The derived line rate is 15.625 kHz.

It is confusing to refer to fields in 576i as odd and even. Use first field and second field instead.

This chapter details the scanning, timing, sync structure, and picture structure of 576i25 (625/50/2:1) video. The scanning and timing information here applies to all variants of 576i25 video, both analog and digital. The sync information relates to component analog, composite analog, and composite digital systems. I assume that you are familiar with 480i component video, described on page 445.

Frame rate

576i video represents stationary or moving two-dimen- sional images sampled temporally at a constant rate of 25 frames per second. For studio video, the tolerance on frame rate is normally ±4 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 comprises a total of 625 horizontal raster lines of equal duration, uniformly scanned top to bottom and left to right with 2:1 interlace to form a first field and a second field. Scanning 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 top field.

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

Table 40.1 576i line assignment

EQ Equalization pulse

BR Broad pulse

‡Burst suppressed if –135° phase

§Burst suppressed unconditionally

¶In 4fSCPAL, line recommended for 1137 STL (“reset”)

†VANC is permitted only on lines 20 through 22 and 333 through 335.

◊The thick vertical bar at the right indicates lines carried in 576i or 576p MPEG-2 according to SMPTE RP 202. (The vertical center of the picture is located midway between lines 479 and 167.) Unfortunately, 576i DV systems digitize a range one image row up from this.

For details concerning VITC in 576i, see EBU Technical Standard N12, Time-and-control codes for television recording.

Table 40.1 opposite shows the vertical structure of a frame in 576i video, and indicates the assignment of line numbers and their content.

Lines 6 through 21 and 319 through 334 may carry ancillary (“vertical interval”) signals either related or unrelated to the picture. If vertical interval timecode (VITC) is used, redundant copies should be placed on lines 19 (332) and 21 (334); 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 the vertical sync lines of each field, each line commences with the assertion of 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, here denoted zero, except for the interval(s) occupied by sync pulses.

Analog 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:

•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 625 halflines in the frame is associated with a vertical sync sequence, as follows:

Line 623 commences with a normal sync pulse and has an equalization pulse halfway through the line. Line 318

commences with an equalization pulse and remains at blanking with no sync pulse halfway through the line.

See Table 13.1, on page 132, and Figure Figure 2.2 and Figure 2.3 in Chapter 2 of

Composite NTSC and PAL:

Legacy Video Systems.

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

•Five preequalization pulses

•Five broad pulses

•Five postequalization pulses

In analog sync, line 1 and 0V are defined by the first broad pulse coincident with 0H; see Figure 2.3 in Chapter 2 of Composite NTSC and PAL: Legacy Video Systems. (This differs from the 480i convention.)

Figure 40.1 opposite shows the vertical sync structure of 576i analog video. This waveform diagram is the analog of Table 40.1, 576i line assignment, on

page 458.

Sync in 576i systems has several differences from 480i sync. There are five preequalization, broad, and postequalization pulses per field (instead of six of each). The frame is defined to start with the field containing the top line of the picture, actually a right-hand halfline. (In 480i scanning, the first picture line of

a frame is a full line, and the right-hand halfline at the top of the picture is in the second field.)

In 576i systems, lines are numbered starting with the first broad sync pulse: preequalization pulses are counted at the end of one field instead of the beginning of the next. This could be considered to be solely a nomenclature issue, but because line numbers are encoded in digital video interfaces, the issue is substantive. In 576i systems, lines are always numbered throughout the frame.

When sync is represented in analog or digitized form, a raised-cosine transition having a risetime (from 10% to 90%) of 200±20 ns is imposed, where 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 576i systems, switching occurs midway through line 6. (If field 2 were dominant, switching would occur midway through line 319.)

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).

40 CHAPTER

VIDEO COMPONENT 576I

FIRST FIELD

EQ EQEQ EQEQ BR BR BR BR BR EQ EQEQ EQEQ

309 310‡ 311 312 313 314 315 316 317 318 319‡ 320 321 ... 335 336 337 338 ... 623

Figure 40.1 576i raster, vertical. This drawing shows the waveforms of the first and second fields, detailing vertical sync intervals. The first field comprises 312 lines, and the second field comprises 313 lines.