Issue link: https://www.e-digitaleditions.com/i/1255991
22 I ICT TODAY FIGURE 6: In a typical analog control signal application, variable dc voltage from a temperature sensor triggers a control processor to turn on a fan at a programmed temperature threshold. Relay Contact Closure: screens, projector lifts, switchers, power controllers, lighting controllers Typically, an electromechanical relay in the control processor performs a single pole single throw (SPST) switching action via a two-wire cable. When the relay contacts close, they complete a low-voltage circuit in the AV device being controlled to trigger a designated action. No data is passed over this interface, and there is no status feedback. Digital Inputs/Outputs: motion sensors Transported over separate two-conductor input and output cables, bi-level voltages representing digital logic high or low conditions initiate control actions or indicate status for the AV device being controlled. Voltage thresholds representing high and low may be fixed or programmable, depending on control- ler/control processor capabilities. Analog Inputs: temperature sensors, limit switches, door sensors Analog inputs can monitor dc voltage that is contin- uously variable within a specified range, typically 0 to 24 volts. Using an A/D converter, the controller creates digital values representing the analog input. The control software uses these digital values to interpret physical conditions into control actions. For example, if temperature rises to a programmed threshold, the controller can turn-on a cooling fan via a relay contact closure (Figure 6). Types of AV Control Signals and Where They are Commonly Used Audiovisual systems seldom have all components sourced from the same manufacturer. Each component has a unique set of control and status signals, as well as control interface protocols. Consider the following list of interfaces that AV control systems use to commu- nicate with system components: Serial RS-232/422/485: video projectors, flat panel displays, DVD, video teleconferencing (VTC) codec, cameras, switchers, digital signal processors (DSPs), lighting controllers Wired bidirectional serial data interface via unshielded twisted-pair cable. The AV control/status message format is manufacturer specific using soft- ware drivers. RS-232 is single-ended and reliable up to 50 feet (15 m). RS-422 and RS-485 are differential and can go up to 4,000 feet. (1,200 m). RS-232 and RS-422 allow communication between two endpoints, while RS-485 allows party-line communication for up to 32 devices. IR/Serial: video projectors, flat panel displays, DVD, VCR, VTC codec, cameras, switchers Wireless one-way line-of-sight interface uses an infra- red light beam to send commands to devices. The AV control message format is manufacturer specific using software drivers. Ethernet: video projectors, VTC codec, switchers, DSPs, control processors, controllers Bidirectional serial data interface over a wired or wire- less TCP/IP network. Audiovisual control/status mes- sage format is manufacturer specific using software drivers and is transported via packet switching. Ether- net is fault-tolerant, interoperable, scalable, and part of the infrastructure already in place at virtually every enterprise where AV systems are used. Audiovisual devices can share the existing IT network or can be placed on a dedicated AV Ethernet network.