Let's design a digital filterDecember 2006Digital filters in the form of DSP-based Fast Fourier Transform solvers are to be found in many commonplace digital electronic products, but don't be fazed by the mathematics. There are many software packages that can help you design the right digital filter for your product with the minimum of effort The performance of digital filters far outstrips that of their analogue counterparts. They offer very low signal-to-noise ratios, can achieve virtually any filtering effect that can be expressed as a mathematical function, and don't require a circuit design involving numerous discrete electronic components. Digital filters have become an essential element of many compact, portable devices that we take for granted today - mobile phones pocket media players, DAB receivers to name but a few.
There are two main types of digital filter based on the Fast Fourier Transform function, which can be solved exceedingly quickly by modern DSP devices. These are the Finite Impulse Response (FIR) filter (inherently stable as they don't require feedback) and the Infinite Impulse Response (IIR) filter (requiring internal feedback but very versatile). Common examples of the latter include Bessel, Chebyshef and Butterworth filters. Fortunately, you don't have to be an accomplished mathematician to design a digital filter, thanks to the numerous software packages that are available - some of which can be downloaded free from the Internet.
Digital filter design often involves engineering trade-offs to balance desired characteristics with implementation limitations. National Instruments' (www.ni.com/labview/design/dfd/) LabVIEW Digital Filter Design toolkit simplifies this task by providing a single graphical development environment to apply all aspects of the design process, including initial filter specification, filter design, design validation, prototyping and final hardware implementation.
Digital Filter Plus from Numerix (www.numerix-dsp.com/dfplus/), now in version 2.30, can be used to design both FIR and IIR type filters. A free demonstrator is available with the complete filter design toolset to help you get acquainted with the software. However, you can only save the filter coefficients of your design to disk when you purchase a developer's licence.
The data analysis and presentation software package, FlexPro (now in version 7) has a new digital filter design option that is simple to use. Available in the UK from Adept Scientific (www.adeptscience.co.uk), the FlexPro Analysis Wizard merges steps for filter design, filtering and presentation into a simple sequential process. Using real time 2D and 3D graphics, the software provides instantaneous visual indication of your filter's characteristics as you change the filter specification. The package can be used to design IIR and FIR filters as well as smoothing and channel frequency class filters.
Kane Computing (www.kanecomputing.co.uk) offers its QEDesign family of filter design software packages in five versions according to your needs. Suitable for both FIR and IIR filter design, these include the QEDesign Lite (student version), QEDesign 1000 ('Pro' version) and QEDesign 2000 ('Premier' edition). In addition, the company offers its QEDesign ADS (advanced design series) packages for FIR filter design, covering sample rate conversion and multi-rate digital filters.
The coefficients produced by Tyder's (www.tyder.com) ONEoverT digital filter design software can be used with all DSPs, and with its add-on module you can create filters for all FPGAs and ASICs. Available in stand-alone or network versions, the software can be downloaded from the Internet. A demonstration version can also be downloaded - all features are the same as the full versions, except that there is no file output.
And what about that freeware I mentioned at the beginning? Well, there are many, but worthy of note are DisPro and WinFilter. DisPro (www.digitalfilterdesign.com) is a professional-level software package for the design and evaluation of digital filters. It was the first commercially available filter design package to run on the IBM PC (it was introduced in 1983!) and is now offered as freeware by its creator, Dr John O'Donnell.
Meanwhile, WinFilter (www.winfilter.20m.com) can be used to design FIR and IIR filters as well as being able to generate the necessary C and VHDL code. The presentation is good, with a dialogue box to the left of the screen and graphic representation of filter performance to the right.
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