5.6 CobraNet

5.6 CobraNet

CobraNet is a proprietary audio networking technology developed by Peak Audio, a division of Cirrus Logic. It is designed for carrying audio over conventional Fast Ethernet networks (typically 100Mbit/s), preferably using a dedicated Ethernet for audio purposes or using a switched Ethernet network. Switched Ethernet acts more like a telephone or ATM network where connections are established between specific sources and destinations, with no other data sharing that 'pipe'. For the reasons stated earlier in this chapter, Ethernet is not ideal for audio communications without some provisos being observed . CobraNet, however, implements a method of arbitration, bandwidth reservation and an isochronous transport protocol that enables it to be used successfully.

It is claimed that conventional data communications and CobraNet applications can coexist on the same physical network but the system implements new arbitration rules, in the form of a so-called 'O-persistent' layer within the data link layer, to ensure that collisions do not take place on the network. All devices have to be able to abide by these rules if they are to be used. The company provides software that can be used to verify that an existing network design is capable of handling the audio information intended (in respect of bandwidth, delay and other critical parameters).

CobraNet can also be used for sample clock distribution and for equipment control purposes (interfacing with RS-232 and RS-485 equipment) and it is becoming popular in venue or live sound installations for transferring audio between multiple locations. It requires a dedicated CobraNet interface to convert audio and control data streams to and from the relevant Ethernet protocol and claims a low audio latency of 5.3 ms. A number of items of audio equipment are already equipped with CobraNet technology, such as microphone preamplifiers, power amplifiers and routers. Users claim considerable benefits in being able to integrate audio transport and equipment control/monitoring using a single network interface.

The CobraNet protocol has been allocated its own protocol identifier at the Data Link Layer of the ISO 7-layer network model, so it does not use Internet Protocol (IP) for data transport (this is typically inefficient for audio streaming purposes and involves too much overhead). Because it does not use IP it is not particularly suitable for wide area network (WAN) operation and would typically be operated over a local area network (LAN). It does, however, enable devices to be allocated IP addresses using the BOOTP (boot protocol) process and supports the use of IP and UDP ( user datagram protocol) for other purposes than the carrying of audio. It is capable of transmitting packets in isochronous cycles, each packet transferring data for a ˜bundle' of audio channels. Each bundle contains between zero and eight audio channels and these can either be unicast or multicast. Unicast bundles are intended for a single destination whereas multicast bundles are intended for ˜broadcast' transmissions whereby a sending device broadcasts packets no matter whether any receiving device is contracted to receive them.

Rather like 1394 (see above) isochronous cycles are initiated by one bus-controlling device (the 'conductor') that sends a multicast packet to indicate the start of a cycle to all other devices ('performers'). In CobraNet terminology this is called the ˜beat packet'. This beat packet is a form of clock for the network and also carries information about the overall network operation, so it is sensitive to delays and must be maintained within a very narrow time window (250 s) if it is to be used for sample clock locking. These packets are typically small (100 bytes) whereas audio data packets are typically much larger (e.g. 1000 bytes). The CobraNet interface derives a sample clock from the network clock by means of a VCXO ( voltage-controlled crystal oscillator) circuit.



Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net