Broadcasters ask, "What should I do to prepare for repack and ATSC 3.0?" The answer is seek advice from the experts.
The future of over-the-air broadcast TV is clouded by a lack of technical planning. There are no national frequency plans for Repack, nor are there any plans for simulcasting or subsidized converters for the ATSC 3.0 transition. Broadcast engineers must be keenly aware of and plan for known future scenarios that have no plan, such as Repack and ATSC 3.0. Every station and market is on literally its own.
Is there a difference between installing a new transmitter and antenna at a new location and participating in Repack and/or ATSC 3.0? If your transmitter and exciter are frequency agile and field upgradable to ATSC 3.0, processes inside the transmitter building are similar. Outside, the effects of Repack and ATSC 3.0 could ripple up the tower.
If your station remains on the same channel, the RF transition to ATSC 3.0 might be no more than an exciter upgrade. That is, if the transmitter and hardware have adequate headroom for the higher peak-to-average power levels of the ATSC 3.0 signal. A new feedline and antenna might be in order.
A TV transmitter puts out less power transmitting DTV than analog because ATSC 1.0 has a higher peak-to-average ratio than analog. The peak-to-average ratio of ATSC 3.0 is higher than ATSC 1.0. According to Rich Redmond with GatesAir, “Studies indicate the approach seems to be that a 5KW transmitter system would need 2dB more peak power headroom.”
A new channel requires a new antenna. If you intend to broadcast on both channels for a while, you’ll need two feedlines and space to mount the second antenna. Your second antenna will likely be side-mounted and could potentially interact with other antennas on the tower.
FCC illustrates digital repack dynamics. Few stations will be able to make all of the needed decisions without help from experts. Image: FCC. Click to enlarge.
Some towers don’t have the capacity for a second TV antenna. FM and other antennas sharing the tower may need to be relocated, and there’s no money in the FCC TV Repack budget for anyone but affected TV stations to reclaim relocation costs. Are tenants on your tower aware that some significant structural changes may be forthcoming?
You’ll also need a building to house two transmitters and enough HVAC capacity to cool both units pumping full power into two tower-mounted antennas 24/7. Does your transmitter building have space for two working full-power transmitters? Will your existing HVAC system be able to keep both systems cool on a sunny hot summer day?
TV RF theories
There are a couple of theories about TV RF. One is to saturate the market with a high-power signal. Saturation generally requires a tall tower, rigid feedline and lots of ERP. The other theory is to focus less powerful signals on market population centers. For some propagationally-challenged over-the-air viewers, focused coverage may be more effective than brute force.
Over the years, many TV stations found that a new transmitting antenna doesn’t resolve all reception issues. Sometimes, a new antenna design mounted on the same tower will make some new or struggling viewers happy, while some long-time viewers’ reception is instantly degraded, and some of those you’ll never hear from. A better idea to fill in DTV holes and gaps is distributed transmission.
The FCC repack plan gives TV broadcasters four choices, so make your bet. Image: FCC. Click to enlarge.
One form of distributed transmission beginning attract attention in the US is known as a single frequency network (SFN). A SFN is a network of DTV transmitters operating on the same channel, synched to GPS, with a unique input delay dialed into each exciter to keep signals in sync.
While the US is auctioning off the 600 MHz band, some of the rest of the world is auctioning off the 700 MHz band. The FCC auctioned the US 700 MHz band in ‘Auction 73’ between 2007 and 2011. All 700 MHz spectrum was sold, except the 10 MHz occupied by channels 62 and 67.
Right now, the UK, Europe and other parts of the world are involved in what’s called the 700 MHz Clearance. Its purpose is to create a unified European Digital Single Market (DSM), designed to remove online barriers across the EU. Like Repack, it too forces some TV stations to move.
Repack and 2nd generation DTV
When asked if his company was prepared to handle an avalanche of Repack orders, Jampro president Alex Perchevitch said, "Yes! We have been increasing our capacity to deliver products for a number of years and improving our line of broadband solutions as evidenced by our broadband UHF slot antenna which can provide bandwidth of more than 200 MHz with various polarizations and power ratings."
In addition, the company’s UK-based Alan Dick Broadcast division “has been busy since last year moving TV stations off the 700 MHz band.” Among other RF projects, the company had a contract to re-engineer more than 100 UHF stations for network operator Arqiva. Alan Dick division provided about 80 % of the gear for the project.
Perchevitch commented, “We've invested more in the last 12-14 years in tools, engineering technology and software than all years combined since Jampro was founded in 1954.”
DVB-T2 is a second-generation, multiple transport stream system “electrically similar to ATSC 3.0 for all practical purposes,” Perchevitch said. It too has a higher peak-to-average power ratio than DVB-T.
