DTV: It's not just for primetime anymore. For police, fire, and EMS personnel, DTV is the Next Big Thing in emergency communications, an ideal broadband backbone for relaying street maps, building blueprints, and realtime command communications. The proof? Four Mbps worth of video and Web content were recently transmitted from WNYE-TVâs DTV transmitter in Brooklyn, NY to the World Trade Center site, plus several fixed and mobile units in Manhattan and Brooklyn. Broadcast on channel 24 with an EIRP of 1.25 kW, the signals were received on SDTV monitors and laptop computers equipped with high-speed wireless modems. Although not exactly broadcast quality, 4 Mbps was good enough to support two video channels (2.5 Mbps live SDTV and 0.5 Mbps streamed), plus IP data.
There's just one catch: The test, which was performed October 1-5, 2001, was conducted using COFDM (DVB-T) digital modulation, rather than U.S. 8-VSB technology. The reason is because 8-VSB, in its current form, can't support mobile emergency communications.
The test itself was organized by Hicks & Associates, Inc. (H&AI), which consults on national security issues for government and industry clients. In this instance, H&AI was acting on behalf of the Department of Defense (DOD) and representing the needs of the Federal Emergency Management Agency (FEMA) and other emergency service groups.
"We wanted to see if crews equipped with wireless laptops could move around Ground Zero, and still have access to high-speed Web content and video," commented Guy Beakley, H&AI's vice president.
But why COFDM, rather than 8-VSB? "DOD and FEMA were looking for a DTV system that was flexible and robust enough to support multiple data rates to mobile receivers," said Mark Aitken, director of Advanced Technology for the Sinclair Broadcast Group, one of the test's participants. "DVB-T," the commercial version of COFDM DTV, "was the only standard proven to have this capability."
"DOD has been a firm advocate of the COFDM standard," added Sinclair Vice President Nat Ostroff. "One reason is because COFDM is an internationally available DTV standard, and DOD works worldwide. Another is that COFDM has been proven to work in mobile applications, where 8-VSB hasn't."
The test partners also included Acrodyne Industries and SkyStream Technologies. Operationally, the video and data streams were packaged for IP using SkyStream Network's Source Media routers, and multiplexed with MPEG-2-encoded live video content. Logistically, Sinclair provided the systems planning and integration, while Acrodyne supplied WNYE's transmitter. Being in Brooklyn, it's one of the few New York sites unaffected by the WTC collapse.
"WNYE's 500-W Acrodyne DTV transmitter was originally configured for 8-VSB," said Ostroff. "We pulled out the 8-VSB modulator and replaced it with a COFDM unit, and that was it." As well, UBS Technologies and Rohde & Schwarz supplied COFDM modulators while Tandberg helped support multiplexers and MPEG-2 encoders. Finally, the signals were received on wireless-equipped Hewlett-Packard laptops and Nokia USB (Universal Serial Bus) receiving units.
At Ground Zero, testers were able to receive streamed video, maps, and selected Internet content in realtime. Moreover, they didn't have to rely on local towers to relay it to them: WNYE's Brooklyn tower, several miles away, was close enough. In short, it worked in downtown Manhattan and Brooklyn as well.
However, some reception problems were experienced in other parts of New York City. These were mainly due to interference from WNYE's NTSC channel 25 signal, which transmits with an EIRP of 2,450 kW from the Empire State Building. As well, limits with the Nokia USB cards meant that the laptops could only receive 1.5 Mbps of data at best. As a result, the 2.5 MPEG-2 live video had to be watched on COFDM-compatible DTV receivers. This said, the test proved that DTV can carry emergency communications, as well as SD and HDTV.
So does that mean broadcasters' 8-VSB transmitters can be adapted to carry emergency communications? In its simplest terms, the answer is yes. Emergency communications are nothing but data streams. As a result, "this emergency system could be supported on either a COFDM or 8-VSB standard, as long as it works in a mobile, urban environment," said Beakley. The downside, of course, is that 8-VSB doesn't work well for mobile applications. In fact, "..right now, broadcasters can't provide an emergency broadcast system to a disaster site, due to the limits of 8-VSB as we know it today," said Ostroff. However, "work is going on right now to enhance 8-VSB," said ATSC Executive Director Mark Richer who, although his organization was not part of the testing, had something to say. "The problem has more to do with the architecture of the system, rather than the modulation choice," he added. "Whether you use COFDM or 8-VSB, you still have to lower the data rate to make the system work for mobile communications, and broadcasters would need to deploy an array of transmitters." Clearly, there's no easy answer to this problem. But nevertheless, the opportunity offered by emergency communications via DTV remains.
James Careless is a contributing writer for DigitalTV.
