(CICATS)
Technical Details
of the Proposed Base-Line Format of the
Computer Industry Coalition on Advanced Television Service (CICATS)
July 10, 1996
Overview
The CICATS technical proposal for the US digital TV standard is briefly
this:
- Adopt ACATS Low Levels: That the FCC adopt all ACATS proposals
for modulation, error correction , data packetization, and compression
for the new digital TV channels.
- No Video Format: That the FCC not specify a video data format.
In other words, adopt all low-level ACATS standardization proposals,
where low-level means all levels except the video data level, which is
not to be standardized by the FCC. CICATS understands that the FCC may
find it impossible to honor the second point above (No Video Format), in
which case we propose an alternative second point:
- One Required Video Format (Alternative): That the FCC specify
a single 480-line (nominal), progressive-scan video format with square
pixel spacing, utilizing a base-layer technology concept. Others could
be implemented but only one would be required.
As will be explained below, CICATS actually couches this alternative
as follows:
- One Required Video Format (Alternative): That the FCC specify
the CICATS Reference Decoder.
This is as opposed to the ACATS proposal of 18 video formats that do
not use a base-layer concept and that include interlaced formats. The CICATS
alternative proposal is cost-effective for consumers, immediately gives
them higher resolution video, ensures smooth and true interoperability
with computers, and is ready for improvements--such as even higher resolutions--as
digital component costs drop.
On Point 1: Adopt ACATS Low Levels
The ACATS proposal (formerly the Grand Alliance proposal) has much to
recommend it. At its most fundamental level, it proposes a completely digital
standard, and we applaud this. Eight or nine years ago, when ACATS began
its deliberations, analog systems were still being contemplated.
ACATS has invested much effort in testing the radio frequency and transmission
system of its proposal. We recommend, in particular, that its vestigial
sideband (VSB) modulation subsystem be adopted. This is the subsystem that
takes a digital stream of bits and modulates the radio frequency transmission
carrier with it. The most time-consuming tests of the ACATS proposal were
those that eliminated all other contenders for this level (the physical
level) of standardization.
CICATS further supports these ACATS protocols and technologies: Trellis
coding, Reed-Solomon error correction with interleaving, and Dolby AC-3
audio.
ACATS also proposes the use of the MPEG2 transport stream system for
packetization of the bit stream. CICATS endorses this packetization level
proposal from ACATS, to within the caveat on error-correction of non-video
data already mentioned and further discussed below.
CICATS proposes that MPEG2 error correction be adopted for video data,
as per the ACATS proposal.
On the Error-Correction Caveat to Point 1
The most general view of a digital channel, and one which CICATS highly
encourages, is that it is a communications medium for arbitrary digital
data. In this view, video data is simply one type of digital data that
can be carried in the new digital channels.
CICATS proposes adoption of the ACATS error correction technology for
video data, but the error-correction level may be too low for use by the
new digital TV channels for transmission of many types of non-video digital
data. The ACATS error correction level is sufficient for transmission of
pictorial data (e.g., video) but not for general data. For example, if
a new channel were used to download a program, then loss of data generally
could not be tolerated. This is especially true in the headers for the
transmitted data.
CICATS proposes that the FCC endorse the ACATS position on the low-level
protocol but with the addition of a mandate to determine what needs to
be done to the standard to increase its error-correction capabilities.
This could take the form of an FCC specified committee to report on the
problem and suggested solutions to it within one year from propagation
of the initial digital TV standard. We believe it is important to officially
pursue the problem with a time limit.
It is important to understand, however, that by delaying this study,
a solution to the problem may be precluded. It may be that the structure
of MPEG2 transport headers do not permit a sufficiently robust solution,
and that a reasonable solution would require changes to the MPEG2 standard.
If this standard has already been mandated and placed into use, then it
cannot be changed.
Point 2: No Video Format
The computer industry is well aware of the astonishing rate of change
in its underlying digital technology. It prospers because of it. It knows
that it is difficult, if not impossible, to predict order-of-magnitude
phenomena that happen regularly every five years in the high-technology
sector (the MooreÍs Law phenomenon, see Glossary). So the CICATS position
is that the FCC specify the fundamental protocols for its new digital TV
channels but refrain from overly restricting what forms the data in those
channels may take.
