ATSC distributed transmission
Feb 2, 2007 8:00 AM
WPSX-DT’s 1kW booster in Altoona, PA, is part of the world’s first Distributed Transmission (DTx) network.
ATSC distributed transmission
Analog broadcasting uses one transmitter and a single frequency per broadcast channel. The geographic area covered by this signal has formed the basis of designated media areas (DMA), which have been the foundation of television’s advertising-based revenue model.
DTV frequency allocations strive to maintain the same distribution contours as a station’s original analog channel. But because of differences in how analog and digital signals propagate, this has not always been possible at acceptable power levels.
By using multiple distributed transmitters at lower power levels, DTV transmissions can minimize co-channel interference and maintain DMA coverage. This technique is called a distributed transmission network (DTxN). Implementations can be made through a single frequency network (SFN) or a multiple frequency network (MFN).
There are three types of multiple transmitter deployments: distributed transmitter network, distributed translator network and digital on-channel repeater (DOCR). Benefits of DTxN include filling gaps in signal coverage and improved reception in areas such as cities where tall buildings often produce multipath reflections.
DTx implementation details
A typical distributed transmission (DTx) system includes a distributed transmitter adapter (DTxA) and distributed slave transmitters. Inputs and outputs are SMPTE 310 streams and transmitter frequencies must be locked to within 1Hz of each other. A GPS signal enables synchronous operation.
Synchronization of trellis coders and sync segments in slave transmitters is crucial. In order to align segments and frames in the DTxN, the 0x47 transport stream sync byte is periodically inverted to 0xB8 to create a cadence sync that acts as a VSB frame sync.
Also assisting in synchronizing slave transmitters is a distributed transmission packet (DTxP), a 188B transport packet (PID = 0x1FFA) that includes trellis code state data and a synchronization time stamp. A placeholder packet is inserted at the service multiplexer and is replaced by the DTxA to form a valid DTxP, enabling all transmitters to send the same symbols at a given instant.
Included in a DTx system is an RF watermark. The signal appears as noise, but the information it conveys enables transmitter identification. This signal is operated at about 30dB below the average transmitter output power. The usual seven–eye pattern created by 8-VSB modulation will now have eight smaller eyes reflecting the presence of a low-level code watermark.
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