Cobranet

CobraNet is a combination of software, hardware and network protocols designed to deliver uncompressed, multi-channel, low-latency digital audio over a standard Ethernet network. Developed in the 1990s, CobraNet is widely regarded as the first commercially successful implementation of audio over Ethernet. [Citation
last = Karagosian
first = Michael
title = Following the Digital Audio Chain
year = 2004
url = http://mkpe.com/publications/theme_parks_&_systems/digitalaudiochain.php
accessdate = 2007-03-19 ]

CobraNet was designed for and is primarily used in large-scale audio installations (for example, convention centers, stadiums, airports, theme parks, concert halls). It is most useful in applications where a large number of audio channels must be transmitted over relatively long distances and/or to multiple locations.

CobraNet is an attractive alternative to traditional analog audio, with one audio channel being transmitted per set of conductors (usually a shielded, twisted pair). Although analog audio transmission involves virtually no latency, its sound quality can be adversely affected by signal degradation due to electromagnetic interference, high-frequency loss, and voltage drop over long cable runs. Additionally, thanks to digital multiplexing, the cabling requirements of CobraNet are almost always less expensive than traditional analog audio.

History

CobraNet was introduced in 1996 by Boulder, CO-based Peak Audio. Its combination of feature set and capabilities proved useful to the commercial audio industry, and it quickly gained popularity. CobraNet was first demonstrated as a proprietary system for Disney; using a 10 Mbit/s Ethernet segment, transmission of six channels of audio in one direction and two in the other was demonstrated.Citation | last=Karagosian | first=Michael | title = CobraNet Boosts Audio Networks | year=1999 | url=http://mkpe.com/publications/theme_parks_&_systems/cobranet_the_link.php | accessdate = 2007-03-19 ] Audio was sampled at 44.1 kHz, 20 bits, and all packets were unicast. CobraNet in this form was deployed by Disney to provide background music throughout the Animal Kingdom theme park. [Citation
last = Karagosian
first = Michael
title = How Theme Parks Work (Part 3:Networks)
year = 2006
url = http://www.mkpe.com/theme_parks/networks.php
accessdate = 2007-03-19 ]

CobraNet was first introduced as an interoperable standard in collaboration with manufacturer QSC Audio Products. QSC licensed the technology from Peak Audio and marketed it under the name RAVE. At this point CobraNet had graduated to Fast Ethernet and used a patented collision avoidance technique [Cite patent|US|5761430] to carry up to 64 channels per Ethernet collision domain. An unlimited number of devices could source up to 16 audio channels or receive up to 16 channels from the network.

The first commercial use of CobraNet as an interoperable standard was during the half-time show at Super Bowl XXXI in 1997. CobraNet was subsequently enhanced to support and eventually require a switched Ethernet network. An SNMP agent was added for remote control and monitoring. Support for higher sample rates, increased bit resolutions and lowered latency improvements were also introduced in an incremental and backwards-compatible manner.

In May 2001, Cirrus Logic announced that it had acquired the assets of Peak Audio. [Citation
last = Doering
first = Christian
title = Fiber in the Whole (House): Cirrus Logic Buys Peak Audio
year = 2001
url = http://www.prosoundweb.com/industrybiz/biz_news/5_01_biz_news/cirrus.shtml
accessdate = 2007-03-19 ] Leveraging Cirrus DSP technology, a low-cost SoC implementation of CobraNet was developed and marketed. CobraNet has been widely licensed by commercial audio equipment manufacturers and installed in thousands of facilities worldwide.