The most recent success in high technology, based on a similar strategy,
is the Internet. Only two years ago it would have been impossible to predict
todayÍs businesses and world-wide impact of the data applications built
atop the fundamental protocols of the Internet. We believe the inventiveness
that industry would bring to bear on the new digital TV communications
channels, to be mandated by the FCC, would be phenomenal and do not want
to see any formatsspecified now in the infancy of the digital worldhinder
that innovation.
Hence the CICATS proposal is that the FCC not specify any video formats
in particular but let market forces and economic considerations dictate
how the channels get used. It is conceivable that the TV and PC industries
would decide to proceed with the ACATS video formats, but we believe that
the standards that would actually arise would be considerably different.
We can support the ACATS standard in many areas, as described above:
VSB modulation, trellis coding, Reed-Solomon error correction with interleaving,
MPEG2 transport and packet protocol, Dolby AC-3 audio and, most importantly,
MPEG2 video compression. In the best of all possible worlds, we would revisit
the adoption of MPEG2 itself (the real meaning of no video format), but
in this proposal we are accepting the ACATS choice. Where we diverge is
that we see no need for the artificial restriction of the flexibility inherent
in MPEG2 to a fixed set of resolutions.
Table 3 of section 5.1.2 in Annex A of the ATSC standard is the heart
of our problem. It constrains the vertical and horizontal size values to
specific numbers instead of simply allowing anything under a maximum value,
as MPEG2 does. ACATS tries to put display constraints in the transmission
standard, where they donÍt belong. When these constraints are removed,
then any aspect ratio image can be sent through the channel. It would then
be up to the receiver to display what it can by either pan-and-scan or
letterboxing, or a combination of the two. This would satisfy the objections
of the film industry, since there would be no difficulty sending 1.85:1
or 2.37:1 or any of the other film aspect ratios as long as the horizontal
and vertical sizes stayed below defined limits.
We address below the possibility that the FCC is under so much pressure
from video broadcast manufacturers that it is unwilling to trust the market
pressures as we propose. Hence we provide our alternative second point.
Alternative Point 2: One Required Video Format
Should the FCC feel it needs to specify a video data format, then CICATS
proposes that it be as simple and extensible as possible, while displaying
an immediate quality advantage over the current analog TV broadcast familiar
to all US citizens. Furthermore, we propose a single format that is fully
compatible with modern computer technology, without the quality or cost
compromises intrinsic to the ACATS proposal.
The format we propose is a base-layer concept (see the Glossary), which
means by definition that it is extensible to higher resolutions and better
quality. We differ from the ACATS proposal in that we do not require that
higher resolutions be standardized at this time. We feel that it is premature
to do so, and that the market does not yet support higher resolutions such
as the interlaced 1920x1080 ACATS format. We do believe, however, that
in a few years, the inexorable MooreÍs Law phenomenon will make the higher
resolutions cost effective, and want a base-layer technology in place so
that logical extensions of it to higher resolutions are possible when the
costs drop sufficientlyshould the market then demand it.
In order not to slow down the FCC decision process, we propose a single
format that is easily within todayÍs capabilities and has been demonstrated
so.
Furthermore, we are very desirous that PCs and TVs become tightly coupled
devices, sharing data and even computing on each otherÍs datatrue
interoperability, not simply side-by-side display. Any video data format
should encourage this possibility, not discourage it or make it costly.
Hence CICATS proposes a video format, by way of a Reference Decoder
(see Glossary), that
- Has 480 lines (nominal) of vertical resolution
- Is progressively scanned
- Has square pixel spacing
- Employs a base-layer design for future, logical, cost-effective extensions
to higher resolutions.
(See resolution, progressive, interlace, square pixel spacing, and layering
in the Glossary.)
The 480 lines is numerically the same as todayÍs analog TV standard
but, because the lines are progressively scanned rather than interlaced,
the quality level is immediately higher than todayÍs analog TV. It takes
about 768 lines of interlaced video to equal the same perceived quality
level of 480 progressive lines.
The base-layer concept guarantees that no digital TV set will ever go
dark as newer and better extension layers are built atop ita set bought
honoring the initial standard will always continue to work even as the
extension layers are added in the future. Any future TV broadcast at higher
resolution would still display at the base-layer resolution on any initial
set. This is because the higher resolution would consist of the base layer
resolution plus an enhancement layer that is added to it to form the higher
resolution. The base layer would simply be stripped off by the older set
and displayed. The newer set would combine the two layers and display the
higher resolution picture.
