The Problem
As stations switch off their analog signals they find themselves facing an unexpected crisis – actual digital coverage falling dramatically short of expectations. Some viewers, even with unobstructed paths to the station towers, are unable to receive the station reliably.
Failure to achieve predicted coverage impacts many homes, not just a few isolated viewers. This is a wide area problem. Whole pieces of DMAs that once received a fine analog picture now report no digital reception. These were areas predicted to retain full coverage after digital conversion.
The Difference between Analog and Digital Coverage
Generations of television broadcasters since the birth of the medium learned to equate coverage with “millivolts per meter.” Managers and salesmen might not know a millivolt per meter from a klystron, but they knew if they didn’t have enough millivolts in an area, they didn’t have any audience.
Signal strength was king. All the other transmitter stuff that the engineers spent time and money making perfect – SNR, the waveform, differential phase, etc, etc all affected the quality of the picture. And most mere mortals, as opposed to engineers, could see little difference in the picture one way or the other.
The trouble is digital television really is different — very different. Many engineers, and even the FCC don’t realize how different digital coverage issues really are. A mistake in adjusting the transmitter or the drift of some setting over time does not degrade quality the way it would for an analog station; for digital it trashes coverage.
And since the FCC recently released coverage maps for all full power digital stations in the country, everyone has a copy of their predicted coverage. More to the point, these maps showed the gain or loss of digital coverage against what analog had delivered. The trouble is, these maps are based on signal strength and interference. They fail to take into account all the factors unique to DTV that impact coverage.
Put in analog terms, the impact of an out of adjustment DTV transmitter is the same as if an analog transmitter were reduced in power twenty times – from, say, 100 kilowatts to 5 kW.
The Cliff Effect
Digital TV reception is an all or nothing affair. It is both a strength and weakness of DTV. The picture is either perfect, or there is no picture. If the picture is gone, it has “fallen off the cliff.” Contrast this with analog TV, where the quality of the picture gradually degrades with the distance viewers are from the transmitter. There is no such thing as a DTV picture with snow or ghosts; it is always either perfect or just not there. The problem is that a lot more factors affect when the signal falls off the cliff than just distance from the transmitter (signal strength).
In a footnote to the report that accompanied the publication by the FCC of the digital verses analog coverage maps, the FCC stated,
“ We recognize that the digital cliff effect can also occur at the fringe areas of coverage. However, this cannot be quantified and for purposes of this report we apply the term “digital cliff effect” only to losses within the service area that can be quantified.”
With that footnote, the FCC dismisses issues that can reduce the coverage of a station by as much as 50%.
The Key is Margin
Rather than signal strength, gauging the coverage of a digital TV station is a matter of figuring out margins.
Let’s say that a perfect transmitted digital TV signal has a margin of 100%. A typical digital TV will receive a perfect picture if the signal at its antenna terminals has a margin of 30%. At 29% the picture fails (falls off the cliff). Depending on the TV set, failure means the picture will freeze, go blank, or break into random blocks.
Decreasing signal strength reduces the margin only by a small amount until it falls below a critical level (a very low level) and then the picture fails. However, if nothing else consumes much of the margin, the distance to the point where the picture fails is typically MUCH further away than the point where an analog picture would be unappealing to watch and much further than the predicted coverage.
In predicting the coverage performance of a digital TV system, its designers assumed that the 70% margin between a perfect signal and the point where a typical TV set fails would be used to deal with degrading influences like multipath. Multipath is when the signal bounces off objects like buildings and mountains. These reflections create additional paths of different lengths between the transmitter and the TV set. In analog TV, multipath caused ghosts. The designers of our digital system expected that multipath would consume the most margin and would control coverage. Other things that consume margin are rabbit ear TV antennas and people moving around the room near the rabbit ears.
Weather sometimes consumes a good chunk of margin. Surprising, it is not often severe weather that interferes with DVT pictures, but rather inversions in coastal areas and hot humid weather on the plains. The good news is that the problems only affect a small area at a time and usually last for seconds to tens of minutes, although they often occur in clusters. It is more of a problem when a cable head-end or a translator is affected, since either of them may be providing signal for tens of thousands of homes.
Margin is a Systems Parameter. It is the sum of the entire signal – what is transmitted by the station, the propagation path between the transmitter and receiver (including any multipath), the receiver antenna, and the digital technology used in the receiver.
Loss of Coverage
The designers of DTV assumed that the DTV picture that the broadcaster sends out would have a near perfect margin, 99 to 100%. In many cases this has proven incorrect.
The consequences can be devastating. Defects in the transmitted signal can eat up 50 to 60% of the margin. With so much of the margin gone before the signal leaves the station’s tower, only viewers in near ideal situations will get a picture. This has proven to be the case for many of the stations that have recently shut off their analog signal.
With many stations having been on-air in digital for more than five years, how could this have gone unnoticed for so long? The answer is, so long as there was an analog signal, no one noticed. If the digital signal was not usable, the viewers stayed with the analog. A number of coverage studies were performed by stations and interested organizations. Some of the studies relied only on signal strength, assuming a perfect transmitted signal. The studies that did check to be sure that there was a usable picture, also usually had an experienced engineer first verify the transmitted signal to insure that it was near perfect. If it wasn’t, it was fixed before the tests were run.
So how do so many stations, maintained by conscientious engineers, transmit such substandard signals?
Education and Economics
It’s difficult to explain to a non-engineer the magnitude of the shift in thinking required to make the transition from analog to digital TV transmission. Everything is different, even the acronyms. It is a whole new bowl of alphabet soup.
With everyone in the same boat – manufacturers, suppliers and consultants – learning as they went along, the station engineer had few places to turn for education. One great resource was Gary Strongly, one of the inventors of ATSC DTV. After he left Zenith, Gary went on the road doing seminars about the “nuts and bolts” of making DTV work. Gary correctly realized that no matter how good the technology, if the people who had to make it work on a day-to-day basis didn’t understand it, DTV would not succeed.
I am certain that Gary’s heroic effort helped, but not everyone could attended his seminars. And those who did attend encountered another hurdle when they got back to their stations – budget.
The million or so dollars that most stations had to spend to go digital offered no promise of return on investment for several years, so the request for many tens of thousands of dollars needed for test equipment to insure that full coverage could be realized was largely not forthcoming. Even today, few stations own the basic test equipment needed to insure full coverage, although a few suppliers, my company (MSI) among them, have introduced much low cost test gear.
The situation remains grave today. The bottom line is that many stations are transmitting signals so deficient that they are reducing their effective coverage by 50% or more. This not only puts the station at risk, but it places the entire medium in jeopardy.
Solutions
Money needs to be found to purchase test equipment. This must be 8vsb transmission test gear. PSIP analyzers or transport stream analyzers, although important, will not help resolve coverage problems.
Beyond the test gear, many station engineers need help in first understanding what the data from their test equipment means, then in how to apply that information in adjusting or repairing their transmitter systems.
In an effort to deal with this crisis, my company is now offering a block of free telephone consulting time with each transmitter analyzer sold. This time can be used to learn to interpret the analyzer’s readings, or to relate those readings to transmitter adjustments, or some of both.