Digital Signal Processing Handbook P56

Tzou, K. “Digital Television”
Digital Signal Processing Handbook
Ed. Vijay K. Madisetti and Douglas B. Williams
Boca Raton: CRC Press LLC, 1999
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56
Digital Television
Kou-Hu Tzou
Hyundai Network Systems
56.1 Introduction
56.2 EDTV/HDTV Standards
MUSE System
•
HD-MAC System
•
HDTV in North America
•
EDTV
56.3 Hybrid Analog/Digital Systems
56.4 Error Protection and Concealment
FEC
•
Error Detection and Confinement
•
Error Concealment
•
Scalable Coding for Error Concealment
56.5 Terrestrial Broadcasting
Multipath Interference

•
Multi-Resolution Transmission
56.6 Satellite Transmission
56.7 ATM Transmission of Video
ATM AdaptationLayerfor Digital Video
•
Cell Loss Protection
References
56.1 Introduction
Digital television is being widely adopted for various applications ranging from high-end applica-
tions, such as studio recording, to consumer applications, such as digital cable TV and digital DBS
(Direct Broadcasting Satellite) TV. For example, several digital video tape recording standards, using
component format (D1 and D5), composite format (D2 and D3), or compressed component formats
(Digital Betacam) are commonly used by broadcasters and TV studios [1]. These standards preserve
the best possible picture quality at the expense of high data rates, ranging from approximately 150 to
300 Mbps. When captured in a digital format, the picture quality can be free from degradation dur-
ing multiple generations of recording and playback, which is extremely attractive to studio editing.
However, transmission of these high data-rate signals may be hindered due to lack of transmission
media with an adequate bandwidth. Although it is possible, the associated transmission cost will be
very high. The bit rate requirement for high definition television (HDTV) is even more demanding,
which may exceed 1 Gbps in an uncompressed form. Therefore, data compression is essential for
economical transmission of digital TV/HDTV.
Before motion-compensated DCT coding technology became mature in recent years, transmission
of high-quality digital television used to be carried out at 45 Mbps using DPCM techniques. Today,
by incorporating advanced motion-compensated DCT coding, comparable picture quality can be
achieved at about one-third of the rate required by DPCM-coded video. For entertainment applica-
tions, the requirement on picture quality can be relaxed a little bit to allow more TV channels to fit
into the same bandwidth. It is generally agreed that 3 to 4 Mbps for movie-originated or low-activity
interlaced video (talk shows, etc.) materials is acceptable, and 6-8 Mbps for high-activity interlaced
video (sports, etc.) is acceptable. The targeted bit rate for HDTV transmission is usually around

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20 Mbps, which is chosen to match the available digital bandwidth of terrestrial broadcast channels
allocated for conventional TV signals.
56.2 EDTV/HDTV Standards
The concept of HDTV system and efficient transmission format was originally explored by researches
atNHK (JapanBroadcastingCorp.) morethan 20 years ago[2] inorder to offersuperior picture qual-
ity while conserving bandwidth. Main HDTV features, including more scan lines, higher horizontal
resolution, wider aspect ratio, better color representation, and higher frame rate, were identified.
With these new features, HDTV is geared to offer picture quality close to that of 35-mm prints.
However, the transmission of such a signal will require a very wide bandwidth. During the last 20
years, intensive research efforts have been engaged toward video coding to reduce bandwidth.
Currently there are two dominant HDTV production formats being used worldwide; one is the
1125-line/60-Hz system primarily used in Japan and the U.S. and the other is the 1250-line/50-Hz
system primarily used in Europe. The main scanned raster characteristics of these two formats are
listed in Table 56.1. The nominal bandwidth of the luminance component is about 30 MHz (in some
cases, 20 MHz was quoted). Roughly speaking, the HDTV signal can carry about six times as much
information as a conventional TV signal.
TABLE 56.1 Main Scanned Raster Characteristics of the 1125-line/60-Hz
System and the 1250-line/50-Hz System
Total scan lines Active lines Scanning Aspect
Format per frame per frame format ratio Field rate
1 1125 1035 2:1 interlaced 16:9 60.00/59.94
2 1250 1152 2:1 interlaced 16:9 50.00
Development of HDTV transmission techniques in the early days was focused on bandwidth-
compatible approaches that use the same analog bandwidth as a conventional TV signal. In some
cases, in order to conserve bandwidth or to offer compatibility with an existing conventional signal
or display, a compromised system—Enhanced or Extended Definition TV—was developed instead.
The EDTV signal does not offer the picture quality and resolution required for an HDTV signal;

