Field Report: Wheatstone Bridge

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As part of the facility installation at Entercom Kansas City (profiled in the April issue of Radio magazine), we installed a Wheatstone Bridge router with G-series control surfaces. We knew that we wanted to build the facility around a router for maximum flexibility with our nine stations. These needs were met with the Bridge.

The system is built around card-configurable frames that can sample any combination of analog or digital audio sources and interface with logic control sources. Two frames can be connected to share as many as 64 sources and destinations using Wheatnet Audio Network cards and CAT-5e cable. As many as 48 frames can be connected using the Wheatnet Hub for a flexible and scalable building-wide system.

It wouldn't surprise me if the North Carolina-based company took inspiration from the hundreds of spans, viaducts and turntable drawbridges that surround the factory. The Bridge interconnects a family of audio interfaces and control devices into a cohesive and flexible system. Like islands, studios are no longer isolated or limited by cabling or router size.

Creating connections, processing audio and making future changes is easy when every audio source, destination or logic control resides in the same digital system. In our installation, Generation-series mixing surfaces take the place of the traditional console, and fader and switch commands are transmitted to their respective Bridge frame. Because no audio passes through the surface, all audio conditioning and mixing takes place within the digital confines of the frame.

Performance at a glance
Eight-channel microphone pre-amp boards
Eight stereo channel analog input and output boards
Eight and 16 channel digital AES input and output boards
Generation Series consoles in a variety of fader and output bus sizes
TCP/IP-based remote control panels
Extensive configuration, monitoring and control with XPoint GUI software
Direct TCP/IP communication with many popular audio automation systems
Wheatnet Hub 48-port centralized network controller

Control logic such as remote microphone on, off, cough and talk back functions are digitized and routed just like its audio counterpart. Recently, Wheatstone developed a TCP/IP-based logic control so stand-alone, multi-button panels and computer automation systems can easily communicate to the Bridge over Ethernet.

At the heart of this system are two classes of interface processors: the Bridge, capable of processing 512 analog or digital audio sources, and its scaled-down cousin the Satellite. The Bridge frame is suited for centralized operations where a high volume of signal processing is concentrated in one or more rooms. The Satellite frame is a cost-effective interface for smaller studio settings with fewer audio sources. Both devices have essentially the same function, but have different audio signal-handling capacities.


Each frame is configured with a CPU card and can be populated with a combination of mic pre-amp, line level analog, digital AES, logic and DSP mixing cards. Physical connections are made with 25-pin D-style, RJ-45 or BNC-type connectors.

The Microphone Card accepts eight mono microphone level (-50dBm) sources and provides eight mono direct outputs. The pre-amplified microphone signal is electronically balanced and introduced to a 24-bit A/D sampler set to the master clock rate. All eight digitized audio signals are inserted into a single Time Domain Multiplexing (TDM) channel on the router bus. The card also supplies 32V phantom power.

The Analog Eight Channel Stereo (16 mono) Input Card is a direct coupled, balanced input with a nominal level of +4dBu. Signals are buffered and digitized using 24-bit A/D converters. The Analog Eight Channel Stereo (16 Mono) Output Card can be configured in mono, stereo or 5.1 output modes. Outputs are direct coupled, balanced at 50Ω with a nominal level of +4dBu.

The Digital Eight or 16 Channel Input Card conforms to the AES-3 standard interface of +5V peak-to-peak balanced signal at 110Ω. Inputs are transformer balanced. The Digital Eight or 16 Channel Output Card also conforms to the AES-3 standard. Sample rates of 44.1kHz and 48kHz are set according to settings on the DSP card. There is an option for selecting an external clock source as well. Embedded channel status information is also transmitted with the audio such as channel mode, word length, sample, time of day and block CRC.

The Logic input/output card is a general-purpose interface (GPI) that hosts 12 independent, opto-isolated, solid-state relay inputs and outputs. Customer supplied +5 to +15Vdc is applied to a logic input. The state of that input can be cross connected to a logic output port or can be programmed as an inverted state of the input. Output logic can be programmed in a normally on, off or current state when disconnected from an input port.

The DSP card (digital signal processor) is used by the Bridge to condition and mix audio. In systems configured for radio, two processor cards are installed, one to pre-condition the incoming audio and the second one to perform the mixing bus function. Larger systems use a dedicated card for the master mix.

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