The name "CobraNet" originated as an acronym for COordinated BRoadcast Audio Network and was approved by Peak Audio's president, a car racing enthusiast. This eventually led to a Shelby Cobra making an appearance at the Peak Audio booth at a Las Vegas trade show. [cite conference
first = John
last = Storyk
authorlink =
coauthors = The Walters/Storyk Design Group
title = Audio Engineering Society November 2004 Meeting
booktitle = Studio Acoustics
pages =
publisher =
date = 11–2004
location = Washington, DC; American University
url = http://www.american.edu/perf_arts/season/documents/Studio_Acoustics.pdf
doi =
id =
accessdate = 2007-03-19 ]

Advantages and disadvantages

Advantages

* Cabling cost – using CobraNet and fast Ethernet, 64 channels of uncompressed digital audio can be sent on a single, inexpensive Cat-5 cable. In the analog world, this would have required 64 separate analog audio cables, each of which cost the same or more than a single Cat-5 cable. Using gigabit and/or fiber optic Ethernet variants, even greater economies can be achieved for large systems. Also, since CobraNet data can coexist with data traffic over existing Ethernet networks, money can be saved by eliminating additional infrastructure costs.
* Flexibility – a well-designed network provides enhanced flexibility for future changes to the system. For instance, audio routing changes can be made in seconds from software, and do not require any rewiring.
* Reliability – use of by CobraNet Ethernet affords many High Availability features such as Spanning Tree Protocol, Link Aggregation and Network Management. For critical applications, CobraNet devices can be wired with a redundant link. In the case that one CobraNet device, cable, or Ethernet switch fails, the other takes over almost immediately.
* Audio quality – audio is transmitted in digital form, meaning that it is immune to signal degradation caused by electromagnetic interference, crosstalk, or voltage drop due to cable resistance. Additionally, CobraNet contributes towards keeping the signal chain digital for as long as possible.

Disadvantages

* Latency – delays over the CobraNet transmission medium itself can be anywhere from 1.33 to 5.33 milliseconds. Further delays are introduced when converting back and forth from analog to digital. For live musical performance, these delays can sometimes be unacceptable.
* Hardware cost – although significant money is saved in cabling, that money is spent on the CobraNet devices which encode and decode the CobraNet signal. Those devices usually have additional integrated DSP for effects which adds to the cost, but since each manufacturer must license CobraNet per device, that cost must be passed on to the consumer.

Transmission

Ethernet

CobraNet is transmitted using standard Ethernet packets. Instead of using TCP/IP packets, CobraNet transfers data using link layer packets, which travel quickly through hubs, bridges and switches, and are not as susceptible to the latency and QoS problems commonly found in streaming protocols using a higher transport layer. However, since CobraNet does not use an IP protocol, its packets cannot travel through routers, and therefore it is limited to use on a LAN; CobraNet cannot be used over the Internet. The network over which CobraNet is transmitted must be able to operate at a minimum of nowrap|100 Mbit/s (also known as Fast Ethernet). All CobraNet packets are identified with a unique Ethernet protocol identifier (0x8819) assigned to Cirrus Logic.cite paper
author = Cirrus Logic, Inc.
title = CobraNet Programmer's Reference
version = 2.5
publisher =
date = 2–2006
url = http://www.cirrus.com/en/pubs/manual/CobraNet_Programmer_Manual_PM25.pdf
format = PDF
accessdate = 2007-03-19 ]

While CobraNet has been shown to function properly over wireless networks under ideal conditions, bandwidth and reliability issues associated with typical 802.11 wireless networks tend to cause frequent dropouts and fatal errors.Citation | last = Cirrus Logic | title = CobraNet FAQ, Question 13 | url = http://www.cobranet.info/en/support/cobranet/faq.html#Q13 | accessdate = 2007-03-19 ] However, wireless communication of CobraNet data can be reliably accomplished using lasers (for example, [http://www.whirlwindusa.com/ebeam.html Whirlwind's E-Beam Laser] ).

Channels and bundles

CobraNet data is organized into channels and bundles. A typical CobraNet signal can contain up to 4 bundles of audio travelling in each direction, for a total of 8 bundles per device. Each bundle houses up to 8 channels of 48 kHz, 20-bit audio, for a total capacity of 64 channels. CobraNet is somewhat scalable, in that channel capacity increases when 16-bit audio is used, and channel capacity decreases when 24-bit audio is used. Specific channel capacity is defined by the 1,500-byte Ethernet payload limit.

