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DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

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255<br />

Full-range code, computing<br />

0<br />

-16<br />

0<br />

FOOTROOM<br />

AVERAGE SLOPE = 255<br />

219<br />

SLOPE=1<br />

109 110<br />

MISSING<br />

CODES<br />

0<br />

Rec. 601 code, black at zero 219 239<br />

16 Rec. 601 code, interface levels<br />

235 255<br />

CHAPTER 27 VIDEO SIGNAL PROCESSING 329<br />

131<br />

128<br />

127<br />

124<br />

Figure 27.3 8-bit Rec. 601 to full-range (computer) R’G’B’ conversion involves multiplying<br />

by a scale factor of 255 ⁄219, to account for the difference in range. This causes the footroom<br />

and headroom regions of the studio video signal to be clipped, and causes the output signal<br />

to be missing several code values. The detail shows the situation at mid-scale; the transfer<br />

function is symmetrically disposed around input pair [109, 110] and output pair [127, 128].<br />

This graph shows a linear relationship from black to white. The linear relationship is suitable<br />

in computer systems where a ramp is loaded into the lookup table (LUT) between the framebuffer<br />

and the display; in that case, R’G’B’ data is displayed on the computer monitor<br />

comparably to the way R’G’B’ is displayed in video; see Gamma, on page 257.<br />

HEADROOM

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