AES47 the AES Has Released AES47, a New Standard for Passing Uncompressed Professional Audio Over High-Speed Networks
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technology AES47 The AES has released AES47, a new standard for passing uncompressed professional audio over high-speed networks. Typically using ATM 25Mbs or 155Mbs interfaces and conventional Cat 5 structured wiring or optical fibre, AES47 can interoperate with other types of data to provide a complete communications package for the most challenging audio and video tasks. JOHN GRANT, NINE TILES NETWORKS UDIO ON TRADITIONAL cables provides a The Internet packs data into conventional Ether- with communications delays like this. A stream, service, but the cables themselves are inflexi- net packets, each with header data to indicate its final maybe, but a ‘live’ stream? Perhaps not. Able, bulky and expensive. Networks, on the delivery address. The useful packet content, or ‘pay- The delay between an audio sample being received other hand, are simpler to install, offer large-scale load’, is usually kept large compared to the fixed size at the transmitting network interface and that same routing inherently, and can be engineered to be fail- header data to keep the administrative overhead audio sample being output by the receiving network safe. So if networks are so great, why has it been so down and to use network bandwidth more efficiently. interface has three components: packetisation delay; hard to use them before for audio? Each of these packets makes its way from router to transit delay; and buffering delay. Almost all computer networks in recent memory router across the Internet to its intended destination. Packetisation delay is the time between the first are based on Ethernet Local Area Networks and the At each router, the packet’s address information is sample being received and the packet being ready for Internet. Not so long ago, networking was a serious read and checked against a list of locally-available transmission. In the first diagram, it’s the time tool used by serious corporates to connect terminals options. The packet is then retransmitted along the between the first person getting on the bus and the to mainframe computers. Typically, each computer most appropriate network segment chosen from the bus being ready to leave. manufacturer had its own network scheme and com- options available, taking into account the needs of If samples arrive at regular intervals and n samples patibility was not considered a driving issue. With the other traffic at that instant. If a packet takes too long are packed in a packet, then the packetisation delay rise of personal computers, the pressure towards net- will reasonably be n sample times. works with standardised interfaces became irre- Transit delay is the time from the packet being sistible. One standardised networking system, IEEE ready for transmission until it is ready to be unpacked 802.3. (more commonly called Ethernet), was just at the receiving end (or from the bus being ready to one contender but in time succeeded in dominating Samples being packed into cell leave until the bus arrives at its destination). This is the office data market and established the largest composed of the time from starting to arrive at each installed base worldwide. Although these standard- switching or routing element in the network until it ised LANs were still mainly independent islands, can start to be transmitted, time waiting to be trans- some were beginning to interconnect. mitted on each link, and the propagation delay along The explosive growth of the Internet accelerated Cells in transit the link (time queuing at road junctions etc, waiting everything very rapidly. Suddenly, it was possible for for traffic lights, and actually moving). all computers to be connected to all other computers The length of the link and the speed of the signal in the world. This was possible because the Internet along it govern the propagation delay, which is about used available technology to appeal to the greatest 1m per 5 nanoseconds or 200km per millisecond. The community of potential users, which were initially length of the cable may be considerably more than the military and academic establishments, then business Unpacking samples from cell straight-line distance between its ends. Buffering and general users. They almost all used Ethernet delay is the time between the packet arriving at the LANs. to reach its destination, or is undeliverable for any destination and the first sample being output. The early Internet applications were limited not so reason, it will simply be dropped and the two com- The receiving device needs to output samples at much by the available computer hardware or soft- puters exchanging that data will need to arrange for regular intervals, and needs to be sure that each sam- ware, but by the low speed of the interconnections. another packet to be sent or accept a hole in the ple will have arrived by the time it is needed. It there- While politicians were making brave statements received data. fore needs to keep some data in hand in case a packet about the ‘Internet super-highway’, the reality was This is the ‘connectionless’ principle that under- arrives late (there needs to be either a queue of buses throttled to the speed of a 14.4kbs modem. This was pins the Internet. The good part is that it doesn’t mat- waiting to unload, or a queue of people who have got fine for transferring text-based documents and sim- ter if routers or network segments are damaged, the off the buses waiting to leave the bus station). The ple Web interactivity but was a far cry from the audio packet can always get through by another route Packet Delay Variation is a measure of how late a visual promises that were being made. The Internet automatically. But the large size of Ethernet packets packet is liable to arrive, and defines the minimum protocol was sufficiently good to get the Internet coupled with the lack of a firm connection is also the buffering delay that will ensure reliable transfer of the going as a useful tool, the ball was rolling, and the root of our problem for live audio. They all contribute audio samples. real killer application was email. to an overall delay, which rapidly becomes unac- To keep packetisation delay small, the packet size Now that we have reasonably fast connections, ceptable. needs to be small, particularly if only one or two audio the ‘super-highway’ is closer to reality, but the net- Look at Internet radio for a demonstration of what channels are being carried. Typically, when convey- work protocol is under increasing strain. We are I mean. Internet radio uses high compression coding ing an AES3 stream, a 48-byte packet would carry six now being offered audio and video streams but, to and it takes significant time to encode at the broad- audio samples per channel, resulting in a packetisa- overcome the limitations of Internet delivery, they caster and to decode at your receiver. So, understand- tion delay of six sample periods. are structured in very different ways to professional ing the relevant coding technologies, you would The time spent waiting to be output on each link streams and they have quite different criteria for expect a delay in the order of 100 milliseconds or so can be kept small by reserving periodic ‘slots’ on the success. to be incurred by the coding process. To find out what output, and by giving audio priority over other traffic. Whether compressed or not, audio needs to arrive actually happens try the following. If the network allows large data packets to be con- in a continuous and unbroken stream. The real-world Tune an analogue AM or FM radio to a convenient veyed in one piece, an audio packet that arrives at an bandwidth restrictions of the Internet make this prac- radio channel, go on-line and find the Internet ver- output just after a large packet begins will have to tical for heavily data-compressed audio, but very dif- sion of the same radio channel, and compare the tim- wait until the end of the packet. ficult for the uncompressed audio data you would ing difference. The delay between an announcement Packet routing within the switch should be a sim- want in your production. In addition, live audio needs on the analogue radio and the Internet equivalent is ple, fast, process that can begin as soon as the first to arrive in a continuous stream with no appreciable not small; it’s typically many seconds and can run to few bytes of the packet have been received. delay. minutes. Imagine trying to hold a two-way interview Buffering delay is minimised by a service that 62 resolution May 2002 technology delivers packets at regular intervals, rather than available network bandwidth but, more importantly of the digital audio signal has been set up, that band- allowing them to become bunched together. for us, it provides guarantees of quality of service width remains completely available until the circuit is We know what a live digital stream is because we (QoS) and can handle all of its bandwidth more effi- disconnected. use AES3 or MADI for digital audio data and both are ciently. The unprecedented success of the Internet and A number of modes are possible, including a designed to emulate simple analogue lines. An ana- Internet Protocol eclipsed this technology for a time, range of multichannel options covering 5.1 surround logue line has no appreciable delay other than the but the technological arguments for its use remain and multiway feeds. Perhaps the most useful mode speed of light. With all digital audio, there is an particularly strong for real-time applications, such as for interchange purposes provides a transparent path unavoidable delay for analogue to digital conversion, video and audio streaming, where timing and band- for an AES3 signal, including all its associated ancil- and another minor delay needed to reclock data, as it width matter.