sync sequence as follows:

•Six preequalization pulses

•Six broad pulses

•Six postequalization pulses

Vertical events are referenced to an instant in time denoted 0V. In the analog domain, 0V is defined by the first equalization pulse coincident with 0H. Line number 1 is signalled by 0V; lines count in interlaced time order (not spatial order) throughout the frame. 480i systems are exceptional in identifying 0V and line 1 at the first equalization pulse: In 576i, and in HD, 0V and line 1 are marked at the first broad pulse.

Historically, in the analog domain, 0V was defined for each field by the 50%-point of the first equalization pulse; lines were numbered from 1 to 263 in the first field and from 1 to 262 in the second field. In the digital domain, the first field contains 262 lines and the second field contains 263 lines.

Figure 39.1 opposite shows details of the sync structure; this waveform diagram is the analog of Table 39.1,

480i line assignment, on page 446.

When sync is represented in analog or digitized 4fSC form, a raised-cosine transition having a risetime (from 10% to 90%) of 140±20 ns is imposed; the midpoint of the transition is coincident with the idealized sync.

Switching between video sources is performed in the vertical interval, to avoid disruption of sync or picture. Switching occurs 30±5 µs after 0H of the first normal line of each field. In 480i systems, switching occurs midway through line 10. (If field 2 were dominant, switching would occur midway through line 273.)

R’G’B’ EOCF and primaries

Picture information is referenced to linear-light primary red, green, and blue (RGB) tristimulus values, represented in abstract terms in the range 0 (reference black) to +1 (reference white).

39 CHAPTER

VIDEO COMPONENT 480I

Figure 39.1 480i raster, vertical. This drawing shows thewaveformswaveformsof firstof theandfirstsecoand lds,fieconddetailingfields, detailing the vertical sync intervals. CC indicates a line that may contain EIA 608 (“line 21”) closed caption data.