of its 2-wire-to-4-wire hybrid circuits. Each of the two independent circuits provides low noise and distortion, good frequency response, and high return-loss (“nulling”), even when presented with a wide range of 2-wire party-line conditions. Unlike tele- phone-line (“POTS”) oriented DSP-based hybrid circuits, the Model 45’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 circuit.
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 require- ment. The Model 45’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 set 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.
Whenever a user presses the Model 45’s auto null pushbutton switch digital circuitry adjusts the analog hybrids to achieve their maximum return-loss. The nulling process takes approximately 12 seconds for each interface channel. And it’s important to
highlight that while the nulling process is automatic, it only takes place upon user request. The parameters obtained during the nulling process are stored in nonvola- tile memory; power interruptions won’t require the auto nulling function to be performed again.
The Model 45 generates a sine-wave audio tone for use during the auto nulling process. The signal’s frequency is soft- ware-controlled to maximize the ability of the hybrid circuits to reach a “deep” null. In addition, at the beginning of each auto nulling sequence a short period of 24 kHz tone is sent to the associated channel of the 2-wire party-line interface. This serves as a microphone disable (“mic kill”) signal for user devices such as the RTS BP325. By automatically disabling “open” microphones the auto nulling process can achieve a better result.
4-Wire Interfaces
Associated with the 4-wire portion of the Model 45’s dual-channel interface are analog line-level inputs and outputs. These are intended to interconnect with a variety of 4-wire devices, including matrix intercom systems, audio-over-fiber trans- mission systems, and specialized audio equipment. The input and output circuitry is transformer-coupled to minimize the chance of hum, noise, or ground “loop” issues. The nominal input and output levels are +4 dBu, helping to ensure compatibility with professional audio equipment. Some digital matrix intercom systems use other nominal levels but with their configuration flexibility they can be easily adjusted to match the Model 45. For example, the Riedel Artist® system has a nominal level of +6 dBu so an adjustment
