Alabama Public Television (APT) is a statewide network consisting of nine television broadcast stations and 30 microwave relay sites constituting 2200 miles of microwave relay. Network operations are located at the APTV studios in Birmingham, AL.
The network had two major requirements for this upgrade. First, the radios had to be digital-ready because APT had to continue to transport analog audio and video during this buildout. Second, APT had to be able to add equipment without discarding any gear purchased for the initial upgrades. The DAR radio from Microwave Radio Communications (MRC) met these requirements and offered additional features. The DAR is digital ready, is capable of transporting NTSC video and audio, and is easily field upgradeable to digital DS-3 (45 Mbits/s). It is also a basic heterodyne design, necessary for long-haul and multi-hop applications. Characteristics such as network management, high stability and low phase noise, local, and remote monitoring capabilities played a role in APT’s decision to use the system. Another strong selling point was MRC’s ability to engineer a multi-hop analog and digital microwave network. Because the DAR radios are easily upgradeable, APT was able to significantly limit the time each station would not be receiving the microwave signal. The feed during this cutover came from a temporary satellite feed.
APT can send two compressed NTSC analog signals simultaneously to receive a live interconnect from any of APT’s remote sites for news programs. One of the NTSC signals is coupled and fed to the NTSC analog transmitter. The transport stream is fed to equipment that inserts the virtual channel table and converts it from ASI to a SMPTE 310M composite signal. That signal is then connected directly to the digital transmitter.
With the 45 Mbits/s (DS-3) bandwidth capability, APT multiplexes a 19.39 Mbits/s ATSC transport stream along with two outbound channels of compressed analog NTSC video and audio to each of the nine broadcast transmitter sites.
APT’s microwave paths are bi-directional and employ ATM, a high-speed packet-switching technology capable of data rates of up to 622 Mbits/s. Depending on a station’s requirement in the network, it may be necessary to insert digital video, data or information that is carried within the vertical blanking interval of the displayed signal.
APT can feed and switch various digital signals throughout the statewide microwave, all completely seamlessly. The 45 Mbits/s data stream is reduced to an approximate 38 Mbits/s payload; the ATM cell header uses the remaining bits. ATM switching is performed on a cell-by-cell basis, based on the routing information contained in the cell header. Barco’s Alcor ATM adapters are used to take in the 45 Mbits/s ASI transport streams and then convert them to ATM signals.
The APT network is a bi-directional system. Once fully operational, it will use the backhaul feeds to monitor each of the statewide transmitter sites. In addition to monitoring off-air signals, APT will have security monitoring at each transmitter site. System operators will also be able to turn the transmitters on an off from network master control as an additional backup to controlling these sites via remote dial-up telephone control circuits. The system also continuously monitors all fault reporting for the entire network via the MRC site-monitoring, alarm-reporting terminal (SMART) system manufactured by Ardax. Network operations in Birmingham has complete control and monitoring capability of all microwave relay and terminal site locations. The network is displayed on a computer video monitor as a map overlay in a user-friendly graphical fashion. Each site and piece of equipment in the system can be interrogated down to the most minute detail to ensure proper operation. This is a great troubleshooting aid that will minimize downtime. A hot spot, or trouble location will be easily identified. Getting back on-air will be quick and hopefully painless.
APT is currently monitoring its transmitter via a 10baseT Ethernet connection over the digital microwave network, which is a portion of the DS3 circuit. But monitoring will eventually be accomplished using T1 circuits, again a portion of the DS3 45 Mbits/s digital microwave.
Windell Wood and Charlie Grantham are broadcast services directors for Alabama Public Television.
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