So I’m back (after a year of hiding )
So rephase is going good a new release is out 1.4.1
It has some new features worth checking out.
However on the topic of phase correction and phase manipulation I would like to chat about all pass filters
Many of your dsps have all pass filters and maybe I can shed some light on there operations
What is an all pass? It’s a filter. There’s a few different types , I’ll talk about 1st and 2dn order all pass (for audio)
So let’s dig in.
First we need to discuss a few things. First the complex plane. The complex plane is an imaginary line that is where each pole on the filter is placed. Poles are exactly what they say. Poles like positive and negative right....not exactly but close. If a input and output signal have a shift in phase to or more than 360° another pole could be said to be added , but in coefficients it’s dosent quite work like that , it’s the calculation that can add poles
For instance a filter with 90deg would have one pole at -1/rc and a zero at rc. Which makes them an inverse of each other. So the delay at the frequency would add the pole.
So in a 2nd order filter for instance the shift is frequency dependent. And the shift center frequency is at 1/4th the wavelength, which is called quadrature. So on your all pass control which is most likely a 2nd order all pass, the frequency you pick is when the input and output are 1/4th wavelength apart or 90° respectively . The Q of the all pass says how fast the input and output rise and fall from 0° to 360° .
So when you add a all pass filter , the signal goes in , and when it reaches its center it is like a mirror effect putting the output to 360° , so if it goes to 360 at lets use a center of 400hz , that means the input and output have a one cycle delay of the wavelength of 400hz right? Almost, it has a delay, but not quite a full cycle, it’s a quarter cycle delay between poles. So what’s the wavelength of 400hz 33.9 inches or 2.51ms a quarter of that is .62ms. So that added group delay can be a added or subtracted or by 1/2 from the input or output to “alignment “ with another crossover or speaker, but in a car , that’s not what ere listening for.
You could certainly measure your car, window the response and do all that playing around with measurements and apply the all pass and it could definitely make things worse sounding.
I would suggest using an all pass filter as something you can listen to real time.
So where do you place these all pass filters ?
You have to get to know how your car behaves. It’s not something rew can listen for it don’t know if your stage is high or the vocal is high and center. Is there that one annoying frequency that pulls your phantom center to the left or right, or is there a severe combfiltering notch that just won’t quit eating up all the ambiance and sucking the impact out of your system. An all pass might be a great help to solve some of these issues for one seat.
Knowing the all pass goes “in phase” at 360 degrees which is essentially zero degrees with added group delay, you can move only some frequencies phase in a line, how steep that line is is the Q. A Q of 50 would move the phase from zero to 360 inmaybe a span of just a few frequencies where a Q of .5 will slowly shift the phase over the span of maybe entire octaves depending on how high or low the center is placed so if you have a deep combfiltering notch at 350hz or 400hz or 500hz (wherever that critical midrange and midbass notch appears) you can add an all pass to one of the two speakers and play with the centering and Q and it may or may not cooperate with what your trying to do. There’s a lot of different things happening) if it does help great! You should now have either reinforcement at that frequency or you have shifted the aural cues on that frequency only to help support imagining and centering. Another trick is to use the all pass in the peaks to drive them out of phase , which will subsequently help fix the dips which are out of phase (that’s why there dips) so if everything is out of phase, it’s actually back in phase, sometimes a simple invert polarity switch along with an all pass is just what is needed.
I know the Macaroni .....I mean masconi and the JLvxi have all pass so does minidsp and a few others. Use the all pass , do it real time and listen.
Another cool thing with all pass, you can apply it at the knee of a crossover, as the crossover rolls off the phase moves , if you add a all pass at a frequency that is let’s say 15° from zero (could be 1db down or so on let’s say a high pass) then it would shift the entire pass Band by 15° +360 . Doing that and a little trickery between inverting the polarity you could shift the phase of the entire left or right channel , by small increments, that along with signal delay can really make them dash pods come together just great....
Or cascade several all passes with delay between them for reverb for that rear fill you always wanted to add space to ..... possibles are endless.....