2-Wire Party-Line Interface
The Model 45’s two-channel 2-wire party- line interface is optimized for direct connec- tion with a dual-channel party-line intercom circuit. In addition, single-channel party- line intercom circuits can also be connect- ed. Many broadcast applications use the dual-channel TW-series from RTS®, which includes their popular BP325 belt pack. The Model 45’s 2-wire interface is configured for a –10 dBu nominal level, exactly matching the level of the TW-series. Other industry- standard single- and dual-channel party- line intercom systems, including those from Clear-Com®, are also directly compatible. While their nominal level is approximately
4 dB lower (–14 dBu) their greater dynamic range tends to compensate for any opera- tional differences.
The Model 45’s 2-wire party-line interface is typically connected to a powered (“wet”) unbalanced intercom circuit. Powered party-line circuits have a DC voltage pres- ent, typically provided by power supplies from RTS or Clear-Com. This DC power, normally 30-32 volts, provides energy for connected devices such as user stations or belt packs. A configuration setting is used to select how the Model 45 will impact the connected 2-wire circuit. In the default mode the Model 45’s circuitry maintains
a high-impedance load and draws no power from the party-line circuit. An alter- nate mode allows the power required by the Model 45’s circuitry to be supplied by the connected party-line circuit.
A significant capability of the Model 45’s
2-wire interface is its ability to create a fully functioning 2-wire party-line intercom cir- cuit, supplying the required DC power and 200 ohm AC terminations. Referred to as the Model 45’s 2-wire (PL) power source
mode the 30 volt, 300 milliampere maxi- mum output can power devices such as user stations and belt packs. In many applications this will eliminate the need for an external intercom power supply. Besides reducing total system cost, this feature can also lower system weight, reduce required mounting space, and decrease the mains energy requirement.
With the 2-wire interface’s ability to supply up to 300 milliamperes of current a typical broadcast application which uses two or three BP325 belt packs can easily be sup- ported. The circuitry’s output regulation is such that little change in the output volt- age will occur over its entire rated output current. The power source is an “industry- standard” 30 volts DC, helping to ensure that applications requiring long intercom cable runs will function correctly. Also, the design of the power supply circuitry helps to minimize the noise and “hiss” often associated with intercom power supplies. Under software control the DC power supply’s output is monitored for over-cur- rent 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.
An auto terminate function ensures that should a party-line circuit not be connect- ed, the Model 45’s interface circuitry will remain stable. This unique feature makes certain that objectionable audio signals, such as oscillations and “squeals,” will rarely be sent to the connected 4-wire device.
Analog Hybrids with Auto Nulling
A key reason that the Model 45 achieves excellent audio performance is the design
