Most people who have used a digital mixer in the last ten years are familiar with incorporating networking technology into their audio application. Remote control over wireless LAN networks, proprietary audio-over-Ethernet protocols, and extensible audio networking platforms have all become relatively commonplace. As networking speed and reliability have increased, and the underlying technology has become more affordable, transporting audio over an Ethernet cable now offers dramatic savings of time and money, making it more attractive than ever.
While there are several protocols currently in use for audio networking, AVB has many unique benefits that have made it the protocol of choice for the latest generation of PreSonus® pro audio equipment. This article explains the basics of AVB networking, and much of the information here is relevant for other IEEE 1722.1-compliant AVB devices, in addition to supported PreSonus AVB products. PreSonus StudioLive® Series III console and rack mixers, NSB-series Stageboxes, and EarMix™ 16M Personal Monitor Mixers are fully compliant with the IEEE 1722.1 standard, which is the protocol for discovery, enumeration, connection management, and control of AVB devices, also known as AVDECC.
Note: Earlier generations of PreSonus AVB products (StudioLive RM-AI and RML-AI mixers, StudioLive CS18AI, and AI-series consoles equipped with the SL-AVB-MIX option card) are not 1722.1 AVDECC-compliant and can only be used with each other. These products are not compatible with IEEE 1722.1 devices like the StudioLive Series III mixers or other third-party AVB products that follow the 1722.1 AVDECC standard.
What is AVB?
AVB (Audio Video Bridging) is an extension to the Ethernet standard designed to provide guaranteed quality of service, which simply means that audio samples will reach their destinations on time. AVB allows you to create a single network for audio, video, and other data like control information, using an AVB-compatible switch. This enables you to mix normal network data and audio network data on the same network, making it easier to create both simple and complex networks. Numerous audio companies have adopted it, and more companies are adding it all the time.
Audio-over-Ethernet has become increasingly attractive in Pro Audio applications especially for distribution in large-scale systems, such as those used in sporting venues, concert halls, and education institutions. The problem is that most solutions are proprietary, making these systems too expensive and too complex for most smaller applications. AVB is intended to change that by providing an open source collection of IEEE standards available for use by the pro audio market and its manufacturing community.
AVB networking offers several features that make it ideal for audio applications:
- Long, light cable runs. A single lightweight CAT5e or CAT6 cable can be run up to 100 meters (328 feet). This makes it easy to have audio I/O located in different rooms (or even different venues in the same building) and run multichannel audio between them in real time.
- Low, predictable latency. AVB provides latency of no longer than 2 ms sending an audio stream point-to-point over up to seven “hops” (trips through switches or other devices) on a 100 Mbps network. With higher speed networks, many AVB devices support lower latencies and additional hops. Please note that while PreSonus AVB products operate at faster Gigabit network speeds, they are currently fixed to 2 ms of latency.
- Scalable, with high channel counts. AVB’s bandwidth is sufficient to carry hundreds of real-time channels using a single Ethernet cable. This offers the future possibility of expanding your system with additional devices that contain different kinds of audio I/O, multiple controllers, and other useful functions.
Guaranteed bandwidth. AVB networks intelligently manage the data traffic giving priority to AVB data. This means standard network traffic, such as Internet streaming, won’t prevent your audio from being delivered reliably and on time.
Integrated clock signal. In a digital audio system with multiple devices, having a master clock is critical to maintaining audio fidelity. The AVB specification defines such a clock to be accurately distributed to all devices in the system.
AVB networks behave very much like an analog audio system. Like an analog audio system, audio networks consist of sources, destinations, and intermediate processing along the way.
Let’s look at a simple live-sound setup:
In the above example, an audio signal goes out of the microphone and into the stage box. It then goes to the mixer, where the microphone signal is amplified, routed to the appropriate output, and sent to an active subwoofer, where it is finally passed through to the full-range loudspeaker. All of this is readily apparent to any audio engineer just by looking at the diagram—a good thing because the skills required to configure an analog system are nearly identical those needed by a network engineer.
Let’s compare the audio system above with components in a network:
In our network example, the microphones connected to the mixer and stagebox can be freely available on either or both, depending on the routing.
