Hi Spirit,
Hope you don’t mind but decided to reply to you here since it seemed more on-topic. In the Audio Exotics Super Hi End Show thread you posted:
My understanding is limited, so take this with a shaker of salt:
Group Delay is a measurement of time (not phase) in which frequency components of a square wave (can) become misaligned relative to one another. A filter (like a crossover) that passes a signal (square wave) in which misalignment is minimised can be considered constant, hence the term “constant group delay”, i.e. all frequencies arrive in time relative to one another, but delayed in time.
Phase Delay however, is a measurement of phase (duh) in which the frequency components of a square wave shift out of phase relative to their initial state. A filter that passes the above with minimal phase shift at all frequencies can be considered phase linear.
Without sounding like a shill (too late?), the Aries Cerat Symphonia’s outboard analog crossover is both time and phase linear relative to the mechanical alignment of its drivers. A speaker featuring a constant group delay filter (the Symphonia has three) also needs to be designed relative to the physical alignment of each of the drivers and vice versa with respect to both time and phase. In and of itself, physically aligning the drivers in space relative to one another achieves nothing other than a nice looking speaker. Fundamentally, physical alignment, time alignment and phase alignment are all interdependent variables, and must be considered together as a whole. That it also achieves a true 101dB sensitivity rating and has a maximum deviation of +/- 0.5dB over six and a bit octaves is kinda impressive (to me). At the very least, I don’t know of any other horn that achieves this, but am open to being wrong, which remains a constant possibility*.
It’s important to note that “linear” is a relative term apropos the real world (i.e. we’re talking about the “least non-linear”), but nevertheless it is indeed possible to build an analog filter that is both time and phase linear, like, for instance, in a Bessel filter (though I don’t know if that’s what AC use in the Symphonia - perhaps Flyer or Stavros can chime in).
“Perfect”, however, is an altogether different thing.
Best,
853guy
*If someone wants to school me on how woefully lacking my understanding may be, please go ahead and begin the lesson. I am but a humble student.