Barco - RHDM-2301 - User manual ____________________________________________________ 97
Video signal processing
5.6 Selection of OETFs in the RHDM
OETF means “Opto-Electric Transfer Function”. An OETF defines the way the input signal
(electric) is transferred to luminance at the display side. The video encoding functions
explained above are examples of OETFs.
The following OETFs can be selected by the user:
•
Rec.709
•
SMPTE 240M
•
sRGB
•
xvYCC
: The xvYCC function is identical to Rec.709 in the legal range of values. How-
ever, xvYCC can encode headrooms and footrooms that give rise to out-of-gamut
colors. With the wide gamut of the display unit, these colors are displayed correctly
on screen.
•
Pure Gamma
•
Extended Gamma
: This option is an extension of the “pure gamma” OETF to head-
room and footroom, with the same intent as xvYCC.
In addition to these possible OETFs, the user can also select the gamma exponent
γ
(ranging from 1.0 to 3.0, with a default value of 2.35 as per EBU Tech.3320). This
gamma exponent can be understood as the resulting
“effective gamma”
of the display.
The simplest function is the “pure gamma”, where L-L
0
~V
γ
. But because of differences
between CRT and LCD technology, using a pure gamma function in LCDs will result in
crushed blacks. The users would need to increase the background offset (black level) in
order to set up the monitor according to the Pluge pattern. But this increases the black
level and decreases the available contrast range of the display.
Therefore the RHDM uses the following OETF concept:
• Invert the encoding equations (Rec.709, xvYCC, sRGB or SMPTE240M) exactly, pre-
serving the “linear toe”.
• Make sure that the “rendering intent” is preserved, which means that the resulting
effective gamma is equal to the gamma selected by the user (default 2.35 as per
EBU 3320 recommendation).
• Take the limited LCD contrast into account so that the dark levels are perceptually
discernable.
This implementation avoids crushing of blacks that would be exhibited by using a pure
gamma function and eliminates the need to set the video offset. The rendering intent
used in the display unit ensures that at a luminance level of 100 cd/m
2
the lowest step in
the Pluge pattern are just noticeable.
Detailed example: Pure gamma
The OETF shown below describes the translation of the electric signal to the video drive
level. This formula applies for each color.
The light output (luminance) Y
i
for each color (i=R,G,B) of an ideal screen is related to
the maximum light output Y
i
max
(for each color) multiplied by the gamma corrected color
factor. The color factor is the input voltage V
i
in
(or Y
i
’) weighted by the reference white
voltage V
i
inrw
. The LCD screen is not able to suppress all the light coming from the
backlight trays. There is still some light (Y
i
0
) emitted by the LCD screen when V
i
in
= 0.
This remnant emission of light is called display black level. The video gain G is a way to
enhance the light output of the image, without influencing the black level of the image.
