Bass Alignment on system using Linear Phase Crossovers

While I was not overwhelmed by the improved fidelity of my system when I started using linear phase crossovers, I already had a good reason to suspect it would not yet have been optimized very well.  I had not done the new time alignment.

And it turned out, that the time alignment was indeed not good.

Without correct time alignment, there could be phase errors even worse than those caused by the nonlinear phase crossovers.  Since phase errors are essentially the only improvement made by using linear phase crossovers, this would make their benefits moot.

Since it takes some time (ha) measured in days or more to do a good time alignment, I hadn't even done the time alignment I needed to do when I started using the miniDSP's as my crossovers, or when I switched the woofer/panel crossover frequency to 125 Hz and slope to LR8.  So by the time I rolled out the linear phase crossovers last week, I was already behind the curve on this.  Here are the previous alignments I developed in June (using ARTA for the first time):

I started by measuring the leading edge of the midrange, tweeter, and bass outputs as I did in June, using the ARTA time record feature (just like a storage scope, no fancy FFT math) and a "transient" impulse of short width like 1 unit for the tweeter and longer like 100 for the subs and panels.  I later determined that the ARTA transient width specification is in samples, and 100 samples is 1/480 second or about 2ms.)  You may notice that the transient starts downwards.  I later determined this has nothing to do with my speakers, but is caused by an issue either in the Focusrite Scarlett interface or the ARTA program.  When ARTA computes a Step Response, the polarity is shown correctly, but the computed step response is useless for determining driver start time.  The bass response has a leading negative portion which needs to be ignored; it is caused by the low pass filter.  But because of the polarity inversion in the time record feature or interface, the leading negative portion of the bass response is shown as positive here.

Leading edge of panel output

Leading edge of tweeter output

Leading (in-phase) edge of sub output

I then proceeded to try to find the maximum cancellation point with the phase of one side (the Acoustats...since that is easiest) reversed.  I did not record the delay times for each as I was going (as I should have!).  However, somehow I ended up with a very impressive cancellation about where it should be.

Oops, no cancellation, forgot to reverse phase, note room notch at 92 Hz

Showing some cancellation just below 125

Better crossover cancellation near 125 Hz

Too far or wrong direction

Looking Good

Accepted!

While I was doing this, I was measuring "close" to 125 Hz.  It was only later that I realized it might have been better to be measuring at the very obvious notch just below 125 Hz.  That's what probably represents the acoustic crossover.  And I should also use that actual frequency as a phase compensation frequency instead of the nominal crossover value of 125 Hz.

Anyway, the last two are about the same, there may have been others in between and I furiously worked to try to repeat the earlier measurement, and while the second may be .02dB worse at 124 Hz, it's so close that the difference may well be random measurement error.

Notice how different the final cancellation result is from the first.  Clearly I've found the new optimum, or close to it.  I strangely seem to have ended up only 2ms of where I had been before the transient measurement, in spite of the 8ms change that the transient measurement suggested would be necessary.  I'm not sure why (lack of notes).  Here are the newly dialed in delay values:

Since both bass and midrange crossovers use the same miniDSP plugin and number of taps and centering method, switching to linear phase filters didn't affect the relative time alignment between the two at all, as I suspected.  (Tentative conclusion #1.)

I suspect it was the change in crossover frequency, from 100 Hz to 125 Hz, that makes the 2ms difference, and/or it could have been the LR8 slope.  (Tentative conclusion #2.)

Turning on the FIR and filling it with about 6000 taps, has made about an 80ms difference in the delay time, indentical to both bass and midrange.  (Firm conclusion #1.)

The center frequency of the steepest relative notch that appears as the correct time alignment delay is dialed in, represents the actual acoustic crossover frequency (which is otherwise fairly hard to determine).  (Firm conclusion #2.)

The best way to construct a linear phase crossover, is to use an ordinary IIR crossover then add a FIR phase compensator for the acoustic crossover frequency, as measured above.  (Hypothesis #1.)