Doug Lung /
04.07.2004 12:00 AM
NAB 2004: RF Expectations
Many manufacturers use the NAB convention in Las Vegas to introduce new products. It also provides a good indication of the direction broadcasting is headed. Can you remember the year the last klystron-based transmitter was shown at NAB? Analog TV transmitters were scarce last year?will they disappear this year? This month I'll outline what I expect to see at NAB and highlight some of the more interesting papers on the NAB Broadcast Engineering Conference schedule.

DIGITAL MICROWAVE FOR DTV & 2 GHz ENG

Broadcasters will be losing the lower two channels of the 2 GHz ENG band soon. The bandwidth of the remaining five channels will be reduced to make room for two more channels to replace the ones lost and provide spectrum for "data return links" (DRL) that could be used for automatic power control and transmission of received signal levels back to the ENG truck. Digital microwave equipment has been available from several microwave vendors (Microwave Radio Corp. and Nucomm are two companies that pioneered digital ENG) for a number of years. Will new companies enter the market for D-ENG microwave equipment to meet the anticipated demand? Will the cost of D-ENG drop? I expect to see new D-ENG radios at NAB, but I'm not as certain of the price drop.

The idea for data return links was floated at NAB last year and became part of the FCC's 2 GHz reallocation plan. It will be interesting to see if any of the digital microwave manufacturers have been able to incorporate it into their existing D-ENG product line or if any new companies have stepped up to fill the need. Most ENG antenna systems?both at the truck and at the receive site?are not equipped for dual transmission/reception. Does it make sense to include a separate antenna on the truck for receiving DRL data that would allow moving the truck's transmitting antenna without losing the DRL?

Although not an RF topic, I expect to see some lower cost MPEG-2 (and perhaps even MPEG-4) encoder/ decoder packages for D-ENG. Today's D-ENG trucks are built using MPEG encoders originally designed for satellite transmission. In many cases, they offer far more capability and complexity than required for everyday ENG operations.

In the analog days, field editing was done on tape. Now, with the availability of low cost IEEE-1394- (FireWire) equipped cameras, inexpensive laptops and inexpensive editing software, new ENG trucks are likely to be editing video that's already compressed to some extent. In this case, it may be desirable to transmit the edited piece back to the studio as a file, complete with metadata, rather than transmit it as video and audio. Some D-ENG systems offer an Ethernet port on the encoder/multiplexer, so it seems there is the potential to use the DRL to allow file transfer over D-ENG microwave.

In many cases, the ENG receive site is not located at the studio, so broadcasters will want to modify or replace intercity microwaves to handle the digital signal rather than attempt to install and operate the decoders at the remote receive site. The simplest approach is to make the connection at IF (intermediate frequency?usually 70 MHz), but this doesn't correct for any distortion in the signal received at the ENG site. An adaptive equalizer can be added to reduce distortion and this may be adequate for many stations. The most thorough approach is to demodulate the received datastream, apply error correction to the incoming data, and remodulate it in the inter-city microwave to the studio. I'll be looking at how different microwave manufacturers handle retransmission of the D-ENG signal.

Of course, broadcasters also have to be concerned with how they get their digital signal to the transmitter site. The "Twinstream" microwave pioneered by Microwave Radio is probably the most common system in use today. The digital data modulates a 16 QAM signal inserted next to the analog FM video signal with FM audio subcarriers in one 25-MHz-wide channel. However, as the cost of MPEG-2 encoders continues to drop, more broadcasters are looking at an all-digital solution?combining an SDTV stream (for the analog transmitter) and an HDTV stream on the same digital microwave. I'm not expecting any major advances in digital STL technology this year, although it will be interesting to see if anyone comes up with an alternative approach for transmitting the NTSC and DTV signals over one microwave.

