How can I attach two powered Two-Wire systems together?: The transformer in Figure 1 is useful if you need to strip DC voltage off of a wet two-wire system. After proper installation one would be left only with audio on the other side of the transformer. This allows one to hook the line up to another system with its own power supply, or to attach the system to another device that would sensitive to voltage on the line.
Figure 1
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How can I get only audio from a powered Two-Wire line?: The transformer in Figure 1 is useful if you need to strip DC voltage off of a wet two-wire system. After proper installation one would be left only with audio on the other side of the transformer. This allows one to hook the line up to another system with its own power supply, or to attach the system to another device that would sensitive to voltage on the line.
Figure 1
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System Components & Their Function: The system components for most party-line intercoms consist of power supplies (or master stations), user stations (e.g. beltpacks, speaker stations, main stations, etc.), interconnecting cable, headsets, panel microphones, push-to-talk microphones, and a system termination.
The power supply (which is normally centralized) generates the DC power for the entire system (with the exception of self powered user stations). The power supply usually includes system termination for the audio channel, 200Ω for RTS® and Clear-Com®, and 300Ω for Audiocom®. This may be as simple as a capacitor and resistor in a series, or, an electronic termination, which is integrated into the power supply voltage regulator.
The user station connects to the power supply and intercom line. The human user connects to the user station via a headset or loudspeaker and microphone or some combination. For a given channel or channels, the user stations are connected to each other in parallel.
The interconnecting cable for most intercoms is standard microphone cable with three pin XLR type connectors. The female XLR connects towards the power supply and the male XLR plugs into the user station. This polarity was chosen to prevent putting DC power onto audio microphones which also use this type cable. There are at least two exceptions to the use of microphone cable: the RTS® TW master stations connect audio with unshielded pairs (12 of the 25 pair in a cable). Another exception is a twisted pair is the only connection between two points. The RTS® TW user stations can connect directly to a twisted pair, while other user stations need adapters of one kind or another, and power may have to be supplied at either end.
The wired systems are of three wiring configurations:
• Separate power, audio, and return conductors (example: Clear-Com®),
• An audio pair which includes phantom power and a common (example: Audiocom®), and
• A conductor that contains one channel and power, a conductor that contains audio with- or without power, and a return (example: RTS® TW intercom system).
The wireless systems usually include an interface to the wired systems. Principal manufacturers include Telex Communications®, Vega® (now part of Clear-Com®), and HME.
Wired intercoms are mostly of the distributed amplifier kind. The distributed amplifier is built into a user station. User stations come in various packages and are of three kinds: headset, speaker-microphone, or both. The various packages include a beltpack (worn on the users belt, and of the headset kind), console mount (headset or speaker-microphone), rackmount (headset or speaker-microphone), deskmount (portable speaker station), wall mount (headset or speaker-microphone), and console/rackmount master station/main station. The distributed amplifier concept allows each user to adjust his/hers own listening level. The user station also includes a microphone amplifier, a line amplifier/buffer, volume control(s), talk switch(es). Some user stations also may have a call light, status indicators, and a channel selector. The microphone may be in the headset, fastened to or plugged into a speaker station, in a handset, or in a push-to-talk hand held unit.
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Beltpack Headset User Station Functional Description: A typical single channel beltpack headset user station has the following connectors:
Intercom Line (XLR-3) and a Headset Connector (XLR-4).
The station has the following controls:
Microphone ON/OFF (sometimes called a TALK switch), and a headset Volume Control.
It may also have a call lamp and a call lamp send button. Examples of this station are an RTS® BP-318 single channel beltpack, or an Audiocom® BP-1002, or a Clear-Com® RS-501.
A typical two-channel headset beltpack user station adds a channel selector switch to the above. Examples RTS® BP-351, Clear-Com® RS-502, Audiocom® BP-2002
Alternately, newer units have two talk buttons, two volume controls, and two status indicators to tell which talk button is engaged. Examples: RTS® BP-325, BP-351, Clear-Com® RS-522-TW, or Audiocom® IC-2B.
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Speaker User Station Functional Description: A typical speaker station can function with either a headset or a speaker/microphone. A power amplifier, a speaker, and a speaker on/off switch are added to the electronics of a beltpack. In addition, a nulling adjustment is easily accessible. The nulling adjustment allows for full duplex operation without unwanted feedback. Also added is a connection or jack for either a panel microphone (rackmount stations) or a push to talk microphone (for deskmount or portable speaker stations).
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Master Stations: The master station allows a user to access multiple channels. This allows different crews to be monitored, cued or updated. If the master station is used for training, again, different crews may be monitored and guided. These master stations have extra features for special tasks such as IFB (Interrupted Feedback) or SA (Stage Announce), relay closures, “hot” microphones, and microphone kill. Master stations can send and receive call light signals on any channel. Two examples of the master station are Clear-Com® Model 912 (12-channel) and RTS® Model 803 (12-channel). Audiocom®’s master station is modular and can be as few as two channels or as many as 22 channels. Master stations allow simultaneous monitoring of any channel, any combination of channels, or all the channels. They can call or “mic kill” on any given channel. In addition, some master stations can monitor a program source.
