RF Technology: Doug Lung

Transmitter Options for Digital Televsion

Now that the FCC has set deadlines for maximizing or replicating coverage, many stations will be looking at upgrading DTV transmitters or in some cases even replacing them.

Last month, I looked at antennas for DTV. This month I'll focus on transmitters.

SINGLE Vs. DISTRIBUTED

I received an e-mail from one engineer who was concerned that his station's management was planning to use multiple low-power transmitters for DTV instead of one high-power transmitter. I got the impression this was for cost-savings. Now that ATSC has approved A/110, the Synchronization Standard for Distributed Transmission, many stations may be wondering if distributed transmission (DTx) might work better for them than a single high-power transmitter.

DTx systems use two or more transmitters, each transmitting exactly the same signal at a specific time. If signals arrive close enough in time, the receiver will treat the multiple signals as multipath, rather than interference.

Figs. 1 and 2 should make this easier to understand. Transmitters A and B are sending the same signals (symbols shown in the resistor color-code sequence).

Look at the areas where the signals are received at the same time and where the timing is quite different. Fig. 1 shows both transmitters sending the same signals at the same time. Fig. 2 shows the effect of delaying the signal from transmitter B by one time unit.

The areas where the signals are in time changes. In a real-world analysis, the signal levels from both transmitters would have to be considered, as well as the location of the population and the terrain between the two transmitters.

If you aren't familiar with how DTx systems work, refer to my articles on Single Frequency Networks in the Feb. 5 and March 5 editions of my RF Technology column, available at www.tvtechnology.com

It should be clear that designing a DTx system is much more complicated than setting up low-power transmitters at convenient locations.

Using a network of distributed transmitters probably isn't the best solution for trying to reach a population evenly distributed over relatively flat terrain.

In such a situation, most viewers will be able to receive signals from multiple transmitters, making it difficult to adjust the timing and power levels to allow reception in one area without causing interference in another.

The Ai analog depressed collector IOT-based transmitter at Baltimore's WNUV-TV

However, if the population is concentrated in a few areas or is separated by terrain features, a DTx system could be the best option. In this situation, the timing and power levels can be adjusted to move interference between transmitters into areas where there are few, if any, people. Terrain can also be used to isolate transmitters.

Another reader asked if distributed transmission could be used to extend his station's coverage. The simple answer is yes, but that depends on how you define "extend." The FCC is unlikely to allow stations to use DTx systems to add coverage outside their already authorized contours without additional interference studies.

Coverage could be extended over a large area using the distributed translator technology described at this year's NAB. Using only two channels in a system, translators in a distributed translator system (DTxR) can receive an off-air signal, which would include synchronization and timing data, and transmit synchronized data to another other channel (including sending synchronization and timing data for the next ring of translators) without the need for a link to the studio.

The FCC has not set rules for DTx systems, but it would likely consider requests for special temporary authority to construct and test systems where they would not cause interference.

A DTx system can be used to provide a stronger signal in areas closer to the limits of the protected contour. If a lower power transmitter is located on a short tower close to the town or population center, much less power is needed to provide a strong signal to that area. With shorter tower height and lower power, the noise-limited contour (41 dBu for UHF DTV) will be greatly reduced compared with a tall tower/high-power transmitter, making it much easier to stay within the authorized contour.

FAR-FLUNG VIEWERS

Distributed transmission could be very effective for stations that serve a large area where the population is distributed in isolated communities far from the main city. Rather than use a high-power transmitter that spreads a signal over unpopulated areas, a DTx system could be designed to deliver a strong signal to these isolated communities.

Is a DTx system cheaper to install and operate than a single high-power transmitter site? Perhaps, but consider that the distributed transmission standard allows for a range of transmitter power and coverage options. If one high-power transmitter site is replaced with four medium-power transmitters, there may be little if any savings.

If the high-power transmitter is replaced with one medium-power transmitter and three or four lower-power transmitters, the comparison gets better. On the other hand, at some point, the cost of building out multiple lower-power sites will exceed the cost of single high-power transmitter. Even if the transmitter power is low, each site will require an 8-VSB modulator with the extra circuitry/software required for the DTx system.

In addition to the cost of the transmitters, the cost of getting the signal to the transmitters has to be considered. Microwave or fiber links to each of the transmitter sites will be needed.

These aren't cheap.

There will also be costs associated with leasing or purchasing multiple transmitter sites and possibly erecting more towers. If there are several widely distributed sites, maintenance could become an issue, although lower-power, solid-state transmitters should require far less maintenance than high-power IOT-based transmitters.

Even if the total power consumption of multiple transmitters turns out to be less than what a single high-power transmitter requires, the fixed costs associated with multiple services at multiple sites could erode the savings.

