Biquad Filtergraph, high pitched sounding

    Sep 04 2013 | 1:43 pm
    Ive been playing around with biquad and filtergraph and just as a test I fed in the same values as in a bell curve on EQ8 (in ableton). Then I sweep from lowest to highest frequency value (using an LFO). However, biquad sounds completely different to EQ8 (really high pitched, no low end, with peaknotch and resonantfilter). How can this be when I have fed the same values into it? Surely it is just sweeping past exactly the same frequencies?
    My guess is that biquad is doing something exponential and spending more time on the higher frequencies, and therefore resulting in a higher pitched sound. Ive tried using logarithmic and linear and linear is the only one that sounds close. Should I be using some kind of exponential curve to focus more on the low end of the frequencies?
    Would it be better to do this type of analysis with a svf~ filter?, or is there something obvious I might be doing wrong with biquad thats causing such different results?

    • Sep 07 2013 | 3:16 am
      Anyone have any opinion on this?
      It seems to imply that biquad filter is just "not as good" as the filters in ableton EQ8. Ideally I would like to rule this out, or continue trying to find a good setting for the use of biquad.
    • Sep 07 2013 | 5:36 am
      Different filters often sound different, even if you feed them the same values. Also keep in mind, that a biquad is a very basic structure for filter design. It is capable of producing all the basic filter types (lp,hp etc.) but I would expect, that for "professional" eqs this is just the starting point. By that I mean, that you use a biquad as a tool for implementing a good sounding eq, but it is not neccesarily a good sounding eq by itself. Still, for simple tasks a single biquad is sufficient and you might get good results, if you just feed other values to filtergraph than to eq8. SVF implements another kind of filter structure, that is very useful for synthesizer filters. I think it behaves nicer when modulated (sweeped), but I'm not really a filter expert. And again: feeding the same values to svf as to eq8 will probably sound different.
    • Sep 07 2013 | 11:47 am
      Hi David,
      Thanks for the reply. Based on this I changed the exponent of the scale object to focus more on the low frequencies and sure enough the filter sweep now sounds just like the eq in ableton.
    • Sep 07 2013 | 3:13 pm
      The scale object can use normal exponents if you give it @classic 0 as an attribute.
      The classic mode (default) uses a very cryptic formula for calculating the curve. With classic mode off, it's y=x^power where x in [0,1].