Skip to main content

TV RF at NAB2003

This month I'll describe some of the new products and technology trends I'll be investigating at NAB2003.

DTV TRANSMITTERS

Last year transmitter manufacturers and broadcasters were focusing on low-power DTV. With FCC deadlines looming, broadcasters were looking for easy, affordable ways to get DTV on the air. Antenna manufacturers offered inexpensive low-power DTV antennas, while transmitter manufacturers offered inexpensive DTV transmitter packages that often included a simple SDTV encoder and some monitoring. What will attendees focus on this year?

Although low-power DTV transmitters will continue to attract attention, I expect to see more interest in high-power DTV transmitters. With the FCC proposing "build it or lose it" deadlines in the second review of the DTV transition, stations are finalizing plans for their high-power facilities. The hot topic in high-power transmitters will be the MSDC IOT technology tube vendors that many transmitter manufacturers introduced last year. The MSDC IOT transmitters on display last year were prototypes; this year we should be able to see some finished products.

The high voltage crowbar gap circuitry used to protect IOTs has often been a source of problems in high-power IOT transmitters. Itelco has for several years offered a high-power IOT DTV transmitter that used a switching power supply and did not require a crowbar or triggered spark gap. Thales introduced a low stored energy linear high voltage power supply with "Soft Arc" protection in its Paragon DTV transmitter last year that eliminated the need for a crowbar. This technology should be covered in the paper "A New Generation, Ultra-Efficient, DTV Transmitter Using the MSDC IOT" presented by Gordon Gummelt and Fred Stefanik from Thales during Sunday morning's NAB Broadcast Engineering Conference.

Astre Systems has "No-bar" tube protection technology in its Digital Plus DTV MDSC transmitter line. Astre says "No-bar" opens the HV power circuit in less than 700 nanoseconds, thus eliminating the need for a shorting crowbar. Acrodyne Industries (Ai) had an MSDC IOT-based DTV transmitter in its booth last year and it will be interesting to see how the transmitter has progressed.

The new "no crowbar" MSDC IOT transmitter designs are not easily adapted for use in analog transmitters, but broadcasters that already have non-MSDC IOT-based amplifiers for DTV may be interested in retrofitting the more efficient MSDC tubes into their digital transmitters. Marconi Applied Technologies (now E2V), Thales and CPI Eimac will be showing MSDC IOTs for both new applications and retrofits. The oil-cooled MSDC IOT from Litton/Northrop-Grumman (now L-3 Communications) attracted the most attention last year and is the one Thales used in developing the Paragon DTV transmitter.

Solid-state transmitters lack exciting components like the 30,000-volt power supplies used in IOT transmitters, but they still have to deal with mundane issues such as cooling and efficiency. IOTs have been the device of choice for high-power amplifiers, although improvements in solid-state amplifiers have increased the power levels at which their use makes sense. At this time, IOTs have the advantage at DTV output power levels over 10 kW or so. Will the cost of solid-state power amplifiers continue to drop, making high-power solid-state transmitters more affordable? I'll be checking the Harris, Thales and Itelco booths, among others, for the answer.

It will be interesting to see if there are any new developments in analog TV transmitters. Although Congress has set 2006 as the year analog broadcasting is to end (assuming certain conditions are met), most of the broadcast engineers I've talked to think analog TV broadcasting will continue well past 2006. The last significant advance I remember seeing in analog transmission was the Rohde & Schwarz exciter used in the Acrodyne Quantum transmitter. The exciter, introduced a couple years ago, generates the VSB signal digitally and applies linearity correction digitally, using a digital-to-analog converter to obtain the analog signal needed for transmission. Will anyone be showing high-power analog transmitters at NAB 2003?

NEW DTV TRANSMISSION TECHNOLOGY

ATSC is considering modifications to the U.S. DTV transmission standard to allow for more robust data-streams and synchronization of DTV transmitters for use in a distributed transmission system. I've described both proposals in previous columns. It appears likely some version of these modifications will be adopted later this year by ATSC and eventually by the FCC.

Current 8-VSB modulators will require modification to allow them to be synchronized for use in a distributed transmission system. If you anticipate building a distributed transmission system, check with your transmitter vendor to see what their plans are for modifying existing modulators. The same applies for transmission of robust datastreams, although the modifications required for this may not be as significant.

