Double Trouble Comes From Undesired Signals
(click thumbnail)Table I from the University of Kansas report presents the maximum U DTV signal power for each offset for each receiver. Numbers in red represent failed states relative to ATSC A/74 Guidelines.Two more reports of laboratory testing of DTV receivers have recently been made public and the news does not portend well. This column in March covered the first of these three laboratory reports. That one was from the University of Kansas. They tested three DTV receivers at a desired (D) DTV signal power of –68 dBm.
The testing determined the threshold of visual impairment of the DTV pictures when the undesired (U) DTV signal was offset from channel n at n–10 to n+15. I commend the wide range of offsets. The results were reported with one undesired signal power in dBm, not the D/U ratio in dB, at which artifacts appeared. The results were compared against the U power implicit in the D/U ratios in A/74, the ATSC Guidelines for DTV Performance.
Table I presents the maximum U DTV signal power for each offset for each receiver in the University of Kansas report.
These are the U DTV signal powers at threshold of interference. Shown in boldface are the ATSC guideline undesired maximum DTV signal powers. Numbers in red represent failed states relative to A/74. My interpretation of this data is that these receivers did not meet the ATSC guidelines.
My further comment is that these ATSC guideline D/U ratios for the tested D = –68 dBm do not provide adequate protection to DTV reception, but more on that later.
(click thumbnail)Table II from the CRC report gives the CRC test data for the maximum U power in dBm the same channels as reported in Table I plus (n+7). Numbers in red represent failed states.The Canadian Research Center published its findings in “Laboratory Evaluation of Five VSB Television Receivers in DTV Adjacent Channel Interference” Jan. 31, 2007. This report covered a wide range of desired DTV signal powers from –15 dBm and included the ATSC D powers –28 , –53 and –68. In addition, data was obtained at a D level near the threshold of noise–limited DTV reception.
This report covers the usual single U DTV signal case, and it examined a limited set of two U DTV signals. The results are nothing less than alarming, as I have been warning my readers for many months. Now a well-known and respected laboratory has reported actual measurements. This 47-page report should be read and studied by anyone whose engineering background and whose professional work involves off-air TV transmission technology. It should be take very seriously.
Table II gives the CRC test data for the maximum U power in dBm (not D/U in dB) the same channels as reported in Table I plus (n+7). Numbers in red represent failed states.
Two receivers were unexpectedly sensitive to an U signal on n+7. The FCC labs also reported on this unanticipated sensitivity in some receivers. The cause is not currently known for certain.
TWO U INTERFERENCE
The CRC broke new ground in reporting tests with two U DTV signals. This topic is by now well known to readers of this column. I believe the CRC report supports the concern I have expressed that DTV-DTV interference tests must include two U signals on channels in the form n+k and n+2k (k is an integer plus or minus), where third-order intermodulation products fall in the desired channel to create interference. It also shows that even more tests with two or more U signals are also needed.
What is most novel in the approach taken by the CRC is the measurement of the threshold power of a first undesired signal (U1) and the threshold power of a second undesired signal (U2) on different channel. Before researchers combined these signals, they reduced the power of U1 by 3 dB so it was well below the known threshold. The power of U2 was varied until threshold artifacts appeared.
For example, U1 was on channel n+2, threshold power –30.5 dBm and U2 on channel n+3 and the threshold was –21.6 dBm. U1 was reduced to –33.5 dBm and U2 added. The power of U2 was found to be –7.6 dBm when threshold artifacts reappeared.
The 3 dB margin was lost by a U2 6 dB below the threshold for U2.
Let’s try that again. U1 was 3 dB below its threshold, and U2 was 6 dB below its own threshold, but when combined, they created interference. Note that each U is below threshold by itself, but when both are present, there will be interference, although neither U is at threshold. This was unanticipated interference. Now we can anticipate such interference and on a broad scale.
In other tests, U1 was of the form n+K while U2 was of the form n+2k, so one of the intermodulation products falls in the desired channel n. The result is that with U1 backed off by 3 dB and U2 backed off by 8.5 dB, the combination reached threshold. Each receiver behaved differently. In one case, U2 was backed off 23 dB below threshold, yet the combination still created interference. Unfortunately there are a very large number of combinations of undesired signals that can torpedo the desired signal.
Consider the situation where a viewer has reliable DTV reception, as the signals of interest are well above –84 dBm at his receiver and none of the U signals is close to its threshold of interference. Now, the guy next door turns on his unlicensed minitransmitter. His signal reaching the receiver is too weak by itself to cause interference, but it lowers the threshold of an U DTV signal to the point where reception becomes erratic.
