RTSTM TW  SYSTEM INSTALLATION  GUIDELINES

by Stan Hubler

INTRODUCTION

The TW Intercom System in its simplest form is a two-wire conference- line intercommunications system that allows up to 50 user stations to connect across a common line (also called a channel). The system operates in a full-duplex mode; simultaneous talk and listen, to and from each user station. Individual user stations may be separated from the power supply as much as 1.6 kilometers (1 mile). The maximum cumulative amount of cable recommended to be used in a system is 3.1 kilometers (10,000 feet). The TW Intercom System power supplies not only provide 26 to 32 volts DC to power the user stations, but also establish a 200 ohm audio impedance on the line. Power may be carried to the user stations along the same pair of conductors that carry the conversations and call-light signals, but in an optionally "local powered" system, only the audio and call signal are transmitted down the pair.

The TW System discussed above is an unbalanced two-wire system, but the equipment can, optionally, be provided in a balanced configuration in both two, and four, wire formats. Also, optionally, the equipment can function in all four modes or in any combination, simultaneously.

MECHANICAL INSTALLATION

Outline and installation drawings for user stations and circuit cards are included in the respective user station technical manuals. These drawings provide mechanical information useful for permanent and other type installations. This information includes overall dimensions, fastener hole locations, console cutout dimensions and weights. Space allowances for control access, cabling and servicing must be determined on a case by case basis by the installer. It is recommended that space be provided for; cabling service loops, reaching XLR type connector locks, local power option power supplies and headset connectors and cables. If the headset connector is remoted, allow space between this cable and interfering sources such as video TV monitors, power supplies and equipment with internal power supplies.

ELECTRICAL INSTALLATION: POWER

The units receive electrical power from either (1) a system power supply (26 - 32 volts DC) or (2) a local power supply option (12 - 18 volts DC). Current requirements for each type of user station are listed in the User Station Specifications sheet. Since, in (1) above, the power and communications signals share conductors, it may be necessary to overcome power losses by increasing conductor size over long runs (over 1 kilometer) Normal conductor size is #22 gage. The maximum allowable loop resistance is determined by the power supply voltage, the loop current and the user station minimum operating voltage. The maximum allowable loop resistance equals the difference of the power supply voltage and the minimum operating voltage divided by the maximum loop current - power required by the user station(s).

Example: a headset station without call light and a dynamic headset (with 25 ohm headphones) plugged in, uses up to 50 milliamperes, the power supply voltage is 26 VDC and the user station minimum operating voltage is 18 VDC.

Then the loop resistance = (26 - 18 ) volts/.05 amperes = 160 ohms. This corresponds to an operating distance of 1.54 km (5000 ft.) for a #22 gage wire pair using a single user station with 25 ohm phones (and no call light option). Data for these calculations are in the specifications in the User Station Specifications sheet and in standard electrical wire tables.

ELECTRICAL INSTALLATION: SIGNALS

The RTS Systems TW Intercom System, in its simplest form is a two-wire system employing just 2 wires for one channel of communications. This means that the number of channels may be increased with the addition of extra wires, i.e., 3 wires = 2 channels, 4 wires = 3 channels, etc. The system utilizes XLR-3 type connectors for interconnection between 2-channel user stations and the power supply. XLR-4 type connectors are used for connection to 3-channel user stations. The best type of cable to use for interconnection depends upon the application and electrical environment. In areas of strong electrical fields (radio frequency, hum, digital circuits), shielded cable should be used; if areas of even stronger fields, it may be necessary to employ balanced user stations and shielded, balanced cables. Some examples of electrical fields to avoid are; high-current primacy power conductors (mains), transformers, transmitters and lamp dimmers. Most 2-channel applications may use either standard microphone cable (for convenience) or 2-twisted-pair cable (considerably less expensive than microphone cable). Standard wire size for the TW System is #22 gauge wire for interconnection. For permanent installations it is recommended that each channel should have individually shielded twisted pair of at least #22 gauge such as Belden #8723 or #9402 for 2 channels, and #8777 or #9873 for 3 channels.

ELECTRICAL INSTALLATION: INTERCONNECTION

Interconnection to the user station's line-input connector can be accomplished using standard 2-conductor shielded microphone cable (for 2-channel stations). The line-input connector on the user station is wired to a loop-through extension connector on the user station for easy cabling between user stations.

Interconnection can be centrally wired ("home-run" cabling), with each cable coming from a central point, or distributed ("daisy-chain" cabling), where all the user stations are looped together from one to another. A recommended cabling scheme involves a combination of both configurations.

TW INTERCOM INSTALLATION:  RATIONALE

Grounding & Shielding/Interference & Crosstalk Reduction

System Grounding

All rack mounted and console mounted user stations should be connected to earth ground or power line safety ground.

The TW Intercom System circuit ground should not be directly connected to "earth" or "chassis" ground (where directly means a connection an ohmeter would show...therefore an ohmic connection). Each user station is bypassed to its own chassis via a 0.1 microfarad capacitor to establish a radio frequency (RF) ground. Establishing a radio frequency ground at the user station reduces the radio frequency voltages below the detection threshold of the analog circuits, since the "RF" ground represents a voltage minimum.

In order to prevent a buildup of voltage across the system capacitance, the power supply has a bleeder resistor to chassis ground (22 kilohms). If the system has no RTS power supply, such a bleeder resistor should be supplied at a central point in the system.

