[opendtv] FCC Spectrum Analysis Doesn't Add Up
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 18 Jun 2010 16:56:37 -0500
http://www.tvtechnology.com/article/102242
FCC Spectrum Analysis Doesn't Add Up
by Doug Lung, 06.18.2010.
This week the FCC released a paper, Spectrum Analysis: Options for Broadcasting
OBI Technical Paper No. 3. I have great respect for the engineers whose names
are on the report. During the 40 years I've been involved with broadcasting,
FCC reports were, with few exceptions, unbiased, accurate and complete. That is
why upon critical reading I was so disappointed to find this attractively
presented Paper to be biased, incomplete, and, in some ways, inaccurate.
Critical information is missing and conclusions are presented as fact when data
doesn't support them. The analysis glosses over data that refutes the National
Broadband Plan's (NBP's) recommendation for taking away broadcast spectrum and
makes unjustified assumptions to support them.
I'll highlight a few examples, but urge you to read the document and draw your
own conclusions.
The Paper notes that the FCC staff and contractors have been developing a new
Allotment Optimization Model (AOM) to achieve the NBP's goal of taking 120 MHz
from the 294 MHz (240 Mhz) after removing three channels for land-mobile and
television Channels 2 through 6 currently assigned for TV broadcasting. Not
surprisingly, when the model was used to see how much spectrum could be
recovered by repacking existing full power stations, only 6-42 MHz of spectrum
was found to be available for reallocation, depending on inclusion or exclusion
of border restrictions.
The scenario did not move UHF stations to VHF or VHF stations to UHF. Cellular
technology is briefly discussed but the only supporting data is Appendix A,
which includes a graph showing 650 MHz vs. 1900 MHz rural cell areas plotted
against maximum allowable path loss in dB. It's an example of the document's
tendency to present impressive looking data with little relevance to the real
world. In this case, multipath and self-interference are ignored, and the table
is intended for rural areas, where there is less demand for both wireless
spectrum and broadcast spectrum.
Appendix D is a cut and paste of parts of FCC OET Bulletin 69, with half a page
devoted to a now out of date table showing desired-to-undesired interference
ratios for analog TV. A close look indicates that only three lines apply to
DTV-into-DTV interference.
Before you argue that many analog LPTV and Class A TV stations are broadcasting
analog, that doesn't matter in this report.
There is a small section on LPTV stations, but as far as I can tell, they were
not included in the studies. The Paper repeats the NBP suggestions for ways to
accommodate LPTV stations, but notes: "To the extent that a reallocation
compresses the broadcast TV bands, non-Class A LPTVs may be forced to move, and
therefore incur relocation costs, and they may find fewer available channels
slots which they can occupy."
Regarding wireless microphones and TV white space devices, the document says
they must be considered, but states, a consideration of any potential effects
and solutions would be "beyond the scope of this paper," as it is intended to
address "particular considerations" that apply to full-power TV. In my opinion,
with no available TV channels where most people live, the options for LPTV
stations and wireless microphone users would appear to be limited at best.
In fairness to the authors, there is some relevant and useful information in
the document's other Appendices. As an example, "Appendix C: Optimization Model
Formulation" provides details on the AOM used to determine the amount of
spectrum reclaimed from repacking and channel sharing.
One thing I found interesting was that "the model does not incorporate terrains
conditions in order to determine the minimum allowable spacing between
stations." Users can selectively relax spacing restrictions between pairs of
stations in a DMA "based on an assessment of terrain or other considerations."
No details are provided on how that would be applied objectively.
Based on the results from the AOM repacking study, the Paper suggests one way
to clear half the broadcast channels would be to put two stations on one
channel. Exhibit G shows the result of this study. If border restrictions are
ignored, the Exhibit indicates that 120 MHz of spectrum can be taken from
broadcasting if 204 stations share a channel. That may not sound like much, but
is an example of where insufficient data is presented for readers to get a true
understanding of the impact.
Only 12 percent of the country's stations sharing channels doesn't sound like
much, but the percentage would have to be higher in markets like Los Angeles
and in areas in the northeast where some stations had trouble finding a
suitable channel for DTV after channels 52-69 were reallocated. In the
scenario, some 707 stations (41 percent) would have to change channels. In
addition, more than half of the stations would lose some service area.
What is interesting is that if the amount of spectrum taken from broadcasting
is reduced to 72 MHz - enough to protect active border allotments - only 38
stations would have to share a channel and another 392 would have to change
channels.
Without more data it's impossible to determine where those 38 stations would be
located. The exhibit shows that the portion of the viewers that would gain
service from stations improving coverage is larger than the number of viewers
losing service from stations with reduced coverage.
Again, the limited data makes it hard to visualize what the real impact is. The
stations gaining population may be those with limited power now, while the ones
likely to be losing population could be the higher power, major market network
affiliates - the ones also likely to have the most viewers.
Look at these columns again, the "Without Border Restrictions" numbers are a
red herring. It seems unlikely the FCC would be able to ignore active border
allotments when repacking the TV band. Unless the Commission is suggesting that
120 MHz may be too much to take from TV broadcasting - which doesn't seem to be
the case - the "Active Allotments" column is also a red herring.
Where will the other 48 MHz come from?
