WPSX-DT Distributed Transmission (DTx)

WPSX-TV is a non-commercial station owned by Pennsylvania State University, serving central Pennsylvania on analog VHF channel 3. WPSX-DT was assigned UHF channel 15 for its DTV signal. The station's city of license is Clearfield, PA, which is separated by mountains from the three major population centers of Altoona, State College and Johnstown, all of which are important to the station's success.

While the propagation characteristics of low band VHF allowed the analog channel to carry over the hills, this challenging terrain promised to block the UHF digital signal from reaching key viewing areas. Additionally, the station's over-the-air signal serves most of the population to the north of Clearfield, as there is little cable penetration in this rural area.

To meet the challenging service demands, the station selected a new technology called Distributed Transmission (DTx) working with their consulting engineer Merrill Weiss. The possibility of relocating the main transmitter to a location that would better serve the three population centers was considered, but no location exists that would allow service to all of the desired areas. Also, any move would have significantly impacted the coverage of the northern region. DTx promised to allow the station to deploy multiple transmitters throughout the region to target all of the desired population centers, and to operate them all on the same channel.

The key to DTx is that the signals from all transmitters in the network are identical. This allows interference received from any one transmitter's signal to the desired signal from another to be treated as multipath. This, in combination with its ability to control the timing of the multiple signals, allows a well-designed distributed transmitter network (DTxN) to take advantage of the equalizers built into every DTV receiver to maximize coverage and minimize interference.

Axcera developed a custom system consisting of a 30kW Innovator DT solid-state transmitter for Clearfield and three medium-power transmitters serving State College, Altoona and Johnstown, plus a distributed transmission adapter called the DTXA2B. The station then applied for an experimental license and placed the order with Axcera.

Axcera delivered as promised, and the system has met the station's goals. In addition to bringing DTV to areas that would otherwise be underserved, the system is currently being used for testing to help promote the DTx standard, which is currently a Candidate Standard at the ATSC.

Design Team