Case Studies
Article for Television Broadcast magazine, 7/15/05 HED: HDTV Image Quality
By Laurence J. Thorpe, Canon U.S.A.
The Two Aspects of HDTV Imagery
There are two aspects to HDTV image quality. The first bears on what
should distinguish the HDTV viewing experience from that of the long-established
analog NTSC world. This encompasses issues of screen size, shape, and
image content. This can be referred to as the Quality of the HDTV Viewing
Experience.
The second aspect of HDTV image quality is the actual technical performance
of the HDTV imagery itself that is reproduced in the home. This encompasses
specific picture attributes of sharpness, contrast, color reproduction,
etc. These collectively define the Quality of the HDTV Image.
To understand the implications of the quality of the viewing experience,
one must return to the superb psychophysical R&D work done in the
late 1970s to establish the basis of a future HDTV system. What emerged
from this R&D was the desire of the average home viewer for a substantially
larger and wider picture than the typical NTSC portrayal. This constituted
the very essence of a radically new viewing experience. Multichannel
audio was later recognized as an additional huge enhancement to that
experience.
The best image that can be implemented in digital 4:3 standard definition
television today is supported by a digital sampling structure of 720
(H) x 480 (V). On the other hand, the 1080-line HDTV system has a sampling
structure of 1920 (H) x 1080 (V). If you multiply the H and V samples
for each of these two systems you get the total pixel count for the
two systems. Dividing that total SD number into the total HD number
will produce the number 6.00. Thus, from a technical viewpoint, HDTV
has six times more spatial samples than SDTV. But, what is the real
significance of that difference?
If all of that substantially greater pixel count is mobilized to merely
produce a sharper and wider version of the current NTSC picture, then
it will become a ho-hum viewing experience from across a large living
room. On the other hand, that six times greater number of HD pixels
can be mobilized to support a significantly larger (and wider) screen.
You can realize a screen having six times the total area of the present
NTSC screen—with the same resolution per square inch on both.
Most important, if the picture content is also increased proportionally
(by opening up the lens angle of view of the HD camera), then a radically
different viewing experience emerges. On the premise that a picture
speaks a thousand words we will address this complex topic and hopefully
help demystify some of the confusion surrounding HDTV.
Figure 1 is a simulation of an NTSC television portrayal. Assume that
it is a large screen—perhaps 32-35 inch. Viewed from across the
living room it portrays a window viewing experience—occupying
very little of the human visual capability. Figure 2 is a simulation
of an HDTV screen replacing the NTSC set, with the viewer remaining
in the same viewing location.
Figure 1: This shows the “window” effect of
the 4:3 NTSC television viewing experience.
Figure 2: HDTV replaces the NTSC screen (at the same viewing
distance) with one having six times the screen area and portraying picture
content that has been dramatically expanded by opening up the lens angle
of view.
This is a difference that the consumer will see immediately! Here, the
picture resolution per unit area has remained unchanged, but a far greater
number of pixels are being mobilized to build out a radically different
picture (than can be sustained on the limited resolution NTSC system).
HDTV has transformed the traditional narrow NTSC “window”
onto a sports field into a stadium experience in the living room. No
consumer can resist this viewing experience—especially when 5.1
channels of digital audio will further augment the experience.
Figure 3 below illustrates the relative mapping of the 4:3 SDTV and
the 16:9 HDTV digital sampling structures. It should be pointed out
that the simulated NTSC image content shown here would actually push
the very limits of that system’s resolution.
Figure 3: This image shows the comparative pixel structures
for the 4:3 NTSC and the 16:9 HDTV images.
The ability of HDTV to sustain such panoramic imagery
has major implications for all forms of program production. Sports will
have the potential to become a truly compelling experience. Drama, natural
history, major events, wildlife, and concerts—all can capitalize
on this.
A most important and misunderstood aspect of this is the potential impact
of such expanded imagery on television commercials. There is broad skepticism
regarding HDTV and its ability to improve the selling power of commercials.
Figure 4 is a simplistic simulation of an HDTV auto commercial compared
to that of NTSC. This picture suggests the ability for HDTV to greatly
enhance environments within which products might be presented.
Figure 4: Suggestive of the HDTV versus NTSC television
commercial.
