From: Richard D Pierce <DPierce@world.std.com>
Newsgroups: rec.audio.high-end
Subject: re: Time Alignment
Date: Tue, 26 Jan 1993 10:02:29 -0500 (EST)
Organization: University of Wisconsin - Milwaukee
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Originator: tjk@csd4.csd.uwm.edu


Stuart Krivis writes:
>If "time alignment" enables the speaker system to properly
>reproduce a pulse signal, I would assume that it is beneficial.
>
>Stepped baffles, ala B&W, high order xover filters, and delay
>networks in the xover probably screw things up enough that proper
>time response is not possible. Therefore, "time alignment," as
>practiced by most designers, is a marketing ploy.
>

No, Stuart, it is precisely the stepped baffles, high order crossover
networks and delay networks in the crossover that, in fact, makes it
possible for a competent engineer to achieve reasonable time alignment. 

Perfect time response is not possible in a multi-driver system where each
driver has its own bandwidth limitations, response anomolies and so on.
But neither does there exists full range drivers (including
electrostatics) that are capable of perfect time response combined with
the requirements of dispersion, power handling and so forth.

             WARNING! Technical term used below! WARNING
   Before I continue, I need to define a term which is often confused
   and misunderstood. That term is "minimum phase." It is a specific
   engineering term, and is not to be confused with "linear phase" or
   any such terms. Minimum phase is a well defined term that means the
   amplitude vs. frequency response and the phase vs. frequency reponse
   are uniquely related by what's called the Hilbert transform.
   "Non-minimum phase" simply means they are not. "Minimum phase" is
   NOT to be taken necessarily as a measure of quality.

   We now return to our regularily scheduled debunking session.

The fact is that the vast majority of drivers exhibit what is called
minimum phase behaviour. That means the the phase response and the
amplitude response are unique transforms of one another. One of the
consequences of this is that any amplitude response anomolies are
accompanied by uniquely characterizable time anomolies. Solve one
properly, and you solve the other. That means that you can use
equalization to flatten both the amplitude and the phase (and thus time)
response of a driver (whether it's done passively or actively makes little
difference). There are some exceptions to this (such as all-pass
equalizers that result in the combination driver/network no longer being
minimum phase).

Multiple driver systems, on the other hand, are general non-minimum phase,
because there are multiple signal paths through the system, each with its
own delay. By equalizing those delay times, using techniques such as
stepped baffles, delays built into the networks, and so on, we can more
closely approach minimum-phase behaviour. We can also use high-order
crossovers to minimize the overlap between drivers and reduce the ranges
in which the speaker exhibits non-minimum phase behaviour. Once we have
achieved a reasonable approximation of minimum phase behaviour, then we
are very close to the poiunt of having a system which has a shot at
reasonable "time alignment."

This, of course, implies a pretty high degree of competency on the part of
the designer, something which, in this industry, is not to be taken as a
given. But the premise that stepped baffles and delays and such are a
crock and nothing work counter to time alignment is, in and of itself,
nonsense and contrary to the physical reality. In the hands of the right
person, they are valuable tools. In the wrong hands, well, all bets are off.

|                Dick Pierce                |
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