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Installation Profile: Legislative Sound

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

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.

Web Expanded
Original 1994 Article


Chamber System Design Comes Full Circle

The recent digital audio upgrade in the Senate Chamber was required to provide the same quality and reliability as the system that had served the Senate for the last 13 years, but at the same time, it took advantage of newer digital audio distribution techniques and DSP technology to provide modern monitoring and control capability.

As documented in Sound & Video Contractor in late 1994, a computer-based audio system was permanently installed into the United States Senate Chamber for the first time.

That work was performed between July 1992 and January 1995 by Colorado’s Peak Audio.

In September 2004, U.S. Senate officials awarded the modern version of that company, Cirrus Logic’s Peak Audio consulting division, a contract to replace that audio system with a newer version that would provide the same level of quality and reliability while taking advantage of newer digital audio distribution techniques and DSP technology. (See sidebar for details on the original design.)

After breaking off from Cirrus Logic in 2005, K2 Audio became an independent audio consulting firm based in Boulder, Colo., but stayed on the ongoing project as the consultant in charge. General Communications (whose Government Services Division is located in Manassas, Va.) served as the contractor responsible for the installation of the upgrade to the audio technology within the historic chamber.

A peek inside the Senate Chamber. Photo Courtesy U.S. Senate Historical Office.

Because the Senate Chamber cannot be taken offline outside of scheduled Senate recesses, the installation had to be done in stages. Initially, systemwide testing was completed and presented to Senate officials at K2 Audio’s office in Boulder. Most cabling was then installed during the summer congressional recess, with additional infrastructure work, including the replacement of most of the loudspeaker drivers in the Chamber’s Gallery, completed during the following winter recess. New desk units, signal processing, and distribution equipment were installed during last year’s summer recess, in time for the first use last September.

The audio system includes the functionality of an advanced delegate conference system, but it also includes support for advance monitoring, control, and processing. In addition to sound reinforcement, it provides the only audio feed allowed outside the Chamber, which is then used by radio and TV networks for broadcast distribution. Thanks to live coverage from C-SPAN any time the Senate is in session, the quality and correct operation of the system impacts millions of viewers each day.


The Chamber itself is a rectangular room that consists of two main areas: the Floor and the Gallery. The Gallery is a balcony that runs along all four walls and seats the audience, the press, and the audio system operator. In the lower center section of the Chamber is the Floor, where legislative sessions take place. On the Floor, a designated desk is provided for each senator. The desks are arranged in four semi-circular rows with each of the two major political parties grouped on either side of a center aisle.

Each senator, the legislative clerk, and the Senate’s presiding officer are provided with individual, always-on microphones. Each one of these input locations has a local mechanism for muting the microphone when it is not in use. Remote control functionality is also provided. Sound reinforcement loudspeakers are distributed throughout the Chamber for high intelligibility and even coverage in every zone. Although the system is typically used for daily legislative sessions, periodic special events, such as the swearing in of new senators, require the provision of many in-room and external auxiliary inputs and outputs.

Desks move frequently due to elections, changes in preference, and many other events that may alter the Senate’s structure. Therefore, the system needs to allow for rearrangement of each desk’s inputs and outputs. With more than 200 cable drops where desks can be located, the system allows the control and audio mix settings to be reconfigured accordingly.

The United States Senate Recording Studio (SRS) is responsible for the production and distribution of all audio and video feeds originating in the Senate Chamber. At press time, the SRS was in the process of relocating its entire facility to the new Capitol Visitor Center (CVC) being built on the northeast side of the U.S. Capitol building. This move would have placed the Senate Chamber equipment racks beyond the cable-length limitations of the technology used in the previous sound system. Although the previous system was extremely reliable — with no failures during its entire life — the move presented an opportunity to upgrade the technology and replace an old system for which some replacement parts were no longer available.

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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.


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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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.


At the center of the Chamber audio system are two sets of 10 MediaMatrix NION n6 DSP stacks that serve as the online and offline audio processing systems, providing full redundancy around the clock. These units handle all signal processing, routing, control, and monitoring. During normal operation, both DSP stacks receive audio from all system inputs. Meters and other monitoring functions on both systems provide SRS engineers with feedback that indicates that the active and offline systems are fully functional. CobraNet outputs from only the active, online system are enabled, leaving the offline system on standby as a backup in case of system failure.

The offline system is always processing audio, and it can be listened to before it is brought online. For analog and AES outputs from CABs, audio is switched using a custom-built relay system. Each system is used on a weekly rotating basis to ensure functionality. In the unlikely event that the backup system is required, the control logic switches output assignments and relays, providing an immediate audio failover to most devices, with a maximum transition time of only 3 to 5 seconds.