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Technical Notes About the Stations Mentioned on this Page: The stations mentioned on this page generally are designed for the dynamic microphones in the headsets to have an impedance of about 150 to 500Ω. The speaker station panel electret microphones are designed to have an impedance of 1000 to 2000Ω and require 1 to 5 volts excitation. In addition, the push-to-talk microphones have around 500Ω. This means the actual input impedance of the station microphone preamplifier will range from 470Ω to 5000Ω. The low impedance of 470Ω minimizes the crosstalk in the headset cord. The headphone impedances expected range from 50Ω to 1000Ω.
The 50Ω headphones along with suitable headphone amplifiers provide enough SPL (Sound Pressure Level) to overcome the interference from loud concerts and sports events.
The headphones also need to have an acoustic isolation of 20dB or more to protect the user. These stations generally have bridging impedance across the intercom line of 10,000 to 15,000Ω. A bridging impedance of 10,000Ω assures that up to 50 stations can be plugged into the systems and the level drop will only be 6dB. The level drop of 6dB corresponds to the level drop when an extension telephone is picked up on an existing conversation-noticeable but the telephone is still usable.
Wiring Notes
• Clear-Com® and Audiocom® two channel stations have 6 pin XLR connectors to connect to the intercom line. Clear-Com® also offers the Clear-Com® RS-522-TW, which has two channels on a three pin XLR.
• Clear-Com® and Audiocom® systems use a female 4 or 5 pin XLR connector on their headsets and a male 4 or 5 pin XLR connector on their user stations. However, RTS® uses a male 4 or 5 pin XLR connector on their headsets and a female 4 or 5 pin XLR connector on their user stations.
• In any system, pin 1 and the shell of the XLR connector should NOT be connected together.
• The pin out of the headset connectors is as follows:
Four pin XLR
Pin 1 - Microphone common
Pin 2 - Microphone “hot”
Pin 3 - Headphone common
Pin 4 - Headphone “hot”
Five pin XLR
Pin 1 - Microphone common
Pin 2 - Microphone “hot”
Pin 3 - Headphone common
Pin 4 - Left Headphone “hot”
Pin 5 - Right Headphone “hot”
• Since the power supply has a limited amount of XLR-3 connectors, splitter boxes are used to expand the system. These boxes have all the connectors wired in parallel.
• Some user stations have “loop-thru” connectors that allow “daisy chaining” stations using a single connection to the power supply.
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How Each System Works: First, please note that although these systems are full duplex and everybody could theoretically talk at once, this is not at all practical or desirable. The usual operation is the director or lead person has their microphone enabled all the time, while all other microphones are switched off. These microphones are switched on only long enough to supply an answer, make a request, or give data. In some cases, especially in noisy environments, all microphones are off and only switched on as required. Because the Party-Line concept has so many signal sources, this operational protocol is the only way the party line can be effective. In addition, this is the reason for the system “mic kill” (microphone turn-off) capability, for the situation where a station is unmanned but has its microphone enabled.
These systems use voltage controlled current sources (or similar electronics) to apply a signal to the intercom line. All the signals applied are summed and converted to a voltage at the single termination resistor or electronic impedance. The current sources (or similar circuits) have output impedances of 10,000Ω or greater. The loading effect of the station on the intercom, say in a 200Ω-terminated system is, worst case, 10,000Ω in parallel with 200Ω. This results in a change of the system termination to 196Ω, a 2 percent change. This, in turn, causes a voltage change of 2 percent or 0.175dB, an imperceptible change. It takes 20 stations across the line to cause a 3dB change, a perceptible but not significant change. The volume controls in the user stations easily adjust for this change. In the “not so” worst-case situation, these systems can work with up to 75 stations, provided enough DC power is available. The work-around in this case, in the RTS® TW system, is a switch on the power supply, which doubles the system impedance. Then, two power supplies can divide the DC load and are coupled together with capacitors to end up with the 200Ω termination and twice the user stations. In the case of Clear-Com®, the system termination is not electronic but a passive resistor. If an adapter is made, the same trick can be done in a Clear-Com® system power supply. In the case of Audiocom® intercoms, paralleling two power supplies with capacitors would result in an impedance of 150Ω which could still be usable in some instances.
Figure 1
Figure 2
Figure 3
Figure 4
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System Powering: Systems can be centrally powered with a power supply or they may be individually powered with “local power” modules, also known as built-in power supplies. The systems can also be a mixture of central and local power. In the cases of Audiocom® systems and RTS® TW systems, the power and signal share the same wire(s). This means, for those two systems, the power supplies DC source must be ultra low noise/quiet, circa -70dBu or better. Most systems can work using main powers of 120 or 240 volts AC. Some individual stations can be powered with two or three nine-volt batteries in series. Venues such as the Rose Parade may have to use a pair of batteries from the telephone company just to cross the street. Since this may involve a mile of copper wire, there is no central DC source that is going to make it. Out come the nine-volt batteries! The RTS® TW power supplies can tolerate only a 5-volt peak-to-peak signal on the powered line. In this system, each station can generate a maximum 2 volt peak to peak signal, so two stations talking simultaneously can add up to 4 volts peak to peak. Therefore, there is just 1 volt of headroom. Clear-Com® specifies a range of signal levels of .5 v p-p to a maximum of 4v p-p, but does not specify the reference (it is probably dBu or dBv). Audiocom® intercoms specify only a nominal level of 1 volt RMS, which is equivalent to three v p-p.
