CDTVS TEST TRANSMITTER PROJECT NOW UNDERWAY
Although only a handful of people have actually observed the results so far, DTV signals from Canadas first high-power over-the-air transmitter are now radiating far and wide within the National Capital Region. These transmissions, which began just before the Christmas holidays, will play a key part in CDTV Incs first major research effort to verify the technical criteria used in the development of Canadas national DTV Channel Allotment Plan.
To paraphrase Winston Churchill, this milestone marks not the beginning of the end, but perhaps the end of the beginning, as far as the overall testing project is concerned. Now that actual DTV signals are available for evaluation, the assessment phase of the project can begin. To this end, the Technical Working Group has approved in principle a test plan developed by the Communications Research Centre (CRC). The objectives of the test plan are to verify, through field tests, that DTV coverage can be at least equal to that of NTSC analog TV. During this process, assessments will be made in both urban and rural locations using a wide variety of reception facilities, including outdoor and simple indoor antennas. It is considered particularly necessary to verify and compare the coverage for analog and digital television at the fringes of the nominal service area, as well as in difficult receiving conditions (e.g. indoor, rough and hilly terrain, etc).
This high-power over-the-air transmitter will also be useful for assessing a number of related cable carriage issues. For example, local cable companies will be able to receive DTV transmissions directly at their local head ends and experiment with either direct carriage of 8-VSB HDTV signals or their transcoding to other modulation forms that may better suit distribution via cable.
The CDTV Inc transmitter installation, while not licensed as a conventional broadcast transmitting undertaking is nevertheless very similar to permanent DTV facilities that have been constructed in most major TV markets in the US. It is authorized to operate with a substantial average effective radiated power (ERP) of 32.7 kW and an antenna height (EHAAT) of 215m. It provides omnidirectional coverage on Channel 67 that is intended to duplicate the Grade B service area achieved by its host analog TV station, CITY-TV-3 Channel 65. The new facility will be receivable over a radius of about 70km, which includes the Ottawa-Hull CMA and stretches south to the Seaway Valley, west to Perth and east to Alexandria.
The facilities have been constructed at an existing TV/FM transmitting site, owned by Rogers Broadcasting Limited, located south-east of Manotick ON. In addition to the CITY-TV-3 and DTV facilities, the site is also used by CFMT-TV-3 Channel 60, CHRO-TV-43 Channel 43 and CHRI-FM.
The DTV facilities were constructed in vacant space, within the CHRO-TV-43 transmitter building, that had been set aside for future DTV facilities for CJOH-TV. In addition to volunteering the use of its reserved building space, CTV will be picking up the tab for the electrical power used by the DTV transmitter. The transmitter, along with the required output filter and 8-VSB modulator, has been provided at no cost by Larcan Inc. It is a solid-state, air-cooled UHF model, and provides 2.5 kW of average DTV power output on Channel 67.
The antenna, transmission lines and the antenna combining device consist of the existing Channel 65 facilities of CITY-TV-3 and are being donated for the duration of the project by CHUM Ltd. A cost-effective method of combining the analog Channel 65 and DTV Channel 67 signals was devised by Dielectric Communications, based upon the use of the constant-impedance output filter used by CITY-TV-3. This short-cut greatly reduced the capital cost of the project and may prove to be an effective means of putting DTV signals on the air in other cases where existing antennas are to be used to carry both analog and DTV services. In the process of making this scheme work, a considerable contribution was made by RFS Broadcast Ltd, the supplier of the CITY-TV-3 antenna, which sent two engineers to the site to help with the tuning of the system.
Contributions to the project have also been made by Davicom Technologies Inc (remote control), Tektronix Canada Inc (test equipment), Bird Electronics (RF load and power metering), LeBlanc Ltd (tuning and adjustments) and G.S. Broadcast Technical Services Ltd (equipment installation). Funding for the electrical and air-handling modifications, as well as other installation expenses, was supplied by Industry Canada.
Attention now turns to the testing process itself. The current study plan calls only for assessing performance using the ATSC 8-VSB DTV system , which has been adopted as the sole standard for over-the-air DTV in both Canada and the USA. However, there has been considerable controversy recently about the performance capabilities of the 8-VSB system. In view of this, the Technical Working Group has left the door open to using the Ottawa transmitter facility for assessment of other systems, should this be deemed appropriate by CDTV
W.A. Stacey, P.Eng.
Chair, CDTV Test Transmitter Project