Phase-linear crossovers and the coherence of the impulse response
Abstract. Conventional crossovers rotate phase around every crossover point, spreading a transient's energy in time exactly where two ways must sum. This note explains the mechanism, the audible consequence, and the FIR implementation behind PhaseTrue.
What IIR crossovers do to phase
The analogue-style filters used in most crossovers are minimum-phase networks: their amplitude response and phase response are tied together. A fourth-order Linkwitz-Riley crossover sums flat in amplitude, but rotates 360 degrees of phase through the crossover region. Group delay varies with frequency, so the components of a transient that span the crossover arrive at slightly different times. On a multi-way system with two or three crossover points, these rotations stack.
The audible consequence
Amplitude response alone does not describe how a system reproduces an attack. A drum hit is a broadband event; if its spectral components are dispersed by a few milliseconds across the spectrum, the reproduced attack is softer than the original even though a swept sine measures flat. The effect is subtle on steady material and most audible on transients, which is much of what live sound is.
The PhaseTrue implementation
PhaseTrue implements the system crossovers as FIR filters in the system DSP. FIR filters can set amplitude and phase independently, so the crossover holds phase linear through the crossover region: every frequency is delayed by the same amount, and the impulse response stays coherent through the handover between ways. The cost is a fixed processing latency, which the system presets account for; the benefit is a multi-way enclosure that sums like a single source on exactly the material where coherence is heard.