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Headset User Stations: The microphone preamplifier has a maximum gain in the neighborhood of 53 dB. Many stations have Automatic Gain Control (AGC), which adjusts the gain according to the incoming microphone signal. Some stations also have a limiter that prevents overloading the intercom line. An electronic switch is placed between the microphone preamplifier and the current source (line driver). This substantially reduces noise on the intercom line. A hybrid connection is necessary to sort out the talk and listen signals (a two wire to four wire converter would work best). The listen signal goes from the hybrid to the listen volume control. The listen volume control drives the headphone amplifier that has a gain in the range of 30 to 40 dB. For a 50Ω headset, the headphone amplifier produces maximum peak sound pressure levels of around 105dB. This is the level needed at concerts and sporting events (along with 20dB acoustic isolation of the headset). In less strenuous situations, a handset instead of a headset may be used with these stations. These stations must have a bridging impedance of 10,000Ω or higher. The current drains range from 30 to 65 milliamperes. Most systems have signal levels that range from -15dBu to 0dBu. In the case of Clear-Com® and RTS® TW systems, the AGC / limiters in the microphone preamplifier tend to keep the level in the -10dB range. This enhances intelligibility and compensates for differences between voices and headset microphones. Usually the headset amplifier has enough gain to make up the differences (by readjusting the volume control).
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Speaker User Stations: Most of these stations can operate in both a speaker/microphone mode and a headset mode. The difference between a headset only station and the speaker station is that a speaker amplifier, switching electronics, and a null pot are added. Usually the portable speaker stations use a push-to-talk microphone, whereas the fixed speaker stations use a panel or gooseneck microphone. The stations that have microphone and speaker on the same panel have less available speaker level because of feedback. The push-to-talk microphone has much better isolation. Speaker stations often have “dimming” or “ducking” which attenuate the speaker output when the microphone is keyed. This allows more gain and less feedback. Speaker stations use a very substantial amount of current, about 120 milliamperes. Therefore, fixed speaker stations are ideally operated with local power, to prevent overloading the central power supply. Some RTS® TW are direct AC powered and do not use central power.
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Cabling: Usually the intercom system’s specifications are based on the use of 22 AWG microphone cable. Microphone cable of 22 gage measures 3Ω per 100 feet or about 30Ω per 1000 feet (round trip resistance). The wire table says 32Ω per 1000 feet round trip, but the shield resistance is much lower than the wire resistance. The Audiocom® system uses both wires and the shield to transport DC so the calculations will be different for DC voltage drop versus distance.
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Outstanding Features of Each System: The Audiocom® system is immune to noise and is a lower cost system. It is used in difficult environments, i.e.: churches, concerts, theaters, and sporting events.
The Clear-Com® system is robust, relatively lower in cost, and rental systems are readily available. It is often used for concerts, rock-n-roll tours, and in theaters. It is also used in remote trucks, uplink trucks, and low budget venues.
The RTS® system is also very robust, reasonable in cost, and rental systems are readily available in most countries worldwide. Because the RTS® intercom has two channels per microphone cable, it is used where many channels are required, such as the Oscar and Emmy award shows. It is also used for events such as the Superbowl. Larger TV trucks carry both a four-wire system and an RTS® Party-Line system. These systems are interfaced together so the four-wire is used inside the truck and the RTS® system is used outside the truck.
In addition to these features, most systems support extra features such as, “microphone kill” and “call light”. The microphone kill feature allows all microphones in a given channel to be switched off. In the case of Audiocom® and RTS®, the signal is an inaudible 24 kilohertz. In the case of Clear-Com®, the power is interrupted for a long enough time to reset the microphones to off.
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Call Lights: The Call Light Signal allows user stations to generate and display a visual signal for attention-getting and cueing purposes. The flashing light of the RTS® and Audiocom® systems alerts the crew to put their headsets back on. The steady light of the Clear-Com® system can also be used for this purpose, however, it has another purpose: when the director holds the call light on, this is a standby signal. When the light goes off, this is the execute signal (raise/lower the scenery, follow spot on, et cetera). Call signals can also be used to key 2-way radios, sound alarms, and activate lighting controls. Audiocom® and RTS® systems use an inaudible 20 kilohertz signal for the call signal; Clear-Com® systems use a DC voltage added to the audio signal. Telex manufactures a call signal detector / display (Model CIA-1000) which provides both a high visibility light and a relay closure when a call signal is sent. The CIA-1000 works with RTS® TW and Audiocom® systems. Clear-Com® and other manufacturers provide similar products. The company VMA supplies a bright strobe lamp that is triggered by the RTS® system call signal. This strobe is powered from the RTS® line but only draws 10 milliamperes. It also supplies a relay closure and a logic signal.
