MOBILE DTV SHOWDOWN IN THE DESERT
April 20th, 2007Earlier this week at the NAB trade show, I had the opportunity to take not one, but two test-drives of VSB-based mobile digital TV systems. One was Samsung and Rhode & Schwarz’ Advanced VSB (A-VSB), first shown at CES 2007. The other was MPH (Mobile Pedestrian Handheld), a joint venture between LG and Harris.
The A-VSB suite piggybacks on an existing 8VSB broadcast, in this case from Sinclair affiliate KVMY-22. The demo I saw required a total bitstream payload of 5.5 Mb/s to transmit two mobile video streams. The first service was recoded from KVMY’s main program and had a bit rate of about 1 Mb/s, using "turbo" half-rate coding to deliver H.264 AVC video at QVGA resolution, which Samsung claims is suitable for highway speeds.
|
SRS vs. Diversity Reception SRS is additional bitstream data that provides a strong “lock” for the VSB receiver to track as carrier levels go up and down and even drop out. It is redundant bits sent over and over. Samsung uses a 10 kB version that chews up 1 Mb/s of the bitstream. MPH doesn’t use SRS. All VSB signals already have a “pilot” at the leading edge of the waveform (low edge of the channel) so that a VSB receiver can lock up the signal faster - see attached waveform. Diversity reception uses two or more antennas to accept a wide range of received signal phases and polarizations, minimizing dropout from echoes and reflections. An active processor is required so the receiver can constantly “vote” to pick the best antenna, or a combined signal from both antennas. Wireless mike systems all use diversity receivers these days to minimize dropout and signal cancellation. |
The second stream also encoded H.264 QVGA video, this time at 550 Kb/s using "turbo" ¼-rate coding, intended for reception aboard high-speed trains. In addition, a 1 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. SFN power was 100 watts and the carrier frequency was UHF channel 38.
LG and Harris took a different approach with their MPH demo, which doesn’t use SRS or diversity antennas. The MPH VSB payload was broadcast from another Sinclair affiliate, KVCW-29. Their primary 8VSB service consumes 15 Mb/s of the available 19.39 ATSC stream. Another 3.4 Mb/s was divvied up across two QVGA MPH programs, also encoded in the AVC H.264 format.
The first service was a real-time mux of KVCW’s standard programming, using ½-rate coding at 557 Kb/s, while the second service provided a mix of content from a server and was ¼-rate coded at 299 Kb/s. LG and Harris claim both modes will provide steady reception at speeds as high as 200 mph.
So, how 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.
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.
The verdict? Both systems worked very well for mobile DTV reception. For now, the LG/Harris MPH system seems to have the edge as MPH-equipped receivers won’t need diversity antennas, keeping them small and portable.
One humorous note: During the Wednesday morning demos, the LG/Harris and Samsung buses wound up next to each other, stopped at a traffic light. Despite the shouts and pleas of the MPH bus riders, the two drivers couldn’t be enticed into a VSB "race" back to the convention center!