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2w-4w Converter

The most surprising thing about a 2w/4w converter circuit is how common they actually are. An RTS BP-325 beltpack actually has two. An RTS 802 has from six to twelve. They are less common as a stand alone device, but with the increased use of four-wire digital matrix system, they are growing in numbers.

 The identifying characteristics of a converter are at a minimum:

  • Four wire input. (A signal injected at this point will be heard on the two-wire system)
  • Four wire output. (This signal is a combination of all talkers on the two-wire system)
  • Two-wire input/output. (connectivity to the two-wire system)
  • A Null adjustment: (Varies the amount of out-of-phase four-wire input signal that is added to the four-wire output)
  •  2w-4w block diagram

    Now, why have a null adjustment? As the block diagram shows, the signal from the 4-wire input, the four wire output and the 2-wire connection are the same exact physical wire. What this means is that the listener on the 4-wire side will hear his or her own voice at the same volume as the voices on the 2-wire side. This condition is called "100% side-tone". Side tone is what causes most of the acoustic feed back that is experienced with intercoms. To reduce the percentage of side-tone, the talkers voice must be subtracted from the talkers listen signal. Subtraction is done by adding a polarity reversed copy of the talkers voice to the talkers listen signal. This subtractive procedure can eliminate most side tone to the point where, in a well set-up system, the listeners voice is removed completely.

     The complication with this simple concept is that the level of out-of phase signal must be exactly the same amplitude and at exactly 180 degrees opposed in phase with the 2-wire signal to produce a 0% side-tone condition. This is almost never possible. The audio amplitude variances that can occur on the 2-wire side can be several decibels. While the capacitance in the wire and other factors can change the phase of the 2-wire system several degrees from 0. That is why the null-pot is so important. The ability to change the phase of the added signal to match the inverse of the 2-wire signal as well as to change the amplitude is extremely important.

    Converters come in many levels of complexity. Some have a simple level control with no consideration for phase. Others will provide adjustments for phase correction as well as level (Clearcom CCI-22) or a multi-band equalization circuit with a null pot for the high, medium and low frequencies can be used. (RTS SSA-324). Every converter is said to have a "window" of adjustment. If the 2-wire and 4-wire systems are out of phase greater than the extreme of the adjustment control, then there will be no way to get the "null" you want. Like wise, if the 4--wire system is feeding the 2-wire system at a level greater than the null adjustment can add, then the side-tone can't be removed.

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    Beltpack Local Power

    First decide why it is necessary to do so. Some of the reasons to try it are:

  • If, due to interference or noise, the TW signal needs to be balanced.
  • If the number of beltpacks in a geographical area are pulling more current than the supply wire can provide.
  • Sometimes,an un-balanced or ground referenced signal run for 500' or more will get a lot of noise or interference. This is especially true if un-shielded wire is used. 25-pair, JKT, etc are examples of this. If you transformer isolate the TW line at each end, the common mode rejection will quiet most of the noise induced in the length of the wire. Since DC voltage will not pass through a transformer, the beltpack will need a source of power at the remote end of the wire.

    There are times when the available power will not fire the number of beltpacks necessary. This is common with the communications circuits in the Telecast® ADDER 162 fiber optic transmission system. The specified current capacity is greater than that required for two beltpacks. However, since it only provides 20vdc, not the typical 30vdc combined with the loss of the supply wire, the voltage at the beltpack may be less than what is required for proper operation.

    Second, decide how to do it

    If you are using an RTS BP-325 beltpack, the manual describes a method of powering through the "Loop/Aux" connector on the rear panel of the beltpack. Unless the beltpack is modified, it is not possible to use the "PGM" input and local power simultaneously because the two functions share one connector.

    The method that is closest to ready-to-use is an RTS power supply (such as the model PS-15) and a 600:600 ohm isolation transformer (RTS part LM-9003). This method only works if there is a source of 120vac available. One strong benifit of this method is that both channels and the program input will work because a standard RTS supply is used.

    There have been custom devices developed which use recharageble batteries for the power source. The FieldPAC (Portable Audio Communications) developed by TNN:The Nashville Network is an example of this. The FP-1 uses a DC power source to power the beltpack on channel one. This simple method of powering allows channel #2 and the program input to function. So the limitation is that channel one can't be used, but you don't need 120vac to power it.

    RTS Dry-out Schematic

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    IFB Earpiece Performance

    Click here for frequency response chart
    Testing conditions
    Signal source: 400Hz 
    Output voltage unloaded: 1 volt AC RMS 
    Distance from transducer exit point to microphone = .125" 
    Testing Equipment
    Audio Precision Portable-One Plus Generator 
    Audio Control Model SA-3050A Real-Time-Analyzer/Sound Preasure Level Meter 
    Fluke Model 8060A True RMS AC volt meter 
    RTS Telex 4030 IFB Beltpack 
    Earpiece Relative Sound Preasure Level Chart
    Part Number. Impedence Currrent Draw @400Hz. SPL.
    RTR-04 15 ohm 60 mA 97dB-SPL
    RTV-04 125 ohm 8 mA 94 dB-SPL
    RTW-04 500 ohm 2 mA 91dB-SPL
    RTX-04 1000 ohm 1 mA 86 dB-SPL
    RTY-04 2000 ohm .5 mA 81 dB-SPL

    Although this test shows that the 15 ohm earpiece will deliver 3dB-SPL more than the 125 ohm earpiece, the 125 ohm is preferable because the current draw on the amplifier is much less. This is mostly important when program material is used, as it has a wide bandwidth and will draw more current at lower frequencies .
    Due mostly to the circuit design in the 4030 beltpack, the extremely low 15 ohm impedence will cause distortion in the amplifier much sooner than the 125 ohm earpiece would. The relative SPL delivery between the individual earpieces would be consistant regardless of the device driving the load. If an amplifier with sufficient wattage was used, then the 15 ohm earpiece would be the better choice.

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    TW Intercom System Termination

    Termination is a mysterious concept to many people. After all, who wants to be "terminated". In early audio systems, particularly in broadcast environments, the inputs on audio equipment were all a fixed input resistance (not impedance: having no relationship to frequency). This allowed the equipment to operate at an elevated level to overcome noise and provided a consistent input load to all signals. Since equipment was manufactured with a variety of input loads, the 600 ohm common denominator value brought most equipment into the same range which simplified system wiring. Many other reasons existed for this and it is really a separate discussion.
    Modern TW intercom systems use a simple but effective little ciruit called a "Bilaterial Current Source". This audio circuit is what allows dozens of intercom user stations to be connected to a single partyline without each beltpack significantly loading the audio which would reduce the level to less than usable. This circuit is really a simple oscillator design which makes it very unstable and prone to a lot of funny business. The stability (they are very stable in practicle use) comes from the addition of a resistance on the output of the circuit. This resistane is provided by a termination resistor typically located in the power supply for the the intercom system. Each channel of the system would have it's own terminator.

    Occasionaly, a situation will call for a TW intercom device to operate without a power supply. One example of this is using a wireless intercom system as a stand alone unit. If the intercom connection of the wireless system is not locally terminated, it will have excessive gain, and be totatly unusable.

    These drawings show the proper method of making a terminater for RTS and Clear-Com TW system.

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