| Advanced Communications In Transportation |
| Advanced Closed Circuit Television Technologies
for Mass Transit |
| By Robert W. Stong, Atlanta, Georgia, 1-404-870-3223,
c/o mjohnson@ogc.itsmarta.com |
| New technologies are changing the way CCTV systems
are being deployed to serve mass transit. This article reviews the
diverse applications for transit CCTV and highlights new applications
that are evolving as a result of technological improvements and enhanced
features. |
|
A 1998 survey1 taken of U.S. transit properties indicated that
the functionality and degree of closed circuit television (CCTV) deployment
varies widely among transit authorities. Typically, each authority
establishes its own set of CCTV functional criteria that define the
requirements for fixed and pan-tilt-zoom cameras, minimum illumination,
real-time monitoring, archival storage and automation. These criteria
lead to the identification of specific hardware required for each
transit property.
CCTV Objectives
Primary CCTV objectives include improving patron and employee safety
and security, and protecting transit properties by reducing vandalism,
fare evasion and theft by patrons. In addition, theft by employees
at fare collection and maintenance facilities is another concern that
has been addressed by certain authorities. The cameras for these purposes
may be either perceptible or covert.
CCTV can also monitor patron flow during peak operating periods and
facilitate patron assistance at fare gates, ticket
dispensers and waiting areas. In certain cases, a transit authority
may also deploy CCTV systems to identify train locations, monitor
bad weather and allow the train operator to observe patron movement
through the vehicle doors.
Technical Advancements
Technological improvements in CCTV have led to higher performance,
improved functionality and lower initial system cost. These advances
include enclosure assemblies, CCD camera pickup devices and storage
techniques.
Enclosure Assemblies. A camera enclosure assembly
equipped with a pre-assembled camera, pan-tilt mount, zoom lens, fan,
heater, and optical transmitter can reduce field wiring, installation
time and cost to a fraction of previous amounts. Reliable performance
is much easier to achieve with an integrated enclosure assembly.
CCD Camera Pickup Devices. Significant technical
advances in low-cost, charge-coupled device (CCD) camera pickup devices
and backlight compensation circuits have reduced the illumination
requirements greatly. Image burn, a common problem with tube cameras,
is virtually eliminated with a CCD camera pickup element. As a result
of these improvements, color cameras can be deployed more widely.
Today, cameras with expendable tube pickups are used only where extremely
high sensitivity is required. Non-intrusive infrared illuminators
can provide covert CCTV coverage from up to 200 meters (660 feet)
away to serve locations where conventional lighting is undesirable.
Such systems are more common in Europe, where CCTV appears to be deployed
more widely than in North America.
Storage Techniques. Conventional analog videocassette
recorders (VCRs) remain the dominant means of maintaining CCTV archives,
but digital storage techniques are evolving rapidly. Currently, the
initial cost of conventional analog VCRs is lower than that of digital
storage equipment, although they require more maintenance than do
electronic digital storage techniques.
Video multiplexers are commonly used to record up to 16 video signals
simultaneously on a single analog videocassette. The digital frame
storage technology used for multiplexing and demultiplexing processes
also enables other beneficial features to be included, such as automatic
camera sequencing, video motion detection and contact closure alarms.
Unlike conventional motion detectors with passive infrared technology,
video motion detectors offer more flexibility because specific areas
of the video signal can be selected or changed by the operator.
Recently, electronic digital storage media have become available for
recording one or more cameras. These systems typically use a hard
disk for short-term storage and digital audio tape (DAT) for archival
storage. Large systems typically used DAT jukeboxes. These mechanical
storage configurations, like the analog VCR,
are expected to require routing maintenance to minimize failures.
Non-volatile digital storage may be the next video storage alternative.
These systems are relatively expensive, however, because of the large
amount of memory required and the need for an open video compression
architecture. Cameras with built-in digital storage are now being
introduced to the marketplace, although the storage capacity of these
early products remains very limited. These deficiencies will be overcome
eventually by lower storage cost and a standard video compression
architecture.
Video Network Evolution
Conventional coaxial cable is used traditionally as a low-cost means
of video interconnection for a relatively short path as, for example,
within a convenience store. In contrast, most transit video applications
require much longer paths, thereby limiting the capability of coaxial
cables. High-frequency signals can become severely attenuated by long
cable runs, resulting in inferior picture detail. Video equalizers
may be installed to compensate for high-frequency signal losses of
long cables, but equalizers add complexity, increase the system cost
and require manual alignment.
Multimode fiber cables are now a suitable alternative to coaxial cable
for short-haul video transmission within transit stations, multi-level
parking decks, office complexes and highway interchanges. Because
of its nonmetallic construction, optical fiber is relatively immune
to electrical transients, a prominent cause of outages in coaxial
cable systems.
Most short-haul systems on multimode fiber use analog transmission
via either amplitude modulation (AM) or frequency modulation (FM)
analog transmission. The video performance is typically higher for
FM systems. Although digital transmission has not been deployed widely
for short-haul video, digital transmission will surely be a significant
contender in the future.
Controlling PTZ Cameras
Remote pan-tilt-zoom (PTZ) control of video cameras can provide operational
benefits to a transit authority, although a system with PTZ capabilities
can be two or three times as expensive as a fixed-camera system. An
advanced PTZ controller can be programmed to cycle between a set of
camera views automatically, thereby enhancing the camera’s functionality.
