All-Digital Anti-Alias Filter Module
Anti-Alias Module Released
Bellevue, WA, October 8, 2012 – Microstar Laboratories, Inc. has announced the release of the Anti-Alias Multichannel Filter software module. This new addition to the timing and machinery software tools for the xDAP data acquisition systems leverages the reserve processing capacity of an xDAP system, which supports the digital signal processing, to provide a completely software-configurable, completely digital implementation of multichannel anti-alias filtering. This will eliminate the need for hardware-based anti-alias filters for most applications.
A Revolution in Sampling Theory?
No. The fundamentals of sampling remain unchanged. When a signal is digitized, frequency bands located at multiples of the sampling frequency are aliased by the sampling. After that, you cannot tell which frequency – or combination of such frequencies – was actually present in the original signal. While intentional aliasing can be beneficial in special cases when used to "undersample" high frequencies in a known band, it can become a serious problem when high frequencies beyond the Nyquist limit unintentionally corrupt data sets that should have been straightforward measurements of speed, pressure, position, vibrations, and so forth.
The traditional solution is to lowpass filter the analog signal prior to digitizing. The filter rolloff must be located well above the frequency band that you want to preserve, avoiding gain and phase distortions, but sufficiently far below the sampling frequency to attenuate aliasing hazards effectively. This is a good, but not perfect, solution. The high-order filter required to do this must satisfy demanding specifications, and it does not come cheap. Even so, the performance is still a compromise.
An Alternative: The Anti-Alias Module
The essential idea is that you can't alias what isn't there. By sampling at a rate that is far beyond the bandwidth that your sensors can drive, 100K to 200K samples per second for example, you capture not only all of the relevant bandwidth but also everything far beyond that, until there is no signal energy left. With everything in the signal fully represented, there is no aliasing. No precise filters are required, typically nothing beyond normal signal line termination. The drawback is a severe excess of samples.
If you naively reduce this sample stream to a lower and more reasonable rate, you are back into the same aliasing problems you would have when sampling at the low rate originally. Rate reduction in the digitized data cannot be done safely as long as frequencies are present above the Nyquist frequency of the reduced sampling rate.
This is where the Anti-Alias Module comes in. One script line will automatically configure digital anti-alias filtering on each channel to eliminate all of the frequencies that could possibly alias to the band that you care about, aligning the filter cutoff to the band you select. There is effectively zero amplitude distortion (less than 1 LSB of 16 bits) and zero phase distortion (less than 0.01 radians) from frequency 0 through 80% of the Nyquist frequency at the output data rate you specify. This is far beyond the capabilities of conventional analog filtering.
The xDAP Platform
xDAP data acquisition systems from Microstar Laboratories have been available since 2010. The xDAP systems can support continuous operation with 16 channels capturing up to 500K samples per second on each channel, driven by an embedded Intel processor – with plenty of capacity for the intensive DSP processing needed to support the new Anti-Alias Module. You can also configure supplemental processing using standard features of the onboard DAPL 3000 software engine that runs the xDAP system – such as "smart" data selection through software-based triggering.
The Anti-Alias Multichannel Module will be distributed via online download at $95 US per license. For order information, contact Microstar Laboratories, Inc. For the next 90 days from the date of this announcement, the module will be provided free of charge upon request from all customers purchasing an xDAP system.
Conclusion and Next Steps
The new Anti-Alias Multichannel Module applies the DSP power of xDAP systems to substitute processing capacity for analog filtering hardware, achieving technically superior results. It is entirely software configured and self optimizing, delivering digitized data at any desired rate, while eliminating aliasing and preserving retained frequency bands to an exceptional degree of accuracy. The xDAP system is premium-grade data acquisition equipment, suitable for demanding multiple-channel applications with rigorous performance requirements. The elimination of all the analog filtering electronics can almost pay for the whole system.
Visit the Microstar Laboratories, Inc. Web site for additional technical details.
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A new Anti-Alias Multichannel Module, for applications within the embedded environment of the DAPL 3000 operating system on an xDAP Data Acquisition Processor from Microstar Laboratories, implements multiple channel all-digital anti-alias filtering, with superior performance and attractive system cost compared to traditional analog filtering schemes. This filtering is completely software configurable with one line of script, and features flat response (within 1 LSB) and zero phase shift (within 0.01 radian) from 0 through 80% of the Nyquist limit at the configured final output rate. Additional information is available at the Microstar Laboratories Web site.
Note to the Editor:
Please go to the editor's section of the Microstar Laboratories Web site to download the associated image. Microstar Laboratories suggests this text as a caption: Digital filtering in the Anti-Alias Multichannel Module provides superior anti-alias performance for the xDAP data acquisition platform.
Microstar Laboratories, Inc. claims Microstar Laboratories, Data Acquisition Processor, xDAP, DAPL, and DAPL 3000 as trademarks. Intel Corporation claims Intel as a trademark.
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