Telex 38109-977 manual How Each System Works, Four pin XLR, Five pin XLR

4The 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”

5Since 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.

6Some user stations have “loop-thru” connectors that allow “daisy chaining” stations using a single connection to the power supply.

How Each System Works

Note Drawings at the end of the chapter depict the systems being discussed.

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. And 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 ohms or greater. The loading effect of the station on the intercom, say in a 200 ohm terminated system is, worst case, 10,000 ohms in parallel with 200 ohms. This results in a change of the system termination to 196 ohms, 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 ohm 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 ohms which could still be usable in some instances.

C h a p t e r 2 - I n t r o d u c t i o n t o P a r t y - L i n e I n t e r c o m S y s t e m s 13