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Limitations of Each System: Cable capacitance, resistance, and crosstalk affect all three systems. The longer cables (over 2000 feet) limit the number of beltpacks at the end. A system with cumulative cables adding up to 10,000 feet will have a reduction in frequency response due to cable capacitance. Both resistance and capacitance affect crosstalk. If all you have is a twisted-pair cable, then the RTS® system is most useful. If you have severe coupling with power cables, the Audiocom® system will help. Some of the information in this chapter is repeated in the next chapter, but in a different context.
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Defining And Meeting Your Needs: Your needs could include buying, renting, assembling or expanding a system. Application Block Diagrams are a good starting place to define a system. In this section, block diagrams of applications in each of the three leading systems will be shown and discussed. These diagrams will range from relatively simple to complex systems. One of these block diagrams could be close to what you need to know, give or take a station or so. If you make a copy of the diagram and mark it up, this could define your system.
Disclaimer
The block diagrams are for instructional purposes, and though every effort has been for accuracy, the manufacturers offerings are often changing. It pays to double check with the manufacturer or rental house to verify the exact system available before buying or renting.
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Generic Single Channel Systems: The first applications are generic single channel systems, see Figure 1. They consist of a power supply, beltpacks, headsets, splitter boxes, and microphone cables. These systems could be used in a small television studio production, a small outside television field production, or an industrial test of a large system. Depending on the detail of the block diagram, you may be able to compile an equipment list from this diagram.
Figure 1
Figure 2
Figure 3
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Two-Channel System: TV, School, Cable: The second application is a two-channel system for a small TV operation (Studio or Truck), school or cable access. The Audiocom® and Clear-Com® systems will require two 3-conductor microphone cables between director, switcher, video, and graphics. The RTS® TW system only requires a single microphone cable for all hook-ups.
Figure 1
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Theater System: The third application is a theater application. See Figure 5. A two-channel system is used in this application. Channel A connects the crew together and channel B is used by the stage manager to cue the actors. This is done using three-wall mount or portable speaker stations. For all three systems, only standard microphone cable is required. In the case of the RTS® TW system, Channel B is available to the crew, but except for rehearsals or setup, they would stay on Channel A.
Figure 1
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Training Systems: The training system consists of an instructor and multiple two-student crews.
In the case of Audiocom®, each of the six two-student groups are independently addressable by the instructor. When the student groups are not talking to the instructor, each two-student group can have semi-private conversations. The call light tells the instructor which group is paging. The balanced Audiocom® system is ideal in hostile electrical noise environments.
It just happens that the Clear-Com® system is the simplest for this application, since the master station, MS-812A has the three pin XLR connectors for 12 channels on the rear panel. The MS-812 has several configurations, and will have to be specified for this application (No IFB, 12 Clear-Com® standard PL channels).
The RTS® TW system for this application is the next simplest, and has added features. The student crews can have completely private conversations, yet are still reachable via the call light paging system. Each BP-325 beltpack can be configured to accept an individual program source (but the loop-through is lost and the two students line connection will be through a simple one to two splitter). The program source is often a training audio/video tape, along with a monitoring computer tests the reaction time and correctness of the students reaction.
Figure 1
Figure 2
Figure 3
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Medium System for Television: This shows an RTS® TW large 12-channel system. This is a system that is in medium trucks that have not yet switched over to a combination matrix and Party-Line system. This system consists of five Model 803 Master Stations, four PS-31 Power Supplies, one SAP1626 Source Assign Panel, a BOP-220 Breakout Panel, a VIE Video Isolate Panel, a four belt pack Telex® BTR-600 Wireless Intercom, and various belt pack and other user station. Also are interfaces to a telephone and a satellite communication link. Many trucks have a similarly configured Clear-Com® system. The master stations are usually for: the Director, Assistant Director, Lighting Director, Audio Mixer and Video operator (the one with the VCP6A isolate panel. No IFB (Interrupted Feedback) is shown in Figure 9, but an IFB system is easily married to the Master Stations. A large RTS® IFB add-on is shown in Figure 10. Note that Model 4020 is now Model 4030. Similar IFB systems are available from Clear-Com® and Audiocom®. The Control Station connects to the “Hot Mic” output of a master station or user station with a “Hot Mic” output. The IFB electronics receives its program audio from the audio mixer board.
Figure 1
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The IFB System (One Way Communications System): IFB is a television acronym for Interrupted Feedback, Interrupted Foldback, and Interrupted Return Feed (IRF). An IFB system permits a director or producer to talk to the talent, typically an “on air” announcer, newscaster, or sportscaster. Normally the talent hears the broadcast program audio. When the director or producer activates the IFB, the director or producer’s voice replaces the program audio. Sometimes the program audio continues in the other ear, sometimes the program audio is reduced instead of completely removed.
