Understanding phase graphs - Home Theater Forum and Systems - HomeTheaterShack.com

Old 06-01-14, 05:39 AM Thread Starter
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Matt

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Understanding phase graphs

When comparing phase of 2 drivers, it seems to be necessary to get the time alignment right first and then shift one impulse to t=0 and the other by the same amount.

If I understand correctly, if this is not done correctly then the phase overlay is basically useless. Is my understanding correct? If so, why is it necessary to move the impulse to t=0?

Thanks
Matt
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Old 06-01-14, 07:42 AM
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Re: Understanding phase graphs

No, that doesn't sound quite right. We shift the higher freq driver IR to near 0 ms first. It does not need to be at "exactly" at 0ms. It just needs to close. I usually adjust it so the so the right phase tail is pretty much horizontal. We can then see the phase trace of that driver clearly on the chart.

Then the lower freq driver IR is shifted as needed so that its phase tracks as closely as possible through the XO range. Once the delay (relative difference in IR shift amounts) is established and entered into the AVR/DCX/miniDSP/?? then the good phase tracking is locked in.

To see why it this is done this way we can just look at what happens if we don't. This will be most clear if we look at extremes (often a good technique). If we have 7.xxx ms TW IR position and a 7.yyy ms MR IR position when initially measured (due the mic distance, XO delay, and speaker delays). We can try to do the phase tracking alignment job without moving both IR positions to near 0ms. We leave the TW IR where it is and try to align the tracking of the MR phase by moving its IR position. If we can do it then the answer will be the same. We will find the same relative IR shift (delay) needed to align them as we would working near 0 ms. It is just almost impossible to do it. The phase chart is so cluttered with lines from all the wraps it is not clear what we are looking at.

The phase chart cannot be easily understood unless the excess delay of the system is removed.
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Old 06-01-14, 02:11 PM Thread Starter
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Re: Understanding phase graphs

Quote:
jtalden wrote: View Post
No, that doesn't sound quite right. We shift the higher freq driver IR to near 0 ms first. It does not need to be at "exactly" at 0ms. It just needs to close. I usually adjust it so the so the right phase tail is pretty much horizontal. We can then see the phase trace of that driver clearly on the chart.

Then the lower freq driver IR is shifted as needed so that its phase tracks as closely as possible through the XO range. Once the delay (relative difference in IR shift amounts) is established and entered into the AVR/DCX/miniDSP/?? then the good phase tracking is locked in.

To see why it this is done this way we can just look at what happens if we don't. This will be most clear if we look at extremes (often a good technique). If we have 7.xxx ms TW IR position and a 7.yyy ms MR IR position when initially measured (due the mic distance, XO delay, and speaker delays). We can try to do the phase tracking alignment job without moving both IR positions to near 0ms. We leave the TW IR where it is and try to align the tracking of the MR phase by moving its IR position. If we can do it then the answer will be the same. We will find the same relative IR shift (delay) needed to align them as we would working near 0 ms. It is just almost impossible to do it. The phase chart is so cluttered with lines from all the wraps it is not clear what we are looking at.

The phase chart cannot be easily understood unless the excess delay of the system is removed.

- what does "the right phase tail is pretty much horizontal" mean? where are you looking at this? I imagine a picture tells a 1000 words here
- I read your reply as "shift the HF driver to near 0 only, now shift the LF driver to establish good phase tracking" but this means we lose information about the existing delay difference between them, hence I thought the approach was "move HF to 0, move the LF by the amount by we moved the HF, now shift LF around to get good tracking & apply this delta to the actual system"
- I had noticed all the phase wraps when it is not shifted, why does this happen?

Thanks
Matt
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Old 06-02-14, 03:53 PM
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Re: Understanding phase graphs

Quote:
3ll3d00d wrote: View Post
- I had noticed all the phase wraps when it is not shifted, why does this happen?
The phase is plotted in REW relative to 0ms. There is no feature in REW to plot phase relative to another time setting. This not the easiest situation for making adjustments to align the phase tracking so we work within this constraint.