“T2 is not new. We’ve been making and installing mask filters and mask filter combiners for T2 since it was launched in 2010,” he said. The ‘700 MHz Clearance’ is scheduled to continue until 2020. The hardware and work T2 and the 700 MHz Clearance requires in other parts of the world isn’t much different than the coming US Repack.
Jampro also delivered more than 1000 broadband panels for approximately 140 sites to cover Singapore in the last two years. Why? “The maximum height for buildings in Singapore is 20 stories, which raises the ground level to 20 stories,” Perchevitch said. “With transmitting antennas on top of buildings there isn’t any signal density inside the buildings below.” Multiple SFN transmitters with flat panel antennas at lower elevations fill in the coverage gaps.
A Single Frequency Network (SNF) requires all transmitters to highly synchronized to minimize interference between RF signals when they arrive at a receive point. Click to enlarge.
The SFN isn’t a new concept. In 2009, the ATSC published a 136-page document ‘ATSC Recommended Practice Design Of Multiple Transmitter Networks’. It details how SFNs work and how DTV receivers respond to them.
The RP document is filled with useful digital UHF RF knowledge such as: “The rule of thumb is that at about 50 miles from a digital UHF transmitter with an antenna at 1200 feet HAAT, it takes approximately an additional decibel of transmitter power to increase coverage by an additional mile.”
The executive summary of the ATSC RP states; “Laboratory tests of newer DTV receiver chips and prototypes have shown that contemporary pre-echo performance has significantly improved.” That was written in 2009 and receivers have certainly improved since then. This is important because even the best designed SFNs can have echos.
The station I do contract transmitter maintenance for happens to be one of the only commercial SFNs in the US. The station recently added a new GatesAir Maxiva transmitter, located much closer to another SFN transmitter than the usual distance between the station’s five other SFN transmitters. In fact, the vast majority of the real estate between all the SFN transmitters is pasture, fields and woods. The station seldom heard reception complaints except from viewers in one populated area on the outskirts of town where viewers lived in a RF shadow from a high hill.
GatesAir installer Carl Williams connects a channel 49 mask filter to a new Maxiva transmitter in Missouri.
The idea was to fill in the shadow from the country-side with a 1KW TPO SFN transmitter and an antenna at about 250 feet. After installation, the net result was some happy new country-side viewers, and a batch of fresh poor reception complaints from viewers in a neighborhood on the city-side that had enjoyed good reception until the second signal signed on the air. It didn’t take many viewer complaints for some at the station to begin wondering if the new SFN transmitter was such a good idea.
That’s when station engineers Art Morris and Brent Bolinger stepped forward with their calculators and some practical logic. Originally, the station was instructed to set the exciter delay based on the distance between the transmitter and its nearest SFN transmitter neighbor. It turns out that predicted transmitter delay offset was confusing receivers that were receiving both signals relatively equally.
In the world of typical DTV receivers, if one signal is 15 dB stronger than another on the same channel, the receiver ignores the weak one. However, there’s a crucial zone where the difference between the two SFN signals received on a single receive antenna is less than 15 dB. That’s where network timing become not only critical, but binary.
More information on the spectrum bidding auction can be found here: http://wireless.fcc.gov/auctions/default.htm?job=auctions_home
If the two SFN signals are within a few microseconds of each other at the receive antenna, the receiver is designed to add them together to produce a signal. Outside the crucial <15 dB zone, timing differences are moot.
When the delay of the new transmitter was redialed-in to align its timing closer to the other signal as they both arrive in the overlap zone, viewers reported success. Viewers who were receiving the station for the first time were still watching, and those in the overlap zone who lost the signal could see it again.
One viewer reporting success had a 12 microsecond difference between signals at the receiver, and the viewer’s TV receiver handled the echo flawlessly. Most modern DTV receivers are said to be able to resolve up to about a 40 microsecond <15 dB difference echo, whether it’s due to multi-path or other SFN transmitters.
UHF timing differences are computed using a delay of 5.33333 microseconds per mile from each transmitter antenna to a local receive antenna. In some unusual circumstances a more directional receive antenna might benefit some viewers, but typically that’s not the case.
Pioneering with a new SFN transmitter isn’t the same as upgrading to ATSC 3.0 or Repack reassignments, but the process raises many mutual issues that demand diligent investigation and design consideration. Planning at the local station level is the key to surviving the dynamic, technical ‘work-in-progress’ modern DTV RF distribution has become.
As Jampro's Alex Perchevitch explained, "The most effective way for stations to actively begin preparing for either or both events is to get your tower surveyed, structurally analyzed and ready for work. Get ahead of the curveball.”
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