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Low cost hardware, software help craft DSP filters without coding

Posted: 17 Feb 2005 ?? ?Print Version ?Bookmark and Share

Keywords:saelig? pc? windows? signal wizard ii? filter?

Trust Saelig to come up with yet another unique cost-effective PC-hosted product. If you're designing audio filters (up to 24kHz) this is a nifty way to go about it. And, if you take advantage of Saelig's offer, you can realize a cost savings on this unique development platform.

On a Windows PC clocking at 400MHz or better, Saelig's Signal Wizard II lets you design and run high performance filters, with realtime control of gain and sample rate. You can even instantaneously switch between a filtered or non-filtered signal at run-time.

No code DSP
Signal Wizard II essentially gives you easy-to-use DSP for any audio-bandwidth signal filtering. A dozen sample rates are selectable from 4kHz to 48kHz, with signal levels between zero and 2V handled at the filter's input, and 0 to 2V signals available at the output.

Also, although the device samples at 48kHz, it's also possible to specify custom-designed versions for sample rates of 96kHz and 192kHz. That would make this system useful for things such as ultrasonic applications.

In addition to audio signal processing, you can likely use this for tasks such as low-speed sensor signal conditioning, or vibration analysis. The software runs under Windows 98, Windows ME, Windows 2000, Windows NT or Windows XP.

In use, the system's high-level PC-based software is what helps you craft the filter performance. Once you're satisfied with your filter's response, you download the filter to the hardware module, and then execute it. The download is performed over an RS-232 serial link running at 115.2kbps. The hardware is based on a Freescale Semiconductor DSP chip.

Invisible firmware
The associated DSP hardware module and its firmware then implement the filtering operations. The hardware even includes a stereo CODEC with 24bit resolution, and the filter module can store up to eight filters in its non-volatile memory.

Although you might be able to do some audio filter design on a PC equipped with a 24bit soundcard, the Signal Wizard II's dedicated hardware kicks it up a notch or two. Once configured, you can even run your filter without the host PC.

This typical screen image shows impulse responses imported and convolved in realtime.

"Filters constructed using DSP offer advantages over conventional analog approaches to filtering," notes Saelig president Alan Lowne. "They're inherently flexible, since changing the characteristics of the filter involves changing only program code or filter coefficients. There's nothing to solder or hook up.

"DSP filtersbased on codeare also entirely immune to the effects of aging and environment. This makes them quite suited for very low-frequency signals, too."

Lowne points out that the Signal Wizard II can create FIR (finite impulse response) filters. FIR filters can be crafted as multiple pass types, stop or arbitrary filters, and as impulse or frequency response import modes. You can experiment with Butterworth filter responses, and all major types of Chebyshev filters (to any order). Single-channel wave (.WAV) files can also be filtered directly using any FIR filter designed by the system.

Impulse and frequency responses are also exportable as text files. That supports things such as post-processing in an Excel spreadsheet.

You can also run passive analog networks, with zero-phase distortion or compose filters with arbitrary phase relationships. The Signal Wizard II will handle rectangular, Bartlett, Hamming, Hanning, Blackman, and Kaiser window functions, too. You can also run de-convolution (inverse) or flipped filters.

The system also supports IIR (infinite impulse response) filters. These can also include Butterworth and Chebyshev types. It will accommodate arbitrary (pole-zero placement) and do IIR to FIR translation.

Adaptive filtering
But wait; there's more. The system will also let you create adaptive filters. You can set up and run true (dual input) adaptive filter for broadband or narrowband noise cancellation. You get true dual-channel 24bit resolution operation, with independent filters for each channel. The system supports channel mixing and inversion as well. You can even crank in channel delays ranging from 21us to as long as 1.8s.

You could even use Signal Wizard II to set up a realtime 2-channel oscilloscope and/or spectrum analyzer, and let it handle data capture, to boot. Very nifty. You can use the system to plot impulse and frequency responses as magnitude, dB, square, root, real, imaginary or phase, using linear or logarithmic frequency axes.

- Alex Mendelsohn

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