13th June 2000 - Sound System Control Networks

Philip Nye gave a well attended June meeting, an insight into the rather unfamiliar world of sound system control networks. Apart from having his own company working in this field, Philip is also Vice-Chairman of the AES Standards Sub-committee SC-10 on Sound System Control and a member of the Advanced Control Network (ACN) task group of the Entertainment Services Technology Association (ESTA).

Control networks are used anywhere that controllable, generation and processing elements are dispersed over an area; where control is integrated with other functions such as lighting, environmental control, pyrotechnics, etc.; and where multiple systems need to be synchronised or integrated under supervisory control. Examples given by Philip included conference centres, auditorium complexes, 'smart' buildings, home entertainment systems, theatres, theme parks and cruise ships. He said theme parks were very demanding in their requirements and as a result they were having a major influence on developments.

Audio control systems are similar to other control systems, however they tend to have fewer nodes compared with say lighting but the nodes are more complex. The timing requirements for audio control may be very tight and the metering necessary can add special requirements and result in floods of data on the network.

Philip then talked about the standards used for control, pointing out that the audio industry was less interested in open standards compared to other industries. Apart from the AES15/PA422 and the IEEE 1394 AV/C standards, proprietary standards still dominate audio control. Examples of such proprietary systems included, QSControl, MIDI, Crown IQ, Nexsys, Medialink, Cobranet/SNMP, Lonworks, Cebus, e-show control and GMICS. These tend to have a vertical structure and define everything from the connector and cable to the protocol.

After a brief description of layering in relation to control networks, Philip outlined the benefits and shortcomings of SNMP (Simple Network Management Protocol). Although SNMP has the advantage of being a proven technology, which has achieved widespread acceptance and is supported with readily available software, there are a number of disadvantages for its use in control systems. There is no provision for multicast or broadcast messaging and it is inefficient for rapid exchange of small get/set operations. SNMP has no control of real time performance or quality of service and in network management the reading of variables dominates the network traffic whereas in most control applications writing is dominant. Additionally all responses, error detection and timeouts are at the application level.

In order to have a control network which meets the needs of the entertainment industry, ESTA embarked four years ago on the development of a standardised control system. Under the working title of ACN (Advanced Control Network), the development is funded and supported by many companies in entertainment lighting and several major mainstream networking companies. PLASA (Professional Lighting and Sound Association) in the UK is also supporting the development of ACN and more recently liaison with the AES has been established. ACN is an enabling technology which will allow manufacturers to incorporate features into their products which were hitherto impossible. It is a protocol-defining vocabulary and syntax which equipment will use to communicate. ACN does not define the physical layers or even the lower transport layers of the network as it is the intention to use existing open standards such as Ethernet, ATM, Sonet, FDDI, etc. In the first instance TCP/IP (Internet Protocol) will be used. The user interface is also not part of ACN and is the responsibility of the equipment designer. The benefits of ACN for the end user are 'plug and play' setup and configuration, optimum control for each device, no arbitrary limits on system configuration, scalability, status monitoring and fault tolerance and recovery. As far as the end user is concerned all the complexity will be hidden behind the user interface.

ACN is a protocol suite consisting of a device description language (DDL), device management protocol (DMP), session data transport ( SDT) and network management protocol (NMP). The SDT sits on top of UDP/IP and manages reliable multicast messaging, on/off-line status, latency and flow control. Above the SDT and NMP are the ACN application protocols but at present the only one being developed is Device Management Protocol (DMP) for direct control of dimmers, automated lighting, audio systems, etc. The DMP provides powerful discovery and auto-configuration with plug and play inter-operability. Devices advertise themselves and for auto-configuration every device carries device description which a controller can retrieve and interpret to 'understand' the device. There can be multiple controllers on one network and efficient use of the network bandwidth is achieved by packaging multiple small data items into messages.

The ACN device description language uses XML (Extensible Markup Language), which is a mainstream internet technology and can be read by machines or humans as it has a text format. The descriptions can be stored in ROM as they are static or alternatively they can be retrieved from the device, imported separately from floppy disc or even downloaded from a manufacturers web site. Philip pointed out that any process interpreting a description does not have to understand the complete description but only as much or as little as its designer chooses.

The development of ACN is now well advanced and Philip said it was the intention to have a reference implementation available with the standard to enable designers to check their systems.

Philip briefly looked at component technologies such as JAVA, CORBA, JINI, etc., before looking at a few pointers to the future. He noted that Intel have assigned $200,000,000 to encourage people to design in their new family of network processors and network processor chips aimed at appliances which include TCP/IP software, are appearing all the time from a range of manufacturers. With the first Web-enabled washing machine having been launched a few months ago Philip concluded that it was now possible to shrink your clothes, turn your underwear grey and flood your kitchen from across the world using your WAP phone!

John Nunn