however, it enhances the picture quality/resolution of conventional TV.
56.2.1 MUSE System
The most well-known early development in HDTV coding is the MUSE (Multiple Sub-Nyquist
Sampling Encoding) system at NHK [3, 4]. The main concept of the MUSE system is adaptive
spatial-temporal subsampling. Since human eyes have better spatial sensitivity for stationary or
slow-moving scenes, the full spatial resolution is preserved while the temporal resolution is reduced
for these scenes in the MUSE system. For fast moving scenes, the spatial sensitivity of human eyes
declines so that reducing the spatial resolution will not significantly affect perceived picture quality.
The MUSE signal is intended for analog transmission with a baseband bandwidth of 8.1 MHz,
which can be fitted into a satellite transponder for a conventional analog TV signal. However, it
should be noted that most signal processing employed in the MUSE system is in the digital domain.
The MUSE coding technique was later modified to reduce bandwidth requirement for transmission
over 6-MHz terrestrial broadcasting channels (Narrow-MUSE) [5]. Currently, MUSE-based HDTV
programming is being broadcast regularly through a DBS in Japan.
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56.2.2 HD-MAC System
A development similar to the MUSE was initiated in Europe as well. The system, HD-MAC (High-
DefinitionMultiplexedAnalogComponent), isalso based onthe concept ofadaptivespatial-temporal
subsampling. Depending on the amount of motion, each block, consisting of8×8 pixels, is classified
into either the 20-, 40-, or 80-ms mode [6]. For a fast-moving block (the 20-ms mode), it is
transmitted at the full temporal resolution, but at 1/4 spatial resolution. For a stationary or slow-
moving block (the 80-ms mode), it is transmitted at full spatial resolution, but at 1/4 temporal
resolution (25/4 frames/sec). For the 40-ms block, it is transmitted at half spatial and half temporal
resolutions. The mode associated with each block is transmitted as side information through a digital
channel at a bit rate nearly 1 Mbps. The subsampling process of the HD-MAC system is illustrated
in Fig. 56.1, where the numbers indicate the corresponding fields of transmitted pixels and the “·”
indicates a pixel not transmitted.
FIGURE 56.1: Adaptive spatial-temporal subsampling of the HD-MAC system. (a) The 80-ms mode

for stationary to very-slow moving scenes, (b) the 40-ms mode for medium-speed moving scenes,
and (c) the 20-ms mode for fast moving scene.
56.2.3 HDTV in North America
HDTV development in North America started much later than that in Japan and Europe. The
Advisory Committee on Advanced Television Services (ACATS) was formed in 1987 to advise Federal
Communications Commission (FCC) on the facts and circumstances regarding advanced television
systems for terrestrial broadcasting. The proposed systems in early days were all intended for analog
transmission [7]. However, the direction of U.S. HDTV development took a 180-degree turn in
1990 since General Instrument (GI) entered the U.S. HDTV race by submitting an all-digital HDTV
system proposal to the FCC. The final contender in the U.S. HDTV race consisted of one analog
system (Narrow-MUSE) and four digital systems, which all employed motion compensated DCT
coding. Extensive testings on the five proposed systems were conducted in 1991 and 1992 and the
testing concluded that there are major advantages in the performance of the digital HDTV systems
and only the digital system shall be considered as the standard. However, none of these four digital
systems was ready to be selected as the standard without implementing improvements.
Withthe encouragement fromACATS, the four U.S. HDTV proponents formed the Grand Alliance
(GA) to combine their efforts for developing a better system. Two HDTV scan formats were adopted
by the GA. The main parameters are shown in Table 56.2. The lower-resolution format, 1280 × 720,
is only used for progressive source materials while the high-resolution format, 1920 × 1080, can
be used for both progressive and interlaced source materials. The digital formats of GA HDTV are
carefully designed to accommodate the square-pixel feature, which provides better interoperability
with digital video/graphics in the computer environment. Since the main structure of MPEG-2
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system and video coding standards were settled at that time and the MPEG-2 video coding standard
provides extension to accommodate HDTV formats, the GA adopted MPEG-2 system and video
coding (Main Profile (MP) at High Level (HL)) standards for the U.S. HDTV, instead of creating
another standard [8]. However, the GA HDTV adopted the AC-3 audio compression standard [9]
instead of the MPEG-2 Layer 1 and Layer 2 audio coding.

