WALA-TV gets creative with digital microwave

WALA-TV, located in Mobile, AL, has been in the midst of a digital upgrade, as well as managing the logistics of moving the studio to a new location.

The righthand rack was customized by Microwave Radio Communications to provide WALA’s STL/TSL with hot standby and space diversity features.

RF link

The upgrade presented a unique set of challenges. WALA had existing STL antennas, but had to make adjustments for the new location. This required a complete path study on the new location. All the path profile and system reliability calculations were done by the Microwave Radio Communications systems engineering group.

Today, WALA’s STL/TSL includes hot standby and space diversity features. The single STL now uses a dual waveguide switch arrangement through the addition of a second STL transmit antenna and two hot standby transmitters each going into a different antenna. The primary STL antenna is connected to the “A” TwinStream transmitter, and the second STL transmit antenna is connected to the “B”, or backup TwinStream transmitter. The A transmitter remains online during normal operation with the B transmitter feeding a dummy load. In the event of an alarm on transmitter A, the A transmitter will switch to a dummy load and the B transmitter will become active through the secondary STL antenna.

MRC recommended a space diversity receive system at WALA’s tall tower STL receive site. This system provides both antenna and transmission line redundancy. The space diversity configuration also provides “five 9s” (99.999%) of digital system reliability.

System components

The STL consists of a dual RF carrier system with the NTSC analog and ATSC digital signals fed to their respective transmitters, but using the same 25MHz channel. The digital signal is a direct SMPTE 310M connection from the receiver to the digital 8-VSB exciter. Two backhaul microwaves on separate frequencies are used to bring ENG feeds back to the studio and to monitor the signal from the transmitters.

As an added benefit, MRC’s TwinStream incorporates T1 onto the microwave. WALA is using part of the T1 capacity for eight channels of four-wire telemetry to and from the transmitter. This provides for off-premises telephone service and Internet connectivity through a LAN/WAN Ethernet connection. The transmitter site can become a hub for monitoring other equipment over the LAN.

ENG backhaul and inter-city relay

WALA uses MRC 13GHz equipment for ENG feeds. One IRC camera feeds a short-haul microwave mounted atop the mast of the USS Alabama battleship, 196 feet above the water. The other camera is on the roof of a hotel, along an ENG pickup. The ENG system can backhaul to the studio using two channels at 13GHz. This enables us to bring back video from the cameras and the ENG feed on the MRC 13GHz radios.

Johnny Reece is chief engineer at WALA-TV.

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