5.1 Introduction to Carrying Audio Over Computer Interfaces

Dedicated audio interfaces mostly carry audio data in a sample-clock-synchronized fashion, often with an embedded sample clock signal, carrying little or no other data than that required to move audio samples for one or more channels between devices. They behave like 'digital wires', acting as the digital equivalent of an analog signal cable, connecting one point in a system directly to another. Computer interfaces, on the other hand, are typically general-purpose data carriers that may have asynchronous features and may not always have the inherent quality-of-service features that are required for 'streaming' applications. They also normally use an addressing structure that enables packets of data to be carried from one of a number of sources to one of a number of destinations and such packets will share the connection in a more or less controlled way. Data transport protocols such as TCP/IP are often used as a universal means of managing the transfer of data from place to place, adding overheads in terms of data rate, delay and error handling that may work against the efficient transfer of audio. Data interfaces may be intended primarily for file transfer applications where the time taken to transfer the file is not a crucial factor as fast as possible will do.

Conventional office Ethernet is a good example of a computer network interface that has limitations in respect of audio streaming. The original 10 Mbit/s data rate was quite slow, although theoretically capable of handling a number of channels of real-time audio data. If employed between only two devices and used with a low-level protocol such as UDP ( user datagram protocol) audio can be streamed quite successfully, but problems can arise when multiple devices contend for use of the bus and where the network is used for general-purpose data communications in addition to audio streaming. There is no guarantee of a certain quality of service, because the bus is a sort of 'free for all', 'first-come-first- served ' arrangement that is not designed for real-time applications. To take a simple example, if one's colleague attempts to download a huge file from the Internet just when one is trying to stream a broadcast live to air in a local radio station, using the same data network, the chances are that one's broadcast will drop out occasionally.

One can partially address such limitations in a crude way by throwing data-handling capacity at the problem, hoping that increasing the network speed to 100 Mbit/s or even 1 Gbit/s will avoid it ever becoming overloaded. Circuit-switched networks can also be employed to ease these problems (that is networks where individual circuits are specifically established between sources and destinations). Unless capacity can be reserved and service quality guaranteed a data network will never be a suitable replacement for dedicated audio interfaces in critical environments such as broadcasting stations . This has led to the development of real-time protocols and/or circuit-switched networks for handling audio information on data interfaces, in which latency (delay) and bandwidth are defined and guaranteed . The audio industry can benefit from the increased data rates, flexibility and versatility of general-purpose interfaces provided that these issues are taken seriously.

Desktop computers and consumer equipment are also increasingly equipped with general-purpose serial data interfaces such as USB (universal serial bus) and FireWire (IEEE 1394). These have a high enough data rate to carry a number of channels of audio data over relatively short distances, either over copper or optical fibre. Audio protocols also exist for these, as described below.