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If you’re a Los Angeles resident who gets their TV through an antenna, you may have noticed lately a few extra stations on the digital TV dial. What you’re witnessing is part of a grand experiment that could determine the future of broadcast television. PBS affiliate KLCS and mutlilingual station KJLA have begun transmitting their programming on each other’s airwaves in LA, essentially packing multiple TV channels into the same broadcast airwaves.

The two stations have volunteered to be the guinea pigs in a trial that will determine if broadcasters can share their TV airwaves. If it’s possible for KLCS and KJLA to deliver all of their combined video content on either Channel 41 or Channel 49 on the UHF band in LA, then the other 6 MHz channel could be sold off to mobile carriers for use in new 4G networks.

By now you’ve probably heard rumblings on an upcoming federal spectrum auction in 2015, designed to re-purpose old TV airwaves for mobile services. It’s an enormously complex process involving broadcasters deciding whether or not to part with their airwaves in a reverse auction, the Federal Communications Commission reconfiguring the 600 MHz band to allow mobile and broadcast TV to co-exist, and then carriers bidding on the resulting licenses.

And at the heart of this process are trials like those being conducted by KLCS and KLJA in conjunction with mobile industry group CTIA. If TV stations determine they can get by with fewer broadcast airwaves, then they’ll be more inclined to pursue the financial incentives in the auction. If they determine they can’t part with their spectrum, then this whole thing will have been a colossal waste of time.

Packing the airwaves

The two TV stations and CTIA will make their initial report this week, but I got a chance to talk to KLCS Director of TV Engineering Alan Popkin as the stations were setting up the trials. For the last month, KLCS have been cramming as many varied video streams onto their airwaves as possible to see what can be supported.

Basically, a 6 MHz broadcast channel can support 19.39 Mbps of capacity, which is way more than necessary for a single HD video stream or a combination of standard-definition streams, Popkin said. The big question is whether you can pack two or more HD streams into that channel, and finding that answer isn’t as simple as it appears, he said.

The MPEG-2 format used by the broadcast industry eats up varying amount of bandwidth depending on the content. Talking-head programs don’t require as much capacity because little is changing in the image frame-to-frame in the video stream. Sporting events, in contrast, have a lot of action going on and demand much more bandwidth to maintain picture quality.

So Popkin has basically been throwing every combination of programming into the same 6 MHz bitstream as possible — both in the lab over live air — to see what flies and what doesn’t. The good news is video and audio compression technologies have improved dramatically in the last few years. When DTV first got off the ground, running two HD broadcasts over the same channel would have impossible in most cases, Popkin said. And as broadcasters explore moving to more efficient formats like H.264, they can squeeze even more into the same bandwidth, Popkin said.

“What is merely possible today, might be more than possible tomorrow,” Popkin said.

Still, broadcasters have little tolerance for grey area. As a PBS affiliate, KLCS isn’t programming many action movies or sporting events, but you can bet that Fox stations aren’t willing to accept the slightest chance of broadcast interruption during the Super Bowl.

A moving target

Even if the trial proves that multiple HD broadcasts over the same channel is viable, some broadcasters may be looking into the future to new higher-resolution technologies like 4K Ultra HD video, which would demand tremendous amounts of bandwidth.

Popkin, however, isn’t bothering with 4K video tests, because he has no assurances that over-the-air broadcasters will ever move to the format. Most TV consumption today is done on cable and it’s increasingly moving onto the internet. 4K may find a home over those pipes, but there’s no guarantee there will be a need or demand for it on broadcast TV, Popkin said. If a broadcaster insists 4K is its future, then it’s probably not parting with its airwaves.

“From our point of view, this incentive auction is going to happen no matter what,” Popkin said. “We can either stick our head in the sand, or we can investigate whether it makes sense to participate. … Our intention is put in the work so we can make an informed decision.”

If the trials do show channel sharing is feasible, the broadcast industry stands to gain considerably by selling extra airwaves. Not only would there be the proceeds from the auction, but as Association of Public Broadcasting Stations EVP and COO Lonna Thompson explained, broadcasters could share their towers and infrastructure as well. Public stations, Thompson said, could take the auction proceeds and operational savings and invest them back into their primary mission: programming.

“It’s not for every station, but I think many of them will look into it,” Thompson said.