Filtered impulse response - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 28 Old 01-25-15, 08:47 PM Thread Starter
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Filtered impulse response

I've only just discovered the benefits of applying filtering to IR measurements.
Setting the filter point as close as possible to the preferred crossover point you can very quickly nudge the waveforms into alignment.
In the phase plots this filtered IR alignment at the crossover point will also result in the closest phase tracking.
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post #2 of 28 Old 01-26-15, 08:01 AM
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Re: Filtered impulse response

Interesting! I hadn't noticed that before. Well Done.

It does work reasonably well. It adds another way to look at and adjust the XO timing.

It seems to add an extra 360 phase rotation at the XO, but that does not impact its utility in aligning the phase tracking. We just need to keep in mind that the actual phase rotation is distorted by the filter, but that tracking between the 2 drivers is still representative.

Thanks for sharing!

If you have the time an example may be helpful to members to understand the process.
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post #3 of 28 Old 01-26-15, 01:34 PM
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Re: Filtered impulse response

Please explain how this is done. I'm planning to tune my new DSP in my car later this week, and would like to incorporate this if possible.
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post #4 of 28 Old 01-26-15, 06:10 PM Thread Starter
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Re: Filtered impulse response

Ill post graphs and instructions for you when I get home this evening.
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post #5 of 28 Old 01-27-15, 07:03 AM Thread Starter
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Re: Filtered impulse response

Hi there. Hope some of the information below can help.
I have performed multiple sweeps from a distance of 1meter, in increments of 0.1ms.
We're looking at 12" bass and 4.5" bass midrange here crossed over at 400 hz with LR8 filters. My processor is the Behringer DXC.
The overlays below include impulse response, wrapped, unwrapped phase and SPL.

Filtered impulse response-impulse-response-unfiltered.jpg

As you can see, the impulse response of the bass driver exhibits a very broad wave form making it somewhat confusing as to where exactly the mid should line up.
The two phase plots give a better picture and show that I'm getting close to alignment.

Filtered impulse response-wrapped-phase-400-hz-lr8-1meter.jpg
Filtered impulse response-unwrapped-phase-400hz-lr8-1-meter.jpg

In order to render a filtered IR view go to the Filtered IR tab and apply filtering to each impulse to value of your crossover point.
Now go to your overlays view and examine how the IR waveforms have changed. As you can see from the graph the filtered Impulses now align more symmetrically.

Filtered impulse response-filtered-ir-overlays-view.jpg


From this point adjust your time delay settings so as to achieve the closest possible alignment.

Filtered impulse response-filtered-ir-alignment-400hz.jpg

This alignment will result in the closest possible phase tracking at the cross over point as is demonstrated in the phase plots.

Filtered impulse response-wrapped-phase-alignment-400-hz.jpg
Filtered impulse response-unwrapped-phase-alignment-400-hz.jpg

Now remove the filtering on the IR and go back to the normal IR view. You will note that the mids properly align with the bass waveform at the very beginning of the impulse, not at the peak.

Filtered impulse response-unfiltered-ir-alignment.jpg

I've attached an spl overlayFiltered impulse response-spl-response.jpg

Take note of the following points.

1- Any adjustment to your crossover point will require you to re-measure and realign. Changing your crossover point changes the delay on the driver. The same can also be said for slope types which add or reduce delay as adjusted.
2- Always choose a crossover point with good spl support either side of the band pass.

Last edited by geopango; 01-27-15 at 07:16 AM.
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post #6 of 28 Old 01-27-15, 11:43 AM
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Re: Filtered impulse response

Thanks geopango! Nicely done.

I will just add some additional comments that may be helpful in some circumstances.

The phase tracking data can become much more difficult to read than this example if the IR offset delay is greater, or the XO freq is higher, or the mic is at the LP rather than near field. The overlay of the IRs will still be reasonably similar to read however so the phase chart is not really required.