The high density of inputs, combined with the architectural challenges of the Chamber acoustics, required the use of advanced signal processing techniques in order to maximize the listening experience. For example, an advanced automatic gain control (AGC) process was put in place to dynamically adjust the gain for each channel based not only on the signal level at the input, but also on the number of open mics. Signal routing and mixing is performed using a 152×144 matrix mixer that allows operators to set an individual mix for each output.

During setup, an optimized mix is set for each senator’s desk unit. Because desk units are relocated fairly regularly, the system allows operators to easily adjust the mix according to the new locations, providing optimum sound quality, intelligibility, and feedback control. During setup, a base mix is established, but it is automatically and dynamically modified by the DSP system depending on gain adjustments from the AGC process. If a soft-spoken senator requires a significant amount of gain to be applied to his or her voice, the mix of that particular input to nearby locations is adjusted to compensate for the gain increase in order to avoid feedback.

At the software level, the audio system is comprised of three individual NWare Projects that interact with each other. Two parallel main projects run on the redundant DSP systems, while a central project running on a PC handles GUI, synchronization, and failover tasks. All critical project file parameters are kept in sync between the online and offline DSP stacks to ensure that if a failover is required, the system will continue to operate correctly.

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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.

SNMP technology and multiple GUIs located throughout the facility allow maintenance staff to monitor the status of every component of the new audio system (main control GUI pictured).


Facilitating preventive maintenance tasks is a key feature of the Senate Chamber audio system. All CobraNet I/O devices, control computers, NION DSPs, QSC power amplifiers, and Ethernet switches are monitored using SNMP technology. Multiple graphical user interfaces throughout the facility display the status of every component to the maintenance staff. The monitoring software is also tied to the SRS engineers’ pagers, and it will alert them within seconds in the event of a failure. Python scripts were developed to process the monitoring information and provide detailed data, allowing support engineers to be highly efficient.

Additionally, it would not be desirable to find out about the failure of a microphone at the moment a senator picks it up in order to speak. Therefore, custom tests were also developed that allow operators to test system inputs and outputs on a daily basis.

One such test provides graphical feedback to the operator and engineers and indicates the proper operation of the microphones and loudspeakers at the senator positions. Another test reports the state of the predictive mic muting mechanism at each desk unit. Because of their graphical nature, these tests can be run from maintenance stations located outside the Senate Chamber and do not require displacement of maintenance staff.

Access to the audio software and GUIs is restricted according to user level. SRS operators are responsible for the daily operation of the system. They sit inside the Chamber any time the system is in use and perform minor adjustments, such as tweaking the master level and turning a senator’s microphone on or off when necessary.

Although the system does not necessarily require any user interaction during use, an operator is always there to verify that things run smoothly. In order to do so, he or she is provided with a hardware control panel containing individual mute buttons for each senator, which can be relocated based on the actual physical position of a senator’s desk. The panel also provides individual on/off controls for the auxiliary microphone and line inputs, as well as basic system health indicators.

A software GUI replicates the functionality of the hardware panel and offers control redundancy. In the event a senator forgets to return his microphone to its holder after use, the operator can override local control and mute the microphone remotely. If a microphone is picked up in preparation for a speech, the operator can place it on standby while the senator prepares to speak. Each microphone can be placed in seven possible states.

Using the scripting language Python, a high-level user management system was developed to complement the control and monitoring functionality provided by Peavey NWare software. It allows GUIs to remain open and connected but locked in functionality until the correct user information is entered. It also allows management of operators’ shifts and preset recalls, as well as online modification of user permissions.

In addition to having access to all the monitoring functionality, support and maintenance engineers from the Senate Recording Studio are provided with a control and setup GUI that enables them to reconfigure the system for use in case of changes, such as the relocation of senator desks. Several other customized user GUIs are provided, including a display-only monitor showing the current active microphones for use by the video producers, signal dynamics metering pages, and advanced configuration screens. Because all of these user interfaces are managed by the audio DSP system, they can be accessed from any location in the facility that is connected to the Senate audio network.

While the upgraded hardware and software at the operator’s position maintained the functionality provided by the original audio system, the upgrade eliminated external control engines by incorporating all mixing and control functionality into the DSP system. The operator’s panel, previously processed using an external dedicated engine, is now controlled directly using custom scripting running on the NIONs. A JL Cooper control surface replaced a complex and underused broadcast mixer, allowing all broadcast mixing and routing to be performed by the digital audio system.