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How an IFB Works: Those in control positions (the director, producer, or assistant director for example) control the interrupt and or announce functions via control stations. Those in receive positions (on-air talent, floor managers, studio or field crew, audience, talent and crew in remote locations) are on the receiving end of the user station feed or on the actual user stations (talent electronics or talent station) via headphones, headsets, earphones, and/or loudspeakers. In the middle, the central electronics unit provides all the necessary inputs and outputs, processing, switching, and power distribution.
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Studio and Some Field Applications: Note: Model numbers of the different parts of the IFB are as follows:
Control Panel
Audiocom®: Built into US2002, ES4000A.
Clear-Com®: MA-4, AX-4.
RTS® TW: Models 4001, 4002, 4003
IFB Electronics
Audiocom®: Built into US2002, ES4000A
Clear-Com®: PIC4000B
RTS® TW: Model 4010
Talent Receiver
Audiocom®: IFB1000
Clear-Com®: TR-50
RTS® TW: Model 4030
Earset
Audiocom®: CES-1
Clear-Com®: (part of Model TR50)
RTS® TW: CES-1
In non-sports activities, the talent normally uses only the interrupt output (mono) of a Talent User Station. The earphone is hidden behind the talent’s back; a plastic tube runs from the earphone to the talent’s ear.
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Field Application, Sports: In the sports broadcasting or sports communication field, the talent uses a noise resistant headset. The microphone on the headset is the “air” microphone; the headphone is double muff, stereo. The talent is plugged into the stereo output of (for example) the Model 4030 Talent Receiver User Station. At the IFB Control Station, each talent’s name is marked on a strip of tape pasted adjacent to the push buttons.
In stadium sports, there is usually little problem in getting a microphone cable from the IFB Electronics to the Talent Receiver. In the case of golf, auto racing, and sports venues over an extended area, the distances may be too great. In this case, a four-wire circuit can be run to the talent location and adapted to the connector on the Talent Receiver.
In some extreme cases, only a single pair of wires may be available. In this case, plug the talent’s stereo headset into the stereo connection on the talent receiver, then connect the high side of the pair to pins 2 and 3 of the XLR-3 connector and the low side to pin 1 (pseudo-stereo mode). This will give a mono feed with each ear individually adjustable and both ears interrupted.
For runs of two miles of number 22 gage twisted pair, at least one talent receiver station should be operable. For a run of one mile, two talent stations should be operable.
Some users have increased the number of talent stations by using higher impedance (300Ω) headsets. In the case of auto racing and similar loud environment situations, low impedance noise isolating headsets will be necessary to overcome the volume and amount of sound. It may be necessary to use a four wire circuit to connect up each talent station, paralleling the pairs, and running the talent receiver in pseudo-stereo mode, using only the interrupt (“wet”) output of the IFB electronics.
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Field Application, ENG (Electronic News Gathering): In this case, the earphone is again hidden as in the studio case above. If the talent has to carry on a conversation with other talent at the studio and other venues, the program feed should be a mix minus feed. The mix minus feed will allow the talent to hear the other talents loud enough without hearing their own self too loud.
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Connecting (Interfacing) to Other Communications Systems: What is interfacing? Interfacing is either:
1 The interconnection of two normally separate communications systems into one system.
-OR-
2 The connection of a communications station or device that is not directly compatible within a system.
To accomplish this, voice and data information is adjusted and then transmitted to the other system. The adjustments include level translation, impedance compensation, mode translation, and compensation for parameters of each system.
Some examples are:
1 System to system: connection of a four-wire matrix system installed on a large mobile unit to two-wire beltpacks outside of the mobile unit.
2 System to terminal: connection of a camera with a built-in intercom to an intercom system, or connection of a radio transceiver into an intercom system.
Why is there interfacing, operationally? From an operations point of view:
1 An operation requires a larger collection of personnel and equipment than normal.
2 A mobile unit is used with a permanent installation to conduct an operation.
3 Coordination between personnel / equipment is required at a remote location.
4 A special part of the operation requires communication with an odd system or terminal.
5 A redundant “backup” path is required.
Why is there interfacing, technically? There are system-to-system, system to terminal or, system to device differences.
Some of these are:
1 Mode differences. There are several not directly compatible modes of operation: two wire mode, four wire mode, full duplex mode, half duplex mode, simplex mode. Examples: the TW System is two-wire full duplex, the ADAM matrix is four-wire full duplex, the telephone is two wire full duplex except some long distance calls are half duplex (both people cannot talk at once), a walkie talkie is simplex, Audiocom® is two-wire full duplex, Clear-Com® is two-wire full duplex, office intercoms are often simplex operation.
2 Level and Impedance differences. System voltage levels range from - 40 dBu to + 21 dBu with peaks to +28 dBu (where 0 dBu = 0.7746 volts). See Table 1, for typical ranges.
There are different modes of intercom operating modes because each mode offers a different advantage for different needs and situations. For example, two-wire is quick and easy to hook up, while four-wire is easier to interface to other systems.
