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. The first widespread commercial deployment of US ATSC digital television began in 1998, with the first early adopters being stations in the largest markets (including New York City, served by transmitters atop the
World Trade Center). Digital receivers of this era, while expensive, were poorly equipped to deal with reflected signals—a severe drawback in urbanized environments. Later generations of receiver design significantly mitigated these limitations; by 2004 technology existed to build receivers capable of detecting and compensating for static multipath interference conditions where a single echo was 10 dB weaker (within a 30 microsecond time difference) or the same strength (the worst case, but within a 12 microsecond range). If the transmitters could be kept at sufficiently precise synchronization and sufficiently close geographical spacing to operate within these limits, a single-frequency network using the new receiver design would be possible even with the existing North American ATSC digital broadcast standards. Tests by
Pennsylvania State University public educational
WPSX-TV (now WPSU-TV) were initially made in 2003 WPSU was in analog a
VHF 3 station which serves
State College, Pennsylvania from a distant transmitter which must also cover
Johnstown and
Altoona. As a digital station, WPSU had used a large
UHF 15 transmitter at the location of the original
low-VHF broadcast tower, leading to localized problems with
terrain shielding which interfered with UHF reception in State College itself. Relocation of the main transmitter would have interfered with the station's ability to serve the other two communities. Addition of a small (50
kW) synchronized digital TV transmitter in State College, on the same frequency as the main UHF 15 signal, proved a means to improve reception; further improvements would be possible by adding small co-channel 50kW transmitters in each community to be served. ATSC released standards on September 25, 2004, as guidance on the design of multiple transmitters, single frequency networks and multiple frequency networks. The new 2004 standards included: • A/110A, "Synchronization Standard for Distributed Transmission, Revision A" • A/111, "Design of Synchronized Multiple Transmitter Networks" Technical issues addressed included that of synchronization between transmitters (
GPS was used to supply a 1Hz and a 10MHz reference frequency, as well as timing information) and precise control of transmitted frequencies (to within 1Hz). Identification for each individual transmitter needed to be embedded in the signal for troubleshooting purposes, yet the main data stream on every synchronized transmitter must be identical; this is done by adding a second, low bit rate
spread spectrum signal 27–30dB weaker than the main signal. As this "watermark" identifier is buried under the stronger main signal, multiple repetitions of this same identifier could be received and summed in order to provide a readable version of the watermark to broadcast technicians. A standard receiver, meanwhile, would see the same signal from all transmitters by design. The generation of non-MPEG data carried as part of the
transport layer (such as the position of transmitted frame sync, or the initial state of trellis encoding devices) would also have to be matched exactly between every synchronized transmitter. Even though this data is discarded after the received signal is demodulated, any mismatch could create interference between the various co-channel signals. An extra "operations and maintenance" distributed transmission packet (OMP, packet identifier PID:0x1FFA) would need to be added to the ATSC data at the studio and used to control various parameters needed for configuration and synchronization of the individual transmitters. The location, directional pattern and power levels for each of the transmitters would also have to be very carefully chosen, as the ATSC system is subject to very strict limits on the maximum time difference between arrival of multiple versions of the same signal at the receiver. In problem reception areas, significant improvements could be obtained, but careful design would be required to operate multiple co-channel transmitters without destructive interference. Further tests run by
Telemundo owned-and-operated station WNJU, Ion TV and broadcast tower owner Richland Towers using one main
New Jersey transmitter and a
Times Square fill-in DTS secondary transmitter in 2007 indicated that, of fifteen test sites for reception of the station in New York City, 40% would obtain a substantial improvement in signal by the addition of a second transmitter to the existing station, while all but one would receive at least the same signal quality as was observed without a distributed transmission system. New York's
Metropolitan Television Alliance was to run similar tests, but on a larger scale, in 2007 and 2008. == Regulatory issues ==