From the viewpoint of the Quality of the Viewing Experience,
just as the SDTV zoom lens is used so effectively to manage NTSC picture
content (to ensure that it remains within the boundaries of that system’s
severe resolution constraints), so too, the HDTV lens becomes the arbiter
of the new form of wide-angle imagery that becomes the most defining
aspect of the HDTV viewing experience.
@Subhed:Quality of the HDTV Image—A Digital Odyssey
And indeed it is an odyssey. Consider that the formulation of the digital
HDTV video within the contemporary HDTV digital camera requires processing
12-bit (and most recently 14-bit) RGB video at data rates in excess
of 3 Gbps. By the time the signal has traversed the entire television
system and has been encoded for digital transmission it will be represented
by a mere 8-bits at a 19.3 Mbps data rate. There are serious issues
of picture quality associated with this transformation.
There are many dimensions to an HDTV image, but four are critically
important to ensure a breathtaking picture in the living room:
• Dynamic Range (exposure latitude)—the total range of scene
lightness levels that the lens can faithfully reproduce at its output
optical port and that the HDTV camera can adequately represent.
• Contrast Range (tonal reproduction)—refers to the accurate
preservation of the picture grey scale over the luminance range to which
the human visual system is most sensitive.
• Picture Sharpness—the visual stimulation of sharpness
perceived by the viewer at normal viewing distance (of three times picture
height) is a complex function of the system resolution characteristics
and system contrast.
• Color Reproduction—the overall range of colors that the
HDTV system can reproduce (referred to as the color gamut).
It is the role of the HDTV lens/camera system to originate all of these
picture attributes at the highest level possible. It is the role of
the HDTV display to faithfully reproduce all of those core attributes.
It is the role of the complex digital system (terrestrial, cable, satellite,
or packaged media) separating the camera from the display to protect—to
the degree possible—the integrity of each of these image attributes.
Properly quantifying these image parameters is the guide to carefully
protecting them through the system. The contrast range of the HDTV camera
(measured from the noise floor to a nominally exposed reference white)
is of the order of 500:1. With the overhead designed into today’s
CCD imagers for handling overexposed signals, the total dynamic range
(or exposure latitude) is in excess of 3000:1. Picture sharpness begins
in the HDTV lens, which must have a spatial frequency response in excess
of 84 line-pairs per millimeter to fulfill the 30MHz bandwidth of the
HDTV video signal. A 1920 (H) x 1080 (V) sampling by an HDTV camera’s
three imagers will produce an excellent aperture in both the horizontal
and vertical domain. With 60 pictures per second it is easy to see where
the requisite 3 Gbps data rate in the camera is required.
But now, the camera signal must be recorded. And here arises the first
serious confrontation with signal processing. Contemporary HDTV recording
alone can entail component matricing, digital prefiltering, bit-depth
reduction, and compression. There is a range of variations on each of
these among contemporary HDTV recorders. If the camera’s four
core image parameters are to be maintained, then great care must be
taken to match the recording format to the particular production and
postproduction application. In terms of dynamic range, care must also
be exercised in the adjustment of the camera’s nonlinear transfer
characteristic in anticipation of the bit-depth of the recording format.
Between that first HDTV recording and the final display in the living
room the digital HDTV video signal must traverse a long sequence of
digital operations. They include: further recording (on tape or on disk),
image manipulation in postproduction, routing through the broadcast
operation, play-to-air, and—ultimately—encoding for transmission
to the home. There are system rules to the application of digital processing
and they need to be strictly adhered to.
Compression—which can be encountered a number of times throughout
the system—is where the greatest dangers can lie. Effectively
applied, it will have a minor effect on subjective picture sharpness.
Carelessly applied, it can woefully deteriorate the HDTV picture. In
addition to system vigilance with respect to the four image parameters,
very adroit application of compression is necessary to ensure that no
related artifacts are introduced.
Depending on the complexities of the overall production, postproduction
and editing, and contribution feeds, the signal may undergo a number
of conversions between different bit-depths and matrix conversions (from
the Y/Pr/Pb color set of the camera to RGB and back gain). All will
take their toll on contrast range and color reproduction. How much that
toll amounts to will be a measure of the careful management of the digital
HD signal throughout the entire broadcast infrastructure.
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