I've got some pro sound guys telling me that Cobranet is just as good for studio builds as Axia and actually was the first system for putting audio on ethernet. I read some things on your FAQ about the difference between the systems, but it doesn't really go into detail. Tell me why I shouldn't just use Cobranet?
Joined: 25 May 2007
Location: San Diego, CA
Well, there's no short answer to this, so grab your favorite beverage and sit back
First of all, when we began the Axia project, we knew we wanted to route audio over Ethernet. We did not intend to create our own format though. We figured we would use Cobranet or an alternative. However, when we investigated the protocols, none were suitable for broadcast. Here are some of the big differences between Cobranet and Livewire, and why Livewire works better in the broadcast environment:
1. Cobranet was designed as a point to point "snake" replacement; sound reinforcement and installed-sound applications. And it works very well for those apps. Perhaps now the designers of CN have modified it to overcome some of its earlier limitations. Livewire was designed from the start for broadcast audio routing.
2. Livewire was built around the features of VoIP class Ethernet switches. Early Cobranet was not compatible with Ethernet switches and hubs. Later, they said that you could use CN with Ethernet switches, but this needs to be explored carefully. Our research found that Cobranet does not support multicast routing and therefore, there may be severe limitations when attempting to scale the system. Livewire uses multicast routing so that point to multipoint routing is all done within the Ethernet switch, saving link bandwidth and allowing for maximum scalability. This is an important distinction. CN may now work over Ethernet switches; Livewire never worked without one! The Ethernet switch is central to our system. Livewire is designed to adapt the capabilities of the modern Ethernet switch to the broadcast audio routing application.
3. Latency can be a problem with Cobranet. Depending on the packet size you choose, latency can vary between 5ms per network hop down to 1.5ms per network hop. A typical mic to headphone path using Cobranet could be as high as 10-20ms while a Livewire mic to headphone path will be less than 3ms. This was one of the major deterrents to using CN for broadcast. In live or installed sound, this latency is not an issue. Only when an announcer is listening to himself in headphones can this delay become a problem. We understand that CN is able to dial down the latency now, but at the expense of link bandwidth.
4. Mixed data on the link. Livewire allows data (such as GPIO) and audio on the same links. This makes for one connection between the network and the networked device. Cobranet must own the entire link. It does not allow for any other data. There is a trick for getting ancillary data to and from the networked device via a CN link, but it requires the developer of the device to embed information into the CN data stream using registers on a CN interface chip. Don't let anyone tell you this is easy. It might work for a snake application where the same company is building both ends of the link, but it's going to be a real mess when multiple devices are connected. Livewire on the other hand, allows TCP/IP data over the same link as the audio. Because we use the prioritization features of the switch, we ensure audio always comes first. Also, we constrain the audio data on the link by the capacity of the networked audio device (node). Because our nodes are never more than 16 streams, it is impossible to consume or produce more bandwidth than is required for 16 streams. That fits well into a 100mbps link with plenty of room to spare for TCP/IP data messages for control and information. Our architecture also means you can plug other non-Livewire devices into your network switch and never have to worry about overloading it or creating congestion. On a 48 port switch, you can plug in 48 audio nodes, each with eight inputs and eight outputs. That is a 384x384 stereo router using a simple sub-$1,000 switch. Of course, much more powerful switches are available and it is possible to build large audio systems with up to 10,000 stereo audio ports.
5. Distributed clock. All digital systems require reliable clocking. Cobranet uses a clock they call the "conductor;" a master clock that serves the entire system. While it can be backed up by a secondary conductor in case the primary fails, Livewire has a distributed clocking mechanism which can automatically heal itself in the event of problems. Each Livewire device can be master or slave. When a network is assembled, the device with the highest MAC address asserts itself as the master and all other devices slave automatically. If the master is lost for whatever reason, the device with the next highest MAC address takes over. This switchover happens in less than one audio sample so it is seamless. You can also use a node as a master clock input and you can use a second node as a secondary if you wish. This concept of distributed features to eliminate single points of failure is central to the Livewire approach, since Livewire was built for broadcast where reliability is essential.
6. Computer connections. Since you cannot connect non-Cobranet devices to the CN network, you must adapt everything to CN before you can integrate. This means you will not see a CN driver allowing you to use a standard Ethernet NIC to send/receive audio via the network. To connect a computer to a CN network, you will need to use soundcards or a specially designed CN card. In the context of the applications for which CN was designed, this is not such a limitation, since sound reinforcement doesn't use a lot of computer audio. But broadcasting sure does. Livewire allows the audio applications to connect directly to the network via the computer's standard Ethernet NIC.
7. Do you know in advance where your audio needs to go? This is an important question. The Livewire scheme allows any input to connect to any output system-wide. The sources all advertise their availability through the switch fabric and the destinations subscribe to the one they want. This is the way broadcasters want to work. From our studies of CN, it appears that there are limits to the direction and the number of routes which can be made. In fact, without multicast support, this limit may be quite severe, i.e., 64x64 max routes possible at once.
8. Automatic Scaling. Plug in a bunch of Livewire devices into 100mbps ports on your Ethernet switch and plug your Ethernet switch using a Gigabit link to another Ethernet switch. Livewire manages the signals across the gigabit link such that only the signals actually needed on the other end are sent. In this manner, not only is Livewire Gigabit capable, it is also very intelligent about using the links so that they only carry needed signals and never are flooded with unneeded streams. Huge multi-switch systems are possible without special handling. The logic of Livewire manages routing.
9. Routable Packets. This is a huge deal. CN packets may travel over Ethernet, but they might as well be proprietary. Only a CN device can send/receive these packets. Livewire packets can be routed just as any other data packet. In fact, our audio packets are RTP which means our streams can be monitored by standard audio players such as WMP or Winamp or Quicktime. You can have computers in your office listening to streams created in your studios without any special hardware or software.
Long winded answer, but I hope it helps explain things for you.
_________________ Clark Novak
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