One of the advantages of the CICATS proposal is that 4-5 times less
memory is required in a CICATS receiver than in an ACATS receiver that
must decode all 18 ACATS formats, including high-resolution formats that
are prohibitively expensive for most people today. They would have to buy
a receiver with all the memory despite the fact that they needed far less.
And they would have to pay for it. We believe this is an unnecessary burden
on the consumer. Our base-layer concept gives higher-resolution to the
consumer when the costs for it have dropped substantially in about 5 yearswhile
increasing the quality of TV even at the base layer initially offered.
All computer displays are progressively scanned, so CICATS believes
it is essential to rid the national standard of old-fashioned interlaced
scanning formats. Unfortunately, the ACATS proposal includes several interlaced
formats. If even one is allowed then all receivers (including all PCs)
would have to provide for the conversions. These conversions are unnecessary
and are difficult to do with quality. Cost pressures would dictate that
they would be done at low quality.
Furthermore, all computer displays have square pixel spacing (see Glossary)
and therefore so does the CICATS video format. The ACATS proposal includes
several formats with non-square pixel spacing. Again, if even one is allowed
then all receivers (including all PCs) would have to provide for the conversion.
Two further specifications are required to fully describe the CICATS
single video format: horizontal resolution and temporal resolution (frame
rate). The format should have:
- Spatial Resolution: A spatial base layer with horizontal resolution
determined by the CICATS requirement for square pixel spacing. For example,
a TV set with an aspect ratio of 4:3 and 480 lines of vertical resolution
would have 640 pixels horizontally.
- Temporal Resolution: A temporal base layer supporting 24, 36, and 72
Hz frame rates.
The notion of a temporal base layer is a new one to these FCC-related
discussions and needs some explanation. For example, it might appear that
we are proposing three video formats here, one each at 24, 36, and 72 Hz.
This is not the case and hereÍs why:
In case of three separate formats, the broadcaster selects one of the
three to transmit and the receiver detects which one is sent and converts,
if necessary, to its local frame rate. Frame rate conversions are the most
difficult, of all the conversions implied by the ACATS proposal, to do
with quality at a low price. In the case of a temporal base layer, all
sets would implement the base layer (by definition of a base layer), hence
all three frame rates would be implemented. Regardless of transmitted frame
rate, a set receiving the proposed temporal base layer signal would operate
at 72 Hz frame rate. It would select and decode the appropriate MPEG2 frames
(I, P, and B frames in MPEG2 terminology) to form the 72 Hz display. The
base layer technology makes this simple to do. It is a selection process
rather than a conversion process.
It is important to note that the CICATS temporal base layer does not
support 30 Hz or 60Hz. 30 Hz is a relic of interlaced scanning so is not
needed in the progressively scanned future. The PC market has determined
that 60 Hz is insufficient so it is not included in the CICATS temporal
base layer.
But CICATS, again, understands that the FCC might have to support 60
Hz under pressure from the old analog world. In this case, we propose an
alternative to the temporal base layer:
- Temporal Resolution (Alternative): 24, 60, and 72 Hz frame rates. Not
a temporal base layer.
This alternative does extend the CICATS proposal to three video formats,
but the three differ only in frame rate. Although we offer this alternative,
we want the FCC to understand that it implies conversion hardware and more
memory in the receiver, hence more cost to the consumer. Furthermore, the
conversions between 60 and 72 Hz are particularly prone to poor quality.
Nevertheless, 60 Hz display displayed on 60 Hz sets and 72 Hz displayed
on 72 Hz sets would suffer no quality loss.
Spatial Resolution: Reference Decoder
One way to look at the CICATS proposal is that it severs the decision
to go digital from the decision to go high-resolution (or ñhigh-definitionî).
We believe that going digital is the fundamental revolutionary step. We
want to concentrate on doing it right. We believe that adding high resolution
is straightforward if the groundwork is in place. We encourage adoption
of a posture that allows this to happen when MooreÍs Law makes it more
economically feasible than now, in about 5 years. We re-emphasize, however,
that the CICATS base layer alone has higher perceived resolution than todayÍs
TV.
The preferred way to think of the CICATS proposal is in terms of a reference
decoder. The CICATS Reference Decoder has a memory capable of supporting
1024 horizontal by 512 vertical pixels. This plus the requirement for square
pixel spacing implies that the Reference Decoder is capable of decoding
any resolution up to and including 1024x512. The following table shows
several examples supported by the Reference Decoder on TV displays of various
aspect ratios:
Rather than propose a single video format, CICATS proposes that the
FCC mandate the Reference Decoder. Then the choice of horizontal resolution
becomes a secondary choice. This choice would be left to industrythat
is, to market demand.