The worst cases are of the form n+k and n+2k. The CRC demonstrated that other channel pairs might also cause interference when neither U is within 3 dB of threshold. In my column, I’ve always stressed this channel pair relationship produces the worst interference, but CRC has demonstrated that there are many other channel pairs that can do the same thing. These results were confirmed in the FCC Laboratory Report dated March 31, 2007.
While this column has considered k values of plus or minus 1, 2, 3, 4, and 5, the FCC report found that much higher k values might also create interference at U powers below threshold. I had assumed that the tuners in DTV receivers would provide enough RF selectivity that strong signals more than five channels from the desired channel would not reach the mixer to generate intermodulation products. I have been proven wrong. Some tuners have either inadequate RF selectivity or their mixer is too readily overloaded, or both.
In case you haven’t just reread A/74, it admits that there could be interference due to more than one DTV signal, but says not a word about the significance of interference due to two U signals. It is also silent on how to measure the effect of more than one significant U signal. That, in my view, is the worst fault of A/74.
I make this strong statement because if the D/U is based on the maximum U signal of –8 dBm, as stated in Sec. 4.2 of A/74, this gives a D/U of –20 dB, where D = –28 dBm. So they are assuming that there is no more than one U signal present. In other words, the guidelines assume there is one and only one significant U signal present, as they have allocated the entire interference budget to one U signal.
While the possibility of multiple interfering signals is acknowledged, no guidance on what to do about it is provided. A further contradiction in A/74 is in Annex D, where examples of DTV received signal power in excess of –8 dBm are cited. There was one DTV signal power shown at –5 dBm, one at –6 dBm, four at –7 dBm and one at –8 dBm, all from the same location in Hollywood, Fla.
NEW FCC REPORT
The other new report on this topic comes from the FCC Laboratories and can be downloaded by going to the reports link at www.fcc.gov/oet/info/documents/. The document is “Interference Rejection Thresholds of Consumer Digital Television Receivers Available in 2005 and 2006.” It was prepared by Stephen Martin of the FCC Laboratories staff. The report exceeds 217 pages and is impossible to deal with properly in a single magazine article.
It is well and clearly written and I have a very high regard for its technical completeness and objectivity. It may be the most important technical document from the FCC ever. If you download the report and read the conclusions, have a look at Figs. 9–3 and 9–4 where D = –68 dBm.
These figures show how interference from pairs of signals, each of which is below the A/74 limits for single U signals, (sometimes by more than 10 dB) become dominant and hitherto unanticipated sources of interference. If you study Figs. 9–3 and 9–4, you will want to know about this wonderful new source of interference at other D levels.
Figs. 9–5 and 9–6 deal with D = –53 dBm. That will arouse your curiosity about stronger D signals. Figs. 9–7 and 9–8 deal with D= –28, the point at which it appears that wideband RF automatic gain control is effective against this new interference.
As you will have noted, I would be hard pressed to say anything positive about A/74. For a long time, most of us have ignored A/74. The test results by three laboratories seem to suggest that A/74 is largely ignored by the manufacturing community. After all, what is a guideline?
Now it is no longer a mere guideline. It has become the basis for the minimum performance standard for digital–to–analog converters from the National Telecommunications and Information Agency. This was recommended by a number of broadcast organizations to the NTIA. Thus the industry now has a de facto standard for receiver RF performance, but it’s busted! Will the ATSC deplore that it is being used for a purpose for which it was never intended? Or will it overhaul this document based on what is now generally known about DTV interference, but was not known at the time A/74 was written?
Both the CRC report and that from the FCC laboratories found an additional interference situation—n+7. The mechanism of this interference has not yet been fully studied.
Here is another unanticipated interference that needs further study and perhaps a change in the FCC rules.
Next month, I will have something to say about how more than two U DTV signal powers at the receiver input may cause interference. This won’t be harmful interference, as there are D/U ratios in the FCC rules only for cochannel and adjacent channel DTV to DTV interference. For there to be more than two U signals, some must be on what were once called UHF taboo channels. Harmful interference, as I interpret the term, cannot occur when the U signals are on UHF taboo channels, as there are no D/U ratios for signals on the taboo channels.
The fact that more than one U signal can result in interference, even when both U signals are below their threshold power levels (measured separately), makes the interference possibilities simply terrifying when unlicensed transmitters begin to share broadcast spectrum on Feb. 18, 2009. Stay tuned.
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