Let’s take a quick look at a microphone signal’s signal flow on a simple AVB network. In the example below, the microphone’s signal is represented by the blue line:
As you can see, tracing a device’s signal path becomes a little more complex because the routing is handled entirely in the digital domain. But because experienced audio engineers understand signal flow and are used to troubleshooting problems in an analog system at the various points of weakness, configuring an audio network becomes that much easier.
How does AVB work?
On the simplest level, AVB works by reserving a portion of the available Ethernet bandwidth for its own traffic. Because packets of AVB data are sent regularly in allocated slots within the reserved bandwidth, there are no interruptions or interference, making AVB extremely reliable.
What makes AVB ideal for audio networking is that it splits network traffic into real-time traffic and everything else. All real-time traffic is transmitted on an 8 kHz pulse. Anything that’s not real-time traffic is then transmitted around that pulse. Every 125 µs, all real-time streams send their data. Other packets are transmitted when there is no more real-time data ready to be transmitted. To make sure that there is enough bandwidth available for all prioritized real-time traffic, the Stream Reservation Protocol (SRP, IEEE 802.1Qat) is used.
Every AVB compliant switch between each talker and listener will then make sure sufficient bandwidth is available using SRP, making it a foundational building block of the AVB standard. Every switch and AVB device on the network must implement SRP and send real-time traffic at the 8 kHz pulse. If one of the devices on the network does not employ this standard, then real-time traffic could be potentially delayed, causing jitter in the output.
AVB Hardware Components
In an AVB network, every device to and from which audio is flowing must adhere to the AVB standard. These devices consist of the following types:
- AVB Talkers. These devices act as the source for an AVB stream, sending out audio onto the network.
- AVB Listeners. These devices are the destinations for the streams sent out by the Talkers.
- AVB Switches. This is the network hub to which every Talker and Listener must be connected. At its most basic level, an AVB Switch analyzes and prioritizes traffic on the network. It should be noted that just like there can be multiple talkers and listeners on the same AVB network, there can also be multiple AVB Switches.
- AVB Controllers. A controller can be a talker, a listener, or neither. These devices handle routing, clock, and other settings for AVB devices using AVDECC.
The most important rule to keep in mind when setting up an AVB network is that the talker (device sending audio) and listener (device receiving audio) must be connected to an AVB-compatible switch. All the AVB devices on the network must share a virtual clock that defines when the AVB packet should be played.
As previously mentioned, devices communicate on an AVB network as “talkers” and “listeners.” An AVB talker transmits one or more audio streams to the network. AVB listeners receive one or more of these streams from the network. It should be noted that an AVB device, like the StudioLive Series III mixers or NSB-series Stageboxes, can be both a talker and a listener. For example, StudioLive 32 can simultaneously “talk” (send channels out to the network) and “listen” (receive channels from the network).
AVB devices stay in sync by selecting the best master PTP clock after the devices connect with one another. This ensures that every AVB device on the network will maintain precise timing, which is critical to audio quality.
The AVB switch guarantees that real-time audio data packets maintain their timing without losing information. AVB switches do this by allowing a maximum of 75% of each port to be used for AVB traffic. This prevents non-AVB data from being delayed or lost.
When an AVB network is configured, the talkers and listeners identify one another automatically.
SRP works with the 802.1Qav Queuing and Forwarding Protocol (Qav) to ensure that once bandwidth is reserved for an AVB stream, it is locked down from end to end. Qav schedule time-sensitive streaming information to minimize latency. Together, SRP and Qav make sure that all reserved media streams are delivered on time.
In this way, the AVB network has some intelligence as to how much non-media traffic as well as how many media packets are on the system at any given time. This means that on an AVB network, the worst case travel time is known throughout the entire system. Because of this, only a small amount of buffering is needed, lowering latency to 2 ms over seven switch hops on a 100 Mbps Ethernet network. On gigabit networks, even lower latencies can be achieved.
Channels and Streams
AVB Streams can be thought of as the pipeline that carries a predefined number of channels between two or more AVB devices. In a PreSonus StudioLive Series III mixer, for example, there are seven input streams and seven output streams available, each carrying eight channels. In addition to audio channels, each stream can carry clocking information from the network’s global clock. Like all digital systems, devices on an AVB network must receive media clock from a master source to maintain proper sync.
It should be noted that each stream can carry any combination of eight channels. The source for the eight channels within each AVB Send stream can be freely routed from any input channel or bus on a StudioLive Series III mixer via the Digital Patching menu.