SMPTE ST 296, 1280× 720 Progressive Image Sample Structure – Analog and Digital Representation and Analog Interface.

24 ≈23.976

1.001

30 ≈29.97

1.001

60 ≈59.94

1.001

This chapter details the scanning, timing, and sync structure of 1280× 720 video, also called 720p. The scanning and timing information in this chapter applies to all variants of 720p video, both analog and digital.

Scanning

720p video represents stationary or moving twodimensional images sampled temporally at a constant rate of 24⁄1.001, 24, 25, 30⁄1.001, 30, 50, 60⁄1.001, or 60 frames per second. The sampling rate is 74.25 MHz (modified by the ratio 1000⁄1001 in 720p59.94,

720p29.97, and 720p23.976). All of these systems have 750 total lines (LT). The number of samples per total line (STL) is adapted to achieve the desired frame rate. Table 41.1 below summarizes the scanning parameters.

A frame comprises a total of 750 horizontal raster lines of equal duration, uniformly progressively scanned top to bottom and left to right, numbered consecutively

Table 41.1 720p scanning parameters are summarized.

tri Trilevel pulse

BR Broad pulse

The vertical center of the picture is located midway between lines 385 and 386.

Table 41.2 1280×720 line assignment

0H precedes the first word of SAV by 256 clocks.

0H follows the first word of EAV by STL -1280-260 clocks.

starting at 1. Of the 750 total lines, 720 contain picture. Table 41.2 above shows the assignment of line numbers and their content.

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.

At a digital interface, video information is identified by a timing reference signal (TRS) conveyed across the interface. (See SDI and HD-SDI sync, TRS, and ancillary data, on page 433.) The last active line of a frame is terminated by EAV where the V-bit becomes asserted. That EAV marks the start of line 746; line 1 of the next frame starts on the fifth following EAV.

Analog sync

Horizontal events are referenced to 0H, defined by the zero-crossing of trilevel sync. Digital samples and analog timing are related such that the first (zeroth) sample of active video follows the 0H instant by

260 reference clock intervals.

At an analog interface, each line commences with a trilevel sync pulse. Trilevel sync comprises a negative

portion asserted to -300±6 mV during the 40 reference clock intervals preceding 0H, and a positive portion asserted to +300±6 mV during the 40 reference clock intervals after 0H. The risetime of each transition is 4±1.5 reference clock intervals.

Vertical sync in the analog domain is signaled by broad pulses, one each on lines 1 through 5. Each broad

470

INTERFACES AND ALGORITHMS HD AND VIDEO DIGITAL

0V

PROGRESSIVE SYSTEM, FRAME

30 H

25 H

5 H

Figure 41.1 720p raster, vertical