|Using IP for audio distribution is an enticing prospect for most broadcasters. It offers the prospect of OPEX economies and opens up a lot of new flexibility for infrastructure design: backup and regionalisation solutions are among the many possibilities.
IP lines can be used for STL (Studio to Transmitter Links), for studio to studio links, mobile studios and OB trucks linking to the studio, or other types of audio over IP streaming. Apart from the lower costs compared to other technologies such as E1 lines, radio line networks, or satellite uplinks, IP as a distribution method has many advantages. The transport network is scalable, and there's a choice of different providers. Lines can be activated and deactivated flexibly, and it's a transparent transport platform based on standard technology from the studio to the transmitter. For small to medium sized networks, it's also simple to implement.
But there are limitations, and these have meant that broadcasters have tended to use more expensive dedicated IP lines with high QoS parameters, which are more reliable, but which do not provide the same flexibility and cost benefits. Ordinary IP links are prone to packet delay, where the branch with the highest delay causes the other branches to be delayed too. Packet jitter - fluctuating packet arrival times - increases the need for buffer management. Packet loss causes audio dropouts, and compensating correction mechanisms reduce the bandwidth-efficiency. And a complete IP line failure means the end of the transmission.
So the apparent attractions of IP are not quite so compelling as they might seem initially, and broadcasters who implement infrastructure using basic IP components have little or no protection against these problems. The key to achieving the best balance between secure performance and the cost-efficiency lies in the use of infrastructure designed to compensate for the limitations of ordinary IP networks. The basic hardware for audio over IP is cheap but will not deliver the robust performance and quality a broadcaster needs. A little more CAPEX expenditure on systems with advanced features can deliver both the quality required for professional broadcast operations, and long-term OPEX savings.
These features include error correction that is sophisticated enough to compensate fully for transmission errors on the IP network, so that packet loss does not result in audio degradation or loss of the broadcast transmission. Dual IP source inputs with automatic selection of the best signal from each source before decoding provides further resilience against transmission errors. Intelligent buffer management lowers transmission delay, and advanced redundancy management guarantees continuity of the broadcast transmission in the event of failure from the IP source.
Because one of the key attractions of audio over IP is the flexibility it allows, it's a mistake to choose IP hardware that is limited and inflexible. For this reason, 2wcom's audio over IP solutions combine capabilities in a way that lets broadcasters adapt and change their network configurations without having to re-invest in new hardware.
An important part of the formula for solidity is good monitoring information, and if the hardware chosen for the IP transmissions provides nuanced and informative monitoring data, it increases the broadcaster's ability to be proactive in anticipating failures.
The solidity provided by dual input streams, intelligent forward error correction, multi format audio encoding and decoding, and advanced redundancy management with automated switchover to alternative sources (satellite, FM rebroadcast, Icecast, and internally stored gap filler content) means that IP can play a major part in the audio distribution without the broadcaster risking loss of quality or service. In case of a failure of the device there should be redundant switch over - a passive loop-through of the device.
One inherent aspect of IP that has direct consequences for audio broadcasting is the lack of precision timing, and this makes it difficult to implement SFNs (Single Frequency Networks). It's important for a high quality AoIP device to provide the timing accuracy required, so that broadcasters have the option of implementing SFNs as required, while still using and benefiting from the IP infrastructure.
These features are not widely available, and basic AoIP devices provide few or none. But to make Audio over IP viable for broadcast distribution, it's necessary to have most of them, as some broadcasters are finding - after trying to do without.