Fred Baumgartner is the former TV Product Manager for Nautel.
Fred started with AM in the 1970s, then FM, and now TV. Of all things in broadcasting, he loves transmission the most. He’s the guy with the 1954 1KW AM rig he’s slowly restoring for 160M Amateur operation; taking up the space normal people might have used for restoring a sports car. Fred loves a good idea, contemplating how to make this industry better, or an engineering puzzle.
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How Big can an SFN be?
– If Regulations Allow
by Fred Baumgartner
In North America, broadcast coverage for VHF (FM and TV) and UHF (TV) has been a simple matter of stacking as much steel as one can in the highest place one can find and feeding it with as much power as one can — then filling in the gaps that terrain and venues create with translators and the occasional on channel repeater – if spectrum and justifiable money is available.
Usually, where the holes were most desirable to fill, the solution was ugly. Translators require the audience to re-tune to the translator. RDS supports alternate freq tables, so in some parts of the world and with a lot of radios, this works rather well. But it’s not a big thing in North America.
Where the terrain is agreeable, on channel repeaters are not at all rare (no re-tuning needed), but the nature of the beast for analog FM and 8-VSB TV is that there are inherent interference zones that don’t always cooperate.
As we’ve mentioned in this column in the past, OFDM is considerably more friendly to SFNs and changes the game completely. Done correctly, an OFDM SFN’s transmitters add their signals without interference. They can also be used where a translator was or where a translator should be, if only there was spectrum available.
HD FM radio is already here and TV is moving towards OFDM with the pending shift to ATSC 3.0. It seems that all Digital “FM” radio (see HD Multiplex) and ATSC 3.0 are inevitable in the future for TV and maybe not so distant future for VHF-broadcast radio as the number of FM HD receivers overtakes analog FM. There are plenty of reasons for TV to go to ATSC 3.0 — even if OFDM and SFNs were not part of the deal.
So, how big can an SFN be? When Qualcomm (Qcom) built out their coast-to-coast SFN OFDM network for MediaFLO TV on cell phones, the regulation was simple. Qcom owned channel 55 from coast-to-coast, so the limitation was only on the ERP of transmitter sites (25 kW in each plane). Qcom could put transmitters anywhere — as close, or as far apart as the design allowed — but FM and TV stations are licensed to a community, not the whole region or country. Each SFN transmitter is individually licensed and restricted to filling in the natural coverage area (the contour) of the primary station. When it comes to placing SFN transmitters at the edge of the coverage area, or to replace translators some distance away, the current rules don’t work well at all.
Consider something like Nebraska Educational TV (NET). NET covers 77,421 square miles from 20 FM and TV sites, using a variety of frequencies. On FM they are relatively easy to find as you drive across the state as they are all in the U.S. Non Commercial Educational (NCE) part of the FM band (88.1-91.9 MHz). NET radio doesn’t send RDS alternate frequency tables, so even if the car radio supported automatic re-tuning, it wouldn’t work. NET TV stations are not so easy to find by dialing around, save for looking for the NET logo on the screen.
If Nebraska were to have all-digital HD FM and ATSC 3.0, a single frequency could be used for each throughout the state. Being all-digital on FM, they could carry a bigger collection of services, and of course ATSC 3.0 allows for all sorts of content. It’s also easier to fill in the gaps that exist in a state that is famously flat with the current network, which is extensive.
Certainly the forces of broadcasting and spectrum conservation will encourage the FCC to come up with rules that are more favorable for SFNs. Don’t underestimate the chilling effects that complex regulation has on building out SFNs. A big piece of the SFN build out process is the complexity, cost and delays of licensing the transmitters. Simple rules that allow stations to do as much build out as they wish, right up to the edges of the market would be a major help in serving the public. Ideally, when a station wanted to add an SFN transmitter, they simply give the FCC the particulars and certify that the transmitter fits into the coverage area.
TV DMAs don’t look like circles. By 7.11brown – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=33183158
Transmitters have a coverage contour that generally looks like a circle… often distorted by terrain, and sometimes intentionally modified to directionalize the transmission. TV’s 210 market DMA’s and radio’s 286 ADI’s don’t look anything like circles. Public TV and Radio often serve states or regions unrelated to DMA’s or ADI’s. In any case, regulation should focus on allowing broadcasters to cover their coverage areas… and not being stuck in an arbitrary contour.
The biggest limitation on SFNs isn’t the technology (once we get to all digital, OFDM radio and TV) it’s the last century’s regulations that can be relatively easily changed. Given the freedom to build out SDN’s… one can only imagine what good will happen.