[pulseaudio-discuss] New Module: module-lfe-lp

[Justin Chudgar]

On Wednesday, March 20, 2013 09:48:56 AM Alexander E. Patrakov wrote:
> 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
> _______________________________________________
> pulseaudio-discuss mailing list
> pulseaudio-discuss at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/pulseaudio-discuss

So, since this discussion is a bit over my head, let me see if I understand. 
The recommendation is to use a fixed 2nd order butterworth biquad. That is, 
remove the lppoles-# option and just allow user selection of cutoff frequency?

If that is correct, I drop the RC filter function, and keep the existing coeffs 
calculations and the existing biquad, removing the code to enable cascading.
Do I understand correctly?

And thanks for being patient with my ignorance.