ATSC M/H basics
Mar 15, 2009 8:36 AM, By Russell Brown
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.
Figure 2: Saving power
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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.)
Digital exciter
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.
Figure 3: Simplified antenna polarization
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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.)
Conclusion
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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