LOUD & CLEAR
Nov 1, 2001 12:00 PM,
By Gary Parks
TWO TYPES OF PRODUCTION INTERCOMS, party-line and digital matrix,are used to coordinate events, performances, news broadcasts and otherapplications. These two systems are quite different, each with its ownapplications and advantages. Additionally, wireless intercoms andinterrupted foldback systems (talent cueing) are often used inconjunction with wired intercoms, and most applications requireinterfaces to connect the intercom with telephones, radios, and otheraudio and communications systems.
PARTY-LINE INTERCOMS
A basic party-line system (see Figure 1) consists of beltpackor rack-mount intercom stations and a system power supply, with thecomponents connected by 2-conductor, shielded microphone cable. Userswear headsets or talk through a gooseneck mic plugged into a station.All of the users on a channel — those connected directly witheach other by microphone cable — can talk and listen to eachother as a group. The name party-line came from the originaltelephone systems where everyone shared the same line and could hearall conversations at once.
Some stations in party-line systems have buttons that allow theusers to talk and listen on different channels, separately or at thesame time. These multichannel stations are usually used by directorsand producers. In a performance event, one channel might go to thefollow-spot/lighting operators, another to the props crew, another tobackstage workers, and yet another to the audio staff.
The intercom stations are powered by a DC electrical current flowingthrough the cabling. Each station contains a microphone pre-amplifierand a power amplifier for the headset or speaker. The two-way audiosignal flows through the cable as well, and the circuitry in thestations allows each user to hear everything on the channel except forhis own voice. (A slight amount of the user’s voice, calledsidetone, is retained so the user knows the unit is working).These systems typically can create a visual signal at the station, toshow that someone wants to talk with the operator.
Wireless intercom systems are a variation on party-line systems.Rather than connecting the beltpacks together with wires, each wirelessbeltpack has both a receiver and a transmitter circuit board. A basestation receives and mixes the talk signals from each of the beltpackusers and transmits the mixed signal back to the beltpacks using acommon frequency. The systems may be used alone or may be linked to awired party-line system or a digital matrix system.
Applications and Advantages
The advantages of party-line systems include relatively easy setup,simple operation, use of common cabling (standard microphone cables),portability, and clear and reliable communications.
Because the typical range of demands on intercom systems isn’t thatgreat, a conventional party-line system is often sufficient. Most usersonly converse on one or two channels, receiving cues from the directorsand talking within their own group. The directors talk with the variousgroups (channels) individually or all together, and there may be anadditional channel for directors and producers only.
Party-line intercoms are standard equipment at performingfacilities, allowing communications among the lighting andstage-production staff. Touring sound and lighting companies will carrythese systems with them for concerts and events. Locations such ascommunity theaters, school and college auditoriums, sporting and racingvenues also use party-line intercom systems.
Smaller TV broadcast facilities and mobile production vehicles,especially those that produce new programs, will have 2-channel or4-channel party-line systems to communicate among camera operators,audio and video technicians, outside broadcast staff, facilitytechnicians and the talent. As the communications needs of thesefacilities become greater and more complex, a point-to-point matrixintercom system is considered.
DIGITAL MATRIX SYSTEMS
Matrix intercom systems consist of central communications framesthrough which all communications and data are routed: multikey intercomstations that allow many channels of communication and interfaces toother systems such as telephones, two-way radios, a party-lineintercom, audio and IFB/cue systems, microwave, and ISDN links andrelays to control other devices. These systems are fully programmableso that any type of point-to-point, group or party-line communicationcan be created, and any or all of these interfaces can be accessed byeach user station.
The central communications frame contains a number of circuit-boardcards that process and route audio signals and control data coming fromthe individual stations and interfaces. Each station and interfaceconnects to the frame via a port, such as an RJ-45 connector, withpaths for audio signals and control data. A typical configuration mightbe nine 8-port circuit-board cards for a total of 72.
The stations themselves typically offer between 4 and 32 keys each,and are often expandable. No matter how many individual keys are used,the stations connect to the central communications frame via a single4-pair cable for analog transmission or single-pair or coaxial cablefor digital transmission.
Digital matrix systems also allow the voice levels of the individualkeys to be controlled at the stations, giving each user his or her ownaudio mix.
Applications and Advantages
Digital matrix intercom systems are ideal for complex communicationsapplications. They allow many channels and types of communication to becombined into a single, integrated system. Because these systems areflexible and programmable, they can be adapted to work in a variety ofsituations, from a television news studio to a large performance venueto controlling a rocket launch. Real-time signal routing changes can bemade without moving any cables.
