As more stations move to digital transmissions, transporting the IBOC signal from the studio to the transmitter is proving to be more of a challenge than anticipated. The manufacturers have identified various problem areas, and solutions are starting to filter out to the field.
Overcoming the STL Bottleneck
A studio-transmitter link must be able to carry Ethernet/IP based traffic along with traditional audio streams, making a digital STL mandatory. Many stations have already transitioned to digital STLs, and most STL vendors now provide upgrade paths to share the existing digital STL bandwidth between audio paths and Ethernet-based paths.
However, since most STLs have traditionally been unidirectional in nature, many digital STLs today only provide a unidirectional Ethernet path.
Digital signal paths are notoriously fragile. Signal fades, ducting, multipath, etc. are challenging enough with analog STLs, but digital streams are unforgiving. A single bit error on the STL can cause an entire data packet to be dropped, which translates to one to two seconds of dead air on all digital program channels (HD dropout).