As I described last month, Axcera has built and tested 8-VSB modulators for a distributed transmission system and I'm sure the company will be showing it at NAB. If your station is interested in distributed transmission or robust data modes, be sure to let your transmitter vendor know. Both of these ATSC modifications will require modification of DTV receiver designs to work effectively and there is a risk neither will gain acceptance in the marketplace. Without customer demand, transmitter companies are unlikely to devote resources to the new standards.

Although the NAB convention is focused more on broadcasting than receiving, in the past, chip manufacturers and receiver manufacturers have displayed their new technologies and products in hotel suites. I'll be looking to see whether the enhanced 8-VSB technologies are gaining acceptance among receiver manufacturers. If you are interested in learning about enhanced 8-VSB, there will be a tutorial Saturday morning at NAB. Tuesday afternoon the Broadcast Engineering conference will focus on DTV reception and enhancements. This session will include papers on field tests of the Linx receiver and other next-generation ATSC DTV receivers, minimum performance standards for DTV receivers, and distributed transmission systems. See the NAB Web site, http://www.nab.org/conventions/nab2003/bec.asp for details.

DIGITAL MICROWAVES

I'm not expecting to see dramatic improvements in digital microwave systems. Any improvements are likely to be in integrated end-to-end solutions for TV links rather than in the underlying technology. There will be more interest in existing digital ENG COFDM products and digital STL links. As broadcasters move to DENG, there will be a need to upgrade relay links to pass the digital signal. For HDTV transmission, digital STL microwaves will be required.

Nucomm introduced the "Analog Coder" a few years ago. It consists of a digital modulator/demodulator that will work with an existing analog microwave link, even if multiple hops are involved. Nucomm will be showing an upgraded version of the product at NAB. Dr. John Payne, Nucomm's president, said the new "Analog Coder" has worked with systems including as many as 14 hops. It takes a 19.39 Mbps ATSC datastream in either SMPTE-310 or ASI format. It also provides a T-1 data channel and up to four audio subcarriers, which are useful for sending IFB audio or other non-broadcast audio to the transmitter site without the cost and time delay involved with encoding it into the ATSC or T-1 datastreams at the studio and decoding it at the transmitter site.

Some broadcasters have used the 900 MHz unlicensed spectrum for remote control data links, and most are familiar with the ubiquitous 2.4 GHz unlicensed wireless LAN systems using the IEEE 802.11b standard. You may not be aware that FCC rules do not restrict use of the 2.4 GHz and 5.8 GHz unlicensed bands to the IEEE Wireless LAN modulation standards. Multiple companies are now offering point-to-point microwave systems, using other spread-spectrum techniques capable of reliable data transmission over distances of 15 miles or more. Last year I found some of them in the smaller booths at the edges of the main exhibit area. If you are looking for an inexpensive way to move noncritical digital data (or compressed video) between two locations, search for them. Would they work for a low-cost DENG system?

TEST AND MONITORING

Many TV stations are broadcasting DTV and using only a set-top box to monitor their DTV signal. The two test and monitoring products that attracted attention at NAB last year were the Rohde & Schwarz EFA-53 receiver and the Sencore "DTV Investigator." Measuring distortions in 8-VSB signals is not easy and I'm not optimistic we will see any lower-cost options at NAB this year. Last year Rohde & Schwarz showed how the ATSC signal could be used with its EFA53 to provide a time domain analysis of transmission line reflections. It will be interesting to see if new applications are added this year.

Both K-Tech and Sencore have improved the capabilities of their DTV reference receivers. These receivers offer a number of tools for measuring 8-VSB signal quality and ATSC datastream compliance at a fraction of the cost of the EFA53 or DTV Investigator. I'll be checking out the latest software for these products at NAB. Several manufacturers have demodulators in their 8-VSB exciters for adaptive correction that are able to provide data on signal quality. The quality of the software used to display this data varies widely among manufacturers and I would hope to see some upgrades at NAB. Z-Technology attracted a lot of interest in its DTV field-test setup last year. I'll be looking for improvements and similar offerings from competitors. This data could be obtained from an existing digital spectrum analyzer with the right software and a calibrated antenna. Will anyone offer software to analyze DTV coverage using location data from a GPS and signal strength data from an Anritsu, HP or Tektronix spectrum analyzer?

This is only a small sample of RF technology at NAB2003. I couldn't mention every company or every product I'll be looking at! If you think I've missed something important, please let me know! Send e-mail to dlung@transmitter.com

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.