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A Typical Interfacing Problem: A television camera uses a triax cable to connect the camera to the rest of the electronic system because a triax cable allows operation over longer distances with more consistent quality. This is because the triax cable uses radio frequencies to transmit information both ways on the cable. This is, in effect, four-wire (two path) communication. The following implementations often need interfacing:
• Television camera intercoms to intercom systems.
• Two-wire systems to four-wire systems.
• Full duplex systems to simplex systems.
• When transmission medias change.
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Interfacing Issues: There are three tasks to interfacing:
• Mode Conversion.
• Level Problems.
• Signal / Data Conversion.
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Modes: The following modes exist in intercom systems:
M2) Two-Wire.
M4) Four-Wire.
The following sub-modes are considered for two-wire and four-wire:
M2F) Two-Wire, Full Duplex.
M2H) Two-Wire, Simplex.
M4F) Four-Wire, Full Duplex.
M4H) Four-Wire, Simplex.
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Level Problems: One problem in interfacing from two-wire to two-wire is caused by the two-wire systems’ use of two- to four-wire hybrids. Interfacing requires conversion from two-wire to four-wire twice to allow level adjustments to and from systems. The quality of the two-wire to four-wire hybrid limits the amount of make-up gain available to match levels in one system or the other.
Another problem with interfacing is that level adjustment is difficult when interfacing from a limiter-controlled system, such as the TW Intercom System, to a non-limiter controlled system, such as some two or four wire systems. The reason for the difficulty is that the perceived loudness is greater on the TW System and much less on the non-limiter controlled system. This difference can be improved or eliminated depending on two limiting factors:
• The headroom of the electronics involved
• The quality of any two-wire to four-wire hybrids in the path.
Interfacing from two-wire to two-wire systems is the most difficult. Interfacing from two-wire to four-wire is easier, and interfacing from four-wire to four-wire is the easiest. The problem in two-wire/two-wire interfacing is getting the levels right and preventing oscillations.
The level of the TW and 800 Series conference intercom systems ranges from -10 dBu to 0 dBu, with an average value of - 6 dBu, and is limiter controlled.
Some other systems are listed in Table 3-2. The objective is to convert the modes and to adjust the levels.
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Signal/Data Conversion: Call Light
Some intercom systems use a “Call Light” signal to illuminate lights in individual stations. This signal may be a 20 kHz tone, a DC level, or a digital logic level. An interfacing device may handle the method conversion to carry the call light signal.
Data
Other systems have data flow via various methods including: contact closure, logic level, RS-485 bus, RS-422 bus, and RS-232 bus. The handling of the RSxxx signals is done best on a case-by-case basis. At this point, system-to-system communications is done via RS-232 communications by wire, fiber optic, or telephone lines via modem. Some system-to system communication is accomplished through user specified hardware imbedded in special products. Some master stations have an RS-232/485 connection that allows control of the station over a terminal or another computer.
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Interfacing Practice: Interfacing Television Camera Intercom Systems to TW Systems
General Camera Configuration Information for Television Cameras (except ENG units)
Television cameras used in broadcast and industry usually have two parts: a camera head and a camera control unit (CCU). The camera head assembly usually contains the lens equipment, camera electronics, and triax adapter (if used). The CCU contains additional electronics for processing video, the other end of the triax adapter, an interface for microphone audio, and the intercom interface. The intercom interface usually incorporates switches and electronics so that the intercom can be two-wire or four-wire.
The Problems in Interfacing to Cameras
There are two problem areas in television camera intercoms:
• The electronics in the camera head.
• The intercom interfacing electronics at the CCU.
Some possible problems with the camera head intercom electronics are as follows:
• Inadequate headphone drive (Not loud enough for athletic contests and studio shows)
• No limiter in the microphone preamplifier (level variations are too much)
• The headphone and the microphone share a common circuit return conductor (headphones oscillate when volume is turned up)
• The Triax Adapter / electronics does not give the camera intercom enough headroom, so there is a trade-off between signal clipping and signal to noise ratio.
• The microphone on/off switch does not disconnect the microphone preamplifier thus adding noise to the system.
• Some possible problems with the CCU intercom interface electronics are:
• An earth ground is applied to the wiring usually in two-wire mode (causes hum loops in the system)
• The four-wire input to the camera is not bridging impedance
• The two-wire “RTS0153 Systems compatible” interface loads the line
• No safety capacitors are installed in the CCU, thus causing burnt transformers if connected to the intercom line
Alternatives for Interfacing to Television Cameras
• Bypass the camera, tape a microphone cable to the camera cable, and plug a TW belt pack in at the end.
• Use the existing camera intercom; interface it to the TW system with a Model SSA-324 or SSA-424 interface (if camera intercom is four-wire).
• In multi-core connected cameras; use the camera wiring to allow a TW beltpack to be plugged into the camera head. This allows the camera operator to use a portable User Station mounted on his belt or attached to the camera body. (Note: This requires significant modification to the camera head and CCU)
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Headset Cable Lengths: The dynamic (low level) headset cable carries signal levels that differ by as much as 34 dB + 52 dB = 86 dB. Ordinarily, there are three types of unwanted coupling possibilities: resistive (through a common ground), capacitive and inductive. Since separate grounds are carried back to the microphone preamplifier and headphone amplifier, the common ground resistive coupling is, in this design, negligible. The capacitive coupling can be made no significant by a 100% shield in the cable. The inductive coupling mode dominates in this design, and can be offset in several ways:
• The distance between the microphone and headphone pairs can be increased, while the mutual inductive coupling is decreased by the use of “ribbed” cable (two cables molded together side-by-side).
