Driving Mobile DTV from Coast to Coast
Stations buy gear, coordinate reception tests
By Glen Dickson -- Broadcasting & Cable, 7/12/2009 1:14:00 PM
The most prominent market test in the near term is the seven-station trial in Washington, D.C. that gets underway later this month and is being coordinated by the Open Mobile Video Coalition (OMVC). OMVC Executive Director Anne Schelle notes that Washington will also be "an interesting test-bed" for mobile DTV reception since the seven stations
are transmitting at different power levels and also include a VHF station in WUSA; the viability of VHF channels for mobile DTV has been questioned by some engineers because of the larger antennas VHF signals generally require.
OMVC, in partnership with Association for Maximum Service Television (MSTV), will also be conducting reception testing in additional markets through August, including San Francisco, Los Angeles, Dallas, Boston and New York.
How the stations juggle the new mobile DTV streams with their existing high-definition and standard-definition services will also be a topic of interest. According to Harris VP Jay Adrick, who is helping the six D.C. stations ready their facilities with mobile DTV-capable exciters and encoders, most stations are devoting about 3.7 megabits per second of
their 19.4 Mbps DTV pipe to the mobile streams, which are encoded using MPEG-4 compression. Because the D.C. stations will be using a lot of forward-error correction (FEC) to ensure robust reception, only about a quarter of that bit-rate is available for transmitting audio and video.
"That translates into about 900 kilobits [per second] of payload, and we can put two or three services in there," says Adrick. "It's working out reasonably well."
Adrick notes that the ATSC-M/H standard allows a station to turn down the FEC and transmit a single video stream using only 900 Kbps in total. And encoder manufacturers like Harris, Tandberg and Harmonic say they
are already working with customers to compress their existing DTV services as efficiently as possible in order to free up room for mobile services.
Vendors say they are already selling mobile DTV product today. Harris has already shipped mobile DTV systems to 10 stations across four station groups, and taken orders for three more stations from another group. Markets include Atlanta, Charlotte, Los Angeles, Oakland, Seattle, Orlando, Fresno and Omaha; Harris also supplied equipment to
WRAL in Raleigh, which began transmitting mobile DTV to a specially-equipped commuter bus in April. And Acrodyne, working in partnership with Rohde & Schwarz, has installed a high-powered mobile DTV system at WLNY, an independent in Riverhead, N.Y.
The station's one-megawatt signal, supported by an Acrodyne IOT tube transmitter and Rohde & Schwarz encoding gear, radiates from a circularly-polarized antenna atop the station's tower to provide coverage over most of Long Island and New York City, as well as parts of southern Connecticut.
WLNY, which broadcasts a single standard-def program stream, is currently using a little over a megabit of its DTV pipe to deliver a 500 Kbps mobile simulcast of that content, says director of engineering Richard Mulliner. It has been doing some informal testing with prototype receivers before conducting formal reception testing later this summer with professional measurement gear. While it's obviously early days for mobile DTV, Mulliner is optimistic about the technology's potential to reach viewers on the go.
"I'm quite impressed with the coverage," says Mulliner. "I can drive on the [Long Island] Expressway at 60 to 70 mph, and the signal never locks out."
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