Python scripts were also developed to communicate with external hardware, taking advantage of the different technologies and functionality provided by each product, including SNMP, Telnet, RS-485, relays, and control voltage inputs. By working hand in hand with manufacturers, the need for an external control system was avoided, and all functionality was integrated to the DSP system.

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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.


The new Senate Chamber audio system has successfully replaced the high-quality and extremely reliable system installed almost 13 years ago. Although the requirements were demanding, the installation, testing, and commissioning were completed on time during scheduled congressional breaks, and they caused no system downtime. The additional monitoring functionality has already paid dividends by allowing the maintenance staff to identify any problems prior to use. The flexibility of a network and DSP-based system has allowed for system reconfiguration, updates, and adjustments to be performed post-installation as required without any interruptions in use, which is an absolute requirement for this system.

There were many key people involved in this project — including Rich Zwiebel, Deb Britton, Glenn Musser, and Ray Rayburn from K2 Audio; Gain Foster from General Communications; Bob Swanner and David Bass at the Senate Recording Studio; and Kevin Gross, Bill Lowe, and the entire Peak Audio engineering team — without whom the success of this project would not have been possible.

Garret MakiandRodrigo Ordoñezare consultants at K2 Audio in Boulder, Colo. Maki was lead designer of the sound system and specializes in acoustics and mechanical design for integration. Ordoñez specializes in network design and custom programming for audio systems. He designed the network monitoring system currently in use at the Senate Chamber.

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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.

Chamber System Design Comes Full Circle

By Rich Zwiebel

The U.S. Senate Chamber audio system described on these pages replaces one that first went online in January 1994. When K2 Audio was awarded the design of the new audio system, I found myself in the unique position of being part of a design team that was charged with replacing a system that I had designed many years ago.

I first became involved as a consultant on the Senate Chamber audio system in 1989 when I was with the Joiner-Rose Group. As we studied the needs of the Senate Chamber, we felt that a new concept was needed to best accommodate the needs of the space. The original system was quite old, and it worked on the principal of muting speakers whenever a nearby person spoke. This caused many problems with image shifting. The system developed at that time is described in great detail in an article that was published in the November 1994 issue of Sound & Video Contractor. The main concept was to use a digital audio system that used a unique mix-minus for each senator. This eliminated the image-shifting issues, provided greatly improved intelligibility, and avoided feedback.

At the time, configurable DSP, which allowed wiring on screen, was in its infancy. In fact, this system was one of the first projects specified using the Peavey MediaMatrix system. The power available in DSP products has grown so much since then that, instead of using a mix-minus for each output as we did in the original system, for the most recent upgrade we were able to provide a full 152×1 mix to each of the 144 outputs, including one for each senator.

The added DSP power available in the Peavey NION n6 units we used also allowed us to replace some of the outboard hardware with custom software-based devices we developed to run within the NION. Another benefit that we realized was the ability to use highly advanced custom algorithms, such as the dynamic mix adjustment described in the accompanying article.

When we designed the 1994 system, audio networking did not yet exist. Yet, we wanted to send a unique mix in a digital format to each of the 100 senators’ desks. Working together, the Peak Audio engineers and David Carroll Electronics developed a unique solution that carried AES3 digital audio over 120VDC power lines to each desk unit. This was indeed a unique solution, and it worked flawlessly for many years. This technique never became a standard practice, however, because during the intervening years audio networking became commonplace.

When we began the new system design in 2004, the entire K2 team felt strongly that we should use a standard technology for distribution, and we chose CobraNet. Using CobraNet allowed us to use products from many manufacturers, which greatly improved flexibility and capabilities.

Finally, one big difference is in the ability to monitor system health and to diagnose past events. We can currently do so at a level that was unthinkable with the old system.

The new system has kept all of the functional aspects that the Senate staff wanted to retain, while making use of current technology to greatly add to that functionality. However, one thing that did not change between the two systems was the need to spend a lot of time carefully adjusting the system, both with instrumentation and by ear to obtain optimal performance.

In the end, any audio system still requires human attention and skill to make it perform at its best. Technology itself does not provide good sound. And that is part of what makes this business so much fun, and what keeps many of the old-timers in the industry around. Perhaps some of my younger colleagues will be writing a similar article on the next upgrade to the Senate Chamber in, say, 15 or 20 years.

Rich Zwiebelis the president of K2 Audio in Boulder, Colo. He served as a consultant on the original U.S. Senate Chamber audio upgrade in 1994 and wrote an extensive article on the project for Sound & Video Contractor that year.

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