Designing an audio network


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Spoke and hub topology

The spoke and hub remains the logical topology for an audio network. It is one of the most basic methods of communication between multiple locations. This method is used by airlines, the post office, the phone company and many others.

Radio stations still need separate studios to carry out specific functions, such as on-air and commercial production. Most stations have a rack room or some other location that is the heart of the facility, as shown in Figure 1. This is where the audio network hub (in the spoke and hub context) is located. Several manufacturers make use of a peer-to-peer variant on the spoke and hub idea, by use of a device commonly known as an audio engine. This is a device that handles the cross-point switching and other functions that would heretofore have been accomplished in a router. A unit such as this would be at the hub (in the spoke and hub context) but also, in some cases, in a studio. Most units such as this are mainframes, and as such have I/O cards to receive and send audio signals, in the analog or digital formats; logic I/O cards; DSP cards for audio processing (including mixing); and finally specific cards for communication with other audio engines (peers) or peripherals (spokes) in the audio network such as control surfaces. Shared audio devices (such as a satellite receiver) located in a rack room would connect to an audio engine located in the same room.

The studios themselves still have specific items that are needed locally (such as a CD player, for example). To avoid a wire run all the way back to the master control room audio (because after all, that was part of the reason for the network, right?) an audio I/O device still needs to play the role of interface between the audio network and the local device itself. Some manufacturers make special I/O devices and some simply specify that a stripped-down version of an audio engine be located in the studio. In some cases, an audio engine located in a studio has specific plug-in cards that allow it to work in conjunction with a control surface located in-studio, thus providing all the necessary and familiar console functions. Each audio engine, whether located in a studio, a rack room or some other place, is a peer. Essentially they are all functionally equivalent, and they communicate with one another. Their specific inputs and outputs become available throughout the entire network.

The audio engines

AEQ's offering is the BC 2000D, a mainframe (and hub of the system) that is built to house the various plug-in modules associated with the typical audio network: I/O cards and DSP boards. The control surface for the AEQ system is made up of combinations of the Arena DM (five input modules plus a monitor module) and an Arena D10, which has 10 input modules.

Figure 2. A mini-engine or other I/O device can be used in a studio to further reduce the cabling needed to attach sources to the audio network.
Click image to enlarge.

Axia's approach is perhaps the one that is most like a computer network. All signaling is routed via Ethernet. The heart of the audio network is an Ethernet switch that ties all the spokes together. Elements that communicate via Ethernet through this switch include the Axia Studio Engine, which is a 2RU device that uses DSP to accomplish all the mixing functions given it via the Smartsurface. The Smartsurface is the user-interface that looks and functions like an audio console.

Harris offers the Vistamax system. It starts with a mainframe that is loaded with the appropriate analog and digital I/O cards and communications cards, and will typically be installed in the rack room. Rack room sources are integrated into the audio network at this location. This particular frame would then make peer-to-peer connections with other frames throughout the facility.

Klotz Digital has recently introduced the Vadis 212, a fanless mainframe with 10 freely assignable slots for interface cards accommodating analog or digital inputs and outputs. With the appropriate DSP cards installed, the unit can perform mixing functions along with real-time audio processing functions such as EQ, compression and limiting. Like the other audio engines mentioned, the Vadis 212 operates in conjunction with a control surface, for example the Vadis DCII. (It should be noted in this case that the communications between the DCII and the Vadis 212 is via a proprietary digital interface.)

Logitek is a long-term player in the audio networking game. Its Audio Engine is a rack-mount main frame that offers the capability of direct connection to multiple control surfaces, and other Audio Engines as well, via fiber. In addition, the Audio Engine is a full X-Y router, and acts as the heart of the audio network. Under commands from the control surface, it performs all the normal console functions, such as mixing, channel on/off and cue. A fully configured Audio Engine can handle as many as 128 mono (64 stereo) inputs and outputs by way of plug-in analog or digital I/O cards. Control, programming and analysis can all be done via a TCP/IP connection.

Sierra Automated Systems offers the 32KD — its digital audio router that performs the requisite routing functions, and others as well, such as mixing, level control, intercom, IFB and mix minus. A single 32KD frame can accommodate 512 inputs and outputs, and multiple frames can be connected together via a fiber-optic link. Separate plug-in modules handle digital inputs, digital outputs, analog inputs, analog outputs and serial interfaces.

Studer offers the Route 5000 digital router system. The core of the 5000 is the mainframe into which the multichannel audio digital interface (MADI) cards are installed. Each MADI input or output card accommodates 28 AES data streams. In addition to the normal routing functions the 5000 offers DSP capability. It communicates with the PC that is the control server via a fiber-optic link. Other control workstations or XY controllers communicate with the control server via Ethernet. The 5000 can communicate with a Studer digital console via a MADI link.

Wheatstone offers the Bridge, an audio engine and routing system that works in conjunction with control surfaces remotely located in studios. The Bridge is a mainframe into which all the necessary cards are installed: analog and digital I/O cards, DSP cards, serial data cards and even one that supports 16 audio streams. Multiple Bridges can be connected via fiber or CAT-5 cables.



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