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Serial Digital Video/Audio Systems 8-33

The standard also offers a limited capability for metadata to be added, providing packet con-

trol information to aid the successful transfer of packets. The specification of the metadata fol-
lows the K-L-V approach of the SMPTE dynamic metadata dictionary and provides extensibility
for future requirements.

Timing Issues

Most packet streams do not have critical timing requirements and a decoder can output packets
in the order in which they were encoded, but with increased packet jitter resulting from the buff-
ering of packets onto SDTI lines [16]. The result of the SDTI-PF packet encapsulation process is
to introduce both delay and jitter to the packet stream. However, MPEG-2 transport stream
(MPEG-2 TS) packets are one case where a relatively small packet jitter specification is required
to ensure minimal impact on MPEG-2 transport stream clock recovery and buffering circuits.
SMPTE 332M contains provisions to allow the packet jitter to be reduced to insignificant levels;
the delay is an issue addressed by the method of packet buffering at the encoder. As a bench-
mark, the specification is defined so that a low packet jitter source can be carried through the
SDTI-PF and be decoded to create an output with negligible packet jitter.

Although MPEG-2 TS packets are the most critical packet type for decoder timing accuracy,

this standard also allows for other kinds of packets to be carried over the SDTI, with or without
buffering, to reduce packet jitter. Such packets may be ATM cells and packets based on the uni-
directional Internet protocol (Uni-IP).

MPEG Decoder Templates

SMPTE Recommended Practice RP 204 defines decoder templates for the encoding of SDTI
content packages with MPEG coded picture streams [15]. The purpose of RP 204 is to provide
appropriate limits to the requirements for a receiver/decoder in order to allow practical working
devices to be supplied to meet the needs of defined operations. Additional MPEG templates are
expected to be added to the practice as the SDTI-CP standard matures. The SMPTE document
recommends that each new template be a superset of previous templates so that any decoder
defined by a template in the document can operate with both the defined template and all sub-
sets.

8.2.5

References

1.

Fibush, David: A Guide to Digital Television Systems and Measurement, Tektronix, Beaver-
ton, OR, 1994.

2.

SMPTE RP 165-1994: SMPTE Recommended Practice—Error Detection Checkwords and
Status Flags for Use in Bit-Serial Digital Interfaces for Television, SMPTE, White Plains,
N.Y., 1994.

3.

SMPTE Standard: SMPTE 305M-1998, Serial Data Transport Interface, SMPTE, White
Plains, N.Y., 1998.

4.

Gaggioni, H., M. Ueda, F. Saga, K. Tomita, and N. Kobayashi, “The Development of a
High-Definition Serial Digital Interface,” Sony Technical Paper, Sony Broadcast Group,
San Jose, Calif., 1998.

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Serial Digital Video/Audio Systems

8-34 Audio Networking

5.

“SMPTE Standard for Television—Bit-Serial Digital Interface for High-Definition Televi-
sion Systems,” SMPTE 292M-1998, SMPTE, White Plains, N.Y., 1998.

6.

“SMPTE Standard for Television—24-Bit Digital Audio Format for HDTV Bit-Serial
Interface,” SMPTE 299M-1997, SMPTE, White Plains, N.Y., 1997.

7.

“SMPTE Standard for Television—Mapping of AES3 Data into MPEG-2 Transport
Stream,” SMPTE 302M-1998, SMPTE, White Plains, N.Y., 1998.

8.

“SMPTE Standard for Television (Proposed)—Vertical Ancillary Data Mapping for Bit-
Serial Interface, SMPTE 334M, SMPTE, White Plains, N.Y., 2000.

9.

“SMPTE Standard for Television (Proposed)—Signals and Generic Data over High-Defini-
tion Interfaces,” SMPTE 346M, SMPTE, White Plains, N.Y., 2000.

10.

“SMPTE Standard for Television (Proposed)—High Data-Rate Serial Data Transport Inter-
face (HD-SDTI),” SMPTE 348M, SMPTE, White Plains, N.Y., 2000.

11.

SMPTE 344M (Proposed), “540 Mb/s Serial Digital Interface,” SMPTE, White Plains,
N.Y., 2000.

12.

Legault, Alain, and Janet Matey: “Interconnectivity in the DTV Era—The Emergence of
SDTI,”  Proceedings of Digital Television '98, Intertec Publishing, Overland Park, Kan.,
1998.

13.

“SMPTE Standard for Television—SDTI Content Package Format (SDTI-CP),” SMPTE
326M-2000, SMPTE, White Plains, N.Y., 2000.

14.

“SMPTE Standard for Television—Element and Metadata Definitions for the SDTI-CP,”
SMPTE 331M-2000, SMPTE, White Plains, N.Y., 2000.

15.

“SMPTE Recommended Practice—SDTI-CP MPEG Decoder Templates,” RP 204 (Pro-
posed), SMPTE, White Plains, N.Y., 1999.

16.

“SMPTE Standard for Television—Encapsulation of Data Packet Streams over SDTI
(SDTI-PF),” SMPTE 332M-2000, SMPTE, White Plains, N.Y., 2000.

8.2.6

Bibliography

“SMPTE Standard for Television—Ancillary Data Packet and Space Formatting,” SMPTE

291M-1998, SMPTE, White Plains, N.Y., 1998.

“SMPTE Standard for Television—Bit-Serial Digital Interface for High-Definition Television

Systems,” SMPTE 292M-1996, SMPTE, White Plains, N.Y., 1996.