Conveniently missing is information on the shortage of channels, the greatly
increased loss of service or the additional channel sharing that would be
required to reclaim that extra 48 MHz. Why isn't that analysis included in
Exhibit G?
Perhaps it's not there as it would conflict with the statements elsewhere from
the FCC that broadcasters will not have to give up spectrum involuntarily, and
that taking away 120 MHz of TV spectrum will not prevent them from multicasting
or transmitting Mobile DTV.
Indeed, the amount of space-with examples, of how even network affiliates
broadcasting HDTV can share one channel-ignores Mobile DTV or multicasting
considerations in channel sharing. The scenarios described for combining two
stations on one channel with HD can work only if some-perhaps all multicast
channels are dropped and Mobile DTV is not transmitted.
The data rate required to transmit video programming, as the document shows, is
highly dependent on content. Statistical multiplexing can be used to
dynamically share bandwidth, allowing maximum spectrum efficiency.
In the case of ABC (one of the examples cited), I think that anyone who's
watched ABC's second HD channel would agree its quality is significantly
impacted by what's airing on the main channel. How would that work in a shared
channel arrangement?
If you look at the situation with regard to WRC-TV in Exhibit L, you can see
how statistical multiplexing works. Programs are given different priorities.
When NFL Football needs more bandwidth, it can take it from the two multicast
channels. When Universal Sports (the "third channel") needs more bandwidth it
can take it from NBCPlus.
Viewers on Universal Sports will see some artifacts (and there have been
complaints) when the HD channel needs the bandwidth to avoid artifacts. The
second channel is primarily used for weather maps and talking heads, and as you
can see, doesn't require much bandwidth.
Now imagine trying to squeeze two HD channels into 19.39 Mbps with mobile and
multicast. Mobile DTV requires a fixed bandwidth as does audio and PSIP. If
video description is required in the future, that will take additional fixed
bandwidth. The lowest bitrate for HD shown in the paper is 8 Mbps. Double that,
add another 1.2 Mbps for PSIP and Dolby 5.1 audio for the two programs, and
you've used up 17.2 Mbps. The 2.19 Mbps left over is enough for one Mobile DTV
program or perhaps two low-action multicast streams (don't forget they need
fixed bandwidth for audio). Which of the stations sharing the channel gets the
single Mobile DTV channel?
I've seen tests using the latest video encoding techniques available - which I
would argue match or exceed the improvement shown in Exhibit I from Tandberg.
These, by the way, according to the footnote are based on a study of "SD
streams extrapolated for HD," and not HD.
The tests I've seen indicated that with challenging sports material running at
1080i, the video data rate had to be greater than 9 Mbps to avoid objectionable
artifacts. While it might be possible to squeeze in low action HD content
together with sports in 19.39 Mbps, if two stations are sharing a channel which
one has priority?
Exhibit N, "Hypothetical Coincident Programming Snapshot Based on Pairing of
Eight Stations in Washington, D.C." is also very interesting. Look at the third
item-NBC-Uni. This is based on the premise that Univision is transmitting SD.
Univision is now broadcasting in HD. Imagine how unhappy all of those World Cup
viewers would be if Univision was limited to SD-only, due to channel sharing.
NBC doesn't do well either.
Even if Univision operates in SD, NBC would have to drop Mobile, its popular
"Universal Sports" multicast, or-more likely-both, since NBC couldn't take bits
from Univision when needed for HD.
The Paper's analysis doesn't allow stations to move from SD to HD, as Univision
has done, nor does it accurately reflect that on a shared channel it's likely
to be difficult, if not impossible, for one station to steal bits from the
other when needed. The analysis should have been based on peak bit rate, not
average bit rate. Some sharing is possible, but I think it's disingenuous to
suggest that it can be done without affecting channel choices and Mobile DTV.
I haven't addressed channel allocation issues in this article, but if you're
concerned about the future of broadcasting, look at Section IV, Reallocation
Mechanism. It says that with a repacking scenario, "every existing broadcast TV
license would go into a 'pool' for determination of channel assignment
post-auction."
As noted at the beginning of this article, I was very disappointed in this
report. I agree there are ways to allocate at least some broadcast spectrum for
wireless broadband, although the amount varies by location. Also, moving to
MPEG-4/AVC compression could provide significant reductions in the amount of
bandwidth required for HD content although at the risk of disenfranchising
existing viewers. Would we need a DTV converter box program V2.0?
If the OBI Paper was less of a cheerleader for the NBP, and more of a detailed
technical analysis of spectrum availability by market and a discussion of
alternatives, it could have facilitated discussions in the upcoming Broadcast
Engineering Forum.
It doesn't help to list potential problems and then ignore them or to dismiss
two of the most promising benefits of the DTV transition - more choice in
programming and Mobile DTV - in favor of an economic analysis that shows using
spectrum for TV broadcasting doesn't pay. Multicast programming has allowed
unique programming to be provided to special interest groups, such as cycling
on Universal Sports or ethnic programming in a wide variety of languages, as
can be seen on multicast channels in Los Angeles. The low cost of multicasting
makes this possible.
Is it really right to argue that just because that Korean multicast programmer
may not pay as much for access to her audience as Verizon would for the same
spectrum, this programming has no place on free TV?
PBS doesn't make a lot of money, but I'd sure hate to see it, or its new
multicast channels, disappear.
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