If we do a TW measurement with REW loopback timing activated using a mic at a 2m LP, we may get an IR delay (arrival time) of 8.000 ms due to the mic distance and any other delays in the measuring system. The phase angle for this measurement is calculated by REW from the 0 ms reference time. 8ms at 20kHz results in; 0.008 s / 0.00005 s/cycle = 160 cycles. Each cycle being 360° of course. So if we plot the phase angle on the wrapped phase chart (360° chart) the it would wrap around 160 times before reaching 20 kHz. That's a very cluttered chart! We want to see what the phase is doing without all the extra wraps so we need to remove the excess delay time by shifting the IR back that amount of time. We actually don't know the exact amount of excess time added so we estimated it. We do that either by letting REW automatically estimate it or do it ourselves manually. REW normally does a good job, but sometimes the IR shape can cause an issue that needs to be manually corrected.

With the excess time removed we can better see the phase tracking on the chart. Any small IR shift will move the right tail up or down according to the phase calculation. The high freqs (right tail) are very sensitive to IR time position. The lower freqs (left tail) is much less sensitive due to the longer wavelength.

Since we are after the relative differences between 2 drivers, we don't really care if the shift is not exactly the excess amount as any extra rotation is present in both drivers. The relative phase tracking is still accurate.

Quote:
- I read your reply as "shift the HF driver to near 0 only, now shift the LF driver to establish good phase tracking" but this means we lose information about the existing delay difference between them, hence I thought the approach was "move HF to 0, move the LF by the amount by we moved the HF, now shift LF around to get good tracking & apply this delta to the actual system"
You are correct in this statement. I didn't understand this to be what you said in your last post. I explain the process in different ways to try to get the concept across. Maybe that just makes it more confusing.

However we get there, good phase tracking can only be found easily with both IRs initial rise near 0ms. Then adjust the LF IR as need to best overlay the traces in the XO range. The time difference in the amount of shift needed for the 2 IRs is the additional delay need in the speaker management box to implement the correction. We can either keep track of all the adjustments in IR positions so we can determine the difference between the 2 or we can use REW to measure the starting IR position and final positions to determine the relative changes.

If we want to see the existing tracking, then yes, both IRs must be shifted the same amount.

Quote:
- what does "the right phase tail is pretty much horizontal" mean? where are you looking at this? I imagine a picture tells a 1000 words here
In 70 Hz XO example below we have an MW driver with XOs at 70 Hz and 1800 Hz we can call that the bandpass range for this purpose. We had REW estimate the excess delay and the resulting SPL and phase charts are shown. The right tail is the high freq stopband tail of the phase trace. In this example the right tail is just before the phase is lost into the noise floor which is ~6kHz.

This is the HF driver of the 70 Hz XO. This IR shift is an okay estimate of the excess time for our 70Hz XO. If the W is shifted the same amount then the current relative phase tracking can be seen without excess rotations. We still have to deal with (look past or ignore) room modes/reflections. There is lots of room modes and reflections present to disrupt the direct sound phase trace. I roughly indicated the direct sound phase. We could use this chart as is or clean it up a little with the use of window settings.

Below I cleaned up the higher portion of the freq range by setting a too small IR window. The phase is only accurate >~500Hz with this window setting. It indicates the general direction down to about 200Hz. Note that the right phase tail is still falling until the noise floor (not horizontal), but this IR position is a perfectly good setting for the target. We would need to increase the right window though so we can see the phase through the lower stopband if we want to align a SW with it. A right window setting time such that the REW "Frequency Resolution" is reported at maybe 10-15Hz will probably work well (1/3 or 1/2 the lowest freq of the XO range).