The CICATS Reference Decoder is a way of specifying a class of video
formats acceptable to the computer industry. It is a hardware specification
to the same degree that an ACATS video format is a hardware specification.
That is, it puts requirements on the hardware but does not specify the
implementation that satisfies them. Following are some example uses of
the Reference Decoder.
There are arguments for the choice to transmit 640x480 pixels: It is
consistent with todayÍs capabilities. Progressively-scanned 640x480 systems
have already been demonstrated. The costs and demands for this resolution
are well known. The aspect ratio of 4:3 is the current one for which CRT
(cathode-ray tube) technology is already well-suited and cost effective.
Computer displays are as comfortable with this format as are TV displays.
The cost of a converter for one of todayÍs analog sets to receive the new
digital signal is minimal for this resolution. CICATS believes this format
to be the one most likely to appeal pricewise to consumers now, encouraging
them to convert to the new digital standard and thereby release the old
analog spectrum.
There are, however, good arguments for other choices within the set
allowed by the the Reference Decoder. Consider, for example, 1024 by 512
pixels, the maximum allowed by the Reference Decoder (base layer only).
The vertical resolution would be higher than todayÍs analog TV because
512 is greater than 480, but more importantly because progressive 512 lines
is equivalent to about 780 interlaced lines. And the horizontal resolution
(on a TV set with aspect ratio 2:1) would be very much higher than todayÍs
analog TV, as well as spread out much wider. 2:1 aspect is considered desirable
by Hollywood. Enhancement later (with an enhancement layer added to the
base layer) to a nominal 2048x1024 resolution would be straightforward.
But there are serious counterarguments against the 1024 by 512 choice.
The most serious is that displays for such an aspect ratio have not been
demonstrated. Even if they were, they would probably be exorbitantly expensive
at this time. So the same argument we levy against the expensive ACATS
array of formats holds against this format too: Only the wealthy would
be able to afford it at first. Sets that displayed in the old 4:3 aspect
would either have to letter-box the wide aspect ratio, or pan-and-scan
in it, or both (MPEG2 supports all of these choices). Both of these are
familiar practices in widescreen films broadcast on TV today. All sets
would implement the Reference Decoder but only those capable of 2:1 aspect
would get full benefit of the signal.
Notice that a format with approximately 16:9 aspect could be chosen
within the parameters of the CICATS Reference Decoder. This is one of the
ACATS proposed aspect ratios. This aspect ratio has some of the same problems
as just discussed for the 2:1 aspect ratio. In particular, sets to display
at that ratio are too expensive for the average consumer. It is not an
interesting aspect ratio for Hollywood. On the other hand, CRTs of that
aspect have been demonstrated. Pan-and-scan or letterboxing would be required
for satisfactory display on sets of smaller aspect ratio, as discussed
above for the 2:1 ratio.
In any case, the new digital TV sets would implement the Reference Decoder.
They would need 4-5 times less memory than the equivalent ACATS-compliant
set so would be optimally cost effective for consumersand at no loss
in quality implied by the conversions required between the 18 ACATS formats
at the receiver. The CICATS proposal would be cheaper and better. Over
time the cost differential between the two types of sets would diminish
(with MooreÍs Law again) but in the meantime, US consumers would have paid
many billions of dollars for unnecessary conversion and suffered unnecessary
loss of quality as well.
Spatial Resolution: Enhancement Layers
Since the submissions to the FCC so far have confused the change to
digital with the change to high resolution, it is important that the CICATS
proposal not be interpreted as sacrificing the push toward higher resolution.
Higher resolution is desirable to all members of CICATS and to all consumers.
We argue that most consumers will not be interested so long as the price
tag is in the thousands of dollars per set, as it is today. We believe
that high resolution will certainly arrive, as costs drop with MooreÍs
Law, and hence are proposing a base-layer / enhancement-layer technology
that paves the way (see layering in the Glossary).
We propose that the FCC suggestas a recommended practicehow
enhancement layers might be used atop the standardized base layer to reach
higher resolutions immediately, if certain market sectors wish to pursue
it now. We emphasize that this should not be mandated.
