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Installation Profile: Legislative Sound

Nov 1, 2007 12:00 PM, By Garret Maki and Rodrigo Ordoñez

Inside the U.S. Senate Chamber’s digital audio upgrade.

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The Senate Recording Studio was extremely happy with the performance of the existing sound system, so one of the main requirements of the new system was that it resemble its predecessor as closely as possible, while providing modern monitoring, control, and backup functionality. This included meeting stringent reliability requirements.

As one would expect for such a historic room, aesthetics and minimum architectural impact were major requirements. Senate officials did not allow any aesthetic changes such as new holes in or modifications to the senators' desks. New microphone holders and desk electronics had to be approved by the Capitol Senate Curator's office prior to installation, which in some cases required matching the existing desk finish on nearly 200-year-old furniture.

Aside from the aesthetic requirements, there were many unique challenges to installing cabling and other infrastructure in a building that is more than two centuries old. Because electrical infrastructure was not a requirement when the U.S. Capitol was built, there were no pre-existing cable paths available. Equipment for the existing audio system was in a different location, which meant that very few of the existing cable paths could be reused. Infrastructure challenges included having to cut through 6ft. of granite foundation in order to get more than 300 cables from the Chamber Floor into the new equipment room.


CobraNet was selected for audio distribution in the Chamber. This provided for the flexibility to dynamically reconfigure desk locations, and it offered efficient distribution of audio, control, and power for the more than 140 inputs and 160 outputs needed. The Senate Chamber installation required close to 170 devices on a single network. The size of the system brought significant challenges regarding the configuration of audio transmitters and receivers, conductor priorities, and system timing. The audio network was optimized by the efficient configuration of CobraNet bundles and the use of multiple unicast distribution. Timing stability between the dual NION systems and the CobraNet network was achieved by incorporating redundant external CobraNet conductors custom made by Whirlwind.

Engineers at Cirrus Logic's Peak Audio group, in conjunction with K2 Audio, designed the senator desk units (SDU) used at each desk position specifically for this project. Using CobraNet, the SDUs provide primary and secondary audio input, audio output, control, and IEEE 802.3as-compliant power-over-Ethernet over a single Cat-6 cable by using all eight conductors.

Custom Senators’ Desk Unit [SDU] and mic holder.

Each unit includes a two-way loudspeaker, a tweeter for microphone proximity sensing, a microphone holder with an optical sensor for un-muting the input and a light ring indicating mute state, an internal DSP for processing and logic control, three power amplifiers, a cable reeler, an RF-shielded microphone, and a network-controllable preamp. All the components are housed inside a relatively small wooden box that sits on a shelf under each senator's desk. Using a star-wired ring network, each desk unit is connected to two separate stacks of Ethernet switches, which communicate with the main DSP and control systems through a ring of routing switches, providing high amounts of fault tolerance at the network level.

Customized Audio-Technica desk-unit microphones can be handheld, placed on podiums, or worn on lapels for minimum visual impact on camera. When the microphone is taken out of the holder, the system automatically un-mutes the input. The senator can then pull out the desired amount of cable to allow for freedom of movement while speaking. The proximity sensor, a custom algorithm that runs inside the SDU's DSP, automatically mutes the microphone fractions of a second before it is returned to the holder, thereby eliminating any mechanical noise resulting from the microphone's contact with the holder. It compares the input signal received by the microphone with a high-frequency signal emitted by the desk unit's proximity tweeter to determine if a microphone is approaching the holder. Once in the holder, a motorized Xedit Servoreeler automatically coils the cable back into the SDU.

Another requirement of the Chamber system was that it had to be RFI-resistant and shielded from phones and other transmitting devices in case a senator inadvertently leaves his or her electronic device on. We tested every component that was susceptible to interference, and we worked closely with the microphone and desk-unit manufacturers to ensure that the units were designed to meet the audio system's stringent specifications.

The legislative clerk and other non-senator positions on the Floor used Rane NM 1 interfaces connected to gooseneck microphones and passive loudspeakers. Because they also use CobraNet and PoE technologies, they share the same infrastructure requirements as the SDUs. For special events, additional auxiliary analog and AES inputs and outputs are provided. They connect to Peavey MediaMatrix CAB hardware, which feeds the audio to the core NION DSP system.

The complexity and particular characteristics of the project required several other pieces of hardware to be custom built or adapted purposely for the Chamber. These include a specialized panel used by the operators for remote control of all system inputs and outputs, custom-built metal work housing equipment at the operator's position, and a modified JL Cooper mixer controller used for broadcast mixing that, at the request of the Senate Recording Studio, has level controls that go up to 11.

For sound reinforcement inside the Chamber, there are more than 550 loudspeakers in use. Because any listening location is typically only a few feet from a loudspeaker, the direct-to-reverberant ratio is quite high to yield excellent speech intelligibility. In addition to the senator desk units described previously, fill locations throughout the Floor and at the Senate Clerk's position use custom-built two-way drivers. In the Gallery and staff areas, there is a pew-back-style distributed loudspeaker system using 4in. coaxial drivers. All loudspeakers were custom-built or adapted models provided by EAW.

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