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92 _____________________________________________________Barco - RHDM-2301 - User manual
Video signal processing
On the side of the display, the video signal is converted back to light. The following steps
are done in the RHDM:
•
Undo step 3
: When the color channels have been encoded, chroma upsampling and
color channel decoding (Y’C
b
C
r
to R’G’B’) is necessary.
The color channel encoding is done automatically (Rec.601 matrix for SD and
Rec.709 matrix for HD). The user only needs to select the type of input signal
(Y'C
b
C
r
/R'G'B’, bit depth, chroma subsampling or field/frame/segment type).
User video settings such as
background
,
gain
,
saturation
and
hue
are also
calculated in this matrix.
NOTE
:
The
Picture
setting does not have any influence on the conversion of the video signal to the image on
the screen. The Picture setting only drives the power of the backlight, and consequently, the brightness
of the image.
•
Undo step 2
: The video signal needs to be converted to the linear light domain by
using an
Opto-Electrical Transfer Function (OETF)
. More details about this step
will be given in the following sections. The OETF can be defined by the user:
gamma exponent: from 1.0 to 3.0 in steps 0.05.
gamma shape: Rec.709, xvYCC, SMPTE 240M, Pure gamma, Extended
gamma, sRGB.
input footroom and headroom levels.
•
Undo step 1
: The linear RGB signal is presented on the screen and must be shown
in a given color space (primaries and white point). Because the display typically has
a wider gamut than the intended gamut, a display
color space
conversion is
needed. The color space can also be defined by the user.
white point selection: D55, D65, D75, D93, custom in x,y.
color gamut selection: EBU 3213, Rec.709, SMPTE RP 145, xvYCC, Native, and
Custom - R, G and B primaries in x,y (Option for P-version).
NOTE
:
The fundamentals of the information presented in this chapter can be found in the following
references:
•
Charles Poynton, Digital Video and HDTV, Algorithms and Interfaces, Morgan Kaufmann Publishers,
2007
•
EBU-Tech 3320, User requirements for Video Monitors in Television Production, Version 1.1, Geneva,
May 2008
•
Alan Roberts: Methods of measuring and calculating display transfer characteristics (gamma), BBC
RD 1991/6.
5.2 General definition OETF
In a video camera, linear light (L) can be encoded into video levels (V) in different ways.
In most cases, light is encoded using a power function (something like V’~L
a
). The
exponent “a” (0<a<1) is chosen to guarantee a conversion to perceptual uniform space.
The signal is “gamma encoded”, and this is made clear by writing V’ instead of V. In
reality, a pure power function, as presented in the form above, is not used because
analog electronics can not cope with an infinite slope at L=0. A more detailed encoding