Preliminary Results from the CDTV Ottawa Field Tests
Communications Research Centre
Field tests to measure how well the DTV 8-VSB system replicates the existing NTSC coverage are now underway in Ottawa. The CDTV transmitter now operates on channel 67 with an average power of 36 kW, and is expected to replicate the coverage of the existing NTSC station on channel 65 (CITY TV, 741 kW peak). Both transmitters feed a 215 meter tall (705 feet) common antenna tower.
Outdoor tests used a 6.35dBd directional antenna at height of 10 meters or 33 feet. Only 4 of the 50 sites were without DTV reception. The NTSC signal could synchronize however the picture quality was around an ITU-R grade 1 and 1.5.
The most significant C/N degradation of the DTV signal due to multipath in the outdoor tests was 2.7 dB for receiver F and 2.9 dB for receiver H, compared to laboratory tests of 1.8 dB and 3.5 dB respectively.
The indoor tests use two antennae: an aperiodical antenna with a 5 to 7.5 dB gain and an active one with up to 30 dB gain. Each antenna is mounted on a tripod and oriented for maximum C/N. At this point, 43 indoor sites have been visited. In 13 sites where there is no DTV reception, there is usually low field strength. In addition, multipath seems to be a problem in regard to spatial availability within a room, evidenced when reception is not possible is some locations but is in others, just few feet away.
The DTV receivers selected for the field tests were first characterized in the laboratory, and were neither the best nor the worse of all the receivers available. It is hoped that new DTV receivers may have a better performance in handling multipath conditions, in particular long pre-echoes.
The results, which have been presented, are only preliminary and based on analysis of a limited number of sites. The Communications Research Centre is still in the process of collecting data from the field tests and doing more analysis.