The FCC Office of Engineering and Technology released a report March 30 showing that none of DTV receivers tested to determine their ability to reject interference from proposed TV-band devices using so-called TV white-band channels and other sources “appeared to fully achieve the ATSC recommended guidelines for interference rejection performance.”
The Technical Research Branch Laboratory Division conducted the tests to help in assessing interference to DTV reception from non-TV use of unused TV spectrum, often called white space use, non-TV use of spectrum adjacent to TV spectrum and other DTV stations.
Significantly, the ATSC interference rejection guidelines for DTVs are less stringent than the assumptions made about receiver performance used as criteria for current DTV interference protection. The tests also show the degree to which the tested DTV receivers comply with ATSC’s voluntary receiver performance guideline standards.
The report presents findings on the type of tuners found in 30 consumer digital televisions, as well as on how susceptible DTVs are to interference levels at a receiver’s antenna input terminal. Assessing the potential for interference will necessitate studying “specific protection scenarios, propagation modeling, and antenna modeling,” the report said.
The report’s findings include:
- All of the 30 sets tested for tuner type relied on single-conversion tuners, not the double-conversion tuner used in the Grand Alliance’s DTV prototype used to establish interference protection criteria for DTV channel allotments.
- DTV receivers are most vulnerable to interference when they operate “at low desired signal levels.”
- At such levels, DTV receivers “are as susceptible to interference from the second-adjacent channels” as from those that are first-adjacent.
- If looking at worst and second-worst performance, the DTV receivers “are actually more susceptible to interference from second-adjacent channels than from first-adjacent channels,” which is a contradiction of OET-69 and ATSC Receiver Guidelines assumptions.
The tests were performed in the UHF band. Most were done with the DTV tuned to channel 30. Some early tests were done on channel 51. According to the report, measurements on channel 51 “generally matched those on channel 30 within about 4dB — and in most instances much closer than that.”
The report found that if channel 30 were designated “N:”
- DTV receivers “tend to be more susceptible to interference from N+2, N+1, N-1, N-2, N-3, N-4, and sometimes N-6 than from the mixer image channel offsets of N+14 and N+15.”
- At moderate levels of desired signal strength, “the receivers exhibit relatively high susceptibility to interference from channel N+7.”
- At lower level of desired signal strength, “other channel offsets become more vulnerable.”
To read the report in its entirety, visit: click here.