Sophisticated interfacing capabilities make digital matrix intercomsystems the choice for communicating among distant facilities, and theymake it easy for distant users on a telephone, cell phone or two-wayradio to participate in the conversation. Digital matrix systems canroute multiple sources of program audio, create as many cue channels asdesired, and control external devices such as lights and monitor levelsvia relays.
Digital matrix intercom systems are the choice of most televisionbroadcast studios, especially those with active news-reportingoperations. The larger mobile production vehicles invariably carrymatrix systems. Cable and satellite broadcasters use them to coordinatetheir multiple programs. Large performing arts centers, especially inEurope, interface a matrix system for directors with party-linebeltpacks and headsets for the lighting, audio and technical staff.Government and aerospace facilities choose digital matrix intercoms fortraining and simulation laboratories, video production facilities, andlaunch and control operations.
WIRELESS INTERCOM SYSTEMS
Wireless intercom systems consist of full-duplex beltpacks (able toconverse two ways) and a base station (the common link among them and aconnection nexus to wired intercom systems or other audio systems).Standard intercom headsets are typically used with wireless beltpacks.These systems generally operate on VHF or UHF frequencies in thetelevision bands, just like wireless microphones.
In most applications, the wireless intercom is an extension of thewired system, used by those staff members who require mobility forsafety or convenience. Because it is a complex technology that’sdifficult to manufacture well, wireless intercom systems are oftenconsiderably more expensive per user than wired systems, even comparedto digital matrix intercom systems. Entry-level units cost about fivetimes more per user than do wired beltpacks.
So that users can talk together at the same time, each beltpack mustoperate on its own unique frequency — again, just like wirelessmicrophones. Each beltpack has a receiver in the base station tuned toits transmit frequency, and the signals from the beltpacks are adjustedin level and placed on the audio bus of the base station. The mix ofall of the beltpack conversation, plus that of any connected wiredintercom or program source, is then re-transmitted from the basestation to receivers in all of the beltpacks — completing theloop. A six-up system with a base station and six beltpacks usesseven different frequencies.
Each wireless beltpack requires a transmitter and a receiver workingtogether closely, plus audio circuitry, a mic preamp and a headphoneamp. This fact makes them inherently more complex than wired beltpacks,and more difficult to manufacture.
Considerations
Before adding wireless intercom systems to an installation, theinstaller must determine what frequencies are being used in thevicinity. First, all VHF and UHF television channels in the area mustbe noted. Analog television channels have their picture, color andaudio carriers and subcarriers on certain frequencies within a 6MHzband, and some areas within the band will be free of signals and can beused for wireless communication. Digital television signals have energyspread throughout the spectrum, so you must avoid putting anythingwithin those channels, or your wireless intercom or microphone userswill experience noise interference.
The next thing to consider is which other wireless devices are usedin the installation: microphones, two-way radios, walkie-talkies and soon. Getting a complete picture of the radio frequency environment isthe first step in frequency coordination so that the best and mostinterference-free frequencies may be chosen. This is typically done bythe manufacturer using a computer program that calculates all potentialinteractions among the existing frequencies and the new ones that thewireless intercom will introduce. This process is essential when usingcrystal-based, single-frequency wireless intercom systems ormicrophones.
Frequency-Agile Wireless Intercoms
Things are getting easier: This year the first frequency-agilewireless intercom systems have been introduced, allowing the installeror the user to select different frequencies over a much wider band— typically equivalent to several U.S. television channels— with the same device. This agility allows the user to move awayfrom interfering frequencies at the touch of a button or with acomputer program, and can reduce pre-planning. However, remember thatchanging a frequency will also change the RF environment and canpotentially cause interference effects elsewhere in the system.
The great advantage of wireless intercom versus other two-way radiotechnologies is that it is full duplex, so that users can maintain atrue conversation, hands-free instead of push-to-talk. Also, they aredesigned to work in conjunction with wired intercom systems with theproper interfaces and connections to do so.
Wireless IFB Systems
A variant on wireless intercoms is the IFB, or cue system. This typeof system is used for listen-only applications such as monitoring anongoing program, cueing talent, and where the return path for theperson’s voice is a microphone (e.g., an on-camera television announcerreceiving cues from a director).
