The FCC has tested a total of 136 DTV converter boxes to determine their compliance with the NTIA performance standard, their ability to reject interference from other channels and their sensitivity. The testing also examined several operational issues, including the ability to handle a variety of ATSC standard format and daylight saving time changes.
Steven Martin from the FCC Laboratory presented the test results during a Friday morning session at the IEEE Broadcast Technology Society Symposium in Alexandria, Va. last week. If you’re interested in the results of the testing, download the 202 page report – DTV Converter Box Test Program – Results and Lessons Learned.
One thing that surprised me was the difference in interference rejection between single conversion and double conversion tuners. The double-conversion tuners were better on adjacent channel interference, but were significantly worse than single-conversion tuners in connection with third adjacent channel interference.
There are also some interesting findings in Chapter 7 of the report “Failures to Satisfy NTIA Requirements.” Of the 136 samples, 12 units failed to operate (dead box, dead remote control, dead RF output, or lockups). Daylight Saving Time (DST) issues were a problem for 16 out of 54 boxes tested. You may have seen this recently—some converter boxes switched away from DST early in October rather than waiting until the officially recognized 2 a.m. Nov.1 time and date, causing all program times to show an hour early.
However, before blaming DST problems on the converter boxes, please be aware that stations have to set the DS_day_of_month and DS_hour fields when the DST transition is less than a month away to notify boxes when to switch. After all time zone transitions, stations must then set the DS_status bit to represent the new DST state and also zero the DS_day_of_month and DS_hour fields. Stations that don't do this could have their program times displayed incorrectly even if the converter boxes worked properly.
Some of the boxes had problems with output video quality, and at least one could not handle some of the ATSC standard formats without problems.
Unfortunately, the FCC did not name the manufacturers or model numbers of the converter boxes that failed the tests. Even without the names there is a lot of useful information in the report. Chapter 3 details the performance of 13 different demodulator chips for single echoes plus or minus 50 microseconds from the dominant signal. This should be helpful for anyone designing a distributed transmission system or digital on-channel repeater.
The FCC Laboratory staff has done an excellent job, and I'm sure this report will become a reference document for ATSC system design and interference analysis.
