System Design: Tips & Tricks
This page provides tips & tricks for system design by experienced Telex Audiocom technicians.
- 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, 200? for RTS® and Clear-Com, and 300? 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 belt pack 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 BP318 single channel beltpack, or an Audiocom BP1002, or a Clear-Com RS-501.
A typical two-channel headset belt pack user station adds a channel selector switch to the above. Examples RTS BP351, Clear-Com RS-502, Audiocom BP2002
Alternately, newer units have two talk buttons, two volume controls, and two status indicators to tell which talk button is engaged. Examples: RTS BP325, BP351, Clear-Com RS-522-TW, or Audiocom IC-2B.
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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 2 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.
Some Technical Notes About The Stations Above
The stations mentioned above generally are designed for the dynamic microphones in the headsets to have an impedance of about 150 to 500 ohms. The speaker station panel electret microphones are designed to have an impedance of 1000 to 2000 ohms and require 1 to 5 volts excitation. In addition, the push-to-talk microphones have around 500 ohms. This means the actual input impedance of the station microphone preamplifier will range from 470 ohms to 5000 ohms. The low impedance of 470 ohms minimizes the crosstalk in the headset cord. The headphone impedances expected range from 50 ohms to 1000 ohms. The 50-ohm 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 a bridging impedance across the intercom line of 10,000 to 15,000 ohms. A bridging impedance of 10,000 ohms 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
1 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 3 pin XLR.
2 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.
3 In any system, pin 1 and the shell of the XLR connector should NOT be connected together.
4 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”
5 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.
6 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 10,000? or greater. The loading effect of the station on the intercom, say in a 200?-terminated system is, worst case, 10,000? in parallel with 200?. This results in a change of the system termination to 196?, 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 200? 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 150? which could still be usable in some instances.
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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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- Cabling
- Usually the intercom system’s specifications are based on the use of 22 AWG microphone cable. Microphone cable of 22 gage measures 3? per 100 feet or about 30? per 1000 feet (round trip resistance). The wire table says 32? 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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- 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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