video demystified

YUV Color Space

The YUV color space is used by the PAL

(Phase Alternation Line), NTSC (National
Television System Committee), and SECAM
(Sequentiel Couleur Avec Mémoire or Sequential
Color with Memory) composite color video
standards. The black-and-white system used
only luma (Y) information; color information
(U and V) was added in such a way that a
black-and-white receiver would still display a
normal black-and-white picture. Color receivers
decoded the additional color information to
display a color picture.

The basic equations to convert between

gamma-corrected RGB (notated as R´G´B´ and
discussed later in this chapter) and YUV are:
Y = 0.299R´ + 0.587G´ + 0.114B´
U = – 0.147R´ – 0.289G´ + 0.436B´
= 0.492 (B´ – Y)
V = 0.615R´ – 0.515G´ – 0.100B´
= 0.877(R´ – Y)

YIQ Color Space
The YIQ color space, further discussed in
Chapter 8, is derived from the YUV color space
and is optionally used by the NTSC composite
color video standard. (The “I” stands for “inphase”
and the “Q” for “quadrature,”

YCbCr Color Space
The YCbCr color space was developed as
part of ITU-R BT.601 during the development
of a world-wide digital component video standard
(discussed in Chapter 4). YCbCr is a
scaled and offset version of the YUV color
space. Y is defined to have a nominal 8-bit
range of 16–235; Cb and Cr are defined to have
a nominal range of 16–240. There are several
YCbCr sampling formats, such as 4:4:4, 4:2:2,
4:1:1, and 4:2:0 that are also described.

4:4:4 YCbCr Format

Figure 3.2 illustrates the positioning of
YCbCr samples for the 4:4:4 format. Each sample
has a Y, a Cb and a Cr value. Each sample is
typically 8 bits (consumer applications) or 10
bits (pro-video applications) per component.
Each sample therefore requires 24 bits (or 30
bits for pro-video applications).

4:2:2 YCbCr Format
Figure 3.3 illustrates the positioning of
YCbCr samples for the 4:2:2 format. For every
two horizontal Y samples, there is one Cb and
Cr sample. Each sample is typically 8 bits (consumer
applications) or 10 bits (pro-video applications)
per component. Each sample
therefore requires 16 bits (or 20 bits for provideo
applications), usually formatted as
shown in Figure 3.4.
To display 4:2:2 YCbCr data, it is first converted
to 4:4:4 YCbCr data, using interpolation
to generate the missing Cb and Cr samples.

4:2:0 YCbCr Format
Rather than the horizontal-only 2:1 reduction
of Cb and Cr used by 4:2:2, 4:2:0 YCbCr
implements a 2:1 reduction of Cb and Cr in
both the vertical and horizontal directions. It is
commonly used for video compression.
As shown in Figures 3.7 through 3.11,
there are several 4:2:0 sampling formats. Table
3.3 lists the YCbCr formats for various DV
applications.
To display 4:2:0 YCbCr data, it is first converted
to 4:4:4 YCbCr data, using interpolation
to generate the new Cb and Cr samples. Note
that some MPEG decoders do not properly
convert the 4:2:0 YCbCr data to the 4:4:4 format,
resulting in a “chroma bug.”

Gamma Correction
The transfer function of most CRT displays
produces an intensity that is proportional to
some power (referred to as gamma) of the signal
amplitude. As a result, high-intensity
ranges are expanded and low-intensity ranges
are compressed (see Figure 3.17). This is an
advantage in combatting noise, as the eye is
approximately equally sensitive to equally relative
intensity changes. By “gamma correcting”
the video signals before transmission, the
intensity output of the display is roughly linear
(the gray line in Figure 3.17), and transmission-
induced noise is reduced.
To minimize noise in the darker areas of
the image, modern video systems limit the
gain of the curve in the black region. This
technique limits the gain close to black and
stretches the remainder of the curve to maintain
function and tangent continuity.
Although video standards assume a display
gamma of about 2.2, a gamma of about 2.5
is more realistic for CRT displays. However,
this difference improves the viewing in a dimly
lit environment.