Preliminary Results from the CDTV Ottawa Field Tests
Communications Research Centre
Canada
May 2000
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.