Audio over Ethernet
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In audio engineering (and now in broadcast engineering), audio over Ethernet (sometimes AoE) is the concept of using an Ethernet-based network to distribute digital audio. It is designed to replace bulky snake cables, and to use the existing wiring infrastructure in a facility, providing a reliable backbone for any audio application, such as for multiple studios or stages.
While on the surface it bears a resemblance to voice over IP (VoIP), it differs in several very important ways. First, AoE is high-bandwidth, intended for high-fidelity and therefore high-bitrate professional audio, rather than voice. In addition, it is also for isochronous and multi-channel use, rather than independent streams. Second, it is designed to have very high reliability, including very low latency (under one millisecond) and almost no packet loss. Because of this, audio over Ethernet is also uncompressed, which prevents both delay and unwanted compression artifacts. Likewise, AoE by definition runs on a dedicated local-area network (LAN), or at minimum on a virtual LAN (VLAN), so that quality of service (QoS) is guaranteed to provide uninterrupted and uncorrupted audio. AoE also does not use TCP or UDP for layer 4 and Internet Protocol for layer 3 (see OSI model), but rather its own protocol that creates data packets and data frames that are transmitted directly onto the Ethernet (layer 2) for efficiency and lack of overhead. The word clock may be provided by broadcast packets.
[edit] Protocols
There are several different proprietary and therefore incompatible protocols for audio over Ethernet:
- CobraNet by Peak Audio (now owned by Cirrus Logic) [1]
- RAVE by QSC Audio [2]
- EtherSound by Digigram [3]
- Livewire by Axia/Telos (mainly used in broadcasting) [5]
- MaGIC by Gibson (non-proprietary) [6]
- M11
- AES50 (non-proprietary)
- SuperMAC, Implementation of AES50 by SonyOxford [7]
Using category 5 cable and 100BASE-TX hubs and switches, each protocol can generally transmit up to 64 channels at a 48kHz sampling rate, with 24 bits per sample. Some can handle other rates, such as 44.1kHz (CD-quality), 88.2 and 96kHz (2× oversampling), even 192kHz (4×), as well as up to 32-bit samples, with a corresponding reduction in channel capacity. On some this is accomplished through channel bonding, while others use individually-scalable channels. Each protocol is also designed for different network topologies, and some use their own media access controllers (MAC) rather than the one native to Ethernet, which could create compatibility issues when encountering traffic from other network interface devices. AoE is not necessarily intended for wireless networks, thus the use of various 802.11 devices may or may not work with various (or any) AoE protocols.
[edit] Similar concepts
The Audio Engineering Society's MADI or AES10, although similar in function, uses 75-ohm coaxial cable with BNC connectors instead. It is most similar in design to AES3, which can carry only two channels (stereo).
In broadcasting and to some extent in studio and even live production, many manufacturers equip their own audio engines to be tied together with Ethernet. This may also be done with gigabit Ethernet and optical fibre rather than wire. This allows each studio to have its own engine, or for auxiliary studios to share an engine. By connecting them together, different sources can be shared among them. Logitek Audio is one such company using this approach.
An audio over IP setup differs in that it works at a higher layer, encapsulated within Internet Protocol. These systems are usable on the Internet, but may not be as instantaneous, and are only as reliable as the network route — such as the path from a remote broadcast back to the main studio, or the studio/transmitter link (STL), the most critical part of the airchain. This is similar to VoIP, however AoIP is comparable to AoE for a small number of channels, which are usually also data-compressed. Reliability for permanent STL uses comes from the use of a virtual circuit, usually on a leased line such as T1/E1, or at minimum ISDN or DSL.
