Sunday, July 16, 2017

Time Alignment using Tact

It's pretty easy to set the supertweeter time alignment using the Tact as measurement tool, using the mid way Behringer DEQ to set the delay because the supertweeters are about 6 inches back.  The Tact stimulus is a high frequency rich "snap", and it's easy to see where the high frequency wiggles of the supertweeter line up with the initial impulse in the panels.  The initial impulse in the panels appears to go down in the Tact display apparently because of some weirdness in the Tact stimulus and display.  I use SpeakerPop to set polarity.

I've determined the correct setting to require 0.2 ms delay in the panels.  However, since Tact uses a 48kHz signal, which is delaying the panel approximately an additional 0.35 ms due to the DVD-5000, the total delay is about 0.55 ms, or about what it physically looks to be.  I actually set the relative delay at 0.55 because I am "optimizing" for 96kHz.  If I am seriously listening to 44.1 I can subtract 0.35 ms delay in the mid way DEQ to compensate for the added latency in the DVD-5000 at 44.1.

Setting the time alignment for the subs is as clear as mud.  The Tact stimulus doesn't produce very much bass energy, and if you play the system normally the "bass" is just endless LF ringing.

What I have historically done, only it isn't working as good anymore (and never worked great) is separate the channels.  To set right time alignment, I disconnect the left panel and sub, reverse the panel channels.  Then both Tact channels play the right side, with the left panel channel actually playing the right channel.  This gives a display with sub in one channel, and panel in the other channel, and you can sort of see how they line up.

But it's pretty hard to tell, because the sub is only making very small and slow waves in the Tact display, and it's not always exactly clear where the "beginning" is.

The Tact doesn't make this any easier.  It does not save the previous measurement(s) so you can see how things changed.  Because the snap stimulus has very little low frequency information, the subs are barely even audible.  This lack of information means that each plot is going to differ from the previous one, randomly, even if you didn't make any changes (unless you do endless averaging, I suppose, I do only 10 trial averaging which already requires long waits).

Anyway, after hours of futzing with this anyway, I finally came to the idea that about 5.4 ms was the correct panel delay time align the subs.  (Because the subs are further back, the panels need to be delayed to compensate, by 5.4 ms?)  This makes no sense because the panels are not that far back.  I made that judgement on the left side, for which the subs may be as much as 3 feet back (they're only as much as 28 inches on the other side).

Anyway, on the other side, things were even less clear.  The number could have been as low as 2 ms (about what it looks) or as high as 5 ms.

Finally I came up with a clever trick in programming the DEQ's.  I indeed do delay the subs (even though they are the farthest back) by 4ms.  Then I delay the panels by 7.5ms and the super tweeters by 6.95 ms and I can leave those two alone.  Then, I can adjust the sub delay simply by turning one knob.

And this control is very nice.  Now it is very clear how the sub delay affects the sound, and exactly the opposite as I had thought.  At the "minimum" measured sub delay of 3.5 ms the sound is a trifle boomy.  If I dial the sub delay more, it tightens up.  I had figured you always want the panels to "lead" and therefore provide the initial more perfect sound.  But it appears what is really important is to not have the subs lag too far behind.

Anyway, now I have a control and I can just keep tuning by ear.  By turning the sub delay up to 7.5 I could in effect make the relative delay 0, or by turning the sub delay down to 0 I can make the relative delay 7.5.  So I have all the range I need in one control.

(Sadly there appears to be no way to add relative delay to the subs.)

With this control, so far I have gravitated to 3.6 ms, which would mean the panels are delayed 7.5-3.6  or 3.9 ms relative to the subs (maybe some different numbers would make this easier).  I had previously been using a 3.3 ms delay as handed down from endless re-thinkings mainly.

I went ahead and put the panels at 10 ms delay.  That puts the super tweeters at 9.46 ms (only 0.02 adjustments available) and the subs at 6.10 ms for the same relative delays as above, and gives me full adjustability down to panel delay of up to 10 ms or down to relative negatives just by turning the sub delay knob which turns out to have a very interesting subjective effect.

Then I discovered that running the Tact test at precisely 6.25ms (relative to 10.00 ms in the panels) show the right channel going down (the sub leading) over 100's of MS, and the left channel going up (the sub lagging).  This has to be the perfect in-between compromise.  At 6.00 ms and 6.5 ms both channels go either up (one more  than the other) or down.  So I've settled on 6.25 as my best guestimate (equal to 3.75 ms difference from the panels which are delayed by 10 ms for convenience now).

Tact Impulse Response at 6.25 ms dub delay

At 6.50 ms both channels "rising"

At 6.0 ms both channels "falling"

When I play 96kHz, I merely dial up the mid way delay from 10.00 (calibrated at Tact's 48 khz) to 10.38.  To play 44.1kHz I should dial down the mid way delay to 9.94 (if I care).  To change the bass alignment I can just adjust the bass.

Actually I'm finding that 6.50 msec delay, relative to 10.38 (for 96kHz sampling).or whatever I've set the panels to, works better than 6.25.  6.50 subjectively lets the bass be bass, to hang a bit.  6.25, what I called the "compromise" position, is too dry, the reverse angle taken by the two sides cancel or something and it sounds dry.

Each time I do these alignments via Tact some trick like this arises and gives me an "angle" to make an objective/subjective judgement.  Who knows I could be off by more than 1 ms.

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