Power and control cabling to distant camera enclosures within a transit
station can be a significant complication and expense. Parallel control
wiring for PTZ cameras may no longer be cost-effective control for
cables more than several hundred feet in length. Instead, a single
multimode fiber cable carrying video and two-way control signals will
greatly reduce the number of interconnections and reduce system maintenance.
A cost-effective approach is to deploy low-voltage power wiring and
multimode fiber cables to each camera enclosure from one of several
distribution points in the station. At each distribution point, a
power distribution enclosure receives 120 volts ac from the power
panel and provides a separate low-voltage branch circuit to power
each camera enclosure. A fiber distribution enclosure is mounted adjacent
to each power distribution enclosure to facilitate splicing from a
multi-fiber trunk cable to the individual fiber cables, which also
fan out to each camera enclosure.
Transmission Media
Certain transit authorities, such as the Washington Metropolitan Area
Transit Authority (WMATA), have deployed CCTV locally within each
station while others, such as MARTA, also transport these signals
to other facilities for monitoring and recording. The cost and complexity
of transporting video within a metropolitan area network can be reduced
by multiplexing a number of different signals on a common transmission
media, such as a broadband trunk cable or optical fiber strand.
These fiber systems digitize and multiplex between 4 and 32 video
channels into high-speed serial data, suitable for transmission on
one single-mode fiber using one optical wavelength. The limitation
of 32 channels will be eliminated as wavelength division multiplexing
(WDM) becomes more cost-effective in the future. WDM systems are already
being developed to enable a single-mode fiber to carry 64 wavelengths,
thereby multiplying the present capacity of each single-mode fiber
by a factor of 64.
A less expensive video transmission alternative is to use a dial-up
telephone circuit for video transmission with a slow refresh rate.
Equipment is available to select, equalize, compress, and transmit
video images remotely to a distant monitoring location. Alternatively,
a video signal can be stored, compressed, selected by remote control
and transmitted on a dial-up telephone circuit, but the video performance
is greatly inferior to real-time transmissions.
Broadband ISDN Telecommunications Networks
for Video
The convergence of video, computing, and telecommunications technologies
promises to be one of the most significant advances of our time. Convergence
will enable a common network to transport data for various applications
and users instead of using a disjointed array of separate networks
with proprietary standards for various applications.
Each digital video signal will be converted to a standard telecommunications
signal format, such as an OC-3 signal at 155.52 megabits per second
(Mbps) for transmission via synchronized optical network (SONET) terminals.
Other possibilities are to compress each video signal into a DS3 signal
at 44.736 Mbps, or into a DS1 signal at 1.544 Mbps. Convergence will
allow common network resources to be utilized more efficiently, and
video can be transported anywhere on existing telecommunications networks
via broadband cable, fiber and satellite.
Video networks of the future can also be dynamic. The video camera
signals and stored video archives can be shared among authorized users
at different locations within the transit agency. Because telecommunications
networks provide two-way links, a reverse channel is available to
transport control signals for the video switch and PTZ unit. Interactive
video services can be made available to local station and parking
attendants, transit police, rail operations, fare collection personnel
and transit management.
Though it is technically possible to serve many users, real-time video
signals require considerably more bandwidth than signals for voice
applications. The data transmission requirements of a single digitized
real-time video signal can approach or exceed that of an entire telecommunications
network. Therefore, video compression is required to reduce the video
data rate to a practical value for transmission and storage. Video
is usually compressed in accordance with standards developed by the
Motion Picture Experts Group (MPEG). Although video compression is
too expensive to enable video deployment to many users currently,
future technological advances promise to reduce this cost significantly.
The broadband integrated services digital network (B-ISDN) forms the
universal digital network that can transport data independent of the
application (i.e., low-speed voice and data, medium-speed data in
accordance with the specifications for X.25 or frame relay, and high-speed
video data). Broadband ISDN can be integrated within SONET as well
as in local area networks (LANs), metropolitan area networks (MANs),
and wide area networks (WANs). The network resources (bandwidth) can
be allocated dynamically to serve all users and applications, and
the quality of service can be specified.
Conclusions
In the near future, it is likely that conventional VCRs and video
multiplexers will be replaced by large, solid-state video servers
with digital storage. In the short run, proprietary video networks
remain a cost-effective choice to serve the transit industry’s
dedicated point-to-point video transport needs, primarily because
video compression remains expensive. As the cost of video compression,
digital storage and WDM is reduced, video will be integrated with
telecommunications networks and new services, such as interactive
video, distance learning and desktop videoconferencing, will become
a reality for the transit industry. |
|
Bob Stong is a communications engineer who has
been designing closed circuit television networks and supervisory
control and data acquisition systems for MARTA since 1989. He has
been instrumental in migrating MARTA·s CCTV systems from monochrome
cameras and broadband cable to color cameras, single-mode fiber and
multimode fiber. Prior to joining PBT-TA, Bob was a member of the
engineering team at Public Broadcasting Service that designed the
first nationwide satellite distribution systems for television and
radio networks.
1 The survey was conducted by Parsons Brinckerhoff Tudor–Turner
Associates, the general engineering consultant joint venture to MARTA
that PB has been a senior member of for more than 30 years. |
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