Delivering Signals to the DAB Transmitter

by Dave Youell

Or How Self-Flagellation Felt Better Than Trying to Get an STL License
The author is chief engineer of CFUN(AM) and CHQM(FM) in Vancouver.

In early 1999, CHUM radio was granted DAB licenses for its two stations, CFUN(AM) and CHQM(FM), in Vancouver. The two signals had to be delivered from the studios in Vancouver to the CBC transmitter site on Mount Seymour in North Vancouver. This is a distance of approximately seven miles.

The CFUN(AM) transmitter site is located in Delta, British Columbia, about 20 miles south of the studios. A T-1 circuit supplied by the phone company delivers programming to the transmitter.

CHQM-FM’s transmitter is also located on Mount Seymour but at a separate area from that of the CBC, where the DAB transmitter is located. An existing 950 MHz composite STL feeds the FM transmitter.

The fact that the AM transmitter is fed via phone line meant that the AM signal needed some way of being delivered to Mount Seymour. The existing FM STL could have been modified to accept two stereo signals.

Several problems, as follows, were associated with this concept:

1. The DAB license allows for 14 hours per week of separate programming. This would have not been possible using this scenario.

2. The modified STL would have had only one data channel. Two data channels would have been required for the program-associated data for both stations.

3. The existing FM processor would have to be moved to the FM transmitter site. This is not the most desirable situation, as having the processing at the transmitter site does not allow for easy changes to the FM sound.

4. The programming source for the two SCA channels would have to be moved to the FM transmitter site. Previous to installing the composite STL, the SCA programs were delivered to the site via phone lines and satellite receivers. There were continual problems with these methods.

5. There are no high-speed data lines on Mount Seymour. This presented difficulties for anyone who wanted to use the data channel on the DAB transmitter.

Faced with these problems, I approached our consultant Doug Allen of DEM Allen and Associates to have him apply for two STL channels in the 950 MHz band.

Industry Canada had expanded the STL band to accommodate links for DAB transmission. Each channel is 350 kHz in size, which meant that we would require 700 kHz of spectrum for the two stations.

E-1 in 950

At about the same time Doug Allen was preparing the technical brief for Industry Canada, I had a discussion with the local Moseley representative, Mike Brennan of Applied Electronics. I asked him for a quote on radios for this project using the traditional approach of digital encoding of an analog signal.

I commented to him that would it not be nice if we could have the source encoders for the DAB transmitter at the studio. This, however, would have entailed sending an E-1 datastream (2.048 Mb) to the transmitter.

At that time, the only way of sending an E-1 signal would have been to use a 2 GHz spread-spectrum system. In the Vancouver area, spread spectrum is beginning to be congested. I commented to Mike that I wished there were some way to use a licensed channel in the 950 MHz band to avoid the problems I believed were going to happen with a spread-spectrum system. Mike thought that Moseley might be able to do something,

David Chancey, national sales manager for Moseley, came up with the NXE1 system, which has the capability of accepting an E-1 signal and can transmit in the 950 MHz band. By using 32 QAM Modulation, the system would use less than 500 kHz of spectrum.

This was fantastic. With this system, five stereo signals could be encoded on the STL plus any data signals that might be needed. All the encoding would take place at the studio, thus allowing for total control of the encoders and processing. The studio is located in a major business area so high-speed data lines are readily available.

Efficient

This system allowed for much greater spectrum efficiency. If all five stations applied for 950 MHz STL channels, a total of 1750 kHz of spectrum would be consumed. With this system, less than 500 kHz of spectrum is used.

What a concept. Our consultant Doug Allen thought so, too.

One of the big pluses of DAB is its spectrum efficiency. Each DAB pod (or transmitter) is capable of delivering five high-quality stereo signals with relative little spectrum. With this concept in mind, Doug approached Industry Canada to obtain the necessary 950 MHz channel.

Everyone thought that this was a no-brainer. Were we wrong!

The local Industry Canada office flatly turned down the request, as this was a nonconforming use of the STL channel. Doug Allen spent the next nine months educating Industry Canada as to the benefits of the concept. This involved not only the local Industry Canada engineers, but also the engineers at the Industry Canada officials in Ottawa. At one point the Canadian Association of Broadcasters technical committee was involved.

Eventually Industry Canada saw the light and licensed a 500 kHz channel on March 16 of this year.

Now the fun began. There was to be an official launch of DAB in Vancouver on May 11. This did not leave much time. Moseley promised a delivery date about two weeks before the launch.

However Industry Canada was not finished with us. On March 31, the officals informed us that their calculations had used the wrong formula. On April 7 the proper frequency was assigned.

After much sweating, nail-biting and consumption of stomach antacids, the radios arrived on May 3. Following a system check, the STL was put into service on May 8 with no problems. It performed as promised.

The author can be reached via e-mail to dyouell@cfun.com

Tell RW readers about your experiences making the transition to digital audio in any part of your facility. Write to us at radioworld@imaspub.com