TABLE 56.2 Main Scanned Raster Characteristics of the GA HDTV
Input Signals
Active Active lines Scanning
samples/line per frame format Aspect ratio Frame rate
60.00/59.94
1280 720 1:1 progressive 16:9 square pixels 30/29.97
24/23.976
30/29.97
1920 1080 1:1 progressive 16:9 square pixels 24/23.976
1920 1080 2:1 interlaced 16:9 square pixels 30/29.97
56.2.4 EDTV
EDTV refers to the TV signal that offers quality between the conventional TV and HDTV. Usually,
EDTV has the same number of scan lines as the conventional TV, but offers better horizontal res-
olution. Though it is not a required feature, most EDTV systems offer a wide aspect ratio. When
the compatibility with a conventional TV signal is of concern, the additional information (more
horizontal details, side panels, etc.) required by the EDTV signal is embedded in the unused spatial-
temporal spectrum (called spectrum holes) of the conventional TV signal and can be transmitted in
either an analog or digital form [10, 11]. When the compatibility with the conventional TV is not
required, EDTV can use the component format to avoid the artifacts caused by mixing of chromi-
nance and luminance signals in the composite format. For example, several MAC (Multiplexed
Analog Component) systems for analog transmission were adopted in Europe for DBS and cable TV
applications [12, 13]. Usually, these signals offer better horizontal resolution and better color fidelity.
There were many fully digital TV systems developed in the past. These systems that used adequate
spatial resolution and higher bit rates were likely to achieve superior quality to the conventional TV
and were qualified as EDTV [14]. Nevertheless, an efficient EDTV system is already embedded in
the MPEG-2 video coding standard. Within the context of the standard, the 16:9 aspect ratio and
horizontal and vertical resolutions exceeding the conventional TV can be specified in the “Sequence
Header”. When coded with adequate bit rates, the resulting signal can be qualified as EDTV.
56.3 Hybrid Analog/Digital Systems
Today, existing conventional TV sets and other home video equipment represent a massive invest-

ment by consumers. The introduction of any new video system that is not compatible with the
existing system may face strong resistance in initial acceptance and may take a long time to penetrate
households. One way to circumvent this problem during the transition period is to “simulcast” a
program in both formats. The redundant conventional TV, being simulcast in a separate channel,
can be phased out gradually when most households are able to receive the EDTV or HDTV signal.
Intuitively, a more bandwidth efficient approach may be achieved if the transmitted conventional
TV signal can be incorporated as a baseline signal and only the enhancement signal is transmitted
in an additional channel (called “augmentation channel”). In order to facilitate the compatibility,
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an analog conventional TV signal has to be transmitted to allow conventional TV sets to receive
the signal. On the other hand, digital video compression techniques may be employed to code the
enhancement signals in order to accomplish the best compression efficiency. Such systems belong to
the category of hybrid analog/digital system. A generic system structure for the hybrid analog/digital
approachis shown in Fig. 56.2. Due tothe interlacing processing used in TV standards, thereare some
unused holes in the spatial-temporal spectrum [15], which can be used to carry partial enhancement
components as shown in Fig. 56.2.
FIGURE 56.2: A generic hybrid analog digital HDTV coding system.
The Advanced Compatible Television System II (ACTV-II), developed by the consortium of NBC,
RCA, and the David Sarnoff Research Center during the U.S. ATV standardization process, is an
example of a hybrid system. The ACTV-II signal uses a 6-MHz channel to carry an NTSC compat-
ible ACTV-I signal and uses an additional 6-MHz channel to carry the enhancement signal. The
ACTV-I consists of a main signal, which is fully compatible with the conventional NTSC signal, and
enhancement components (luminance horizontal details, luminance vertical-temporal details, and
side-panel details of the wide-screen signal), which are transmitted in 3-D spectrum holes of the
NTSC signal. The differences between the input HDTV signal and the ACTV-I signal are digitally
coded using 4-band subband coding. The digitally coded video difference signal and digital audio
signal require a total bandwidth of 20 Mbps and are expected to fit into the 6-MHz bandwidth by
using the 16-QAM modulation. The enhancement components of the ACTV-I signal are digitally

processed (time expansion and compression) and transmitted in an analog format. Nevertheless,
they could be digitally compressed and transmitted, which would result in a hybrid analog/digital
ACTV-I signal. For users with conventional TV sets, conventional TV pictures (4:3 aspect ratio) will
be displayed. For users with an ACTV-I decoder and a wide screen (16:9) TV monitor, the wide-
screen EDTV can be viewed by receiving the signal from the main channel. For those who have an
ACTV-II decoder and an HDTV monitor, the HDTV picture can be received by using signals from
both the main channel and the associated augmentation channel.
The HDS/NA system developedby Philips Laboratories is another example of hybridanalog/digital
system where the augmentation signal is carried in a 3-MHz channel [16]. The augmentation signal
consists of side panels to convert the aspect ratio from 4:3 to 16:9, and high-resolution spatial
components. The side panels from two consecutive frames are combined into one frame of panels
and are intraframe compressed by using DCT coding with a block size of 16 × 16 pixels. Both the
horizontal and vertical high-resolution components are also compressed by intraframe DCT coding
with some modifications to take into account the characteristics of these signals. The augmentation
signals result in a total bit rate of 6 Mbps, which is expected to fit into a 3-MHz channel using
modulation schemes with efficiency of 2 bits/Hz. However, the HDS/NA system was later modified
intoananalogsimulcastsystem, HDS/NA-6, whichoccupiesonlya6-MHz bandwidth andis intended
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