More important than a specific enhancement layer is a process for adding
enhancement layers and how they work. Following is one of many possibilities:
Suppose that the 1024x512 resolution discussed above has been chosen
with the framework of the Reference Decoder. Suppose further that memory
prices have dropped substantially so that now a memory of 2048x1024 pixels
is as cheap as 1024x512 pixels is today. MooreÍs Law tells us this will
happen in a few years. At that time the PC and TV industries might decideor
the FCC might decidethat it is time to add an enhancement layer to
the base layer already in place in the national digital TV standard. An
example enhancement layer that would work in this case is this:
The base layer is expanded by 3/2 to get a resolution of 1536x768 pixels.
The difference is sent in an enhancement layer. 3/2 is a straightforward
multiplication factor for digital images . Notice that 768 lines, progressively
scanned, is perceptually equivalent to about 1180 lines interlaced, greater
than the 1080 lines interlaced in the ACATS highest proposed resolution.
CICATS has determined, from the work of Gary
Demos of DemoGraFX, that all of this fits into a given 6-megaHz digital
TV channel using MPEG2 compression technology.
Temporal Resolution: Temporal Base Layer
This is an example of another new idea that goes beyond simply digitizing
the analog video world as currently understood (the idea of a reference
decoder being the other). The notion is to apply the base-layer / enhancement-layer
concept in the time dimension as well as in the space dimension. The receiver
of a digital TV signal would select among the various entities provided
by the MPEG2 bitstream.
DemoGraFX has proposed a temporal base layer that supports either 24
Hz or 36 Hz frame rates and a temporal enhancement layer that goes up to
72 Hz. Since CICATS proposes 24, 36, and 72 Hz frame rates, it is convenient
to think of the DemoGraFX temporal base layer and enhancement layer to
72 Hz as a single CICATS temporal base layer.
As explained above, regardless of transmitted frame rate, a set receiving
the CICATS temporal base layer signal would operate at 72 Hz frame rate.
The Reference Decoder, which incorporates the CICATS temporal base layer,
would select and decode the appropriate MPEG2 frames (I, P, and B) to form
the 72 Hz display.
This technology has been demonstrated by Gary Demos of DemoGraFX and
formally reported to the SMPTE (Society of Motion Picture and Television
Engineers) . Displays capable of 72 Hz are commonplace today in computers.
Just as the Reference Decoder supports many more spatial formats than
will actually be used, it also supports more temporal formats than will
probably be used. For example, the 36 Hz format might not typically be
used, but it comes ñfor freeî with the CICATS Reference Decoder (just as
the other spatial formats do).
Temporal Alternative: 3 Frame Rates
CICATS understands that there is immense pressure from the existing
analog TV industry to maintain the current frame rate of 60 Hz. CICATS
would prefer to see new and legacy material at this frame rate converted,
at high quality, at the head-end before transmission over the new digital
channels. However, we are willing to expand our one simple format (expressed
in terms of the Reference Decoder) into three formats that differ only
in frame rates: 24, 60, and 72 Hz. This does not have near the elegance
of the temporal base layer proposal but it does allow continued use of
60 Hz.
Use of three formats requires interconversion. Interconversions have
been our argument against the ACATS proposal because they typically imply
loss of quality in order to meet realistic consumer price expectations.
However, the very difficult conversions between interlace and progressive
scan are not asked for in this alternative 3-format proposal, and spatial
conversions are not required either. Only temporal conversions are required.
The most tricky conversions are between 60 and 72 Hz because they are
relatively so near one another. Conversions tend to show artifacts related
to the 12 Hz difference, a very visible frequency for the human eye.
Furthermore, conversions imply more machinery and hence higher costs.
Finally, the three-format scenario is not a base layer concept so there
is no clear enhancement path to higher frame rates in the future.
Nevertheless CICATS would compromise to three video formats differing
only in frame rate so long as the FCC understood the interconversion problem
and its consequences.
Summary
CICATS proposes that the FCC mandate the ACATS low-level protocols for
the new national digital TV broadcast channels.
CICATS proposes that the FCC not mandate video data formats for the
new digital channels. CICATS proposes that the FCC institute a study group
chartered to return a finding within one year on how to improve the low-level
digital TV protocols by several orders of magnitude to accommodate error-free
transmission of non-video or non-pictorial data. It is understood that
a satisfactory solution might be precluded by proceeding with the low-level
protocols before this study is made.
