Phase response - looks weird - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 7 Old 03-20-12, 06:09 AM Thread Starter
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Jonas hansen
 
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Phase response - looks weird

I would like to measure the phase of my system, and have hooked up a USB soundcard, made loopback connection etc (and checked this option in the settings).

My measurements shows this phase response:


I dont think it looks anything like other phase responses I've seen? Does it look right to you?
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post #2 of 7 Old 03-20-12, 10:52 AM
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Re: Phase response - looks weird

It appears that the measurement gives more than 360 degrees of phase shift in going from 40 to 60 Hz, and over 25,000 degrees change up to 1.3 kHz. This is not what I would expect to see. There appears to be a phase bias, or calibration error, that must be removed from the measurement.
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post #3 of 7 Old 03-20-12, 10:57 AM Thread Starter
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Re: Phase response - looks weird

Can you help me troubleshoot what the problem could be? I will try some new measurements tonight. When I use loopback, do I need a calibration? I didnt think that was necesary.
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post #4 of 7 Old 03-20-12, 02:13 PM
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Re: Phase response - looks weird

Jonas,
The phase response is calculated based on the positioning of the IR. The IR need to placed near 0 ms for the phase to make any sense. The loopback setting places the IR at the total delay time instead of shifting it to 0 ms. So you have 2 choices to see the phase.

1 - Turn off the loopback option and remeasure. [REW will move the IR to near 0 ms.]
2 - From the "Impulse" chart of the current measurement open "Show the Graph Controls" and select "Estimate IR Delay" and then "Shift IR" (just accept the value offered). [REW will shifth the It to near 0 ms just as it would with out the loopback set.]

There still may be some of the sudden jumps in phase, but most of the curve should be pretty smooth. The jumps (which become more prevalent at higher frequencies) are caused by the effects the room and do not reflect the phase of the direct signal from the speaker.

The actual phase is of little general value, but can be used as one method of evaluating the delay timing (distance setting) for the handoff between the SW and the main speaker. Proper timing/distance results in a smoother SPL response through the XO range and eases the amount of EQ needed to smooth things out in that region.
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post #5 of 7 Old 03-20-12, 05:52 PM Thread Starter
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Re: Phase response - looks weird

Quote:
jtalden wrote: View Post
Jonas,
The phase response is calculated based on the positioning of the IR. The IR need to placed near 0 ms for the phase to make any sense. The loopback setting places the IR at the total delay time instead of shifting it to 0 ms. So you have 2 choices to see the phase.

1 - Turn off the loopback option and remeasure. [REW will move the IR to near 0 ms.]
2 - From the "Impulse" chart of the current measurement open "Show the Graph Controls" and select "Estimate IR Delay" and then "Shift IR" (just accept the value offered). [REW will shifth the It to near 0 ms just as it would with out the loopback set.]

There still may be some of the sudden jumps in phase, but most of the curve should be pretty smooth. The jumps (which become more prevalent at higher frequencies) are caused by the effects the room and do not reflect the phase of the direct signal from the speaker.

The actual phase is of little general value, but can be used as one method of evaluating the delay timing (distance setting) for the handoff between the SW and the main speaker. Proper timing/distance results in a smoother SPL response through the XO range and eases the amount of EQ needed to smooth things out in that region.
Thank you - now it looks a little better. THe phase measurements are exactly for aligning SW to main and aligning LF to HF section on my mains. They use active crossover.

When I see examples with software like Smaart, it looks a little easier for measuring phase and making the adjustments real time. But if REW can provide the same functionality, it would of course be better as it does not cost $800.

What about the LF/HF alignment when I am shifting the IR's etc? Shouldnt they be shifted the same amount?
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post #6 of 7 Old 03-20-12, 07:30 PM
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Re: Phase response - looks weird

May I suggest one peruse this paper by Charlie Hughes who takes the issue one step further and corrects for the oh so common assumptive errors that affect ALL measurement platforms, regardless of cost...

Phase Response and Receive delay
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post #7 of 7 Old 03-20-12, 10:07 PM
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Re: Phase response - looks weird

Quote:
Jonas_h wrote: View Post
Thank you - now it looks a little better. THe phase measurements are exactly for aligning SW to main and aligning LF to HF section on my mains. They use active crossover.

When I see examples with software like Smaart, it looks a little easier for measuring phase and making the adjustments real time. But if REW can provide the same functionality, it would of course be better as it does not cost $800.

What about the LF/HF alignment when I am shifting the IR's etc? Shouldnt they be shifted the same amount?
Yes, that's right. Both IR's need to be delayed the same amount and this can be done by:
1 - Use the loopback function.
2 - Measure the SW
3 - Measure 1 Main speaker
4 - Have REW offer the IR offset needed for the main speaker (not the SW).
5 - Copy or write down the IR offset value, then go ahead and shift the IR that amount.
6 - Manually offset the SW IR the same amount.
7 - The phase of the 2 measurements can now be compared in the overlay window.
8 - The delay for that main speaker can be adjusted as needed to align the phase at the acoustical XO.

Each additional main speaker can be adjusted in the same manner.

Here is a link with a litte more detail:
Aligning Driver Phase REW-V5 Example

The mic will need to be located at the LP and you may run into room modes that make the phase a little difficult to interpret. I think most people opt for a different method to adjust timing.
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