The windowed sinc technique of designing FIR filters is a useful technique that is understandable and usable. The results may need some amount of trimming, but all in all a useful technique to know. The free javascript available from the Signal Processing Group Inc website provides a working prototype to use for windowed sinc design. The available javascript uses a Blackman window, but other windows can easily be used in it. There are only two other parameters specified by the user. M, the order that should be even and fc, the cut off frequency. Once run, the script gives the impulse response. which in turn can be used to plot the frequency response. Please visit the SPG website for more technical articles and information.