DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

PDP and DLP devices are
commonly described as
employing PWM. However, it is
not quite the width of the pulses
that is being modulated, but the
number of unit pulses per frame.
representation requires 4 bits in excess of 10 on the
nonlinear scale – that is, 14 bits per component.
If RGB or XYZ tristimulus components were conveyed
directly, then 16 bits in each component would suffice
for any realistic image-reproduction purpose. Linearlight
16-bit coding might be practical in a decade, but
for now, for most purposes, we exploit the nonlinear
characteristics of perception to achieve an efficient
image data coding.
Gamma in emerging display devices
Emerging display devices, such as liquid crystal displays
(LCDs), have different transfer functions than CRTs.
Plasma display panels (PDPs) and Digital Light Processors
(DLPs) both achieve apparent continuous tone
through pulse width modulation (PWM): They are intrinsically
linear-light devices, with straight-line transfer
functions. Linear-light devices, such as PDPs and DLPs,
potentially suffer from the “code 100” problem
explained on page 12: In linear-light, more than 8 bits
per component are necessary to achieve high quality.
No matter what transfer function characterizes the
display, it is economically important to encode image
data in a manner that is well matched to perceptual
requirements. The most important aspect of Rec. 709
encoding is not that it is well matched to CRTs, but that
it is well matched to perception! The performance
advantage of perceptual coding, the wide deployment
of equipment that encodes to Rec. 709, and the huge
amount of program material already encoded to this
standard preclude any attempt to establish new standards
optimized to particular devices.
A display device whose transfer function differs from
a CRT must incorporate local correction, to adapt from
its intrinsic transfer function to the transfer function
that has been standardized for image interchange.
CRT transfer function details
To calibrate your monitor, or to determine the transfer
function of your CRT, you must be familiar with the
270 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES