Operation Manual
I-Tech HD DriveCore Series Power Amplifiers
page 54
I-Tech HD DriveCore Series Power Amplifiers
Operation Manual
page 55
12.2 A Closer Look at CobraNet
Licensed by Cirrus, CobraNet is a protocol, firmware and hardware that lets you trans-
mit digital audio over a 100Base-T Fast-Ethernet network.
This section provides an overview of CobraNet. Cirrus Logic has provided a number of
technical papers that describe CobraNet in detail. These papers are available at:
http://www.cobranetinfo.com.
12.2.1 Fast Ethernet
(100Base-T)
Fast Ethernet (100Base-T) runs at 100Mbps (Megabits per second). It uses a hub or
network switch to interconnect all network nodes (components). This forms a star
network (see Figure 9.1). In a star network, all PCs are connected to a centralized hub
or switch. Messages are sent from the originating PC to the hub, which then transmits
the message either to the whole network or, if a switched hub (network switch) is used,
to the destination device only.
In larger Fast Ethernet networks, additional hubs, concentrators, and other network
hardware are used to form a larger network, as shown in Figure 12.4.
Today, commonly available networking cards are 10/100Base-T capable, which allows
them to be used on either 10BaseT or 100Base-T networks.
The maximum length of cable for CobraNet is the same as for Ethernet:100 meters over
CAT-5 copper cable, 2 kilometers over multimode fiber. Proprietary “Fast Ethernet via
single mode fiber” networks can reach even further.
12.2.2 Audio Specs
Sample rate:
CobraNet can handle sample rates of 48 kHz or 96 kHz. I-Tech HD is
switchable between 48 kHz and 96 kHz.
Bit depth:
Set each channel to transmit 16-, 20- or 24-bit audio data as desired.
Latency:
The transmitter and receiver buffers required for reliable transmission are
set to 256 samples as a default. This gives a network transmission latency of 5.333 mS
(256 samples at 1/48 kHz per sample). I-Tech HD latency is switchable between 1.33,
2.66, and 5.33 mS.
12.2.3 Bundles and Audio Channels
You assign all audio channels to groups called Bundles for transmission over the
CobraNet network. Bundles were formerly called Network Channels.
A bundle can carry up to 8 audio channels. Up to 8 bundles can be sent in a 100 Mbps
line, depending on the signal’s bit depth and sampling rate. Only 4 bundles are avail-
able when 96 kHz sampling rate is in use.
The usual assignment is 8 channels at 20 bits. You can use fewer channels per bundle,
but maximum size bundles are suggested for the most efficient use of network band-
width. If 24-bit data is desired, then only 7 audio channels can be loaded into a single
Bundle. With a 96 kHz sample rate, only 4 channels of audio are available.
Figure 12.3 Star Topology
Figure 12.4 Multi-Star Topology
In System Architect, you create audio connections between sending
devices (transmitters) and receiving devices (receivers). For example,
a mixer could be a transmitter, and a power amp could be a receiver.
You assign each connection a
Bundle number.
The Bundle number
indi cates which devices are communicating with each other.
Also in System Architect, you address each connection as
Multi-
cast
or
Unicast
. A Multicast (Broadcast) connection is from one
trans mitter to multiple receivers. A Unicast connection is from one
transmitter to one receiver. For example, if you send a mixer’s digital
audio signal to sev eral power amps, the signal would be Multicast.
Unicast is most used on “switched” networks requiring more than 64
channels.
You specify a connection as Multicast or Unicast by the delivery
address it you give it in System Architect. 255 and lower is Multicast
(Broadcast); 256 and up is Unicast.
The “Receiver Count” monitor indicates how many devices are
receiving the digital audio bundle. The “Active” indicator indicates
whether the par ticular Bundle is being actively transmitted onto the
network.
12.2.4 The Conductor
A CobraNet system is coordinated by one audio device in the network
called the
Conductor
. It regulates the CobraNet digital audio traffic
on the CobraNet network. That is, the Conductor is the master clock
for the rest of the network.
You choose one device to be the network conductor based on a
priority scheme. The Conductor indicator will light on the CobraNet
device that is serving as the conductor.
The Conductor handles time-division multiplexing and clock recovery
on the network. The Conductor periodically broadcasts a well-defined
“beat” packet to all components to recover synchronous timing
information. The “beat” also specifies which addresses can transmit
at certain time slots within the “beat” period.
Each transmitting device is allowed to transmit in a given time-slot
within the Conductor-controlled isochronous cycle time. The time-
slot is deter mined by the assigned bundle priority. Higher bundle
priorities receive lower-numbered positions and are transmitted first.
12.2.5 Switched Networks
A more complex CobraNet network can be built using Ethernet
switches. Switches do not simply broadcast each and every packet
to all nodes. Instead, they check each incoming data packet to
determine its destination and (very quickly) transmit the data to only
that destination port. This allows for more network data flow, more
Bundles and more audio channels.
In effect, each network port in an audio component has 100MB of
band width. The network can be as large as 100MB times the number
of ports on the network.
Another advantage of switched networks is “full-duplex” connections
between components. A full-duplex link allows simultaneous send
and receive over the same Ethernet connection.
