What is the Adjustable Low Pass Filter "Step" mean?

Hello guys,

Can someone help me to understand what is the " Adjustable LPF " function block which has one parameter call "step", what is this actually work at?

I am using ADC to adjust the Low pass frequency, so I choose this block, but I search the wiki, I get nothing explain on the step definition. 

here is my Adjustable LPF block: (I am using ADAU1701)  (Step is in the function block below the curve ( the step parameter is 12 ))

Best regards,

Alvis

  • Hello Alvis,

    It is the number of steps it will use to slew between two filters when the input changes from one curve to the next.

    It looks like you are using 40 curves. This will use up a lot of memory and you probably do not need to use the step size at all. Think if it this way. You are using 40 curves to make it smooth.

    If you used the default of four curves, with the step set to 12, there would be 12 steps between the four curves, so 3 x 12 = 36 plus the four curves themselves will also give you 40 curves with much less memory usage but the devil in the details that this is 40 curves when changing curves. Once you select the next filter index and let it sit it will only sit at one of the filter curves and not in between. So you have many steps for the transition but not once the filter is changed. After all, the input is an integer.  

    So if you want 1dB of resolution then you need to have enough curves to give you that resolution. The step will do a good job of slewing between the settings making the transition smooth. So if you have 40 curves in your design with the thought that you want it to be smooth then you can lower the number of filters.

    Let's see... from your screenshot I see you are going between 40 and 120 Hz. That is 80 Hz difference so 40 curves is 2Hz between curves. I don't think you will be able to hear the difference between two of the filter settings so you can probably make the number of curves be much less. Generally, if I think of a frequency knob in most HPFs I have seen, often there are only three of four choices. I would think 10 or 11 would be tough to hear the difference between two of the curves. Going to 20 will for sure be very fine control. 4Hz! So I think you can go down to 10, (or 11 if you need it to go to 11) That would give you 8Hz between curves. Then set the step to something like 10 and you will get a smooth transition. 

    So lower the amount of curves a lot and then listen to it. Try to compare them and I think you will find the slew works quite well.

    Dave T

  • Hello Dave,

    Thank you for answering my question.

    So I think you can go down to 10, (or 11 if you need it to go to 11) That would give you 8Hz between curves. Then set the step to something like 10 and you will get a smooth transition.

    From this quote, if I reduce the curve number, and to try your way, from the above relation which you explain the curve and step, curve: 10 and step: 10 between the 40Hz to 120Hz, there will be 1 x 10 = 10 curves between 40 to 120 so there will be 10 + 2(40Hz & 120Hz) = 12 curves or is 8 + 2(40Hz & 120Hz) steps?

    Thanks!

    BR,

    Alvis.