There are three types of bundles: multicast, unicast, and private:
* Multicast bundles are "broadcast" from one CobraNet device to all other CobraNet devices in the network using Ethernet multicast addressing. Each CobraNet device individually determines if it will use the bundle or discard it. Therefore, multicast bundles are more bandwidth-intensive than other bundle types. Bundle numbers 1–255 are reserved for multicast bundles.
* Unicast bundles are sent from one CobraNet device to any other device or devices configured to receive the bundle number. Unicast bundles are much more efficient because they attempt to travel only to devices which actually want to receive them. Despite their name, unicast bundles may still be sent to multiple devices, either by transmitting multiple copies of the audio data or using multicast addressing. Bundle numbers 256–65279 are reserved for unicast bundles.

* Private bundles may be sent with unicast or multicast addressing. Bundle numbers 65280–65535 are reserved for private bundles. Private bundle numbers are associated with the MAC address of the device that transmits them. To receive a private bundle, both the bundle number and the MAC address of the transmitter must be specified. Because private bundles are associated with the transmitters there is no hard liming on the number private bundles.

As long as multicast bundles are used sparingly, it is virtually impossible to exceed the bandwidth of a 100 Mbit network with CobraNet data. However, there are limitations to the maximum number of bundles that can be sent on a network, since the conductor must include data in its beat packets for every bundle on the network, and the beat packet is limited to 1,500 bytes. If each device is transmitting one bundle, there may be up to 184 transmitters active simultaneously (for a total of 184 bundles). If each device is transmitting a full four bundles, then only 105 transmitters could be active, although they would be producing a total of 421 active bundles. The use of private bundles does not require any additional data in the beat packet, so these network limitations can be sidestepped by using private bundles.Citation | last = Cirrus Logic | title = CobraNet FAQ, Question 28 | url = http://www.cobranet.info/en/support/cobranet/faq.html#Q28 | accessdate = 2007-03-19 ]

ynchronization

The CobraNet network is synchronized to a single CobraNet device known as the conductor. A conductor priority can be configured to influence selection of the conductor. Among devices with the same conductor priority configuration, the first to establish itself on the network becomes is elected conductor. All other devices are known as performers. In the event that the conductor fails, another CobraNet device will be chosen to become the conductor within milliseconds. CobraNet cannot function without a conductor.Citation | last = Cirrus Logic | title = CobraNet FAQ, Question 24 | url = http://www.cobranet.info/en/support/cobranet/faq.html#Q24 | accessdate = 2007-03-19 ]

Four main types of packet are used in the transmission and synchronization of CobraNet:
* Beat packets – the conductor outputs a beat packet to all other CobraNet devices on the network, at a rate of 750 packets per second. All other CobraNet devices on the network synchronize their audio clock and their data transmissions to the beat packet. The beat packet contains network operating parameters, clock data and transmission permissions for multicast and unicast bundles.
* Audio packets – also known as "Isochronous Data Packets", these packets are sent out by all CobraNet devices after they receive a beat packet. At standard latency settings, one audio packet is sent for each beat packet received, and each audio packet transmits 64 samples of audio data per channel. However, at lower latency settings, audio packets may be sent twice or four times for each beat packet received. Bundles do not share packets; separate packets are sent in sequence for each bundle transmitted from the same device.
* Reservation packets – these packets are transmitted as needed or typically once per second at minimum. Their function is to control bandwidth allocation, initiate connections between CobraNet devices, and monitor the status of CobraNet devices.
* Serial bridge packets – asynchronous serial data may be sent between CobraNet devices on the same network. Many standard asynchronous serial formats are supported, including RS-232, RS-422, RS-485 and MIDI.