CICATS recognizes certain political realities that may cause the FCC
trouble in pursuing the proposals above. The following alternative proposals
are aimed at alleviating these problems, so long as it is understood that
CICATS believes them to be inferior positions. CICATS alternatively proposes
that the FCC mandate the CICATS Reference Decoder that handles up to 1024
pixels horizontally and up to 512 lines vertically (without enhancement),
is progressively scanned exclusively, has square pixel spacing exclusively,
is a spatial and temporal base-layer technology, and supports frame rates
of 24 Hz, 36 Hz, and 72 Hz (without enhancement).
CICATS alternatively proposes that the alternative immediately above
be further modified only by substitution of three frame rates for the temporal
base layer concept, the three rates being 24 Hz, 60 Hz, and 72 Hz. It is
understood that by so doing conversions are required in the receiving sets,
implying cost and quality penalties (but far less than those associated
with the 18-format ACATS proposal).
CICATS further proposes that the FCC suggest, as recommended practice,
how the CICATS spatial base layer might be enhanced to higher resolutions.
These enhancements are not to be mandated at this time.
CICATS proposes that the FCC recommend that old analog content be used
only on old analog channels, or else be converted at high quality at the
transmission head-end to the new digital signal for use on the new digital
TV channels.
Glossary ACATS: Advisory Committee on Advanced Television Service,
to the FCC.
Aspect ratio: The ratio of the width of a picture to its height.
Standard (current) TV has an aspect ratio of 4:3 ("4 to 3") =
1.333. The ACATS proposal mixes 4:3 with 16:9 aspect ratios. 16:9 = 1.777
is a strange aspect ratio that is wider than current TV but is not a Hollywood
compatible aspect ratio. Hollywood films are most often in 1.85 ("academy")
aspect or in 2.37 ("scope") for very wide-screen films. Hollywood
would apparently be content with a 2:1 aspect ratio, but not with 16:9.
Base Layer: See layering.
CICATS: Computer Industry Coalition on Advanced Television Service,
representing 10 leading personal computer companies (hardware and software).
FCC: The Federal Communications Commission.
Frame rate: The number of video pictures displayed per second.
The goal is to seem continuous. Film's frame rate is 24 frames per second,
where each frame is repeated 2 (or sometimes 3) times by a film projector
to give the equivalent frame rate of 48 frames per second (or sometimes
72). The word "Hertz" is used often to abbreviate "frames
per second". The highest ACATS frame rate is 60 Hz ("60 Hertz"
or 60 frames per second), whereas computer consumers rejected 60 years
ago in favor of 70 or more frames per second to avoid objectionable flicker.
(Looking at a TV or PC screen out of oneÍs peripheral vision reveals the
flicker.) 72 Hz is an attractive frame rate because it is computer friendly
and an easy multiple of film rate (film is a major source of all TV content).
Hertz (Hz): One Hertz is short for one cycle per second, or one
frame per second. Frequencies were formerly expressed in cycles per secondfor
example, a radio station might broadcast at 98.1 on the radio dial, meaning
at 98.1 megacycles per second. Today this would be expressed as 98.1 megaHz,
in honor of electromagnetic pioneer Heinrich Hertz. In a related usage,
the ñwidthî of a TV channel is measured in Hz6 megaHz per channel.
Interlace: Current analog TV scans each frame by first drawing
every other horizontal scanline across the face of the TV set, then starting
over at the top and drawing all the skipped in-between scanlines. The first
set, called a "field", is said to be interlaced with the second
set, or second field. Interlaced scanning is opposed to progressive scanning.
Layering: A layered system is a logical system of related frame
sizes, rates, and resolutions (as opposed to a grab-bag of unrelated formats
as in the ACATS proposal). A layered system has a "base layer"
that must be honored plus "enhancement layers" that may be added
to the base layer to make it higher resolution. A good example of a layering
scheme is that used by Kodak's PhotoCD. Snapshots are taken to a photo
house from which they are returned in digital form on a CD, Kodak's PhotoCD.
Each of the snapshots will appear on the CD in several resolutions. The
base resolution is 768x512 (approximately video resolution), but the CD
also contains enhancement layers that are added to the base resolution
to make it into 1536x1024 pixels or 3072x2048 pixels. So one CD contains
at least these three resolutions. Similarly, a layered TV channel could
contain several resolutions simultaneously so long as they were layered
logically. ACATS misuses the term "layering" to simply mean a
TV picture is layered atop a string of digital bits, which is layered atop
a radio frequency modulation technique. Theirs is a much more generic use
of the term than the CICATS (or Kodak) use.