Below, to create the right horizontal tail that was mentioned in the last post the IR would need to be moved another +0.040 ms. This will have no significant impact to the left tail and lower bandpass region the we use to match with the SW phase. This fine tuning is not needed as we are close enough when the right tail is within ±180° of horizontal.
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Old 06-04-14, 09:30 AM Thread Starter
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Matt

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Re: Understanding phase graphs

Thanks for such a detailed reply, v v useful and much appreciated. A couple of further questions/comments

Quote:
jtalden wrote: View Post
We still have to deal with (look past or ignore) room modes/reflections. There is lots of room modes and reflections present to disrupt the direct sound phase trace. I roughly indicated the direct sound phase. We could use this chart as is or clean it up a little with the use of window settings.
would an alternative be to look at a near field measurement of the speaker or do we *need* a listening position measurement here?

Quote:
jtalden wrote: View Post
Below, to create the right horizontal tail that was mentioned in the last post the IR would need to be moved another +0.040 ms. This will have no significant impact to the left tail and lower bandpass region the we use to match with the SW phase. This fine tuning is not needed as we are close enough when the right tail is within ±180° of horizontal.
I'm still not clear on why you want the tail to be horizontal? is it just to make it more obvious that that it an area of little interest to you or is there some other reason?

Another Q concerns unwrapping the phase. Here's an example, this is my sub

the unwrapped view, intuitively, seems to make sense to me in that I am experiencing ~240 degree phase shift between 45 and 65Hz (and this region corresponds to where EQ is most active dealing with room modes). I interpret this as meaning the lower registers of the sub will be somewhat delayed relative to the upper bass. In contrast, I find the wrapped view somewhat confusing.

Is my interpretation correct? Are there reasons for/against unwrapping that I am missing?
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Old 06-04-14, 01:44 PM
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Re: Understanding phase graphs

Quote:
3ll3d00d wrote: View Post
would an alternative be to look at a near field measurement of the speaker or do we *need* a listening position measurement here?
It depends on the situation. With the speakers in the application position and the mic at the LP, it is always correct for phase tracking. It can be more difficult to read the chart however. If we have a 2 drivers on a baffle reasonable close to each other, we can move the mic toward the speaker staying on the listening axis. This works well. For a small speaker 1m would be a minimum for me. For a large MR horn and TW 1.5-2m would seem a better choice. The phase chart will be much cleaner the closer we move. If we are using a SW then the LP mic position is pretty much a necessity unless there is a closer mic position that retains the distance differential of the main vs the SW. I can't think how that would commonly occur, but if we have a stereo setup with 2 mains and 2 stereo SW placed close to inline with them then any error would be very small if we moved closer.

For my SWs-mains XO I like to see the room modes and adjust the XO point, the phase tracking, and EQ setting all with the mic at the LP position.

If we just want see the natural phase responses of a single driver with any XO or EQ active then we can put the mic as close as we want without touching the diaphragm. This is a good way to understand what we are looking for as we measure further back for the purpose of phase tracking between drivers. Up very close, we will find the phase very smooth indeed. All the irregularity from greater distance is due to diffractions/reflections.

Quote:
I'm still not clear on why you want the tail to be horizontal? is it just to make it more obvious that that it an area of little interest to you or is there some other reason?
I stated that right tail horizontal is a good target and if it curls up or down 180° then the result will still be easy to read.

If we have an IR with a large peak that trails the initial smaller IR peaks then REW sometimes adjusts that large peak to 0ms. That can make the chart more difficult to read. We are adding in additional wraps for no reason. I think I mentioned (maybe in another thread) that theoretically someone could do the phase tracking process at any offset from 0ms. I would not even attempt to do it away from 0ms as it can get very difficult to tell which lines on the chart are the ones we want to align. We can't just align any two lines! If there is a particular situation where it easy to read the chart with some other setting then, no problem. I am just trying to give advice that works well accounting for most all situations.

Quote:
Another Q concerns unwrapping the phase. Here's an example, this is my sub
...
the unwrapped view, intuitively, seems to make sense to me in that I am experiencing ~240 degree phase shift between 45 and 65Hz (and this region corresponds to where EQ is most active dealing with room modes). I interpret this as meaning the lower registers of the sub will be somewhat delayed relative to the upper bass. In contrast, I find the wrapped view somewhat confusing.