Latency

The buffering of audio data into Ethernet packets typically incurs a delay of 256 samples (or 5.33 milliseconds). Additional delays are introduced through A-D and D-A conversion (typically 10–50 samples). Latency can be reduced by sending smaller packets more often. In most cases, the programmer can choose the desired CobraNet latency for a particular CobraNet device (5.33, 2.67, or 1.33 milliseconds). However, reducing audio latency has consequences:

* Reducing latency requires more processing power.
* Reducing latency places additional demands on network performance, and may not be possible in some network configurations if the forwarding delay is too great.
* Since reducing latency means sending smaller packets more often, more high resolution (i.e. 96 kHz, 24-bit) audio channels can be sent per bundle without exceeding the 1,500-byte payload limit for Ethernet packets. See the table below for bundle capactity limits:

It may seem from the table above that more information can be sent at a lower latency. However, that is not the case. More channels can be sent per bundle, but less bundles can be processed simultaneously by one device. So, while eight 24-bit, 96 kHz channels can be sent in one bundle at 1.33 ms latency, due to processing constraints, the CobraNet device may only be able to send and receive one bundle instead of the usual four. The bundle capacity of CobraNet devices are unique to the particular device, and are not always the same. However, below is a table illustrating the bundle capacity for a Biamp AudiaFLEX-CM DSP device. The Rx and Tx columns indicate the absolute maximum number of channels that can be received or transmitted. The Rx/Tx column represents the maximum number of channels that can be received and transmitted simultaneously. [cite paper
author = Biamp Systems
title = Audia Operation Manual
date = 2007-02-14
url = http://www.biamp.com/audiaflex.php?act=log&dlid=18&type=prod&tid=3
format = PDF
accessdate = 2007-03-19 ]

Hardware and software

CobraNet network cards

CobraNet network cards come in several varieties, some of which can support more channels than others. Additionally, CobraNet network cards have two Ethernet ports labelled "primary" and "secondary". Only the primary Ethernet port needs to be connected, but if both ports are connected they become a redundant failsafe. That is, if the primary port loses communication, the secondary port immediately takes over with no packet loss. Careful network design and topology which takes advantage of this feature can provide extremely high reliability in critical applications.

The typical CobraNet network cards provided by Cirrus Logic are the CM-1 and the CM-2: [Citation
last = Cirrus Logic
title = CobraNet Networked Digital Audio
url = http://www.cirrus.com/en/products/pro/techs/T9.html
accessdate = 2007-03-19 ]

* CM-1 – the standard CobraNet card, provides 32x32 simultaneous I/O channels.
* CM-2 – compact, low-power, lower cost design provides 8 or 16 simultaneous I/O channels.Both cards are designed to be added to audio products by the manufacturer.

oftware

Cirrus Logic provides a software application known as CobraCAD, which assists in the design of the network on which the CobraNet system will run. It helps to identify if there are too many routers between two CobraNet devices, if a certain latency is possible given the network configuration, and other tasks. However, Cirrus Logic does not provide software to manipulate their hardware. In fact, in the simplest of cases, no software is required by the end user. For instance, a simple breakout box which converts a CobraNet signal to eight analog audio signals would require little or no configuration by the end user (apart from possibly selecting the bundle number). If configuration is required (for example, in a DSP box with integrated CobraNet I/O), then the manufacturer of the device typically supplies proprietary software for that purpose.

Licensed manufacturers

Manufacturers who wish to integrate CobraNet connectivity into their devices must license the technology from Cirrus Logic. Many audio equipment manufacturers have included CobraNet in their products. Below is a partial list of notable examples, sorted by device type: [Citation
last = Cirrus Logic
title = CobraNet Community
url = http://www.cobranet.info/en/support/cobranet/logos.html
accessdate = 2007-03-19 ]

DSP

One of the most popular and useful devices that integrates CobraNet is the audio DSP. These devices typically receive audio from CobraNet (and often from other digital or analog sources simultaneously), process the audio using digital filters and effects (for example, volume control, EQ, compression, delay, crossovers, etc.) and then output the audio via CobraNet (or other digital or analog outputs). Some DSPs even have an integral telephone hybrid, and can incorporate CobraNet and other sources into a teleconferencing application.

