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Signal processing issues

Using current state of the art technology [221,352], a typical resolution of 0.02 mm and an overall accuracy of 0.1 mm can be achieved. Taking only into account the bandwidth of handwriting movements [338], a Nyquist frequency of 10-14 Hz would indicate a sampling rate of 20-28 Hz. However, since this means that there are only two sample points per stroke in a character, interpolation (e.g. by convolution) is absolutely necessary to recover the shape, and also the delay between samples will be annoying in interactive use. Using higher sampling rates is generally a cheaper solution. The most often used sampling rate is 100 Hz for handwriting, 200 Hz for signature verification, and the minimum for interactive use is probably about 50 Hz, yielding a delay of 20 ms, which happens to be 20% of a stroke movement. Another solution is to use spatial sampling, generating coordinates only if a position change above a given threshold is detected. This is similar to most types of sampling in the mouse. However, the disadvantage of spatial sampling is that it is not a proper time function, such that operations like digital filtering are ill-defined. This drawback is sometimes counteracted by adding time stamps to each detection of position change.

Thu May 18 16:00:17 MET DST 1995