Saturday, March 23, 2013

Using the Tact to adjust crossover time delays

At long last I'm firing up the living room Tact 2.0 program (which runs on a Windows PC) to adjust the time delay that time aligns my Acoustat panels and SVS subwoofers.  I am looking at frequency response too.

Unfortunately for me the Tact program is oriented around room correction (not really measurement) so much that it's unclear to me whether the impulse shown after measuring my system is a full cycle (0, +1, -1, 0) because the tact is simply showing what it recorded, an image of its own test signal, or whether it shows a full cycle because of group delay in my Acoustat speaker, and if the Acoustat had no group delay the Tact would be showing a standard impulse that goes from 0 to +1 and then back to 0.  When I use the Liberty Audio Suite, it plays a signal that looks nothing like a simple pulse, but LAS then mathematically transforms it into a perfect step pulse graph if indeed the unit being tested has perfect pulse response (as most solid state electronics does, or comes close to, while speakers are usually far from it). I think that's called deconvolution, but it might be called convolution (I get the two confused).

Worse, I had a specific idea for adjusting the time delay between the Acoustat panels and the subs.  I reversed the amplifier connections for the acoustats.  Then I mute every speaker driver except the Left channel Acoustat (which is actually playing on the right, because I reversed them) and the right channel sub.  Then when I run Tact in full stereo mode, it plays both drivers for the right channel but as if they were left and right full range speakers.  Then, in principle, I could verify and adjust time delay, by comparing the onset of left and right channels in the impulse picture the Tact produces.

However, because the Tact is so oriented around measurement, it uses some trigger to put both channels into alignment even if they are not.  Or at least that is what it seems.  Whenever I do this measurement, the bass looks like it starts about 6.0 ms (millisecond) later than the panels.  Which is very strange because the speakers are only about 3 feet apart, and I was already applying about 3.4 ms of delay to the panels to compensate.  So the expected error, or difference, should have been less than 1 ms.  But as this measurment appears, there is also a box showing the relative time difference between the channels (which the Tact is compensating for).  And it shows a really big number, like 20 ms.  So once again I don't know what the impulse picture means.  Is it showing the raw measurement before the 20 msec correction it is ultimately applying in the chosen correction number?  Or is it showing the two channels adjusted, as it thinks they should be?  If it is showing them after correction, I can't use this measurement.

Because the Tact seemed to be showing about a 6ms gap with the subs starting later, I increased the time delay on the panels, but for some reason I don't now remember, or possibly just lapse of memory, I increased the delay adjustement (in the Behringer 2496 DEQ) for the Acoustats from 2.47ms to 5.53ms.  I would have made the second 5.47 but the adjustment is course and that exact number is not available.  Anyway, that seemed to have little effect on the graph.

So then I tried bigger adjustments, 9ms, 12ms, 10ms, and in every case it wasn't clear whether it made the bass alignment with the treble impulse better or worse.  Those numbers made no sense at all, the required delay should be between 1 and 4 ms.  But in every case it still looked like 8-12 ms of additional adjustment was needed.

One problem is that the bass may start more slowly, for various reasons, especially the crossover, but also it's limited high frequency response.  It should, I believe, begin moving the instant current is applied, but at first slowly, then building up to a full wave.  The resolution of the display is low enough, and there is also noise, so it is ambiguous where the subwoofer output really begins.

After messing around with the primary "Measurments and Correction" page for hours, the only one that actually allows you to do measurments (and you must have a non-bypass correction number selected, it won't let you do measurments in bypass mode) I finally went over to the dual-domain page, where you can load the previously made measurement, and there it seemed I could get a pretty clear image of the treble impulse, and see that it started around 11.8 ms in the right channel.  (This was with all crossover settings restored to original settings.)  Then I played the bass.  Well, it could have been correct, you could see some rising, maybe, in the bass response at 11.8 ms, where the red line is in the graph below.  But it could also be as fast as 2ms, or as slow as 15 ms, it's hard to be sure, because there is noise and the initial start might be slow, very slow.  True, at about 15 ms it really starts going, it's clearly going at that point, though not as strong as later, there's a cycle of reduced output before the full response builds, it's typical for speakers, especially bass speakers being crossed over, to respond that way.  I have to think that on a higher resolution plot, the very initial slow part, corresponding to the limited high frequency response of the subwoofer, there would be a more clearly visible starting point, and it would be very close to the red line at 11.8 ms, because I was already compensating for the delay with digitial delay in the crossover which should have been accurate to a few inches, which would correspond to about 0.1ms or so, because sound travels 1foot in about 1 ms.  On the other hand, frustratingly, I can't be sure, because it may well be that even with the crossover and all filters in the SVS subwoofer turned off (as usual, except the sub 15 Hz filter I am required to run with only one port filled) there is time delay in the electronics, that's different between the sub amp and the acoustat amp.  I can't explain that much difference in delay happening from analog elctronic processing.  I can explain the 11.8 ms delay as about 6 ms from distance to the microphone, and about 6 ms fixed in the digital processing from the output of the Tact through the Behringer DEQ.  But that should not vary between the treble and the bass.

Anyway, as the graph below shows, the Tact measurement system is useless for setting the delay on the bass, because the starting point of the bass is ambiguous with such limited resolution.  And the graph below was made with crossover turned off, and the bass does have HF response to about 300 Hz, so I think a good measurement system would clearly show the starting point better than this.  Part of the problem here...the Tact has a simple impulse that has limited low frequency information, hence, limited low frequency resolution.  A Maximum Length Sequence system, like liberty audio suite, uses chirps, which have a better spectral distribution than clicks, allowing greater bass resolution.

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