Moore's Law: The "law" that says computers get twice
as fast every 18 months. In general, anything digital gets twice as good
every 1.5 years. For example, memory doubles or the processor gets twice
as fastfor a fixed costevery 1.5 years. To understand how stunningly
fast this is, let's restate it as 10 times faster every 5 years (that's
the same as 2 times faster every 1.5 years). During the 8 years that ACATS
has been working on its proposal, personal computers have increased in
speed and memory by a factor of 50 to 100 times (at the same cost). At
the beginning of the ACATS process, PCs weren't powerful enough for TV,
but now they are. There is good reason to believe that Moore's Law will
continue to operate for another 15 years - thus for another improvement
of 1000 times over what we have today! This incredible digital revolution
is what makes CICATS encourage the FCC not to freeze any digital standards
now that it could better make 5-10 years from. We are simply incapable
of predicting what an "order of magnitude" (10x) change means
conceptually. Any standards made now will look foolish 5 years from now,
so only the minimum should be done now. (Two years ago there was no Netscape,
and Microsoft was not an Internet company. Things change very fast in the
digital world. The old analog modes of thinking do not work.)
Order of Magnitude: One power of 10. So 100 is two orders of
magnitude larger than 1. The term ñorder of magnitudeî means more than
simply a larger number, however. It implies a conceptual change as well.
MooreÍs Law says that computers get faster by an order of magnitude every
5 years (at a fixed cost), but more importantly it also means that we require
a different level of understanding every 5 years.
Pixel: Short for picture element. In the digital world, a picture
is represented by an array of tiny samples or picture elements - so many
per line and so many lines. (Pixels, by the way, are single points, not
little squares or rectangles as popularly described. We are careful to
say ñsquare pixel spacingî, not "square pixels".)
Progressive: Current PC screens draw each scanline in order from
top to bottom. They are said to be "progressively scanned". This
is opposed to interlaced scanning. Reference Decoder: CICATS proposes a
layered video format scheme by way of a reference decoder, which is a specification
of the decoder of the new digital TV signalseparate from the display
of that signal. This concept permits a degree of freedom not formerly present
in these FCC-related discussions. For example, instead of specifying a
specific horizontal resolution, which depends highly on the capabilities
of a particular display, the CICATS Reference Decoder says only that the
format must have 480 progressive lines (nominally) and square pixel spacing.
So, if the display device has electronics and width enough to handle a
2:1 aspect ratio, then the Reference Decoder will honor a width of 960
pixels (assuming the industry has agreed to broadcast this signal). If
the display device can only handle 4:3 aspect ratio (as the affordable
ones today do), then the Reference Decoder would dictate a horizontal resolution
of 640 pixels. The same decoder circuit, at the same parts cost, would
handle either situation. The Reference Decoder is not hardware. It is a
way of specifying a class of acceptable video formats rather than a single
video format. Within this class there is no conversion required, but wide
signals (wide aspect ratio) would have to be either letterboxed or pan-and-scanned
to a display with smaller aspect ratio.
Resolution: The number of pixels per line and the number of lines
- equivalently, the number of horizontal pixels and the number of vertical
pixels. Thus a resolution might be given as 2048x1024 pixels, meaning 1024
scanlines with 2048 pixels on each scanline - equivalently, a rectangular
array of 2048 times 1024 pixels (about 2 million pixels). Standard (current)
TV has 480 lines vertical resolution and about 700 horizontal. But it is
interlaced, which brings down its effective vertical resolution to about
320 scanlines. The ACATS "high-definition" format of 1080 vertical
lines is really about 700 lines since the 1080 lines are interlaced at
the 60 Hz frame rate.
Spectrum: Simply all the channels used for TV, cable TV, AM radio,
FM radio, ham radio, and so forth - treated as a single entity. The full
electromagnetic spectrum is vast, including X rays, heat, and even ordinary
light. The FCC has dominion over only the "radio frequency"those
uses listed in the first sentence of this paragraph. That is, the radio
frequency spectrum is a subset of the full electromagnetic spectrum. One
TV channel, whether old analog or new digital, is a slice of the radio
frequency spectrum.
Square pixel spacing: This just means that the horizontal spacing
between pixels is the same as the vertical spacing between pixels (or between
scanlines). Although the conversion from non-square pixel spacing of many
of the ACATS formats to square pixel spacing is straightforward, there
are about 200 million PCs in existence that assume square pixel spacing
and do not have the software for doing the conversion.