“SMPTE Standard for Television—Serial Data Transport Interface,” SMPTE 305M-1998,

SMPTE, White Plains, N.Y., 1998.

Turow, Dan: “SDTI and the Evolution of Studio Interconnect,” International Broadcasting Con-

vention Proceedings, IBC, Amsterdam, September 1998.

Wilkinson, J. H., H. Sakamoto, and P. Horne: “SDDI as a Video Data Network Solution,” Inter-

national Broadcasting Convention Proceedings, IBC, Amsterdam, September 1997.

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Serial Digital Video/Audio Systems

8-35

Chapter

8.3

Video/Audio Networking Systems

Jerry C. Whitaker, Editor-in-Chief

8.3.1

Introduction

Video networking depends—to a large extent—on system interoperability, not only for basic
functions but also for extended functionality. Interoperability has two aspects. The first is syntac-
tic and refers only to the coded representation of the digital television information. The second
relates to the delivery of the bit stream in real time.

Broadcast digital video systems, specifically DTV and DVB, support bit streams and services

beyond basic compressed video and audio services, such as text-based services, emergency mes-
sages, and other future ancillary services. The MPEG-2 transport packet size is such that it can
be easily partitioned for transfer in a link layer that supports ATM transmission. The MPEG-2
transport layer and the ATM layer serve different functions in a video delivery application: the
MPEG-2 transport layer solves MPEG-2 presentation problems and performs the multimedia
multiplexing function, and the ATM layer solves switching and network-adaptation problems.

In addition to ATM, a number of networking systems have been developed or refined to carry

digital video signals, including IEEE 1394, Fibre Channel, and Gigabit Ethernet.

8.3.2

Architecture of ATM

Asynchronous transfer mode is a technology based on high-speed packet switching. It is an ideal
protocol for supporting professional video/audio and other complex multimedia applications.
ATM is capable of data rates of up to 622 Mbits/s.

ATM was developed in the early 1980s by Bell Labs as a backbone switching and transporta-

tion protocol. It is a high-speed integrated multiplexing and switching technology that transmits
information using fixed-length cells in a connection-oriented manner. Physical interfaces for the
user-network interface (UNI) of 155.52 Mbits/s and 622.08 Mbits/s provide integrated support
for high-speed information transfers and various communications modes—such as circuit and
packet modes—and constant, variable, or burst bit-rate communications. These capabilities lead
to four basic types of service classes of interest to video users [1]:

•

Constant bit rate (CBR), which emulates a leased line service, with fixed network delay

•

Variable bit rate (VBR), which allows for bursts of data up to a predefined peak cell rate

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Source: Standard Handbook of Audio and Radio Engineering

8-36 Audio Networking

•

Available bit rate (ABR), in which capacity is negotiated with the network to fill capacity
gaps

•

Unspecified bit rate (UBR), which provides unnegotiated use of available network capacity

These tiers of service are designed to maximize the traffic capabilities of the network. The CBR
data streams are fixed and constant with time; the VBR and ABR systems vary. The bandwidth
of the UBR class of service is a function of whatever network capacity is left over after all other
users have claimed their stake to the bandwidth. Not surprisingly, CBR is usually the most
expensive class of service, and UBR is the least expensive. Figure 8.3.1 illustrates typical pack-
ing of an ATM trunk.

One of the reasons ATM is attractive for video applications is that the transport of video and

audio fits nicely into the established ATM service classes. For example, consider the following
applications:

•

Real-time video—which demands real-time transmission for scene capture, storage, process-
ing, and relay—fits well into the CBR service class.

•

Non-real-time video—such as recording and editing from servers, distributing edited masters,
and other operations that can be considered essentially off-line—can use the ABR service.

Figure 8.3.1

 The typical packing of an internodal ATM trunk. (

After [2].)

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Video/Audio Networking Systems

Video/Audio Networking Systems 8-37

•

Machine control and file transfer—such as sending still clips from one facility to another—
find the VBR service attractive.

ATM is growing and maturing rapidly. It already has been implemented in many industries,

deployed by customers who anticipate such advantages as:

•

Enabling high-bandwidth applications including desktop video, digital libraries, and real-time
image transfer

•

Coexistence of different types of traffic on a single network platform to reduce both the trans-
port and operations costs

•

Long-term network scalability and architectural stability

In addition, ATM has been used in both local- and wide-area networks. It can support a variety of
high-layer protocols and will cope with future network speeds of gigabits per second.

8.3.2a

ATM Cell Structure

It is worthwhile to explore the ATM channel format in some detail because its features are the
key to the usefulness of ATM for video applications. ATM channels are represented by a set of
fixed-size cells and are identified through the channel indicator in the cell header [2]. The ATM
cell has two basic parts: the header (5 bytes) and the payload (48 bytes). This structure is shown
in Figure 8.3.2. ATM switching is performed on a cell-by-cell basis, based on the routing infor-
mation contained in the cell header.

Because the main function of the ATM layer is to provide fast multiplexing and routing for

data transfer based on information included in the header, this element of the protocol includes
not only information for routing, but also fields to indicate the type of information contained in
the cell payload. Other data is included in the header to perform the following support functions:

•

Assist in controlling the flow of traffic at the UNI

•

Establish priority for the cell

•

Facilitate header error-control and cell-delineation functions

One key feature of ATM is that the cells can be independently labeled and transmitted on

demand. This allows facility bandwidth to be allocated as needed, without the fixed hierarchical

Figure 8.3.2

 The ATM cell format. (

After [2].)

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Video/Audio Networking Systems