As we approach the end of analog transmission, having a reliable DTV transmission facility will become more important. Many TV stations do not have full-power analog backup capability, instead depending on transmitter redundancy (multiple amplifiers/devices) or older, lower-power transmitters.

CLIFF EFFECT

Unlike what some station salespeople think, even half-power analog operation does not mean half the audience disappears. Most viewers would not notice the power reduction if they had a clean picture at full power. We've been warned, however, that due to the digital "cliff edge," a very small difference in signal level can be the difference between having a picture and no picture. A 3 dB reduction in power could leave some DTV viewers with an intermittent picture or none at all.

Clearly the safest approach for DTV backup is to have a full-power backup transmitter or to design a single transmitter with sufficient power margin to deliver full power even if one amplifier fails. This may not be difficult with solid-state transmitters, but for tube-based transmitters, four amplifier cabinets could be required to maintain a balanced combining system. For most stations, this will be difficult to justify until there are a larger number of people receiving DTV signals.

If full-power backup isn't practical, but some useful backup capability is needed, what power level is required?

The answer will depend on whether interference from high-power stations on adjacent channels is an issue and how much margin there is at full power. Note that this can be difficult to determine as a stronger signal will be needed in areas with more multipath or where indoor reception is desired.

I've often said that a low-power TV station that provides excellent coverage in rural west Texas wouldn't even be noticed in a city like Houston or Dallas with a lot of strong full-power signals that people can pick up easily using an indoor antenna.

Here's a simple guideline for estimating required DTV backup power--if your station's backup DTV power is close to that of other popular (widely viewed) DTV stations transmitting from the same site on the same band (low VHF, high VHF or UHF), many viewers will have installed a sufficient antenna to receive the other stations and shouldn't have a problem with the power reduction. However, if your signal level drops too far below that of other stations in the market, you may start receiving complaints.

A more complete analysis involves looking at antenna patterns, not only for your station but the competition as well. Last month, I showed how the signal from a low-power DTV station with a lower elevation gain antenna could equal or exceed that from a high-power station using an antenna with higher elevation gain in some areas.

Of course, given the performance of most DTV receivers being sold today, if there is a high-power station (analog or digital) on an adjacent channel, it is likely to cause problems for a low-power operation, and there is little the viewer can do to fix it even if they are willing to make the effort.

At NAB this year, I was surprised at how much interest there was in analog TV transmitters. With analog broadcasting expected to end in five years, why buy a new transmitter?

Many stations that deferred buying replacement analog transmitters in the past are finding that it is becoming difficult and expensive to keep the old transmitters on the air. As analog broadcasting is responsible for the bulk of the revenue at most stations, having a reliable analog transmitter is still critical. Most transmitters sold today can be converted from analog to digital operation in the future, so if the station plans to move back to its analog channel or if the analog and future digital channels are in the same band, the new analog transmitter can become the main or backup DTV transmitter in 2009.

The introduction of analog UHF TV transmitters using multistage depressed collector IOT amplifiers provided another reason to consider a new analog transmitter--a significant reduction in power consumption.

Acrodyne Industries (Ai) installed the first analog depressed collector IOT-based transmitter at Baltimore's WNUV in late spring 2004. Recently Ai released data on the actual power savings achieved by replacing a five klystron pulsed 240 kW transmitter with an Ai Quantum transmitter equipped with four e2v water-cooled ESCIOT multistage depressed collector amplifiers.

Comparing power bills from July 2004 with ones from July 2003, the cost of supplying power to the transmitter was reduced by more than 62 percent, for a savings of more than $28,000 per month, according to Ai. Ai notes that when analog operation ends, the transmitter can easily be converted to digital operation.

When specifying a new analog transmitter, be sure to find out how much the analog-to-digital conversion will cost. Most manufacturers will, at a minimum, require replacement of the modulator, although with some digital modulators, this could be as simple as installing new software/firmware and reconfiguring the modulator.

Don't overlook the RF system. Most RF systems are optimized for a single channel, and while some components will work over a portion of the band, any channel change will require retuning. Filter manufacturers are offering mask filters for analog transmitters that can be easily modified for DTV bandwidths on the same channel. If you plan to convert a new analog transmitter to digital on the same channel, make sure the filter will do an adequate job for DTV operation as well.

DON'T FORGET E-VSB

Finally, when purchasing a DTV transmitter, it is worthwhile to get a guarantee that the DTV modulator you are purchasing can be easily (and inexpensively) converted for E-VSB operation and possibly distributed transmission as well. If reception of ATSC DTV on handheld, portable or mobile devices becomes popular, E-VSB could be needed to provide them with a reliable signal. As described at the start of this month's column, a DTx system, consisting of a single high-power transmitter and a few lower-power units, could be useful in filling in holes in coverage or extending strong signal coverage, also important for reception on handheld and portable tuners.

I hope you found this month's discussion useful!

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