Mobile DTV
Oct 1, 2008 12:00 PM, By Jay C. Adrick and Wayne E. Bretl
Find out how to integrate ATSC M/H technology into your station.
The ability for instantaneous turn-on and acquisition allows power saving in handheld, battery-powered devices. Various programs can be carried in separate parades of groups, and the receiver RF front end only needs to draw power during relevant times. (See Figure 2.)
ATSC-M/H system architecture
Figure 2. The receiver RF front end only needs to draw power during relevant times.
Click to enlarge
Integrating ATSC-M/H systems into an existing ATSC broadcast chain is relatively easy, as was demonstrated during the OMVC Initial Demonstration of Viability (IDOV) testing that took place earlier this year.
At the studio end of the system, an M/H multiplexer is integrated into the ATSC stream after the existing ATSC multiplexer. Depending on the manufacturer, the multiplexer could include an integrated IP encapsulation system and also integrate operational and maintenance data packets to control the various M/H modes created in the M/H exciter. The multiplexer may also perform some packet timing functions to prevent buffer overload at the receiving end. The multiplexer output format is a conventional 19.4Mb/s stream based on either ASI or 310M format. No changes are required for the station's STL.
Each real-time M/H program stream will require an IP-based mobile program encoder that will send the mobile signals to the encapsulator and multiplexer. Non-real-time (NRT) content requires an authoring station and server for storage and a management system to schedule and release the NRT content. Because the connectivity between these content sources and the encapsulator/multiplexer is IP-based, a reliable IP link can allow the content sources to be separated from the transmission system. The ESG system, CAS and other advanced applications systems all connect to the M/H encapsulator/multiplexer using IP as the transport. (See Figure 3.)
Figure 3. The electronic service guide (ESG) system, conditional access system (CAS) and other advanced applications systems connect to the M/H encapsulator/multiplexer using IP as the transport.
Click to enlarge
At the transmitter, the ATSC exciter needs to be replaced with one that can support M/H. In recent months, manufacturers have introduced new software-based exciters that can be upgraded to ATSC-M/H service. Older exciters are not upgradable because of the amount of processing power required to create the M/H signal.
There are several major RF considerations that a station must review when entering the mobile business. ATSC-M/H is an RF-delivered wireless business with no delivery help from cable or satellite. If the transmission system fails, the signal is off-air, making redundancy crucial. Redundancy should cover the entire chain from encoders through to the transmitter.
While UHF works best for mobile DTV, high-band VHF has also been tested and works quite well. There are receiving antenna shortcomings at VHF that introduce some reception limitations.
Transmit antenna polarization may also be a factor because most portable and mobile receiving devices have vertical, polarized receiving antennas. At least one broadcast antenna manufacturer reports that there has been a recent increase in interest by broadcasters looking to add vertical polarization to their signal.
Finally, broadcasters should consider maximizing their power and providing as much signal coverage as possible under the FCC rules. During the recent FCC window of opportunity to maximize, more than 600 stations applied for maximization.
With the ATSC hoping to complete the standards process by early 2009, new broadcast equipment coming this fall and consumer devices planned for later in 2009, this coming year will be busy for broadcasters looking to enter the mobile DTV business.
Jay C. Adrick is vice president of broadcast technology for Harris, and Wayne E. Bretl is senior principal engineer for Zenith.
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