The basic benefit of not "earth" grounding the RTS System circuit return is to permit continued operation during an accidental system ground fault. This accidental grounding can happen as the result of a pinched wire or a scraped cable that has been pulled across a sharp edge. A single accidental ground can be tolerated by the system until the fault can be cleared and, with luck, before a second ground fault can cause noise or overload or bring the system down.

Another benefit of not "earth" grounding the circuit return is to prevent noise introduction through "earth" currents from other equipment. If the RTS circuit ground conducts these currents, it is likely that they will be heard as interfering noise on the communication line.

Crosstalk Through A Common Circuit Ground

Since, in the unbalanced version of the TW Intercom System, 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. 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 "horne-run" 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.

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 2000 ohms. Referred to the system impedance of 200 ohms, 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.

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.

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 through the use of large enough wire, shielded cables and central connections.

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 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 3 is usually limited by current 5.

Currents 4 and 5 are listed in the Power Supply Specifications.

Current 5 can be used to calculate the system start-up time:

 
The system start-up time = (approximately) (NC/i) dV

where:

N is the number of stations.
C is the the capacitance per station = 4 millifarads
i is the power supply foldback current
dV is a change in voltage across tbe capacitors, (say 10 volts)
 

For a 20 station system, a 1 ampere foldback current, and a 10 volt change on the capacitors:

The time = (20 X .004 X 10)/i = 0.8 seconds

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 it is on the order of several seconds.

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 possible; 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 non-significant by a 100% shield in the cable. The inductive coupling mode dominates in this design, and can be offset in several ways: 1) The distance between the microphone and headphone pairs can be increased (and the mutual inductive coupling decreased) by the use of "ribbed" cable which is two cables molded together side-by- side; 2) Both the microphone pair and the headphone pair can each be tightly twisted; 3) Two separate cables can be run; and/or 4) A balancing transformer may be used.

Operating estimated 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 to100 feet depending on cable used.

WIRING PRACTICES / WORKMANSHIP STANDARDS

The two most significant wiring practice/workmanship areas are:

Line noise due to an intermittent connection attributable to:

  • Poor solder joint
  • Corroded connector
  • Loose screw terminal
  • An uninsulated mylar shield touching the metal shell of the connector
  • Unintentional grounding, phase reversals (channel reversing) and power reversal:
    1. Cable shields must not touch connector shells or be tied to the connector shell lug.
    2. Cables (especially the vinyl insulated type) must not be pulled tight around sharp edges.
    3. Portable user stations should not arbitrarily be taped or fastened to metal structures.
    4. 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
    5. Phase reversals are most common with portable microphone cable that has not been checked with a standard cable tester after fabrication or repair.
    6. DC power reversals are usually not harmful to user stations since standardly there is a protective diode in the circuit. The station simply doesn't work. Remember: negative is ground in this system.
    7. Always clear all earth grounds from the RTS TW System circuit return ground. The only ground should be the 22,000 ohm resistor in the power supply.

UNBALANCED VS BALANCED

The TW System, in the standard, unbalanced configuration has been operated at distances of up to two miles with acceptable system noise levels. Routing the TW System cables along the same ductways and pathways as the main power cabling can increase the noise and hum levels in the system. If TW System cables have to be routed in this manner at distances over 300 meters, a balanced conversion should be made. Two channels can be balanced on two pairs and the DC power can also be phantomed. 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.

EXTENDED RANGE ON PART OR ALL OF SYSTEM

If a station is locally powered, operational range can be extended up to five miles, using two transformers to step up the long impedance to 800 ohms (for lower losses). When the users station has the four wire/800 ohm option installed, operation is possible up to 20 miles along telephone company pairs. Operation over longer distances (3000 miles) is possible using dial up or minimum loss dry lines and the SSA series of interfaces.

INTERFACING

To interface the TW System to another communication system, it is necessary to identify the characteristics of the other system, compare them to the TW System, then adjust levels, impedances and formats via an interfacing device to make the systems transparent to one another. It is also good practice to electrically isolate the two systems so that no ground loop type problems occur.

The RTS Systems Model numbers of these units are:

  1. SSA324
  2. SSA424

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, dryed and sprayed with several light coats of plastic spray. This will lessen the noises generated by leakage currents which occur when the moisture and any dirt or film combine. Cleaning can be accomplished with a rinse of 1,1,1, trichlorethane, a very mild detergent (saponifier type) wash and 2 or 3 thorough rinses with distilled water. This routine is to first wash off the nonpolar soluble substances, then the polar soluble substances.

NOTE:

IT IS NECESSARY TO PROVIDE SUITABLE SKIN AND LUNG PROTECTION DURING TILE USE OF THE TRICHLORETHANE SOLVENT.

COOLING REQUIREMENTS

In general, only the power supplies require cooling consideration. Normally, leaving 2 inches 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 inches 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 the knobs.

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 fairly 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 RTS TW System line. If it is likely that the equipment is continually to be exposed to contaminating liquid, suitable plastic covers should be employed. It some cases, it may also be necessary to add a plastic coating.

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.

MAGNETIC FIELDS: HUM AND RELAY PROBLEMS

When the balanced type of TW System equipment is used, it is 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.

The speaker user stations used in the TW System have strong magnetic fields near the speakers. These fields can cause reed relays to malfunction. Some of the TW Intercom System and Series 4000 IFB equipment has reed relays installed. Keep these facts in mind when mounting equipment very close to one another.

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