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.
DIGITAL SIGNAL PROCESSING
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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