CobraNet

File:Connected by cobranet.gif

CobraNet Logo

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.

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

Cobranet is generally used as an alternative to traditional analog audio, with one audio channel being transmitted per cable (usually a shielded, twisted pair). While analog audio transmission involves virtually no latency, its sound quality can be adversely affected by signal degradation due to electromagnetic interference, and voltage drop over long cable runs. Additionally, the cabling requirements of Cobranet are almost always less expensive than traditional analog audio.

Cobranet is a pioneering technology in the field of audio over Ethernet, and as such it has prospered with little or no competition for many years. However, several companies have recently begun to introduce different methods for transmitting audio over Ethernet (most notably, EtherSound), each with their own pros and cons.

History

Cobranet was introduced in 1996 by Boulder, CO-based Peak Audio. While it was not the first of its kind, its combination of feature set and capabilities proved useful to the commercial audio industry, and it quickly gained notoriety.

In its initial version, Cobranet 0, 10 Mbit/s networks were utilized to send six channels of audio in one direction and two in the other. Audio was sampled at 44.1 kHz, 20 bits, and all packets were unicast. The first extensive use of Cobranet 0 was by Disney to provide background music throughout the Animal Kingdom theme park.

Cobranet I followed not long after, prompted by a collaboration with manufacturer QSC Audio Products. QSC licensed the technology from Peak Audio and marketed it under the name RAVE. Cobranet I added the ability to use multicast Ethernet packets to "broadcast" audio channels to multiple devices. Additionally, the sample rate was changed to 48 kHz. Cobranet I upgraded to 100 Mbit/s Fast Ethernet networks, and subsequently increased its channel capacity to 64 on a single cable. However, extensive use of multicast packets limited the usefulness of Cobranet I to smaller, less sophisticated networks.

Finally, Cobranet II added support for mixing unicast and multicast packets, support for 20- or 24-bit audio, and a variable number of audio channels per bundle. Additionally, increased addressing capabilities paved the way for private bundles and more flexible routing options through complex networks.

In May of 2001, Cirrus Logic announced that it had acquired the assets of Peak Audio, which would remain a standalone business under the umbrella of Cirrus Logic. Since that acquisition, Cobranet has been widely licensed by commercial audio equipment manufacturers and installed in thousands of facilities worldwide.

Allegedly, the name Cobranet was chosen by a Peak Audio investor who had a great interest in Shelby Cobra race cars. This eventually led to the original #1 car making an appearance at the Peak Audio booth at a Las Vegas trade show.

Advantages/Disadvantages

Advantages

• Cabling Cost - Using Cobranet, 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. 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 additional cabling to be provided.
• Reliability - For critical applications, Cobranet devices can be wired with a redundant link. In the case that one cable or switch fails, the other link takes over immediately.
• Audio Quality - Audio is transmitted in digital form, meaning that it is virtually invulnerable 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 1⅓ to 5⅓ 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 - While significant money is saved in cabling, this money is spent on the Cobranet devices which encode and decode the Cobranet signal. These 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

Simple block diagram of an audio system employing Cobranet technology. Red lines indicate analog audio signals, while green lines indicate standard Ethernet signals.

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 latency and QOS problems that are 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 100 Mbit/s (also known as Fast Ethernet). All Cobranet packets are identified with a unique Ethernet protocol identifier (0x8819) assigned to Cirrus Logic.

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. However, wireless communication of Cobranet data can be reliably accomplished using lasers (for example, Whirlwind's E-Beam Laser).

Channels & 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 1500 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 device which is "listening" for that 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. Bundle numbers 256–65279 are reserved for unicast bundles.
• Private bundles are sent along with the MAC address of the destination device, and therefore they are only sent to one specific device. Intermediate Ethernet switches interpret the MAC address specified in the packet, and route the packet accordingly. Bundle numbers 65280–65535 are reserved for private bundles. However, private bundle numbers are unique to the device which sends them, so essentially there are an infinite number of possible private bundles available.

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 send more data in its beat packets for every bundle being sent, and the beat packet is limited to 1500 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 network limitations can conceivably be sidestepped by using private bundles.

Synchronization

The Cobranet network is synchronized to a single Cobranet device (usually chosen randomly at system startup) known as the conductor. In the event that the conductor fails, another Cobranet device will be chosen to become the conductor within milliseconds, since Cobranet cannot function without a conductor. There are four main types of packets that are used in the transmission and synchronization of Cobranet:

• Beat Packets - The conductor regularly 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 to that beat packet. The beat packet contains network operating parameters, clock data and transmission permissions.
• 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; that is, separate packets are sent simultaneously for each bundle being 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, SNMP and more.

Latency

The buffering of audio data into Ethernet packets typically incurs a delay of 256 samples (or 5⅓ 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 network (5⅓, 2⅔, or 1⅓ 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 large.
• 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 1500 byte payload limit for Ethernet packets. See the table below for bundle capactity limits:

Latency	Channels per Bundle
	16 bit, 48 kHz	20 bit, 48 kHz	24 bit, 48 kHz	16 bit, 96 kHz	20 bit, 96 kHz	24 bit, 96 kHz
5⅓ ms	8	8	7	5	4	3
2⅔ ms	8	8	8	8	8	7
1⅓ ms	8	8	8	8	8	8

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 1⅓ ms latency, 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.

Channels per Bundle	5⅓ ms latency			2⅔ ms latency			1⅓ ms latency
	Rx	Tx	Rx/Tx	Rx	Tx	Rx/Tx	Rx	Tx	Rx/Tx
8	32	32	32/32	32	32	32/32	32	32	16/16
7	32	32	32/32	32	32	29/29	28	32	14/15
6	32	32	32/32	32	32	29/29	24	32	12/13
5	32	32	32/32	32	32	25/27	21	32	12/13
4	32	32	32/32	32	32	24/24	20	28	12/12
3	32	32	32/32	32	32	20/21	15	24	9/11
2	32	32	28/29	27	32	16/16	12	18	6/7
1	16	16	16/16	16	16	9/10	7	10	4/4

Hardware & Software

File:CobranetBreakout.jpg

A Cobranet input module for Renkus-Heinz loudspeakers.

Cobranet Network Cards

Cobranet network cards come in several varieties, some of which can support more channels than others. Additionally, all 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:

• 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.

Software

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 no configuration by the end user. 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:

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 DSP's even have an integral telephone hybrid, and can incorporate Cobranet and other sources into a teleconferencing application.

Manufacturers of Cobranet-equipped DSP's

• Ashly
• Biamp
• BSS
• ClearOne
• Creative Audio
• dbx
• Digigram
• DigiSpider
• Peavey Mediamatrix
• Symetrix
• Whirlwind

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

• Crest Audio
• Crown Audio
• Electrovoice
• Lab Gruppen
• 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

• EAW
• JBL
• QSC
• Renkus-Heinz

Mixing Consoles

Mixing consoles with integrated Cobranet are a great 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

• D&R Electronica
• Mackie
• Midas
• Soundcraft
• Yamaha

Future Plans

Cirrus Logic has announced 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).

See also

• Audio over Ethernet
• EtherSound
• Cirrus Logic
• Ethernet

External Links

• Cobranet FAQ
• Cobranet Programmer Manual
• Cirrus Logic Cobranet Page
• ProSoundWeb's Cobranet & EtherSound discussion forums