PXI-Based Audio Matrix Switch with Touch Screen Control

Author(s):
Ken Lawton - Government of Alberta, Alberta Infrastructure

Industry:
Electronics, Electromechanics/ Electrotechnics

Products:
Switches, PXI/CompactPCI, LabVIEW

The Challenge:
Finding a backup sound solution to a 12-year-old custom-built operator console for a digital audio reinforcement system for the fall session of the Alberta Legislature

The Solution:
Implementing a complete backup solution using a PXI/SCXI combo-chassis, a 21-inch LCD touch screen, and five NI PXI-2501 FET switch modules to form an audio matrix switch.

Senates and legislature broadcast audio applications feature a large number of microphones controlled by an operator at a console. The state-of-the-art solution today typically involves digital audio technology incorporating digital signal processing. This paper describes an application using National Instruments hardware and software to produce an analog audio matrix switch controlled by a 21-inch LCD touch screen console. The end result is a system for the lowest cost and considerably less complexity.

In 1992, the Government of Alberta invested $500,000 in a state-of-the-art digital sound system for the legislature. The universal digital audio processing system, or UDAPS, digital signal processor array performed the sound system function. The installation included a custom-built operator’s console for selecting the microphones. 12 years later, this custom, one-of-a-kind console failed. Though the console was repaired, the likelihood of a repeat failure necessitated a backup plan. We hired AVW-TELAV to set up and operate on standby a distributed conferencing system that cost $16,000 to rent for the remaining week of session.

Our project team was in the process of drafting the executive summary for the project to replace the UDAPS when the failure occurred. Attempting to shorten the project by so many months would force the new design toward a modified conferencing system. Instead, a backup solution was proposed that would buy more project time and protect against potential UDAPS failure for the coming fall and spring sessions. The final performance of the UDAPS, with the backup system in place, would be the Alberta centennial celebration in May of 2005, during which Queen Elizabeth II was scheduled to speak to the legislature.

In the end, the National Instruments standby backup system was built for less than the $50,000 budgeted, saving $100,000 overall – not to mention the potential embarrassment of a system failure during the royal visit.

Hardware

The existing UDAPS audio system provided for up to four active microphones at one time. Using National Instruments PXI-2501 FET switch modules configured as 2-wire quad 6 x 1 multiplexers to route audio worked perfectly when only one input was active per bank. With two or more channels active at a time, the output level was attenuated proportionally. This attenuation effect could be compensated for using an automatic gain control or other means, but there was another alternative.

With some simple changes, each NI PXI-2501 could switch 24 channels of line level audio without a signal attenuation. Although the PXI-2501 has an operational amplifier on board, an Analog Devices OP275 operation amplifier with a feedback resistor per side was used in an external summing amplifier chassis for the best sound performance.

The complete computer control package includes an NI PXI combo chassis with the five FET boards, and an NI PXI-6052E analog input multifunction data acquisition device for fault and override inputs; an analog input and output are used for the volume control in the external summing amplifier. In the SCXI portion of the chassis, there are four NI SCXI-1127 modules, each with 32 individual relays that are used for indicator LED’s on each desk. The processor is an NI PXI-8186 running Windows XP, a 21-inch LCD touch screen by Stealth; and an SV-2100 with RS232 mouse emulation. The LCD was located at the console operator position using an NTI XTENDEX KVM with RS232 over CAT5.

Sound Reinforcement

Legislature members’ desks each contain a microphone, speaker, and LED. Obviously, connecting to 100-plus speakers for hot swapping was unlikely. Instead, four Frazier 1482 speakers would be set up in the public galleries for reinforcement in the event of a main system failure. Using only one speaker in each corner generated acceptable reinforcement without sound localization issues. The design worked perfectly for several functions when the UDAPS became unreliable.

LabVIEW

The first step was to decode the protocol between the console and the UDAPS, then build a LabVIEW program to emulate the original console. Once the protocol was determined, then the final VI was created to emulate the UDAPS, the console, or both depending on the mode of failure of the existing system. The concept allowed both systems to run in parallel, providing hot standby for television broadcast and secondarily audio reinforcement after a short recess.

The virtual console buttons are read into an array of button values; on a change in console switch data, the sequencer steps through each bit and carries out the correct response to a change in switch value. While the switch pattern is not changing, the software tests the current FET and relay switches to confirm that only the correct points are active. Special functions such as the speaker's override and manual and automatic modes increased the complexity of the system considerably beyond the basic switching function.

For more information, contact:

Ken Lawton

Government of Alberta, Alberta Infrastructure

Room 122, 10800 – 97 Avenue

Edmonton, Alberta, Canada

Tel: (780) 427-4939

E-mail: Ken.Lawton@gov.ab.ca

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