In 480i systems with setup, the term picture excursion refers to the range from blanking to white, even though strictly speaking the lowest level of the picture signal is 7.5 units, not

0 units.

Back porch is described in Analog horizontal blanking interval, in Chapter 2 of Composite NTSC and PAL: Legacy Video Systems.

Video levels

I introduced 8-bit studio video levels on page 42. Studio video coding provides headroom and footroom. At an 8-bit interface, luma has reference black at

code 16 and reference white at code 235; colour differences are coded in offset binary, with zero at code 128, the negative reference at code 16, and the positive reference at code 240. (It is a nuisance that the positive reference levels differ between luma and chroma.) I use the term reference instead of peak; the peaks of transient excursions may lie outside the reference levels. This range is known as “studio-swing.”

All modern studio interfaces accommodate 10-bit signals, and most equipment today implements 10 bits. In 10-bit systems, the reference levels just mentioned are multiplied by 4; the two LSBs provide additional precision. Reference black is at code 64, and reference white at code 940.

Video levels in 480i systems are historically expressed in IRE units, sometimes simply called units. IRE refers to the Institute of Radio Engineers in the United States, the predecessor of the IEEE. Reference blanking level is defined as 0 IRE; reference white level is 100 IRE. The range between these values is the picture excursion.

In an analog interface, sync is coded at voltage level more negative than black; sync is “blacker than black.” The ratio of picture excursion to sync amplitude is the picture:sync ratio. Two different ratios were standardized: 10:4 is predominant in 480i and computing; 7:3 is universal in 576i and HD, occasionally used in 480i, and rarely used in computing.

Setup (pedestal)

In 480i composite NTSC video in North America, reference black is offset above blanking by 7.5% (3⁄40) of the picture excursion. Setup refers to this offset, expressed as a fraction or percentage of the picture excursion. In a 480i system with setup, there are nominally 92.5 IRE units from black to white.

Blanking level at an analog interface is established by a back porch clamp. However, in a system with setup, no signal element is present that enables a receiver to accurately recover black level. If an interface has poor

Consumer video equipment purchased in Japan has zero setup. Some Japanese CE equipment has setup confugurable 0/7.5; sometimes the 0 setting goes by marketing terms like “enhanced black.”

tolerance, calibration error, or drift, setup causes problems in maintaining accurate black-level reproduction. Consequently, setup has been abolished from modern video systems: Zero setup is a feature of EBU N10 component video, all variants of 576i video, and HD. In all of these systems, blanking level also serves as the reference level for black.

480i video in Japan originally used setup. However, in about 1987, zero setup was adopted; 10:4 picture- to-sync ratio was retained. Consequently, there are now three level standards for SD analog video interface. Figure 31.2 shows these variations.

The archaic EIA RS-343-A standard specified monochrome operation, 2:1 interlace with 60.00 Hz field rate, 7 µs horizontal blanking, and other parameters that have no place in modern video systems. Unfortunately, most PC graphics display standards have inherited RS-343-A’s 10:4 picture-to-sync ratio and 7.5% setup. (Some high-end workstations have zero setup.)

The term pedestal refers to the absolute value of the offset from blanking level to black level, in IRE units or millivolts: Composite 480i NTSC incorporates a pedestal of 7.5 IRE. Pedestal includes any deliberate offset

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Full-swing code, computing

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