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XMOS for AVB Ethernet-based Networking for A/V






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XMOS for AVB  Ethernet-based Networking for A/V XMOS for AVB Ethernet-based Networking for A/V Document Transcript

  • 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 XMOS for AVB: coming for Audio/Video (AV) networking, as AVB (Audio Ethernet based networking Video Bridging) products that run over the same network, for Audio/Video begin to enter the market. 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 the description of the page into packets, throws Audio/Video has become more complex, 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 the printer. The higher the number of switches and speakers are in phase in multi-speaker, large 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 2010-10
  • 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 (endpoints, switches/bridges) are synchronized. are called endpoints, which communicate with 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 2010-10
  • sold in very large quantities to recover the Since an XCore is entirely programmed in a development costs, there is no scope for C-based high-level programming language, any individual companies to design-in additional device can be re-configured during the features that can differentiate their product in development process, or even when deployed the market. in the field. This is perfect for building devices Another approach is to use FPGAs (Field when a specification is evolving, or where there Programmable Gate Arrays), general purpose is a requirement to meet a standard in the final chips that can be programmed to carry out stages of development, just as the situation is specific functions and reprogrammed to cope with AVB. with changes. The design process for an FPGA, This flexibility and low price means that XCore however, is complex (it can be as complex as devices are already being used as the basis of a creating an ASIC), individual devices are number of AVB designs from some of the expensive and they can consume large leading AV companies. quantities of power. The XMOS solution XMOS has developed a new approach to   AVB Bodies designing electronic systems. The XCore Two bodies are deeply involved in Audio Video processor is an event-driven multi-core Bridging (AVB). The technical standards, which architecture used as the platform for define how AVB actually works, are being created developing systems entirely in software. within the IEEE by the IEEE 802.1 Audio/Video Bridging Task Group. The work required to turn XCore processors combine the best features of the standards into commercial products is being three normally distinct classes of driven by the AVnu alliance. semiconductor devices. The most familiar The AV Bridging Task Group devices are microcontrollers, the cousin of the Like other IEEE standards activities, the job of the processors used in a PC but tailored for use in task group is to create a set of standards which are control applications. Microcontrollers are then implemented in the commercial sector. programmed using conventional software Members of the Task Group are drawn from a development tools but suffer from inflexible range of organizations, but within the IEEE architectures and interfaces. A second class of environment they are expected to act as objective individuals. While the Task Group is working on AV device is the Digital Signal Processor (DSP), a Bridging as a technology independent of the specialized class of processors designed for underlying network, the most attention has been dedicated processing of data streams, such as on Ethernet, as defined by IEEE 802.3 documents. audio processing. Like processors they are There is also work on AVB for 802.11 (Wireless relatively easy to program but, again, are LANS or Wi-Fi). inflexible in their predefined hardware The IEEE standards most directly used within AVB capabilities. Both processors and DSPs can be are IEEE 802.1BA Audio Video Bridging Systems, costly, but not as expensive as the third class IEEE 802.1AS Precision Timing Protocol, of devices, Field Programmable Gate Arrays IEEE 802.1Qat Stream Reservation Protocol, IEEE 802.1Qav Queuing and Forwarding Protocol, as (FPGAs), which are the reverse of processors in well as IEEE P1722 and IEEE P1733. that the underlying architecture is very flexible. AVnu Alliance An FPGA can fulfil a multiplicity of applications, but the design process is complex (often very The AVnu Alliance is a group of leaders in silicon chips and high-quality professional/consumer complex with larger and more powerful FPGAs) audio, who promote Ethernet AVB. Currently it is and unpredictable. concentrating on the automotive, professional and The XCore combines the underlying flexibility consumer electronics markets. of FPGA with the simple design process of the Members of AVnu are developing the tests and processors, but at a much lower price than any procedures that demonstrate that different devices of these. work together. They organize plug-fests, where members bring AVB equipped equipment and demonstrate that it works with other manufacturers’ products. 3/4 XMOS for AVB: Ethernet based networking for Audio/Video 2010-10
  • The AVB Endpoint Reference Design with two boards, two Ethernet cables, power Developers building an AV product to attach to supplies and a XTAG2 adapter for JTAG an AVB network generally want to concentrate debugging. A PC/Windows configuration utility on the product. They require a controller that is is also available for setup. low-cost, flexible and easy to implement, rather The software included in the reference design than a device that requires them to engage with means that developers don’t need to have the details of AVB standards. detailed knowledge of Ethernet or the AVB To meet these requirements, XMOS and the standards to create a product that meets these design and engineering company Attero Tech standards and operates efficiently—with the have created a reference design for an AVB reference design, a developer can rapidly and audio endpoint. easily incorporate an audio endpoint into a product, while retaining the flexibility to The reference design uses a low-cost XMOS change the firmware to incorporate any XS1-L2 device as the basis for running a changes in the hardware or software software-only implementation of an AVB audio specification. Refining the design through endpoint, capable of both talker and listener firmware changes provides the speed and modes, running up to 8 duplex channels of predictability of an implementation that has audio. It integrates the full range of AVB been proven both through plug-fests and field protocols, digital audio interfaces and control experience. software, in a single board with a range of analog and digital I/O. Developers can buy a kit     More about XMOS and AVB XMOS is transforming the way in which electronic products are designed. Its revolutionary event-driven multi-core processors make it easy to build systems entirely in software. XMOS designers enjoy the fastest route from idea to product, using standard programmable chips to create complete systems combining interface, DSP and control software. For designers in leading electronics brands, entrepreneurs and researchers, XMOS enables innovation and dramatically reduces costs.  XMOS Website http://wwww.xmos.com  Attero Tech Website http://www.atterotech.com  XMOS AVB Product Brief  AVnu Alliance: http://www.avnu.org/ 4/4 XMOS for AVB: Ethernet based networking for Audio/Video 2010-10