THE FRONT LINE: APRIL 22, 2007
Mobile DTV Showdown In The Desert
At this year’s NAB trade show, most of the “buzz” appeared to be about mobile digital TV reception. Several companies have been proposing systems and solutions for mobile DTV, and one (Qualcomm) now has its MediaFLO service running in several markets on UHF channel 55, serving up streaming video and clips to MediaFLO-equipped phones from Verizon.
Two mobile DTV systems that were demonstrated at NAB employ the ATSC terrestrial vestigial sideband (VSB) modulation system. The first was Samsung and Rhode & Schwarz’ Advanced VSB (A-VSB), while the other was MPH (Mobile Pedestrian Handheld), a joint venture between LG and Harris.
Figures 1a and 1b. The LG and Samsung mobile DTV demo buses.
The A-VSB demo rode on an existing 8VSB broadcast from Sinclair DTV station KVMY-22, and required 5.3 Mb/s to transmit two mobile video streams. The first stream was KVMY's main program, recoded as MPEG4 with QVGA resolution (160x120). It had a bit rate of 2 Mb/s, using “turbo” half-rate coding for viewing at highway speeds.
The second stream (also 2 Mb/s) was custom MPEG4 video content, using “turbo” 1⁄4-rate coding and intended for reception aboard high-speed trains. In addition, a 1.2 Mp/s (10 byte) supplementary reference signal (SRS) was broadcast as part of the main SDTV 8VSB stream to ensure that all A-VSB receivers used in the demo stayed locked onto the transmission. Diversity antennas were used to receive all three signals.
Samsung also demonstrated a single frequency network (SFN) with three nodes — one atop the Stratosphere Hotel tower, one at the Paris Hotel, and a third inside the Las Vegas Convention Center, using 100 watts of transmitter power on UHF channel 38.
Figure 2. Samsung’s real-time display included the main 8VSB channel (left), the two secondary channels (right), and live spectrum analysis of UHF channel 22.
Figure 3. Samsung’s A-VSB prototype receiver worked well, but is pretty bulky.
The MPH demo didn’t use SRS or diversity antennas and was broadcast from another Sinclair station, KVCW-29. 4.4 Mb/s of the total ATSC stream was divided between two QVGA MPH programs, also encoded in the AVC H.264 format.
The first service (2.2 Mb/s total) was a real-time mux of KVCW’s standard programming, using 1⁄2-rate coding with a program payload of 557 kb/s, while the second service (also 2.2 Mb/s total) provided a mix of content from a server and was 1⁄4-rate coded with the program payload at 299 kb/s. LG and Harris claim both modes will provide steady reception at speeds as high as 200 mph.
How did do they compare? I rode the LG/Harris bus on Monday and we drove around the Las Vegas Convention Center, through the Desert Inn Road convention center underpass, down the Las Vegas strip, and along Tropicana and Paradise Roads, hitting speeds as high as 60 miles per hour.
The main 8VSB service took the expected hits, but the ½-rate and ¼-rate services were quite robust, with the odd hit here and there. One potentially troublesome location was near Caesar’s Palace where several strong analog carriers were seen around the UHF 29 signal, but both MPH services remained steady.
The only place these signals failed completely was in the middle of the Desert Inn Road underpass between the Central and South Halls of the convention center, and only for a few seconds until we cleared the buildings. At one point, the ½-rate program appeared to have dropped out completely, but that was quickly determined to be a locked-up receiver.
Figure 4. (Clockwise from upper left) MPH services 1 and 2, the main 8VSB program from KVCW-29, and real-time spectral display of the UHF 29 carrier.
Figure 5. This tilted antenna was all LG and Harris used to demonstrate MPH.
Samsung’s tour followed on Wednesday and got out onto Interstate 515 in short order, hitting speeds of 70+ miles per hour. Both A-VSB streams were steady along the route, again with a quick hit from time to time. We did notice a problem with breakup on the main 8VSB broadcast while stopped at traffic lights, but that turned out to be a loose RF connection.
Our subsequent travels took us along main roads near the Las Vegas Hilton, where Samsung switched the main monitor over to their UHF 38 SFN. At that point, we once again traveled westbound through the Desert Inn Road underpass and saw all three programs drop out, recovering quickly as we cleared the buildings.
Samsung also handed out prototype handheld diversity receivers to watch the ¼-rate A-VSB program. These use a pair of extendable whip antennas, which I left collapsed and shorted together. Even so, I had steady reception better than 90% of the time as we drove on local streets and on the Interstate.
Both systems worked very well for mobile DTV reception. If anything, the MPH system has the edge in that it doesn’t need the diversity antenna system, which means MPH receivers can be a lot smaller and more portable. While the A-VSB demo worked well, the prototype Samsung receiver and antennas are too large to carry in your pocket.
It’s important to remember that the video delivered to these systems is very low resolution and not HDTV. The QVGA resolution makes it possible to use low bit rates so that either system can piggyback into an existing 8VSB broadcast. 160x120 pixels may not sound like much, but they’re more than adequate for handheld viewing of video and basic graphics.
Both A-VSB and MPH also look more attractive as secondary revenue streams for broadcasters, most of which aren’t deriving any extra bucks at present from their second, third, and fourth minor channels. These channels often feature weather, local news, and syndicated services like The Tube’s music videos.
In contrast, mobile DTV could turn out to be a multi-million dollar business with relatively low implementation costs for each local broadcaster. The mobile VSB streams aren’t detected by conventional set-top boxes or integrated DTV sets and have little if any impact on 8VSB coding and electronic program guide data.
At present, neither A-VSB nor MPH are working products that you can buy at retail. But they could be, if broadcasters show enough interest and if they can create local market demand for these robust secondary VSB services. With the proliferation of handheld media players such as iPods and new TV services from Apple and Qualcomm, it’s a safe bet there will be…