The audio from an intercom line, a program source or both is inputinto a transmitter, typically housed in a 1RU or half-rack enclosure.One or more beltpack receivers with headphones or ear buds are tuned toreceive the signal from the transmitter. Some of these beltpacks havetwo receivers on different frequencies, so that the user can switchbetween two different audio sources.
Wired IFB systems are also commonly used as additions to wiredintercom systems. These units can have an additional feature thatallows the incoming program material to be interrupted or ducked when avoice cue is introduced.
INTERCOM INTERFACING
Intercom systems are rarely used in isolation; they are thecommunications and coordination systems behind a production that oftenincludes other audio and video systems. In larger events, theproduction communications must include people who are elsewhere withina facility or in a distant location. So devices must be used to matchthe audio levels, impedance and operating characteristics of thesedifferent communications means: telephones, ISDN, two-way radio andwalkie-talkie, paging systems, audio consoles, camera intercoms,etc.
Also, not all intercom systems and brands are compatible, yet insome settings they must communicate together. At least three differentparty-line standards exist, with variation in levels, pin-outs andcall-signal methods. All of the digital intercom systems have their ownproprietary communications protocol. And often party-line and digitalmatrix systems are used together, requiring further interfacing.
Putting It All Together
Though it is often an afterthought in an audio and/or videofacility’s design, the intercom communications system is the“glue” that holds the production together. The elements ofparty-line, digital matrix, wireless intercom and interfacing can bemixed and matched to create a communications system totally customizedto the user. It can be as simple as a couple of beltpacks and headsetsfor the community theater or sophisticated enough to run the largestperforming arts facility, broadcast complex or Olympics ceremony.
Gary Parks is marketing manager and wireless products manager forClear-Com Intercom Systems of Emeryville, California. Thanks to BobTourkow, Clear-Com senior applications engineer, for additionalinformation.
PLAYING WELL WITH OTHERS
Interfacing with Telephones &Two-Way Radio
- TelephonesNearly every production must connect the intercom system to one ormore telephone circuits, providing either a one-way or two-way audioconnection between the system and the circuit. This is especiallylikely with larger, broadcast-oriented productions, which are moreprone to use digital matrix intercoms. Interfaces are available bothfrom intercom manufacturers and from independent manufacturers, foreither analog 2-wire or ISDN digital circuits. The connection totelephone circuits provides three primary functions:Almost all telephone interfaces can provide the correct audioconnection to the intercom, but not all can send and receive thecorrect control signals required by digital matrix systems to enableall of these functions. To fully realize the capabilities availablethrough telephone line-to-matrix connections, the telephone interfacemust be able to send and receive control signals to and from thematrix. This is most easily done by using an interface specificallycreated by the intercom system manufacturer.Analog telephone interfaces can be effective in older telephonesystems because of the limited audio bandwidth (300 Hz to 3.5 kHz) andthe low audio levels needed by a telephone handset. They are alsotypically used with party-line intercom systems because the controlsignals are not part of the communications protocol. But modern digitalintercoms provide full-bandwidth audio (at least 100 Hz to 12 kHz).While there is still an audio bandwidth limitation in standard analogtelephone circuits, digital technology is continuously increasing theaudio quality of phone circuits.The increased audio bandwidth makes designing telephone interfacesmore difficult, and superior telephone interfaces all use digitalsignal processing controlled by sophisticated algorithms to provide themaximum null on the four-wire circuit. Telephone interfaces that useDSP are known as auto-nulling.
- Allowing a station or stations to answer incoming calls and initiateoutgoing calls, as with a standard telephone.
- Permitting a remote location to dial into the system andautomatically establish a preprogrammed IFB feed
- Allowing a remote location, or any other telephone, to dial into thesystem to communicate with specific stations or choose betweendifferent IFB feeds.
- Two-Way RadioThe seamless connection between an intercom and a two-way radiosystem is one way different communications systems can be integratedfor increased flexibility. This can be a difficult interfacingchallenge because many manufacturers build two-way radio equipment, andno standard exists for audio input/output levels, nor for the specificmethod of push-to-talk transmitter keying. The most effective radiointerfaces are those that provide complete electrical isolation betweenthe intercom system and the radio system. They also should not requirethat any additional circuitry be wired between the interface and theradio.The desirable features in a radio interface are easy send andreceive level adjustment; both line-level and microphone-level output(enabling direct connection into a microphone input on the radiowithout requiring external attenuation circuits); relay contacts fortransmitter keying; and control/signal input. Very few radio interfaceshave all of these desirable capabilities, so when considering whatinterface to use, check the specifications carefully.
