As with any new technology, mobile TV, based on the soon to be adopted ATSC M/H (mobile/handheld) standard, has its own learning curve, and any station that plans to broadcast to mobile/handheld devices will need to start scaling that curve soon. ATSC M/H is a real departure from the stream of programs that are part of what is now called main channel ATSC (or legacy DTV), and there are many challenges that need to be addressed to successfully transmit ATSC M/H.
Bursts of data
One of the most fundamental differences in broadcasting mobile TV is that it uses a burst mode of data transmission. This was developed to save power in the mobile receiver. This means that the programs sent are not streamed to the viewer but sent in small packets, or bursts of data, that are stored and put together to form a continuous video or audio program. The entire M/H system is based on the IP data format to allow for future changes and expansions. The standard is also able to deliver data files to mobile devices, such as for programs to be viewed later.
Encoding and multiplexing
Standard MPEG-2 encoders will not work for M/H broadcasting, because MPEG-4 or H.264 is used to compress the video and audio signals. The new M/H encoder produces an IP-encapsulated MPEG-4 signal that is connected to the next new piece of gear, the M/H multiplexer/preprocessor. The encoder’s output is connected to the multiplexer via a 100BASE-T network cable (Cat 5/RJ45). (See Figure 1.)
This multiplexer combines all of the M/H signals, including the electronic service guide (M/H speak for electronic programming guide). One of the main functions of the M/H multiplexer is to add forward error correction to the combined signals. Because of the nature of mobile reception, a great deal of FEC is required, and the multiplexer/preprocessor is where this is added. Although statistical multiplexing could help conserve the bandwidth required for M/H, the FEC required occupies much of the bandwidth that could be saved. Stations can control the picture format (the default is 416 x 240, cell phone size) as well as how much FEC is used, which depends on their particular situation and equates to what the topology of the targeted area is.
The multiplexer/preprocessor combines the main service (legacy ATSC TS) with the new M/H service. It does this by interweaving M/H packets and main service packets in the final SMPTE 310 transport stream.
To combine the two signals, room must be made in the main ATSC TS for the ATSC M/H data. This is accomplished with the use of null packets in the main ATSC TS. But it is important to leave enough room for the M/H data or a data collision will occur, the signal will be corrupted and all programs will be lost.
The electronic service guide is the program and system information service for ATSC M/H. Any M/H services (e.g. video, audio, data) that are available will be announced through the announcement subsystem using a service guide, which is a special M/H service that is located in the service signaling subsystem. The mobile unit determines available service guides by accessing the guide access table for M/H. This table lists the service guides present in the M/H broadcast and gives information about the service provider for each guide, as well as access information for each guide.
The service guide is delivered using one or more IP streams. The main stream delivers the announcement channel, and other streams are used to deliver the guide data. A data service may be used to fill in the program information such is done now with main ATSC EPGs. Of course, many new opportunities may be available, such as having viewers respond to program content using their cell phones, and that would be controlled and monitored using the ESG encoder at the station.
In the output transport stream, the IP-encapsulated M/H data are converted to fit within the normal ATSC TS for compatibility with legacy 8-VSB receivers. The M/H service data is encapsulated in special MPEG-2 transport stream packets called M/H encapsulation packets. The M/H transmission system, which feeds the M/H exciter, can carry many types of encapsulated data including MPEG-2 video/audio, MPEG-4 video/audio and other types of data. Legacy DTV receivers will see this new M/H data as null packets and ignore them.
Time-division multiplexing of main and M/H data introduces changes to the timing of the main service stream packets. Changes are necessary to compensate for time displacements where the two data sets are combined so the transmitted signal complies with the MPEG and ATSC standards so as to protect legacy receivers. These functions are performed by the packet timing and PCR adjustment block in the M/H multiplexer. The data manipulation of the M/H data is divided into two stages: a preprocessor, contained in the multiplexer, and a post-processor, within the DTV exciter.
Data for a particular M/H service is carried in what is called a parade, and these, in turn, are broken into groups of packets within the transmitted signal. These data groups are arranged into a predictable pattern that allows mobile receivers to only power on their receiver circuitry when the data groups they want are present. Of course, this only happens after it has acquired an ATSC-M/H signal. In this way, about 20 percent of battery power is conserved. (See Figure 2.)
A new digital ATSC M/H exciter is required to handle the new ATSC M/H TS. This new exciter outputs the same RF signal as the legacy DTV exciter, but the way it handles the new M/H data is different. The main ATSC TS and the M/H data are routed within the exciter to different preprocessors and then combined again before trellis coding and the modulator. Special flags within the ATSC M/H TS inform the exciter that M/H data are present.
Transmitter and antenna
The actual transmitter required for ATSC M/H stays the same — the bandwidth requirements and power levels do not have to change. But because the target audience is mobile (sometimes very mobile at 60m/h), the antenna may need to be changed. It has been found that an elliptical polarized transmit antenna pattern is best for mobile TV reception. Some broadcast facilities that are currently upgrading or switching channels are switching to an elliptically polarized antenna, while others may have to pull down their current DTV antenna to achieve the best possible mobile TV reception.
Typically, an elliptically polarized antenna for mobile TV diverts from 10 to 30 percent of the power to the vertical and the remainder to horizontal. This, in turn, requires more transmitter power equal to the amount diverted to the vertical polarity. (See Figure 3.)
While ATSC M/H holds a great deal of promise for broadcasters, it also presents a great deal of challenges as well. ATSC M/H is still new, and both station engineers and management have much to learn about what is and is not possible. ATSC M/H may require even more changes to the broadcast facility than mentioned here to reach as many viewers as possible.
Jay Adrick of Harris, Richard Schwartz of Axcera and Richard Fiore of Thomson contributed to this tutorial.
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