Studio Technologies Model 46A Dual 2-Wire to 4-Wire Interface

Studio Technologies Model 46A Dual 2-Wire to 4-Wire Interface

2-Wire Interfaces

The Model 46A's 2-wire interfaces are optimised for direct connection with 2-channel party-line intercom circuits. In addition, single-channel party-line intercom circuits can also be connected. Many broadcast applications use the 2-channel TW-Series from RTS®, including their popular BP325 beltpack. Other industry-standard single- and 2-channel PL intercom systems, including those from Clear-Com®, are also directly compatible. To provide optimal signal matching and interface performance the nominal level of the Model 46A's 2-wire interfaces can be selected. Using DIP switches, accessible on the front panel, the nominal levels can be selected to be either –10 or –14 dBu, settings that should allow compatibility with virtually all party-line systems. For example, for applications that use equipment from RTS the –10 dBu setting is appropriate. For other equipment, such as that from Clear-Com, the –14 setting is correct.

The Model 46A's 2-wire interfaces can correctly function with powered ("wet") or unpowered ("dry") intercom circuits. Powered circuits have a DC voltage present, typically provided by power supplies such as the RTS PS31 or Clear-Com PS-232. This DC power, normally 30-32 V, provides energy for connected devices such as user stations or beltpacks. In this type of application the Model 46A is configured to operate in its external 2-wire power source mode. In this mode the Model 46A's circuitry maintains the required high-impedance load and, as in all cases, draws no power from the party-line circuit. An auto-terminate function ensures that should a "wet" circuit not be connected, the Model 46A'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 46A's 2-wire interfaces is their ability to supply DC power and 200 ohm AC termination to the connected intercom circuits. Referred to as the Model 46A's internal 2-wire power source mode, the 30 V output can power devices such as user stations and beltpacks. In many applications this will eliminate the need for external intercom 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 supply up to 315 milliamperes (mA) of current, a typical broadcast application that uses two sets of three BP325 beltpacks can easily be supported. Each interface's 30 V DC intercom power source ensures that systems requiring long intercom cable runs will function correctly. The circuitry's output regulation is such that little change in the output voltage will occur over its entire rated output current. Also, the unique design of the power supply circuitry virtually eliminates the noise and "hiss" associated with typical intercom power supply performance. Under software control the output of the DC power supplies are monitored for over-current and short-circuit conditions. 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 46A's interfaces achieve excellent audio performance is the design of the 2-wire-to-4-wire hybrid circuits. They provide low noise and distortion, 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 46A's analog circuitry provides extended 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 measured in dB. A high value is important, especially in applications where multiple 2-wire-to-4-wire interfaces are used together. Remote sports broadcast applications are especially sensitive to this requirement. The Model 46A's sophisticated auto nulling function uses analog circuitry under microprocessor control to achieve significant trans-hybrid loss. This return-loss "null" is achieved by making a series of adjustments to account for the resistive, inductive, and capacitive conditions that are present on the connected 2-wire party-line circuit. The party-line's conditions are the sum of the impact made by the type and quantity of cable, the connected user-devices, and the intercom power source.