XMOS for AVB Ethernet Based Networking for Audio/Video
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Only a few years ago, computer networks were complex beasts tended by special acolytes and running on different standards. Today they have become commonplace in many homes and offices, simply plugged together using Ethernet technology. The same revolutionary change is coming for Audio/Video (AV) networking, as AVB (Audio XMOS for AVB: Video Bridging) products that run over the same network, Ethernet based networking begin to enter the market. for Audio/Video Putting together networks of AV equipment for professional and consumer use, or for use in How Ethernet Works vehicles, is about to become simpler while also Within Ethernet, data is transmitted between delivering better quality. No longer will devices (such as a computer and a printer) in specialist connectors and cables be needed to packets. Each packet carries one or more create a rats' nest of connectivity. Instead addresses for its destination. Like a postal packet traversing the postal system, the network has no Audio Video Bridging (AVB), a set of knowledge of what is in the packet, but uses the international standards, will make setting up address to pass the packet to the next point in the and managing networks almost as simple as network. just plugging together the different elements. In an Ethernet based network, each endpoint Sound and video sources will be mixed and (computer, storage element, printer etc.) is distributed to screens and speakers, with high identified by a unique address and has a single quality, low latency and tight synchronization. connection to the network, through an Ethernet Furthermore, the connectors and cables are switch. The switch may have multiple devices cheap, widely available off-the-shelf and easy to connected to it, as well as connections to other switches. route. Whilst a computer previously used a dedicated link Problems with Audio/Video to print a document, now, with Ethernet it breaks Audio/Video has become more complex, the description of the page into packets, throws the packets into the Ethernet and gets on with its particularly in the areas of professional AV in activities. Each packet is labeled with the network studios, concert venues, consumer AV and AV address of the printer and the electronic in automobiles. Two significant problems equivalent of “Packet 3 of 17”. In a simple network particuarly standout; the rats’ nests of wiring the packets go from the computer through a caused by point-to-point communication and switch to the printer. In a more complex network the great care required to ensure that all there may be more steps between computer and speakers are in phase in multi-speaker, large the printer. The higher the number of switches and the longer the distances between them, the longer venues. the packets take to traverse the system. In a There have been several attempts to re-create complex network different packets of the same the simplicity of today’s computer networks for document may be routed through different routes, AV, but these have been only partly successful. arriving out of sequence at the destination. The Few people understand the complexities of printer reassembles the page description and prints it; if a packet is missing, the printer asks for successfully building an AV network and there a resend. As packets travel through the network are no widely accepted standards. The physical they are interleaved with other network traffic, elements of a proprietary network, such as the including computer to computer communication, controller chips for each element in the emails, and so on. network, can be expensive as they are made Ethernet gives no guarantees; packets may not only in small quantities and often don’t meet arrive, may arrive out of order or take a long time the standards required for high quality audio. to cross the network. For printing pages of a document this doesn’t matter greatly—resending a The AVB standards take widely used computer packet is not a big deal and timing measured in networking technologies, particularly Ethernet, seconds is adequate for most computing needs. and add additional capabilities to make them For sending AV, however, timing is crucial. better suited to AV networking. 1/4 XMOS for AVB: Ethernet based networking for Audio/Video 2013-07 Ethernet technology How AVB Works Ethernet is the dominant form of wired network, and has more or less completely Existing Ethernet cabling and Ethernet endpoints (computers etc) can continue to exist on an AVB replaced other versions of networking in Ethernet, but the switches and bridges responsible computing and communication. It is a simple for routing the traffic have to be AVB compliant. and low-cost way to build an easy-to-use basic Any new AV equipment added to the network must network, yet can be scaled to serve large also have AVB functionality. AVB co-exists with networks of computers, printers and other other AV Ethernet protocols, including CobraNet, peripherals. EtherSound, Audinate Dante, AES50 and Q-LAN. At first glance Ethernet might seem unsuitable AVB network endpoints are described as talkers, listeners or both. A talker might be a CD player for AV networks as it includes no concept of while a listener might be a loudspeaker. A mixing time (see How Ethernet Works callout). Ethernet desk both listens and talks. Within the endpoint is can, however, assign priorities to packets and a dedicated AVB controller chip. In a talker, the set up dedicated links—rather like those in an controller takes the digitized audio signal through old fashioned telephone network, where a a standard interface, such I2S, and converts it to dedicated circuit was created at the beginning an AVB Ethernet data stream, with timing data and of a call and torn down at the end. the destination address. It then passes the stream to a Physical Interface Device (PHY), which moves The new approach, Audio Visual Bridging (AVB) the Ethernet out of the endpoint through a exploits these techniques and adds timing to standard RJ45 connector and on to the network. At make Ethernet a suitable base for creating AV the listener the process is reversed; the signal networks. travels from the network through the PHY to the dedicated controller chip, which unpacks the AVB Ethernet signal and carries out the relevant timing AVB is a collection of standards developed by operations before passing it to the audio circuitry. IEEE that build on Ethernet and other AVB adds three new properties to Ethernet and networking technologies to make them suitable other networks: for the demands of all applications, even high- § The concept of time end AV (see How AVB works callout). AVB is § A reservation protocol being promoted commercially by the AVnu § Queuing and forwarding rules alliance (see AVB Bodies callout). AVB also adds a master clock to the network, to With AVB, AV products attached to the Ethernet which all AVB elements within the network are called endpoints, which communicate with (endpoints, switches/bridges) are synchronized. Endpoints can then carry out operations each other through specialist endpoint simultaneously, for example, speakers can be kept controllers. The endpoint controllers sit in phase, or audio and video streams can by between the AV dedicated electronics and the synchronized (lip-synch). network interfaces, handling the timing and While Ethernet is packet based, AV data must be traffic priority issues needed for AVB. streamed—AVB provides a reservation protocol for creating streams of packets. A talker tells the Choosing endpoint controllers network that it wants to send a stream of data to Many companies are contributing to the one or more listeners, and reserves a path and technology for implementing endpoint resources through the network for the time controllers. Developers must make some needed for the stream. difficult judgements as they assess the To make sure that this stream works, there are controller options available, including ease of queuing and forwarding rules that allow priority to the stream while making facilities available to use, cost and how well they cope with the other traffic on the network. inevitable changes in future standards for AVB. With these techniques, the delay through the Using a classic dedicated chip (an ASIC) network (network latency) can be accurately requires a specification that is tightly locked predicted. Even with 100 Mbps Ethernet, delay is 2 down. Once the chip has been designed and milliseconds for traffic going through seven the first samples manufactured (itself an switches, and lower still for Gigabit Ethernet. enormously expensive job with today’s Synchronization of streams and of devices can be technology) any changes will be time as accurate as 1 microsecond. consuming and expensive. Since ASICs must be 2/4 XMOS for AVB: Ethernet based networking for Audio/Video 2013-07 sold in very large quantities to recover the in the field. This is perfect for building devices development costs, there is no scope for when a specification is evolving, or where there individual companies to design-in additional is a requirement to meet a standard in the final features that can differentiate their product in stages of development, just as the situation is the market. with AVB. Another approach is to use FPGAs (Field This flexibility and low price means that xCORE Programmable Gate Arrays), general purpose devices are already being used as the basis of a chips that can be programmed to carry out number of AVB designs from some of the specific functions and reprogrammed to cope leading AV companies. with changes. The design process for an FPGA, however, is complex (it can be as complex as creating an ASIC), individual devices are expensive and they can consume large quantities of power. The XMOS solution XMOS has developed a new approach to AVB Bodies designing electronic systems.