Spectrum Repacking and Channel Scans

In the wake of last year’s big spectrum auction, the FCC is chopping even more spectrum away from UHF TV stations and expecting (somehow) to jam all the remaining TV stations into low band VHF (2-6), high band VHF (7-13), and truncated UHF (14-36) channels.

In my neighborhood, stations are already packing up and moving. While conducting a recent test of a “smart” indoor UHF TV antenna, I grabbed some spectrum analyzer plots of all three television frequency bands. As expected, the RF spectrum from channel 56 to 87 (channels 2-6) was largely unusable due to high levels of impulse and main-made noise.

The high band VHF spectrum wasn’t much better, with some continuous RFI kicking up the noise floor by almost 20 dB. But it was the UHF spectrum I was interested in, and several former broadcasters were noticeable by their absence. Channels 29, 35, and 39 – previously in use for Univision, independent, and PBS stations – had all gone dark.

To get around the lack of available channels, TV stations are “channel sharing,” something the FCC frowned on as recently as a decade ago. What that means is that stations divvy up the available bits in an MPEG2 encoder and multicast several minor channels on one physical RF channel. This technique was almost impossible to pull off twenty years ago when digital TV broadcasts and HDTV were just getting started.

Now, thanks to very powerful processors and tricks like adaptive variable bitrate encoding and statistical multiplexing (a/k/a “stat muxing”), it’s not difficult at all, even though the jury is still out on the quality of HD and SD video using much lower bit rates that were not possible in 1998. NBC has done this in Philadelphia and New York, combining Telemundo channels with NBC programming and making room for one HD service from each.

Locally, an independent station in Allentown (WFMZ) will relinquish its 5-megawatt signal on UHF-46 and move to VHF-9, sharing bits with WBPH and the Lehigh Valley PBS station, WLVT (formerly on channel 39). This will have happened by the time you read this column and I’ll be curious to see just how much image quality has deteriorated for each minor channel after the new transmitter lights up.

Keep in mind that many stations auctioned off their channels in return for a nice pay day. Public stations in particular pocketed some serious change, money that went into facilities upgrades and balancing their budgets. If their multicast services hold up well with the latest in MPEG2 encoding, then they’ll come out of this smelling like a rose.

What this means to you as an OTA viewer is that you will need to re-run channel scans to catch all of these moves – otherwise, you’ll tune to a channel that has gone dark and will be standing there, scratching your heads in bewilderment. I’d perform a channel scan twice a month from now through the end of the year. (You might also pick up some newer, low-power translators and repeaters along the way, and you may find some channels are gone for good.)