If we do need to read the phase more clearly in those conditions it is helpful to:
> Manually shift the IRs back to near 0ms using the same offset value for all IRs. It's important to shift both driver IRs the same amount as this keeps the relative phase relationship of the 2 drivers.
> Use no smoothing or 1/48 smoothing for the best readability of the phase.

Below is a difficult example using my FL speaker MW/TW 1700Hz XO measured at my 13 ft LP. IRs delays are significant at roughly 94.15ms.

Filtered impulse response-1-irs.png

Below, when the IRs are filtered with 1600Hz, 1/3 octave, the close IR overlay is apparent. This to enough to confirm the close phase tracking through the XO.

Filtered impulse response-3-irs-filtered.png

Below, the phase chart is unreadable however.

Filtered impulse response-2-phase-unshifed.png

Below, the IRs are now both shifted exactly 94.15ms to place them near 0ms.

Filtered impulse response-4-irs-shifted.png

Below, the close phase tracking can now be seen reasonably well when no smoothing is applied. That is, it's barely readable if we are experienced with reading phase charts with lots of reflections that tend to hide/smear the direct sound phase.

Filtered impulse response-5-phase-tracking.png

Below, the IRs are windowed as shown in order to eliminate as much of the late arriving reflections as possible while keeping enough of the early IR response as possible to still see some detail. The window settings were adjusted via trial and error. We can see the phase path is similar to that in the unwindowed phase chart above. Had the measurements been done with the mic at 1m, the chart would be very smooth.

Filtered impulse response-6-phase-tracking-windowed.png

Just additional hints for those dealing with reading phase tracking charts in difficult circumstances.
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post #7 of 28 Old 01-28-15, 06:24 AM
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Re: Filtered impulse response

This is brilliant! Thanks so much for the explanations. I look forward to delving into this on Thursday-Saturday with my new Helix DSP Pro. It has time alignment in 3.5mm steps and Phase Angle adjustment in 11.5 degree steps.

I do have a question about measuring mic location. You mention the mic at 1m yields cleaner phase plots due to reduced reflectional influence. But isn't it more important to measure at the LP to properly dial in the time alignment?


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post #8 of 28 Old 01-28-15, 08:00 AM
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Re: Filtered impulse response

In a car there may be no other good options. The LP is a safe choice.

For a small home "bookshelf" size speaker as I measured above, both drivers are closely stacked on the baffle in the traditional way. In that case we could have moved the mic closer while keeping it on the listening axis. At 1m or even a little closer the relative delay timing would still be accurate. We are just removing some of the excess distance/delay without changing the relative delay needed between the 2 drivers. The closer mic position reduces the magnitude of the reflections and results in a much cleaner phase chart. The closer the mic is placed though, the harder it is to be sure the mic is still on the listening axis.

Often SWs are placed away from the mains and thus the LP is the only option for the mic placement. That is why I added the hints about how to clean up phase charts when needed.

Just choose the mic position appropriately for the situation.
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post #9 of 28 Old 01-28-15, 09:43 AM
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Re: Filtered impulse response

Wizards at work!

You guys knock me out!

geopango and jtalden, great work, another reference thread for sure.

Edit:
For those who are following along, here are some interesting links:

jtalden's Alignment Driver Phase reference guide

similar threads:
Time Alignment
Understanding Phase Graphs
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post #10 of 28 Old 02-26-15, 02:17 PM
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Re: Filtered impulse response

Quote:
geopango wrote: View Post

Take note of the following points.

1- Any adjustment to your crossover point will require you to re-measure and realign. Changing your crossover point changes the delay on the driver. The same can also be said for slope types which add or reduce delay as adjusted.
2- Always choose a crossover point with good spl support either side of the band pass.

Just to be clear, we need to pick a crossover point first and then measure as above to determine the delay?

Or should we get a ballpark delay, and then set crossover and fine tune the delay as above?



My situation is a little different because it's a car, with listening position in the driver seat. So I need some large delays on the left midbass and the subwoofer in particular. Furthest speaker is my right compression horn under the passenger side dash.
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