Does the report on the first formal tests of a TV channel-sharing arrangement really say what FCC Chair Tom Wheeler says it says? YOU make the call.
At the recent NAB Show in Las Vegas, Chairman Wheeler came on like a cheerleader at a pep rally, touting the upcoming incentive auction program. (For readers who weren’t there, it was something like Darth Vader trying to sell the Rebel Alliance on the obvious benefits available to Empire participants.) According to Wheeler, the auction presents “a terrific financial opportunity for broadcasters” – and that’s because of the possibility of certain cooperative agreements between TV licensees. This is, of course, the same Chairman Wheeler who, just days earlier, had put the kibosh on cooperation between TV licensees in the shape of joint sales agreements (while also raising a critical eyebrow at shared services agreements and joint retransmission consent negotiations).
But those types of cooperation are different.
In Vegas, Wheeler was talking about channel-sharing arrangements in which two stations use common transmission facilities, arrangements which can provide “under-considered and under-appreciated” benefits. And how is he so sure about that? It turns out that a report has been prepared describing the first formal test of channel sharing and, to hear the Chairman tell it, the report makes a “compelling case” for that practice. He said that he hopes broadcasters “closely study” the report.
Trouble is, the report itself doesn’t appear to have been widely circulated. So we figured we’d take a look at it and let our readers know what we found – and also give them a direct link to the report so they can read it themselves.
The subject of the report is the channel-sharing test my colleague Davina Sashkin wrote about in February. The test was initially proposed by two LA-area TV stations – KLCS and KJLA – in late January. (CTIA-The Wireless Association® was also involved.) Apparently fast-tracked from the get-go, the plan was approved by the FCC in less than a week. The test was initially expected to take up to six months to complete, but that was apparently an overestimate: the report was filed with the FCC less than two months after the proposal was approved by the Commission.
As Chairman Wheeler reported in Vegas, the report does indeed indicate that KLCS and KJLA were able to operate compatibly through the same transmission gear on the same RF channel. But a close reading of the technical details reveals that, to get to the Happy Ending, some interesting basic assumptions had to be made. So we fully agree with Wheeler when he encourages all concerned to read the report . . . carefully.
For example, all of the tests were conducted using the latest-and-greatest encoder hardware and software. The newer gear apparently made a difference in the efficiency of the compression process, thereby producing better results than the encoders currently used at most TV stations. Encoder gear represents a “substantial capital investment” for broadcasters, according to the report. Also according to the report, some stations’ original encoders may now be reaching the end of their useful lives, so some upgrade in the near future might be expected, and it may not be unreasonable to rely on latest-and-greatest hardware for test purposes. Still, to the extent that a positive channel-sharing experience may hinge to some degree on upgrading some pricey equipment, broadcasters should be aware of that.
Second, the testers decided to use a 720p HD format, as opposed to the more data intensive 1080i format. Currently, FOX and ABC use 720p, while CBS, NBC and PBS favor 1080i. Historically, the Commission has steered clear of identifying either 720p or 1080i as preferred in any way. Instead, as the move to DTV developed, the FCC relied on a definition of HDTV drawn from the private sector – a definition which expressly embraced both 720p and 1080i. So the report’s suggestion that channel-sharing may favor one format over the other seems to run counter to the Commission’s historical approach. Whether channel-sharing might ultimately impose any new burdens on broadcasters or broadcast networks already committed to 1080i is unclear – another factor to be borne in mind in any assessment of the report’s conclusions.
Third, for audio streams, the testers followed advice from Dolby Labs that a bit rate of only 64 kilobits per second (kbps) per channel is necessary to reproduce sound “without any
perceptible audio degradation”. That’s a much lower rate than the 192 kbps (for stereo, or 96 kbps per channel) and up to 512 kbps (for surround sound) that broadcasters commonly run. The tests were run using the low-end 64 kbps rate.
The goal was to pick up some data capacity from the audio side in order to increase the capacity on the video side. Fewer kbps devoted to audio streams means more kbps for video data. The trade-off: at 64 kbps per channel, broadcasters should not expect to “exceed audiophile expectations” with their audio. According to the report, the tests were intended only to meet – not exceed – such expectations. Again, this seems to cut back some on the initial sales pitch for DTV, which touted a high-end audio experience.
Another consideration: The testers analyzed the quality of the resulting combined video channels using test equipment that calculated the Quality of Experience (QoE) based upon sophisticated algorithms. That calculation supposedly produces a subjective rating of those factors corresponding to the rating that would have been given by real live audience members. In other words, a machine judged whether the picture quality was acceptable, not a focus group of actual people. Yes, a few engineers also viewed combined streams side-by-side with the original input, but for the most part the conclusions reached in the report about the relative quality of the shared channels’ signals are based on numerical scores produced by a machine. A very sophisticated machine, but a machine nonetheless. (The report acknowledges that this type of QoE testing equipment is not available to most TV stations.)
And then there’s the question of channel selectivity: will Joe Public’s TV set be able to correctly tune to each separate station when two stations’ signals are combined in a single 6 MHz RF channel? A DTV signal is often transmitted on an RF channel that is different from the “virtual” channel reflected on a TV receiver. Thanks to the Program System Information Protocol (PSIP) component of the digital transmission, TV receivers know to determine a station’s virtual channel because the PSIP stream contains, among other information, the station’s virtual channel number. Regardless of the RF channel on which a TV set receives an incoming signal, the set’s decoder reads the PSIP information and displays the incoming signal on the virtual channel indicated in the stream.
But what happens when a single RF transmission is shared by two different stations with two different virtual channels? According to the report, all of the TVs and tuners tested were able to receive and correctly interpret the paired TV signals. However, most of the TV sets had to be rescanned in order to find the newly combined station information. This suggests that stations will need to engage in considerable consumer education, possibly akin to what we all went through in connection with the 2009 DTV transition, when they’re ready to begin channel-sharing operations. Otherwise, ordinary viewers might wonder why some of their favorite stations have suddenly disappeared.
In his remarks in Las Vegas, Wheeler also talked in optimistic terms about the potential of a new technical standard for TV modulation to replace the current ATSC-2 standard. This new standard is referred to as OFDM or ATSC-3. The channel-sharing report addresses in passing the future use of OFDM modulation.
OFDM could result in a significant increase (possibly up to 50% – from 19 Mbps to up to 28 Mbps) in the bit rate, and thus the data-carrying capacity, of each 6 MHz TV channel. An increase in bit rate would obviously increase the capacity of the channel to carry video and audio. That’s the good news.
The bad news is that practically no TV sets currently on the market or currently in use in the U.S. can receive OFDM-formatted transmissions. In order for OFDM to become practically useable in the face of the lack of capable receivers, viewers would have to obtain and install separate TV converter boxes – a process many recall from the 2009 DTV transition. Over a billion dollars were spent on vouchers towards the cost of the DTV converter boxes back then. Presumably more money than that would be needed to upgrade everyone’s TV to be OFDM compatibility. Not a very practical solution.
So you be the judge. The channel sharing report is relatively short (21 pages of actual text and 10 pages of technical appendices and glossary). It may be that some, many, even most broadcasters won’t find the technical fine print and related testing assumptions and conditions objectionable. But it is important that everybody know both (a) that the fine print, assumptions and conditions are there and (b) how they might affect the channel-sharing experience.
We join the Chairman in encouraging all TV broadcasters to read the report, discuss it with their engineers, and form their own conclusions about whether it convincingly demonstrates that channel-sharing is ready for prime time. (Here’s the link to the report again, in case you missed it earlier in this post.) We also encourage you to let us know your thoughts about it.
