Addressing Interference Problems Using FAAF
In the April 2, 2008
, issue of TV Technology
, I proposed a solution to DTV reception problems. This proposal could address interference problems including interference from unlicensed transmitters operating on “vacant” broadcast TV channels and weak signal problems. I call this a Frequency Agile Active Filter (FAAF). Signals from the antenna would be fed to this active filter, which would pass signals on the desired channel, and attenuating undesired signals. The desired signal would also be either automatically amplified if that is needed, or attenuated, before being passed to the DTV receiving appliance—either a DTV downconverter such as those available to keep NTSC receivers in operation after Feb. 17, 2009, or to DTV receivers, etc.
I am encouraged that more than 80 DTV downconverters are now available under the NTIA Coupon Program, which subsidizes their purchase. With an NTIA coupon the out-of-pocket cost of such downconverters is less than $20. This suggests that if produced in volume, my active filter might also be available at a very modest cost to those whom need it.AUTOMATIC TUNING OF THE BANDPASS FILTER
You will have noted that I said this device would operate automatically. So, let us examine how this could be possible. The FAAF has a tunable bandpass filter, so it must be told the DTV channel number to which the set owner has tuned his receiver. You may have heard about “smart antennas” for DTV. The Consumer Electronics Association, which is a sector of the Electronic Industries Alliance, produced an Antenna Control Interface Standard, EIA/CEA-909, in June 2002. This is described in the ATSC receiver performance guidelines document A/74, which can be found at www.atsc.org
Modern DTV receiving appliances including many of the NTIA downconverter boxes have intended this interface to control a smart antenna. By now you realize that if these DTV receiving appliances can control a smart antenna they must know which DTV channel the receiver is tuned to.
To clarify this, suppose the viewer wants to watch Channel 4. His receiver knows that the desired DTV signal is being broadcast on Channel 48. Therefore the channel information available to a smart antenna is Channel 48. That clever bit is possible because of the PSIP data, which is required to be transmitted by all broadcasters over their DTV channel.
So, if a FAAF is connected to the EIA/CEA-909 interface jack on the back of the receiving appliance, it knows how to tune the frequency agile filter and how to adjust the required desired DTV signal amplification or attenuation. CEA-909 describes how this can be done in section B.6 of the ATSC A/74 standard. The FAAF acts like a smart antenna controlled by the receiver, automatically, as was the intention of the designers of the antenna control interface standard (CEA-909).
Section B.9 of CEA-909 suggests that some antenna controllers may be used with analog TV sets that do not have the CEA-909 interface. This is possible if the FAAF is equipped to detect the infrared signal from the downconverter’s remote control unit and thereby control the tuning of the bandpass filter in the FAAF. The viewer selects the analog channel using the downconverter’s remote control. This is because viewers typically do not even know the DTV channel number of their local TV stations. The downconverter receives the analog channel number the viewer desires to watch, so it must contain a look-up table to translate from the analog channel to the corresponding DTV channel in order to tune. This look-up table is automatically created in the downconverter when it is first installed and scans the broadcast bands looking for DTV signals. The user can repeat that process to add or even delete channels. Now the look-up table can communicate with the FAAF in terms of telling the FAAF which DTV channel the bandpass filter (in the FAAF) is to be tuned to.
Of course, if the viewer has a DTV receiver that has the CEA-909 interface, he won’t have or need a downconverter. His DTV set will do just what a downconverter would have done as described above. In this case, the CEA-909 interface of the receiver is connected to the CEA-909 input of the FAAF and controls it as described above.AMPLIFICATION CONTROL IN THE FAAF
The CEA 909 Interface provides for a means to control the signal level fed to the receiver (or downconverter). Just as a small look-up table stores the DTV channel number associated with each local analog channel number, it also stores the gain control data, which has previously been found is best for reception of that DTV channel. This information is fed from the CEA-909 compatible DTV receiver or CEA-909 compliant downconverter fed by the FAAF.
| Table I: Authorized ERP of UHF DTV Stations above 500 kW in the United States|
Just for fun, I recently assembled a two-stage RF amplifier combining two off-the-shelf IC low-noise, variable-gain amplifiers. Each stage can provide a gain up to 9.6 dB, or it can attenuate the signal by up to 19 dB. With both stages controlled by a signal from the CEA-909 interface, the signal to the receiver can be varied over a range of 38 dB. Assuming the maximum received DTV signal power (per signal) at the device input is –5 dBm the signal power (per signal) to the receiver can be attenuated to –43 dBm. At this signal level the receiver front-end should be linear, any third-order distortion products would be well below the noise floor of the receiver. In a weak signal environment, the minimum usable DTV signal will be –84 dBm within the station’s service area. The gain available from this two-stage low-noise amplifier (greater than 19 dB) will completely swamp the receiver generated noise. The Noise Figure (at maximum gain) equals 5 dB, about 2 dB less than the FCC planning factor at UHF and 5 dB less than at VHF. Therefore reliable reception at the 41 dU uV/m contour is assured for those with highly directional rooftop antennas. The FAAF works best in weak signal areas where it is installed on the mast of the rooftop antenna because line losses increase the receiver’s effective noise figure.
Many LNA are available which can equal the weak signal performance of this two-stage LNA, but most are fixed gain amplifiers so they may be subject to overloading by multiple strong undesired signals. This amplifier should be able to avoid this problem.WHO MAY NEED THIS GADGET?
As my April 2 column indicated, viewers intent on receiving a distant station may need the pre-amplification, while those living close to some DTV stations may need the variable attenuation feature. Folks whose receiver is overloaded by some signals can benefit from the tracking filter feature.
More broadly, this gadget may be much more widely needed when unlicensed low-power transmitters are allowed to operate on vacant TV channels. It is safe to say that unless the FCC reduces the allowed radiated power of these devices to a small fraction of the 100 mW for portable transmitters or 1 watt for fixed location devices that have been proposed by the FCC for them, that sooner or later everyone will live within a few hundred feet of an unlicensed transmitter operating on a vacant channel. In that event, I believe that DTV reception with receivers whose interference rejection is about the same as those receivers tested by the FCC Lab in a report dated March 30, 2007, will need some sort of active device to boost their interference rejection and that is what the FAAF is intended to do. That would mean that the improvements in front-end dynamic range and tracking filters would be incorporated in future designs of DTV receivers once the breadth of the problems is appreciated.DTV RADIATED POWER IN POST TRANSITION
Many stations have already maximized their DTV facilities, increasing their ERP as shown in Appendix B of the FCC doc. 07-138 (Aug. 1, 2007).
Others will file for maximization now that the FCC re-opened its window for such filings on May 30. I have counted the number of DTV stations whose ERP is 500 kW to 1 MW. These are all UHF DTV channels. (See Table 1.)
Not all of these maximizations have been put into effect yet so their effect upon DTV-DTV interference is still unknown.
Even on Feb. 18, 2009, the effect of these and other maximizations which are approved later this year will not be fully known as there are probably going to be implementation delays in obtaining high-power transmitters, feed lines and antennas, to say nothing of installation crews to change antennas. But by late 2009, the effects should become clear. By then, there should be several tens of millions of DTV downconverters in actual daily use.
This column has long warned that maximizations to ERP above 200 kW require beam-tilt to protect co-channel and first adjacent channel DTV stations from interference. Under the current FCC rules, there is no such thing as DTV-DTV interference from stations offset by more than +/–1 channel from each other. So I “predict” there won’t be any new and unanticipated interference because there are no protection ratios for such channel pairs. Taboos, for example, have become “history.”