Is my interpretation correct? Are there reasons for/against unwrapping that I am missing?
The unwrapped view is always easier to read if there are no modes/reflections that cause 360° offsets to the chart. You have 6 of those <180Hz. The unwrapped phase is therefore dropping off the chart and would be useable. You were not interested in that range so it is no problem in this case.

The phase trend and subject wrap at 54 Hz doesn't look right. It is not obvious as to the cause however. My first guess is that this is a SW-main XO issue? The trend of the phase above and below the disruption do not seem to smoothly align. Maybe the delay/distance setting was not correct and then EQ was used to smooth the SPL and hence the 240° phase offset of this chart? Or, maybe the mic was moved away from the LP for this measurement? It is impossible to say with so little info and no data to work with.

If you want to post an .mdat with SW and main measurements from the LP with loopback timing activated, I may be able to better understand. If there is large EQ setting in this area it would is best to disable it for this purpose. Modest EQ filters do not normally create a problem.
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Old 06-04-14, 02:24 PM Thread Starter
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Matt

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Re: Understanding phase graphs

Thanks for the further clarification

Quote:
jtalden wrote: View Post
The phase trend and subject wrap at 54 Hz doesn't look right. It is not obvious as to the cause however. My first guess is that this is a SW-main XO issue? The trend of the phase above and below the disruption do not seem to smoothly align. Maybe the delay/distance setting was not correct and then EQ was used to smooth the SPL and hence the 240° phase offset of this chart? Or, maybe the mic was moved away from the LP for this measurement? It is impossible to say with so little info and no data to work with.

If you want to post an .mdat with SW and main measurements from the LP with loopback timing activated, I may be able to better understand. If there is large EQ setting in this area it would is best to disable it for this purpose. Modest EQ filters do not normally create a problem.
XO is a linear phase 2nd order neville-thiele at 120Hz so this range is the subwoofer only & the mic had not moved between generating the XO/correction and measuring the end product.

I have not found a way to get a loopback to work with REW when measuring the output of a jriver hosted convolution. I had another idea on how to do this though so will try again tomorrow. Having said that, the same phase wrap is visible in a similar measurement taken in acourate directly so I'm pretty sure it's not a measurement error. The same "problem" appears to be present in the uncorrected measurement though which confuses me further.
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Old 06-04-14, 04:04 PM
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Re: Understanding phase graphs

SW with Port on the back?
If not, what is the SW box design?
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Old 06-04-14, 04:55 PM Thread Starter
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Matt

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Re: Understanding phase graphs

Quote:
jtalden wrote: View Post
SW with Port on the back?
If not, what is the SW box design?
15" Fi SP4 in a 65L sealed cabinet, theoretical Q=0.71, LT applied that should produce Q=0.5 however the the driver has an inductance hump so the real life Q != theoretical Q. I do EQ out the hump (via a near field measurement and this gets embedded in my XO and is combined with the room correction to produce the final correction filter). The hump is centred around 60Hz, this seems a rather large coincidence if it isn't the source of that phase shift....
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Old 06-04-14, 06:33 PM
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Re: Understanding phase graphs

The LT is very likely the cause. Its phase will drop down quickly just after the early part of the shelf kicks in. It then stays a little below where the speaker phase would otherwise be.

So both charts posted are incorrect in the way it connects up across that 45-70Hz region. If you look at the wrapped chart, the phase connection between the upper an lower portion of the trace should actually go the other way. So the offset going from 70Hz to 45Hz is -115° as you show. The line should connect in that way, not as it shown now.

Another way to say it is that looking at the unwrapped trace the lower portion of the trace (9-45 Hz) is offset 360° too high. If that portion was shifted back down where it should be then the connection between it and the upper portion would slope up 115°.

Windowing the IR would probably show this. If we set a right window a little too narrow and increase it in steps. The wrapped phase trace may first show the correct connection path of the phase across that range and then at some point just a little wider will jump to the opposite (wrong) connection.
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