• Both the microphone cables and the headphone cables can each be tightly twisted.
• Two or four separate cables can be run. A balancing transformer on the microphone circuit may be used. Estimated, Safe Operating Distances are as follows:
• Single cable, two shielded twisted pair: 10 feet.
• Dual ribbed cable, two shielded twisted pair: 30 feet.
• Separate cables, shielded twisted pair in each: 50 feet and more.
• Balanced microphone input: up to 100 feet depending on cable used.
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Headphone Impedances: Low impedance headphones are louder, causing the user station to draw more current from its power source. High impedance headphones are not as loud, drawing less current. Many user stations have a headphone impedance range from 25 - 600Ω. Headphones up to 2,000Ω will function but greatly reduced levels. In a double muff headset such as a Beyer DT-109, there are two 50Ω headphones connected in parallel resulting in an impedance of 25Ω.
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Wiring Practices/Workmanship Standards: The two most significant wiring practice/workmanship problems are as follows:
• Unintentional grounding, phase reversals (channel reversing) and power reversal. Cable shields must not touch connector shells or be tied to the connector shell lug. Cables (especially the vinyl insulated type) must not be pulled tight around sharp edges.
• Line noise due to an intermittent connection:
Poor solder joint.
Corroded connector.
Loose screw terminal.
A non-insulated cable shield touching the metal shell of the connector.
Portable user stations should not arbitrarily be taped or fastened to metal structures.
Grounding the case of the user station to an arbitrary structure may introduce large noise voltages due to local ground currents or due to the completion of a “ground loop antenna”.
Phase reversals are most common with portable microphone cable that has not been checked with a standard cable tester after fabrication or repair.
DC power reversals are usually not harmful to user stations since there is normally a protective diode in the circuit. The station simply does not work. Remember: negative is ground in this system.
Always clear all earth grounds from the RTS® TW System circuit return ground.
The only ground should be the 22,000Ω resistor in the power supply.
Unbalanced vs. Balanced
Intercom systems such as the TW System, in the standard, unbalanced configuration have been operated at distances of up to two miles with acceptable system noise levels. Routing the intercom cables along the same duct ways and pathways as the main power-cabling can increase the noise and hum levels in the system.
If intercom cables have to be routed in this manner at distances over 300 meters (1,000 ft.), a balanced conversion should be made.
Alternatively, the entire system can be operated in an optional balanced mode and be powered at each station with the “local power” option. This is sometimes called “dry line, balanced” operation.
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Extended Range On Part Or All Of The System: If a station is locally powered, operational range can be extended up to five miles, using two transformers to step up the line impedance to 800 ohms (for lower losses). When the users station has the four wire / 800Ω option installed, operation is possible up to 20 miles along Telco dry pairs. Operation over longer distances (3000 miles) is possible using dial up or minimum loss dry lines and the TW series of interfaces.
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Crosstalk: Use shielded cable to interconnect user stations in areas of possible electrical interference, (areas such as those near: digital equipment, high current primary power conductors “power outlets”, transformers, transmitters, and lighting dimmers. Do not run TW Intercom System cables along the same duct ways and pathways as these cables.
Standard wire size for the an intercom system interconnection is #22 gauge shielded cable, such as Belden 8761, 8723, 9406.
In permanent installations, to reduce both capacitive and resistive crosstalk and to afford a degree of RF and electrostatic shielding use a cable that has a shielded twisted pair for each channel, such as Belden 8723. Each pair consists of a conductor for the channel, a conductor for circuit ground return and shield around the two conductors. The shield is accessed via a drain conductor. This drain conductor and the shield can augment the circuit grounds and thus lower the ground resistance. Do not tie the shield to chassis, earth, or connector shell ground.
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Crosstalk Through A Common Circuit Ground: Since, in the unbalanced version of a TW intercom, all channels share a common circuit ground return, crosstalk due to common ground resistance can occur. This crosstalk is proportional to the ratio of the common ground resistance to the system terminating impedance, 200 ohms. This occurs when a talker on one channel is heard by a listener on another channel due to the common ground resistance (see Figure 8-4). Reduction of this crosstalk can be accomplished by reduction of the circuit ground resistance. Reduction of the ground resistance can occur as a side benefit of using shielded cable, since the shield drains can be tied together and electrically parallel the circuit ground.
Another way of lowering this kind of crosstalk is to “homerun” all interconnecting cables to a central or “home” location. This causes the common circuit ground path to be very short, and other things being equal, makes a low common ground resistance.
