Reviewing RF Technology At NAB2007: Part I
This month, I’ll look at RF technology at NAB2007. Some of the technology is available now; some promised for later this year or next. I’ll also look at the competing mobile TV technologies demonstrated at NAB, starting with transmitters.
While many manufacturers improved their high-power transmitter lines, I didn’t see any revolutionary changes. CPI said it plans to have a new MSDC IOT design optimized for DTV operation available later this year. Perhaps we’ll see it in a high-power UHF transmitter at NAB2008.
Many LPTV stations have filed for digital companion channels or for authority to flash-cut to DTV on their existing channel. Several transmitter manufacturers showed LPTV DTV transmitters and analog-to-digital conversion kits. Larcan showed a complete LPTV DTV transmitter solution in a rack. Their Larcan Plus includes the Octane encoder/multiplexer package. Octane supports multiple baseband inputs, dynamic PSIP data and ATSC (EIA-708) closed captioning. In partnership with Burst, Larcan can offer a complete turnkey DTV system.
Single frequency networks are already being used in Europe, and interest in SFNs is growing in the United States.
In many areas, multiple transmitters will be needed for reliable ATSC DTV reception on mobile and portable devices. Until recently, ATSC standard A/110 was the only technology available for ATSC SFN systems. As far as I know, Axcera is the only company that has A/110 equipment in the field.
Rohde & Schwarz and LG demonstrated another SFN option using their A-VSB technology. The demo used three transmitters—one on Black Mountain, one on top the Paris Hotel and a very low-power transmitter at the ATSC Hot Spot in the South Hall.
I used my laptop and Pinnacle HD Stick Pro to check DTV reception of the SFN on Channel 38. From where I checked in at the convention center and the Venetian Hotel, SFN reception required less fiddling with the antenna than other Las Vegas DTV stations needed.
This illustrates the advantage of a SFN—you can place transmitters closer to the viewer and provide multiple signal paths in the event one path is blocked. One problem, however, is that it can get expensive to feed the DTV signal to many transmitters by fiber or microwave, especially in remote locations. Last year Hiwave showed an ATSC low-power booster (or transposer) using echo-cancellation technology developed in Korea at ETRI.
The cancellation system allows less isolation between the booster’s transmit and receive antennas. Tests showed it was possible to mount both antennas on the same structure.
Unfortunately, I didn’t find Hiwave at NAB this year, but was pleased to see Rohde & Schwarz working on modifying echo-cancellation technology developed for DVB-T to work with ATSC signals. We may see a Rohde & Schwarz ATSC transposer with echo cancellation before next year’s NAB.
Experienced broadcasters will remember when most FM stations only transmitted horizontally polarized signals. FM broadcasters switched to circularly polarized, or CP, antennas to provide a more reliable signal after FM radio grew in popularity and car radios started including FM tuners. If mobile and portable reception becomes an important part of TV broadcasters’ business plans, they will want CP and EP (elliptically polarized) TV transmit antennas. Antenna manufacturers now offer narrow-band (one or two channels) CP and EP antennas, but broadband antennas with true CP or EP performance are rare.
RFS said it would have a broadband (more than 200 MHz bandwidth) CP/EP antenna by the end of this year. It will be based on RFS’s existing PHP broadband UHF antenna. The horizontally and vertically polarized elements will be fed separately, making it possible to vary the percentage of H and V power. In a multichannel system, it would allow each station to choose how much of its transmitter power it wants to use for vertical polarization.
(click thumbnail)ERI SlimWing sidemount antennaFor stations moving to a VHF channel after the transition, ERI introduced the SlimWing radiator element for low- and high-band VHF. SlimWing has a low-profile mechanical configuration, which weighs less and has less wind load.
The SlimWing radiator is used in ERI’s top and sidemounted XWing antennas and in a high-band VHF sidemount antenna for standby or LPTV applications.
Dielectric introduced a low-RFR antenna for rooftop applications possibly useful for SFN. The antenna is designed to limit RF exposure to a person standing on a roof to 50 percent of the FCC limits for public exposure, even when the antenna is mounted 15 feet above the roof and operating at 50 kW ERP. This performance is acheived anywhere in the UHF band (300 to 1,500 MHz) at all azimuth and elevation angles.
Dielectric also showed a system for recording and logging reflected power from an antenna and transmission line system. The RF Scout will sell for around $5,000.
There are now two groups competing to become broadcasters’ choice for mobile TV. Just before NAB, Harris and consumer electronics manufacturer LG announced their MPH (Mobile-Portable-Handheld) technology. It competes with the A-VSB format demonstrated by Rohde & Schwarz and Samsung at NAB last year and at the Consumer Electronics Show in January.
I had an opportunity to witness the A-VSB and MPH mobile demonstrations. Both technologies performed well on the trips. My impression was that the MPH route was a bit more difficult than the A-VSB, going along the Strip and under the connector between convention center buildings. The robust MPH signal worked well in this environment. The A-VSB route focused on reception at higher speeds in less congested areas but also worked well. Unlike the MPH demo, riders on the A-VSB bus were handed prototype receivers so they could check reception and play around with the antennas. As at the CES demo, the handheld receivers provided reliable A-VSB reception inside the bus.
Both systems touted reception at signal-to-noise ratios around 4 dB. Both systems promise mobile performance at speeds faster than anyone is likely to drive on the highway. Both systems allow reception of nonrobust streams on existing ATSC receivers. Neither system will work with existing DTV exciters and both require a special adapter or multiplexer to insert the robust streams.
Were the mobile demos an apples-to-apples comparison? Experts I talked to urged me to “do the math.”
My notes indicate the MPH demo used 2.2 Mbps for the 229 kbps robust stream and the same data rate for the 557 kbps mixed-rate coding. For the A-VSB demo, 2.2 Mbps was used for a 1,000 kbps half-rate coded stream and 2.2 Mbps for the robust quarter-rate 500 kbps stream.
Supplemental reference signal adds an additional overhead of 1.24 to 2.89 Mbps. If the MPH robust data bandwidth requirements are proportional to what was shown in the demo, it would require 4.8 Mbps to transmit a 500 kbps stream.
Using the minimum amount of SRS, A-VSB would need 2.2 Mbps plus 1.24 Mbps for SRS, or only 3.44 Mbps. Using the maximum amount of SRS, the total would increase to 5.09 Mbps. For the robust MPH stream, the required data rate appears to be 9.6 times the program data rate, including all overhead, indicating eighth-rate rather than quarter-rate coding.
For a robust A-VSB stream also providing signal-to-noise performance of about 4 dB, the required data rate is 4.4 times the program data rate plus a fixed 1.24 to 2.89 Mbps overhead for SRS.
You can do the math to see where A-VSB becomes more bandwidth efficient than MPH.
Next month, my RF at NAB2007 coverage will continue with a closer look at mobile TV and—sending video in the other direction—advances in microwave technology for ENG operations.
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Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.