Technical Notes
Our VXI Products Make Bose Speaker Tests Sing
THE CHALLENGE
No self-respecting audiophile (or weekend CD warrior, for that matter) needs an introduction to the Bose name or the sparkling array of speaker systems the company produces. One pictures a factory automatically stamping out housings, affixing coils and magnets to paper and metal, inserting the finished products into boxes, and shipping them out. But HOLD ON! There is one very important step to add to this process in order to guarantee those endless hours of listening pleasure: every Bose speaker must be 100% tested before those boxes get sealed.
Thus, the challenge: how to translate "sounds great" into a consistent, reliable testing process which doesn't bog down that assembly line and deprive awaiting listeners of their little piece of audio heaven. This was the concern of Frank W halen, a Test Systems Design Engineer at Bose Corporation, when gearing up for production of the Bose Lifestyle ® 20 audio system featuring the new Jewel Cube ® speakers. "We used VXI before on another line of products, but realized that this prior implementation was not appropriate for the Jewel Cube line. We knew VXI could do the job, but some important features of VXIbus were not supported by the vendor we had chosen previously." Testing of the Jewel Cube transduc er (the actual cone/magnet/coil assembly itself) would be more complex, and the testing cycle would need to be many times faster.
THE SOLUTION
Bose Corporation has long enjoyed the enviable position of being both leader and innovator in its industry. Its products reflect a uniqueness born of Dr. Bose's theories on psychoacoustics, the science of the human perception of sound. It's not surprising then that Bose would stick with the theoretical favorite, VXIbus, for Jewe l Cube testing. They would simply need to find VXI instruments which properly utilized the unique system features that VXIbus offers.
The system architecture for the VXI test system is illustrated in Figure 1.
The transducer Unit Under Test (UUT) is placed in a specially constructed acoustic environment and monitored at some number of specially designated locations wi th specially selected microphones-the details of which represent untold years of optimization research and are of immense competitive advantage. (Sure, they could have divulged more details, but then, of course, they would have to silence me...with real silence, and I know they have the anechoic chambers to do it!)
The UUT is driven with audio waveforms from an Arbitrary Waveform Generator (ARB). The waveforms-different for each subtest-may comprise up to 4 Mbytes of 16-bit sampl e points, representing anything from simple sine waves to multi-period, evolving, complex waveforms. The output of the transducer UUT is measured by the microphones and makes its way back through an Inter-Connection Assembly (ICA) to a VXI signal conditioning module for gain and anti-alias filtering. The conditioned audio signals are then applied to an Analog-to-Digital Converter (ADC) and the digitized data transferred through a Slot-0 controller to a SUN Sparc5T workstation for digit al analysis. In addition, a small number of TTL control signals pass between the VXI hardware and the buttons, LEDs, and custom electronics of the Test Booth (via our V387 Digital I/O module).
MORE CHALLENGES
Several potential difficulties must be overcome if this system configuration is to meet all of the testing criteria:
- Some subtests must run at alternate sampling rates. For these tests, a different set of microphone channels must be automatically selected for routing to the ADC. NO wiring changes are allowed from one subtest to another.
- Transfer of ADC-sampled data to the computer must not interfere with the provision of stimulus sample points to the ARB waveform generator.
- Relative timing information between the excitation waveform and the digitized signals from the microphones is important in some cases. Thus, clocking output points at the ARB generator must be "locked" in step with the AD C sampling.
- Allowance must be made for possible expansion to include real-time digital signal processing of sampled data.
- Speed, speed, SPEED! The tests must run faster than on previous production lines; the development cycle for the test stand-hardware and software-also must be much shorter.
ULTIMATE SOLUTIONS
The automatic selection of channels to be routed to the ADC module is not only handled without wire changes but without wires altogether. The KineticS ystems Corporation V207 ADC and V252 Gain/8-pole Filter modules cards are part of a larger family of analog modules which employ MUX-busT communication. This is our implementation of an analog multiplexing/communication mechanism over the VXI "Local Bus"-a private intermodule, daisy-chained bus created for just such purposes. The choice and order of channel selec tion is programmed into Scan Tables of these modules before each subtest.
The analog signals are then transferred over environmentally isolated, Local Bus backplane lines. "We're always happy to minimize labor-intensive front-panel wiring," says Whalen of the MUX-bus alternative. (Amen to that!)
As for point 2 above, potential data transfer conflicts are avoided by utilizing the extensive buffering capabilities within the V285 A RB and V207 ADC. Each has onboard memory options large enough to avoid having to fill or drain data over VXIbus while the tests are running. "Though the V285 and V207 can execute 'on-the-fly' VXIbus transfers, we were able to utilize the conceptually simpler, basic transfer modes in this application. The advanced multi-buffer transfer capabilities will be useful in future applications requiring additional speed and performance."
The issue of most concern in developing the Jewel Cube test platform was the need to "lock step" the ARB output and ADC sampling from an asynchronous start-like a button push. VXI does provide Trigger Bus lines to communicate timing, but the degree of implementation is vendor dependent. All of the KineticSystems modules in this system have sufficient programmability to pass and receive pertinent timing information over all eight TTL Trigger lines. Thus, a "Start" digital control signal is passed to the V207 ADC over one Trigge r line, and, in response, the ADC sample clock is passed across a second Trigger line to the ARB to be used as its output clock.
This VXI setup is even flexible enough to take on such jolts as real-time digital signal processing (currently under consideration) without disturbing present module communications. A V165 DSP could grab data directly from the V207 ADC using a private Digi-busT path-the digital equivalent of our MUX-bus protocol over the VXI Local Bus.
Finally, regarding the speed issues, Mr. Whalen cites several material points. "The use of register-based VXI instruments was a big factor. Direct access to registers and dual-port waveform memory means lightning-fast data transfers as well as much simpler programming which shortened the development cycle. On top of that, devices like the V252-with filtering, programmable gain, and multiplexing all wrapped into one cohesive unit-save on space, programm ing, and the need for custom electronics. Also, the KineticSystems concept of multiple communication options between modules gave us the flexibility to tailor the setup to our needs instead of wasting a lot of time rethinking our concept to fit the hardware." The end result: a 2-to-1 reduction in the development cycle compared to previous test systems!
Bose Jewel Cube testing is representative of most acquisition and control applications in that its success ultimately depends on the many detailed interactions of its component parts. The choice of platform-in this case, VXI-can only promise the potential for this success. It is incumbent upon the designers of the component instruments to deliver on that potential with devices that flesh out system architectures in consistent, powerful, and creative ways. "The key to the success of VXI for Jewel Cube testing," says Whalen of Bose Corporation's latest foray into VXIbus, "is how well these modules work together as a family."
Modules Used In This Application
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