Matthew Robbetts <wingfeathera at gmail.com> wrote: > On 19 Mar 2013, at 18:34, "Alexander E. Patrakov" <patrakov at gmail.com> > wrote: > > > 2013/3/19 Justin Chudgar <justin at justinzane.com>: > > > I've created a module to ensure that only low frequencies are sent > > > to devices at the end of an "lfe"/"subwoofer" channel. This module > > > allows the user to select the master channel, the low pass cutoff > > > frequency (aka corner freq, -3dB freq) and the number of filter > > > poles. > > > > Sorry, I cannot sign off this filter implementation. > > The filter implementation looks fine to me. Do you mean the coefficient > calculation? I don't really know, that was a blackbox test. Will look again today. > > First, the filter currently fails the "attenuation must be 3 dB at the > > cut-off frequency" test that all Butterworth filters must pass. > > [That's not a very clear way to describe things. A filter is either a > Butterworth or it isn't, and that is determined by ts transfer function. > Specfic aspects of its frequency response are really side-effects of its > Butterworthiness.] Well, what is implemented is certainly not a Butterworth filter of Nth order with a given cut-off frequency. > Anyway: in what way does the filter not roll off correctly? I have not tested the roll-off. Only the statement about the -3 dB frequency. > The coefficient calculation looks to me like a quick swipe from the Audio EQ > cookbook (I haven't verified that). Those formulae work pretty well, so > if this filter isn't working right then presumably Justin has made a > small error in that somewhere. Quite possible. Or it may be that I have misunderstood the expected result. Looking again at the code, in do_filter() I see a strange loop over the poles that, essentially, filters the signal through the same biquad (and not through N/2 biquads with independent history) over and over again. > Justin, a relatively easy way to verify your coefficient generation is > to use Matlab's (well, Octave's!) 'butterworth' function. This will get > you the coefficients you need directly. > > > > Second, Q=sqrt(2) is only valid for a 2nd order Butterworth filter. > > Well, to be clear, it *makes* it a Butterworth filter. The Q in the > transfer function is the only thing distinguishing a Butterworth from > any other filter you can make with a biquad. Higher-order Butterworth filters use more than one biquad, with different values of Q. But indeed, a 2nd order Butterworth filter takes one biquad with Q=2. > Although, since you want him to use a cascade of Butterowrth filters, he > surely *should* use Q=2? Yes, for Butterworth filters of 2nd order. However he wrote some code that pretends (if I understood the intent correctly) to do arbitrary order lowpass Butterworth filtering with a given cut-off frequency. Given the request to use a fixed-order filter, I'd expect Justin to rewrite that code anyway, and I hope that the resulting filter will be correct. As for LR4 vs LR6 vs LR8, I don't care for now which one is chosen, as long as the choice is documented. -- Alexander E. Patrakov Sent from Nokia N900
