• Modelling Digital filters (Z transforms/difference equations) with the

    From robin48gx@21:1/5 to All on Wed Sep 11 00:39:47 2019

    I have been able to model the frequency response of digital filters on my HP48 for years now but thought I would share
    this info, make sure you are in radians mode (RAD)

    First make a directory, called say, DSP

    Go into the directory and make the following


    This means e to the power of -i is substituted if you use 'Z' in equations

    Create the following RPL program

    << ABS LOG 10 * >>

    This program can be use to produce a logarithmic response
    for a filter

    Now type in a Z transform for a digital filter.
    Lets say the old simple LAG filter, the 0.1 of the new value plus
    0.9 of the old. In 'C' this would be

    lag = lag * 0.9 + newval * 0.1;

    But as a Z transform is:


    Put the Z transform on the stack
    Apply ABS

    Store the new equation to 'EQ'

    Go to plot

    Set xrange to 0 3.142
    Set y range to -40 40

    Plot the magnitude response in DBs of your lag filter

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