DTV TRANSMITTERS

I'm not expecting to see any significant new technology in DTV transmitters at NAB; what I do expect to see is more use of the MSDC-IOT. Engineers from Ai presented a paper at the IEEE Broadcast Technical Symposium in October showing that a depressed collector IOT could offer power savings in analog operation as well as digital, and I'm sure they will be highlighting that at their booth. The Thales Paragon MSDC DTV transmitter was introduced at NAB2002 and is now in use at several stations. Although there will probably be few changes to the amplifier cabinets, Thales will most likely upgrade its DTV exciter design to accommodate modified ATSC standards and make it easier to operate.

Merrill Weiss will be reporting on a distributed transmission system he designed using transmitters and modulators from Axcera. Other manufacturers will be showing equipment compatible with the ATSC candidate standard for distributed transmission systems, and I'm sure they will be looking to determine how much broadcaster interest there is in the technology. Designing a distributed transmission system around available transmitter sites is not easy.

E-VSB TRANSMISSION & RECEPTION

I'm hoping this will be the year we see some enhanced VSB (E-VSB) hardware not only in the suites, but also on the convention floor. At least two papers are being presented at the NAB Broadcast Engineering Conference that focus on the technology. E-VSB will require modifications to existing DTV modulators, additional monitoring equipment capable of decoding the robust datastream in E-VSB, and changes to ATSC encoding equipment to generate the PSIP information receivers will need to deal with the robust stream. Although the final E-VSB standard has yet to be published, I would not be surprised to see some manufacturers showing prototype equipment to judge customer interest in the technology.

OTHER RF ITEMS

I'm not looking for any significant developments in TV transmitting antennas, although I have been surprised before. Combined antenna systems are attracting a lot of attention lately and we may see some improvements in wideband antenna and transmission line performance, as well as in combiner performance.

ENGINEERING CONFERENCE PAPERS

The lineup of papers for this year's NAB Broadcast Engineering Conference is one of the strongest I've seen in a couple years. Many DTV reception problems can be traced back to incorrect PSIP (Program and System Information Protocol) data. Incorrect PSIP can prevent DTV sets from finding your signal or from decoding it properly. The ATSC Digital Television University, taking place all day Saturday at NAB, has nine papers on PSIP, starting with the fundamentals. The effect of PSIP on DTV receivers is covered, as well as generating and disseminating PSIP. Papers in Tuesday morning's "Cable Issues for Broadcasters" discuss PSIP from the network through to the cable headend and PSIP in distributed environments. PSIP for E-VSB is covered in Wednesday morning's "DTV Receiver Technology" session.

Distributed transmission systems and E-VSB are covered in Sunday's "State of the Art in Television 2004" session Sunday morning and in Monday afternoon's "Television RF and Transmission Developments" session.

As expected, the Television RF session has a number of papers that attracted my attention. Victor Tawil is presenting a paper on "Post Transition DTV" allotment planning, which should interest many stations, especially those that aren't happy with either their analog channel or their allotted digital channel?a Channel 2 analog and a Channel 63 DTV for example. What options will they have post-transition? I mentioned Bill Meintel's work on real-world DTV interference ratios in my IEEE Symposium report. Bill will discuss "Computing Interference under Moderate and Strong Signal Conditions?Proposed Changes to the FCC TV Analysis Model," which should interest the many broadcasters finding their DTV or analog channels surrounded by new DTV stations. Two papers in that session (one from Lewis Steer at RFS and another from Ken Brown at KJLA) will focus on multistation transmission facilities.

If you have only been attending the exhibits, you are missing an important part of NAB. Although some of the papers have been presented in slightly different form elsewhere and some repeat topics from previous years, a lot of new information is presented at the NAB Broadcast Engineering Conference. Some of it comes up in the question and answer period after the paper is presented. If you can't find time to attend the sessions, pick up the NAB Broadcast Engineering Conference Proceedings at the NAB Bookstore at the convention. Be aware, however, that the latest information won't be in the proceedings. I'll report on the major developments in future RF Technology columns, but cannot go into the detail present in the proceedings.

As always, your comments and questions are welcome. Drop me an e-mail at dlung@transmitter.com. Your question may become the basis for my next RF Technology column!


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