Studio Technologies Model 47A Dual 2-Wire to 4-Wire Interface

2-Wire Interfaces

The Model 47A'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 47A'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.

The Model 47A'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 PS20. 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 47A is configured to operate in its external 2-wire power source mode. In this mode the Model 47A'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 47A'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 47A's 2-wire interfaces is their ability to supply DC power and 200 ohm AC termination to both channels (pins 2 and 3) of the connected intercom circuits. Referred to as the Model 47A's internal 2-wire power source mode, the 30 V outputs can power devices such as user stations and beltpacks. With independent power sources provided for both pins 2 and 3 of each interface, applications that include flexible party-line intercom channel routing will be directly supported. As such, in applications that include both source assignment panel (SAP) and Model 47A units the need for external intercom power supplies can be eliminated. Besides reducing total system cost, this capability may also lower system weight, reduce required mounting space, and decrease the mains energy requirements.

With the Model 47A's 2-wire interfaces able to supply up to 315 milliamperes (mA) of current on each channel, a typical broadcast application that uses four sets of three BP325 beltpacks can easily be supported. And the sophisticated 30 Vdc intercom power sources ensure that applications that require 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 each of the four DC power sources is monitored for over-current and short-circuit conditions. This allows protective shut-down of individual DC outputs, as well as providing an alert by way of a visual indicator.

Analog Hybrids with Auto Nulling

A key reason that the Model 47A'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 47A'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 47A's sophisticated auto nulling function uses analog circuitry under microprocessor control to achieve significant trans-hybrid loss.

For further information please view the attached Datasheet.