The Model 46’s 2-wire interfaces can correctly function with powered (“wet”) or unpowered (“dry”) intercom circuits. Powered circuits have a DC voltage pres- ent, typically provided by power supplies such as the RTS PS31 or Clear-Com PS-232. This DC power, normally 30-32 volts, provides energy for connected devices such as user stations or belt packs. In this type of application the Model 46 is configured to operate in its external 2-wire power source mode. In this mode the Model 46’s circuitry maintains the required high-impedance load and, as in all cases, draws no power from the party-line circuit. An auto-terminate func- tion ensures that should a “wet” circuit not be connected, the Model 46’s interface circuitry will remain stable. This unique feature makes certain that objectionable audio signals, including oscillations and “squeals,” will infrequently be sent to the connected 4-wire device.
A significant capability of the Model 46’s 2-wire interfaces is their ability to supply DC power and 200 ohm AC termination to the connected intercom circuits. Re- ferred to as the Model 46’s internal 2-wire power source mode, the 30 volt output can power devices such as user stations and belt packs. In many applications this will eliminate the need for external inter- com power supplies. Besides reducing total system cost, this feature can also lower system weight, reduce required mounting space, and decrease the mains energy requirement.
With each 2-wire interface’s ability to sup- ply up to 300 milliamperes of current, a typical broadcast application that uses two sets of three BP-325 belt packs can easily be supported. Each interface’s 30 volt DC
intercom power source ensures that sys- tems requiring long intercom cable runs will function correctly. The circuitry’s out- put regulation is such that little change in the output voltage will occur over its entire rated output current. Also, the unique de- sign of the power supply circuitry virtually eliminates the noise and “hiss” associated with typical intercom power supply perfor- mance. Under software control the output of the DC power supplies are monitored for over-current and short-circuit condi- tions. This allows protection shut-down of the output DC, as well as providing an alert by way of a visual indicator.
Analog Hybrids with Auto Nulling
A key reason that the Model 46’s inter- faces achieve excellent audio performance is the design of the 2-wire-to-4-wire hybrid circuits. They provide low noise and dis- tortion, good frequency response, and high return-loss (“nulling”), even when presented with a wide range of 2-wire conditions. Unlike telephone-line (“POTS”) oriented DSP-based hybrid circuits, the Model 46’s analog circuitry provides ex- tended frequency response. With a pass band of 100 Hz on the low-end and 8 kHz on the high-end, natural-sounding voice signals can be sent to, and received from, the 2-wire party-line circuits.
A hybrid’s ability to isolate the transmit signal from the receive signal in the 2-wire- to-4-wire interface is critical. The quality of this isolation, technically known as return-loss or trans-hybrid loss, is mea- sured in dB. A high value is important, especially in applications where multiple
2-wire-to-4-wire interfaces are used togeth- er. Remote sports broadcast applications
