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.
Put to the test
Distributed transmission for a DOCR implementation was first demonstrated in 1998 by the Advanced Television Technical Center. The DOCR DTxN used the signal radiated by the main transmitter to feed the on-channel repeaters.
In 2003, DTx field trials were conducted at WPSX-DT, a non-commercial station owned by Pennsylvania State University. The station's transmitter is located in Clearfield, PA, which is separated by mountains from the three major population centers of Altoona, State College and Johnstown. Its analog signal is on VHF channel 3, while its digital signal is on UHF 15. Local mountains blocked the UHF signal causing the digital coverage to be less than the existing analog contour.
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, the DTxA2B. The station then applied for an experimental license. Outputs of slaved modulators were successfully transmitted to standard and advanced DTV receivers.
Standards & regulation
The ATSC has produced a standard and recommended practice guide for engineers who are designing and deploying compliant DTxNs. Both documents, developed by the ATSC Specialist Group on Transmission (T3/S9), were issued in 2004:
- A/110A, "Synchronization Standard for Distributed Transmission, Revision A"
- A /111, "Design of Synchronized Multiple Transmitter Networks"
The FCC issued an NPRM in 2005 to clarify interim rules for DTx, but not to mandate compliance with a synchronization standard; therefore, the ASTC can modify the DTx standard without requiring FCC action.
Working toward practice
Even with the successful testing of DTxN in the field, a DTx-capable receiver must be designed to handle severe multipath distortion from SFN transmission conditions or delays of more than 50u seconds can occur. Adjacent channel interference must be also be rejected.
The architecture of a DTx-capable receiver includes:
- An adaptive equalizer;
- A channel-matched filter, employed as a pre-filter so the signal energies from different echoes are combined optimally and the signal-to-noise ratio of the equalizer input is maximized;
- Feedforward filters to minimize the pre-cursors or convert them into post-cursors, and feedback equalizers to eliminate the post-cursors;
- Initial tap coefficients that are computed to speed up the convergence of these two filters.
Tools & equipment
The development of computer-based applications that support engineering calculations is necessary to shorten the design cycle and ensure optimal specification of parameters. A DTx SFN is dependent on a complex relationship between filters to minimize the interference between symbols.
For equipment, Axcera introduced its new Axciter at NAB2006, a second-generation, adaptive digital TV exciter-modulator system. The Axciter is designed for field upgrades through a USB interface. Axciters include Axcera adaptive correction technology; AXACT, a continuously adaptive linear and nonlinear pre-correction technology; and DTVision integrated DTV signal analysis, which samples the transmitter output and measures critical transmitter performance parameters.
What’s to come
As more DTV stations crank up to full power and shutdown their analog transmitters, they may discover loss of coverage in fringe regions. Null pockets may also appear within the service contour. This could lead to increased deployment of distributed transmission systems.
Recommended reading
Rhode & Schwarz Power Point presented by Mike Simon
www.webe.org/papers04/Rohde.pdf
E-mail your thoughts, experiences and suggestions for future topics to editor@broadcastengineering.com with the subject line T2D.
