Distributed transmission system

In North American digital terrestrial television broadcasting, a distributed transmission system (DTS or DTx) is a form of single-frequency network in which a single broadcast signal is fed via microwave, landline, or communications satellite to multiple synchronized terrestrial radio transmitter sites. The signal is then simultaneously broadcast on the same frequency in different overlapping portions of the same coverage area, effectively combining many small transmitters to generate a broadcast area rivaling that of one large transmitter or to fill gaps in coverage due to terrain or localized obstacles.

History

While the idea of a single-frequency network of multiple transmitters broadcasting the same programming on the same channel from multiple transmitter sites is not a new concept, the ATSC digital television standard in use in North America was not designed for this mode of operation and was poorly adapted to these applications. The restrictive timing requirements and poor multipath interference handling of early ATSC implementations would have precluded multiple synchronous transmitters on the same frequency at the time of the first wide-scale commercial ATSC deployment in 1998; these restrictions eased somewhat as receiver design advanced in subsequent years. By 2004, technology existed to provide digital television receivers with the means to detect static (not mobile or changing) multipath interference (subject to certain timing constraints) and compensate for its effects on the digital signal.

Tests have been run by various individual broadcasters or broadcast groups, including the Metropolitan Television Alliance (MTVA, a consortium of New York City television stations). A series of initial tests involving four distributed transmission sites and over 100 test measurement sites in NYC and New Jersey were completed in June 2008, along with smaller-scale tests in New York in 2007. The New York market is uniquely problematic for multipath reception due to the large number of man-made obstacles which prevent adequate digital coverage of the entire city from the main broadcast facilities atop the Empire State Building.

Technical issues

To the receiver, a signal from a single-frequency network appears as a single broadcast with strong multipath interference; in the worst case, it is detected as a main signal and a reflection both of equal strength as signals arrive from multiple transmitters to the same intermediate location at slightly different times.

The ATSC standard used for digital television in North America, unlike the DVB-T standard in Europe and other nations, uses 8VSB instead of OFDM—a modulation which allowed a station to transmit at lower peak power levels, but which historically has been far inferior in handling multipath reflections and RF interference.