On Sat, Mar 27, 2010 at 11:47:04AM -0700, Ken Restivo wrote: > http://www.playgroundstudio.com/blog/?p=60 I've been looking a bit deeper into the peculiar shelf filter shown on that page. What is shown there is actually a second order shelf filter - the sum of the input + a second order highpass with some gain. This correspons to the blue curve, the one that 'dips' the most before starting to rise. The other curves are the result if you move the two sections of the highpass apart in frequency, gradually turning it into 1st order for the part that matters - this is the yellow curve. Now what is the origin of this 'dip' ? For a 2nd order highpass the part below the cutoff frequency has a phase that tends towards 180 degrees as frequency goes down and the slope of the filter approaches 12 dB/oct. At the point where the highpass_with_gain curve intersects the 0dB line, the phase shift has not yet reached 180 degrees, but it is well beyond 90 degrees so the real part is in antiphase with input signal it is added to, and that causes the 'dip'. In an analog implementation this would be difficult to avoid. So this 'dip' may well be an unwanted side effect rather than a wanted feature, with the real feature being the 2nd order nature of the filter giving it a slope that is almost double the normal value for a shelf filter. (look at the blue line: 11 dB difference from 2 to 4 kHz). This is relatively easy to implement, maybe I'll do it one of these days. Ciao, -- FA O tu, che porte, correndo si ? E guerra e morte ! _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/listinfo/linux-audio-user