Manufacturers of CobraNet-equipped DSP's

* [http://www.ashly.com Ashly]
* [http://www.atterotech.com Attero Tech]
* [http://www.audioscience.com AudioScience]
* [http://www.biamp.com Biamp]
* [http://www.bss.co.uk BSS]
* [http://www.clearone.com ClearOne]
* [http://www.dbxpro.com dbx]
* [http://www.digigram.com Digigram]
* [http://www.digispider.net DigiSpider]
* [http://www.dolby.com/professional/Live_Sound Dolby]
* [http://www.lancedesign.com Lance Design]
* [http://mediamatrix.peavey.com Peavey MediaMatrix]
* [http://www.symetrixaudio.com Symetrix]
* [http://www.whirlwindusa.com Whirlwind]
* [http://www.yamahaproaudio.com Yamaha]

Amplifiers

Amplifiers with integrated CobraNet help keep the signal chain digital for a longer span. Amplifiers with CobraNet inputs often also have limited DSP and monitoring capabilities built-in.

Manufacturers of CobraNet-equipped Amplifiers

* [http://www.crestaudio.com Crest Audio]
* [http://www.crownaudio.com Crown Audio]
* [http://www.electrovoice.com Electrovoice]
* [http://www.labgruppen.se Lab Gruppen]
* [http://www.qscaudio.com QSC]

Loudspeakers

Loudspeakers with integrated CobraNet help keep the signal chain digital for an even longer span. In a typical unpowered speaker application, the amplifier would be housed far away from the speaker, and a long speaker cable (analog) would be run between the speaker and the amplifier. The speaker cable would be subject to interference and cable loss. However, a powered speaker with integrated CobraNet inputs eliminates that long (and relatively expensive) speaker cable. Since a speaker will only use one audio channel out of the bundle, many speakers with CobraNet will also have a number of analog outputs for the rest of the channels in the bundle, which is useful in speaker cluster applications.

Manufacturers of CobraNet-equipped Loudspeakers

* [http://www.eaw.com EAW]
* [http://www.jblpro.com JBL]
* [http://www.qscaudio.com QSC]
* [http://www.renkus-heinz.com Renkus-Heinz]

Mixing consoles

Mixing consoles with integrated CobraNet are one example of cost savings with respect to cabling. Whereas a normal analog mixer would typically have hundreds of cables plugged into it, a mixer with integrated CobraNet I/O would have 1–4 Ethernet cables plugged into it. These digital mixers usually use DSP chips to perform all audio mixing functions, including volume control, EQ, routing, and effects.

Manufacturers of CobraNet-equipped Mixing Consoles

* [http://www.d-r.nl D&R Electronica]
* [http://www.mackie.com Mackie]
* [http://www.midasconsoles.com Midas]
* [http://www.soundcraft.com Soundcraft]
* [http://www.global.yamaha.com Yamaha]

Future plans

Cirrus Logic has announced for several years plans to produce CobraNet devices that make full use of Gigabit Ethernet. This would theoretically allow a single CobraNet device to handle 10 times the audio (640 channels) at a tenth of the latency (0.5 ms). [Citation | last = Cirrus Logic | title = CobraNet FAQ, Question 14 | url = http://www.cobranet.info/en/support/cobranet/faq.html#Q14 | accessdate = 2007-09-19 ]

ee also

* Audio over Ethernet
* Audio Contribution over IP
* EtherSound
* Cirrus Logic
* Ethernet

References

External links

* [http://www.cirrus.com/en/pubs/manual/CobraNet_Programmer_Manual_PM25.pdf CobraNet Programmer Manual]