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Crosstalk Through A Mutual Capacitance Of Two Conductors: Two conductors such as a twisted pair can accumulate a large mutual capacitance over long distances. Using a figure of 100 picofarads per meter and a distance of 1 kilometer, results in a total capacitance of 100 nanofarads or 0.1 microfarad. The reactance of 0.1 microfarad at 800 hertz is 2000Ω. Referred to the system impedance of 200Ω, the apparent crosstalk is about 20 log (200/2000) or about -20 dB. Separating the two channel conductors by a shield greatly reduces the capacitive crosstalk, so that the resistive crosstalk discussed above dominates.
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A Low Crosstalk Approach To Interconnection: To reduce capacitive and resistive crosstalk and to afford a degree of “RF” and electrostatic shielding, a shielded, twisted pair per channel type cable can be used. Each pair consists of a conductor for the channel, a conductor for circuit ground return and, of course, the shield as a conductor and the shield drain conductors. These drain conductors and the shield can augment the circuit grounds and, thus, lower the ground resistance.
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Distances/Conductor Sizes/Distributed vs. Central Connection: Systems that stretch over distances of kilometers are more subject to power losses and crosstalk. These problems can be minimized with large enough wire, shielded cables and central connections.
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System Current/System Capacitances/Loading: The system currents are determined by several parameters:
1 The current required to supply standby current for each user station.
2 The current required to supply the dynamic current to generate line signal, headphone signals, speaker signals and call lamp signals.
3 The current required to start up a system (inrush current) by charging up to (50) 4000-microfarad capacitors or 0.2 farad.
4 The current limit imposed by the power supply to protect itself.
5 The secondary current limit imposed by the power supply when a fault is close to the power supply (little or no circuit resistance). This limit, called the foldback current, further protects the power handling electronic devices in the supply and determines the system start-up time.
Currents 1 and 2 can be calculated by multiplying the number of user stations times the user station current data in the Complete User Station Specifications. Current 5 usually limits current 3. Currents 4 and 5 are listed in the Power Supply Specifications.
Current 5 can be used to calculate the system start-up time: where:
T is the start-up time (approximated) in seconds.
N is the number of stations.
C is the capacitance per station = 4 millifarads
I is the power supply foldback current dV is a change in voltage across the capacitors, say 10 volts.
For a 20-station system, a 1-ampere foldback current, and a 10-volt change on the capacitors:
The actual system start-up time will be longer since voltages in each user station have to stabilize before audio can be transmitted. This time is approximately several seconds.
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Temperature Range Consideration: All of the elements of the TW Intercom System have been designed to operate over the temperature range of 0 degrees Celsius (32 degrees Fahrenheit) to 50 degrees Celsius (122 degrees Fahrenheit). The high temperature range is extended another 15 degrees Celsius if the units are not operating at full capacity or some other worst-case condition. The low temperature range is extended another estimated 20 degrees Celsius if the full system gain range is not required. The major operating problem at lower temperatures will be the dew point and the resultant condensation. If this is the typical operating environment, then it is recommended that the equipment be opened, cleaned, dried and sprayed with several light coats of plastic spray. This will lessen the noises generated by leakage currents that occur when the moisture and any dirt or film combine. Cleaning can be accomplished a rinse of alcohol, a very mild detergent (saponifier type) wash and two or three thorough rinses with distilled water. This routine is to first wash off the nonpolar soluble substances, then the polar soluble substances.
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Cooling Requirements: In general, only the power supplies require cooling consideration. Normally, leaving 2" clearance above and below the rack-mounted supplies is adequate. Portable supplies should not be left in the sun and these supplies should have clearance of 6" from five of the six surfaces. All other elements of the TW Intercom System require no special consideration. It is important to note that belt packs and other equipment left in the sun can cause burns to human flesh, due to the large amount of heat transfer possible. The user stations will normally continue to operate if one can only figure out a way to flip the switches and touch only the knobs.
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Moisture / Contamination Protection: If, in the field, a soft drink or something like it is spilled into the equipment, the equipment can be dismantled and cleaned gently with clean water. After the equipment is dry, it can be returned to service. If this happens often, residues in the water can be deposited on the equipment. It should be noted that a build-up of contaminates and humidity can cause audible noise on the intercom line. If it is likely that the equipment is continually to be exposed to contaminating liquid, suitable plastic covers should be employed. It may also be necessary to add a plastic coating as described above. When using equipment in the rain always protect the equipment with plastic covers - also, make sure all cable connectors are lifted out of the mud or snow and protected with plastic bags. Rain, mud and snow in connectors can cause considerable audible noise in any communications system.
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Magnetic Fields: Hum Problems: When the balanced type of intercom equipment is used, it is still possible to induce hum into the system by placing or locating user stations or system interconnects near a hum source, such as, power transformers or electrical switch panels or lamp dimmers. When the microphone switch is turned on and a dynamic microphone headset is used, the dynamic microphone is a sensitive antenna for magnetic fields. Often, operating personnel will go on a break, leave the microphone on and lay the headset on equipment with power transformers or near TV cameras or monitors with vertical deflection yokes. This is the reason for the system microphone turn-off scheme (Mic Kill).
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RTS Two-Wire System Design Tips & Tricks

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