8 Digital Interfaces for Broadcast Signals
It is common to talk of a ‘stack’ when referring to interfaces and protocols for transfer of IT data across a computer network. This is a reference to layers of processing within software and perhaps associated hardware, necessary to provide a reliable information link between computer applications. Some communications engineers refer to an idealised ‘OSI seven-layer model’. Transmission of data on the broadcast industry standard
interfaces described here is in many ways simpler than transfer of data over IT style packet networks. These broadcast interfaces carry digital video, audio and related data in one direction, usually over a dedicated point-to-point or point-to-multi-point circuit. Such data interconnections offer very high reliability, freedom from errors, stable reference clocks and consistently low latency. The use of dedicated physical media data pathways is suited to operational practices that make it straightforward to distribute, route and monitor video and audio. The physical routes are robust and reliable, but if they do fail it is usually quick to isolate a fault. Standards for these broadcast interfaces are well known, stable and widely supported by equipment manufacturers. Having sung the praises of specialist broadcast interfaces over
dedicated links there is an increasing trend towards use of IT networks to carry broadcast data. The attraction of less specialist, and therefore lower cost, fast Ethernet network infrastructure carrying broadcast media essence (sound and vision) as well as metadata (data about that sound and vision, e.g. programme scripts, broadcast rights, etc.) is obvious. Broadcast media is increasingly stored on fileservers, alongside the file servers hosting automation and playout systems and more traditional business ‘back-office’ packages. It will rapidly become the norm to interconnect systems outside the studio or operational island with whatever high-speed networking technology the IT industry has to offer at the time. The layered approach that I am using to take the reader through this chapter is a very common way of understanding and structuring IT or Telecomm style networks
(see Figure 2.8.1). The Internet Protocol stack reigns supreme! Asynchronous Transfer Mode with its well-established and inbuilt control Quality of Service offers advantages, albeit at a cost, in theWide Area Network, with proven gateways allowing IP traffic to be carried on ATM on a teleco (telecommunications service provider) long haul Synchronous Digital Hierarchy bearer.