Mobile DTV 101
| This vehicle was used in the recent IDOV tests conducted by OMVC and MSTV.|
During the last few years, exhibit space at the NAB Show devoted to TV transmitters has shrunk. While this might appear to be an indication that terrestrial TV is dying, the packed technical sessions focusing on ATSC mobile-handheld TV and the capacity crowd at th is year’s Open Mobile Video Coalition’s “Mobile TV: Opportunity at 100 MPH” breakfast showed it may be too soon to write-off over-the-air TV.
TV broadcasters pioneered “wireless TV,” but the majority of viewers now watch content delivered by cable, satellite or the Internet. The new ATSC mobile/handheld standard (ATSC M/H) will provide a way to reach untethered TV screens using the transmission infrastructure broadcasters already have in place. Although mobile DTV systems compatible with the ATSC standard have been demonstrated at NAB and the Consumer Electronics Show for a few years, the question has remained as to whether it will work in the real world.AND THE VERDICT IS....
The report on the Independent Determination of Viability (IDOV) conducted by the OMVC Technical Advisory Group (OTAG) and submitted to ATSC on May 15 provided the answer—a resounding yes.
ATSC, OMVC and OTAG did not release details on how individual systems performed in the IDOV, but some of the key findings in the report were made public: Mobile reception is possible at distances up to 40 miles and mobile TV does work on high-VHF channels as well as at UHF frequencies.
The day before the IDOV report was sent to ATSC, two proponents (LG and Samsung) that had publicly demonstrated mobile DTV agreed to cooperate in creating an ATSC M/H standard. This should make it easier for ATSC to adopt a standard that includes the best elements of both systems and, ideally, offers a better solution than either individual technology.
Did the IDOV reflect real-world conditions?
In my opinion, it did.
The IDOV evaluation was done in San Francisco. Three transmitters at two different locations were used in the testing: KNTV, (Channel 12) and KKPX (Channel 41) transmitted from San Bruno Mountain, near San Francisco, and KICU (Channel 52), transmitted from Monument Peak, which is north of San Jose and on the other side of San Francisco Bay. The IDOV test vehicle, shown at NAB, took grid and driving measurements around the Bay area, in urban canyons and in suburban areas throughout the market.
Reception was checked using antennas mounted outside and inside the vehicle, which was supposed to simulate someone watching TV on a portable device inside a car or van. By using a long coax, pedestrian reception was checked in different locations.
In Las Vegas, similar tests were done using two UHF and one VHF station. However, Las Vegas does not offer the range of propagation environments (no over-water paths, for example) available in the San Francisco area.
OTAG also did a laboratory evaluation of the systems. It was important to make sure the systems didn’t “break” reception of conventional ATSC DTV on legacy receivers. The laboratory tests showed the systems did not interfere with DTV reception, although, as I’ll describe next, adding a mobile stream will impact the data bandwidth available for legacy DTV.
| The interior of the IDOV field test vehicle reveals the test gear used to collect DTV reception data. The van traveled more than 1,400 miles during the test period.|
Listening to questions at the weekend engineering sessions at the NAB Show focused on mobile DTV, I realized that while there was a lot of excitement about the medium, many broadcasters do not understand the tradeoffs involved in putting it on the air. I’ve already discussed the LG/Harris MPH and Samsung/Rohde & Schwarz A-VSB systems in detail in these pages and will skip some details and provide a high-level outline of how mobile DTV works.
First, it is important to understand that the current ATSC broadcast standard does not work for mobile DTV reception. ATSC receivers use adaptive equalizers that cancel out effects of multipath reception. However, the current standard is designed for fixed reception and does not provide enough known data sequences (training signals) for the equalizers to adapt fast enough to rapidly varying multipath conditions encountered in a moving vehicle. Another problem is that ATSC requires a signal-to-noise ratio (SNR) of at least 15 dB. While this is better than COFDM (DVB-T) at an equivalent data rate, it may be difficult to achieve in an indoor environment or with a handheld receiver inside a vehicle. Even when the received signal is strong, multipath the equalizer can’t correct will degrade SNR.
Making mobile DTV work requires more than advanced receivers. The transmitted signal has to be modified as well and there’s no such thing as a free lunch.
First, in order to be able to perform while the receiving antenna is in motion, it is necessary to add additional training signals at the ATSC transmitter site. These signals take a portion of the 19.39 Mbps broadcast stream. A-VSB, MPH and the Thomson-Micronas system all use training signals, but they differ in how they are added to the existing ATSC signal. These ancillary signals could also be used to benefit reception of conventional ATSC signals. Samsung and Rohde & Schwarz demonstrated this using the SRS (supplemental reference signal) that’s part of their A-VSB technology.
Second, the required SNR has to be better than 15 dB to allow reception in areas with weak signals, excessive multipath, or interference from computers, appliances and other household or office devices. This could be accomplished using a more robust modulation system—2-VSB instead of 8-VSB, for example—but it wouldn’t be compatible with existing receivers.
While there are ways to position data to take advantage of the 8-VSB constellation, it is easier to make the data itself more robust. This takes extra data bits. Both A-VSB and MPH use quarter-rate, half-rate and mixed rate coding. Quarter-rate refers to transmitting four bytes for every one byte of data. A receiver has four opportunities to receive that critical bit of data. Half-rate coding uses two bytes for every one byte of data. As you can see, this can eat up a lot of the 19.39 Mbps available to U.S. broadcasters. Transmitting a mobile video stream at 1 Mbps will consume 4 Mbps of the 19.39 Mbps, and that’s before we consider the data required for the training signal. Indeed, there’s no free lunch.
The good news is this allows reception with SNR in the range of 4 dB, well below the 15 dB SNR required for legacy DTV.
In addition to adding extra bits to make the signal more robust, the data is spread over a longer period of time using a process called interleaving. This makes reception possible even if some portions are lost due to dropouts or noise bursts. While interleaving itself doesn’t require extra bits, it only works if there is enough redundancy to allow the signal to be reconstructed.
So adding mobile TV will affect what can be transmitted to conventional ATSC TV sets. It will limit the number of program streams sent out, especially if “difficult” programming such as sports in HDTV is on the schedule. However, the new standard will likely use the most advanced video and audio encoding systems—MPEG-4-AVC (H.264) versus MPEG-2, providing higher quality with lower bit-rates. This could open up other opportunities.BEYOND MOBILE DTV
If you have a 50-year-old television receiver that works, it will still receive today’s analog NTSC signals. Add a DTV converter box and it will even display DTV signals. However, if you have a computer that was made 20 years ago, how useful is it today? For broadcasters to compete, compatibility with legacy hardware may be a luxury.
ATSC M/H opens the door to new compression technologies such as H.264. This allows transmission of video to small screens using considerably less than 500 kbps, not counting the bits added for robustness and training signals. While off-air HDTV viewers may lament the loss of their 19 Mbps 1080i HDTV signal as bandwidth is devoted to mobile/handheld services, a move to H.264 for HD transmission could allow 1080p HDTV broadcasts with as little as 6 Mbps of data bandwidth. Broadcasters would have to keep at least one MPEG-2 stream running to support legacy TV sets, but as we’ve seen with the DTV converter boxes, adding an outboard adapter to allow reception of the 1080p H.264 streams shouldn’t be that difficult or expensive.
Mobile DTV will require giving up some bandwidth, but it will open new opportunities for TV broadcasting that should allow off-air TV to survive for at least another 50 years.