Since my article in the Oct. 19, 2005 issue of TV Technology, ("Developing a 24/7 Digital EAS System"), some real progress has been made by the FCC in the matter of the Emergency Alert System.
While broadcasters are required to transmit EAS messages from the White House over analog channels, they are not required to transmit EAS messages over digital channels. With the analog sunset pending, the FCC seeks to extend EAS not just to DTV channels, but to all electronic media, including cell phones.
This column has suggested one technique by which a DTV channel can provide a 24/7 emergency alarm capability, something the present analog system cannot do. It also showed how a unique EAS header can be transmitted from the White House to all local TV stations, providing 24/7 continuous testing to confirm the EAS is actually operational. That could replace the weekly EAS testing, which is an annoyance to the public and to broadcasters.
I am not suggesting that my previous articles played any part in the actions of the FCC. The need to include DTV channels is self-evident. I believe the need for a 24/7 EAS is also obvious since 9/11 and the Katrina disasters. Last fall, I sent copies of my articles about EAS to the Advanced Television Systems Committee, which would play a crucial role in creating a 24/7 EAS over DTV channels by assigning specific ATSC headers for implementing an EAS over DTV channels. The ATSC is sponsored by broadcasters and by representatives of consumer electronics manufacturers.
One problem with implementing a 24/7 EAS over DTV channels is that so far, this has been an unfunded, voluntary effort largely by the Society of Broadcast Engineers.
However, the FCC soon may have funding from the Congress for this effort. It is rather hard to imagine how something like a national EAS can ever succeed on a voluntary basis, as it would require receiver designers to provide for 24/7 monitoring of the ATSC digital datastream for EAS headers, and sounding of both audible and visual alarms when an actual EAS header is received from over-the-air broadcasters. So it is an uphill struggle to implement this on a purely voluntary basis.
While I believe the incremental cost in receivers to provide this function is well worth it, it is not going to be free--it requires a battery backup function should AC power fail. I separate the alarm function from the delivery of emergency messages of what to do in a specific emergency that, as Katrina showed, may rely on battery-operated receivers, i.e. radios. Consumer electronic manufacturers are very sensitive to cost and to governmental regulations of how they design products.
The first step at the ATSC may occur this month. I was invited to attend a committee meeting in Washington, D.C. to explain my concept of how to provide a 24/7 EAS over DTV channels, but alas, that will not be possible since I have moved back to the great Northwest.
A lot has been said about the recent proposal by the FCC to permit further sharing of the remaining broadcast spectrum (Channels 2-36 and 38-51).
There is a proposal to allow unlicensed transmitters to operate in the so-called "white spaces" of the TV spectrum. These are channels not allocated to the community in which unlicensed transmitters will be permitted to operate. In many cases, these white-space channels are adjacent to channels in use in the given community.
MSTV and others have expressed their concern that such unlicensed transmitters might interfere with the reception of broadcast TV signals. This sounds to me like a digital citizens' band--DCB--within Channels 5-51.
WHOSE OX IS GORED
At least one paper has been published which supports the FCC view that such interference will not be significant, but then whose ox is being gored is the question.
The field strength at 1 mile from the antenna with an effective radiated power of 1,000 watts is 102.8 dB above 1 microvolt per meter. For the 1 watt ERP limit of unlicensed transmission, the field strength at 1 mile = 72.8 dBµV/m.
The power intercepted by a resonant dipole aimed towards the signal source can be determined from the field strength by means of the dipole factor, which at the center of the UHF band is -130.8. The maximum power available at a resonant dipole antenna from one unlicensed transmitter at 1 mile is 72.8 - 130.8 = -58 dBm. This would not generate third-order intermodulation or cross modulation in the front-end of DTV receivers.
Those are the mechanisms by which adjacent channel interference to DTV is caused, not poor IF selectivity, as was the case when the analog TV system was developed circa 1940.
At one-half mile, the received power will increase by 6 dB to -52 dBm; at one-quarter mile, it will increase to -46 dBm; and at one-eighth mile, to -40 dBm. One DCB transmitter will not interfere with reception even on nearby DTV receivers.
Please note that I am assuming line-of-sight transmission. There is no reason why rooftop directional antennas may not be used for DCB, is there?
MULTIPLE RECEIVE SITES
But we are talking about one transmitter into one receiver. DCB is expected to be a very popular new wireless service. At four to six residences per acre in a suburban area, there may be 3,000 homes in a square mile, many of which will have these DCB transmitters. None of these would be more than a mile from a DTV receiver in this little cluster of homes. Now this is a very different matter.
The total of say, 2,000 transmitters = 2 kilowatts ERP loose in the neighborhood! Interference may extend outside of this particular 1-square-mile neighborhood as the field strength decreases rather slowly with increasing distances.
Will these all be operating at the same time? Yes (they might) and/not no, not usually.
Will these transmitters be required to observe strict sideband splatter limits into adjacent (non-white) channels?
Will some of these DCB transmitters be moved to another community and no longer be in a white channel?
Will some operators add a power amplifier, which is available on the market today boosting power 10 - 20 dB?
Now for the billion dollar question:
What if intermittent and harmful interference at the site of a CATV headend due to DCB causes the CATV operator to discontinue carriage of some over-the-air broadcast signals?
And finally, who is going to police this new DCB wireless band?
It would be extremely difficult to organize a field test of this kind of interference, as it would involve a large number of these unlicensed DCB transmitters. However it should be possible to create a model of the situation and to analyze the noise at the receiving antenna from a multitude of 1 watt transmitters evenly distributed over several square miles around the receive site. If this has been done, this author is unaware of such calculations and would like to know of it.
My initial calculations show that the noise power in a white channel will vary erratically from nil to -12.5 dBm, which I believe may overload receivers. Details to follow.
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