Smaart Phase Trace Question

[NSMTsound]

I have a specific question about the phase trace in Smaart. Lots of reading and a Smaart training seminar with Jamie have given me a pretty good understanding of how to read it, but one aspect has puzzled me for a while now.

Feeding identical signals to both of Smaart’s inputs gives you a horizontal trace at 0 degrees on the graph. This logically tells you that the two signals are aligned in time at all frequencies. Makes sense.

If I understand correctly, the following points are true when looking at the phase of the pass band of a driver (the typical smile-shaped trace).

1 – Where the trace is roughly horizontal (the bottom of the smile) the two signals are aligned with each other as described above.

2 – Where the trace is angled downward from left to right, the measurement signal is lagging behind the reference signal.

3 – Where the trace is angled upward from left to right, the measurement signal is leading the reference signal.

Okay…now what does it mean when you have a horizontal portion of the trace (the bottom of the smile described above) that does not fall at 0 degrees on the graph? How does the graph’s degree scale relate to the trace? Does it mean anything at all for a single reading, or is it just there so you can compare the relative positions of two separate traces? If I understand correctly, you can’t compare phase angles for different frequencies anyway. Someone please enlighten me!

[Harry Brill Jr.]

Okay…now what does it mean when you have a horizontal portion of the trace (the bottom of the smile described above) that does not fall at 0 degrees on the graph? How does the graph’s degree scale relate to the trace? Does it mean anything at all for a single reading, or is it just there so you can compare the relative positions of two separate traces? If I understand correctly, you can’t compare phase angles for different frequencies anyway. Someone please enlighten me!

The phase angle is what we are concerned about. If it's flat the reference and measurement are hitting smaart's engine at the same time. It's all relative. It's been said Smaart can't measure absolute phase, but I contest that it is irrelevant for our purposes. In order to measure absolute phase the software needs a known source. That would no longer be source independent.

[Arthur Skudra]

What Harry said. That is the reason why the phase display can be wrapped into a 360 degree window, you want to watch the slope of the trace, and watch out for any 180 degree difference between saved traces--a clear indicator something is out of polarity somewhere in your system. True story: I was called in to fix a line array that kept blowing woofers. I compared each separate LF section of the system using captured magnitude and phase traces, and found that one half of the line array LF was out of polarity with the other half (you can see a jump in phase trace by 180 degrees between the two outputs), causing cancellation when both were on. They cranked the volume to compensate, toasting woofers in the process.

Come to find out, a home-made XLR Y cable was found in the drive rack, one output with pins 2 and 3 reversed! That was the most expensive Y adapter I've seen yet!

[Jamie]

The slope of the phase trace shows relative timing, it's position on the graph shows relative phase.

So a downward slope (left to right) indicates the Measurement signal is lagging behind the Reference signal at those frequencies. Horizontal/Flat slope indicates simultaneous Meas and Ref arrival, and upward slope indicates Meas leading Ref.

Further, the severity of the slope indicates the amount of relative time offset. So two measurements with downward slopes at freq X both indicate Meas lag, and the trace with the steeper slope at freq X indicates a greater amount of lag.

So what about a flat slope that doesn't occur at 0 degrees on the Phase graph's scale? That just shows that the Meas signal is timed to the Ref signal at that frequency, but they are X degrees shifted from one another. The most obvious example of this is in looking at a polarity reversal. The polarity rev changes the position of the trace by 180 degrees on the graph's scale, but does not change the relative slope (since a polarity reversal has no time component.)

The phase trace includes information about both the relative time arrival and phase between the Meas and Ref signals - and hopefully by extension, provides a view of what the system you are measuring is doing to the signal passing though it in regards to timing, polarity and phaseshift.

-j

[PhillipIvanPietruschka]

So I'm curious, what sort of scenario might produce a phase trace that was flat or at least, substantially flat at values other than 0 or 180?

[Arthur Skudra]

So I'm curious, what sort of scenario might produce a phase trace that was flat or at least, substantially flat at values other than 0 or 180?

To be honest, I don't pay as much attention to the numbers as I do the slope or spacing between saved phase traces. Apart from measuring a mic cable in or out of polarity, I don't see a flat or substantially flat phase trace at 0 or 180 too often, especially if it's unwrapped. So to answer your question, there are many scenarios where you won't find a flat line at 0 or 180 degrees, 20 to 20K. Depending on where your delay locater is set, where your speaker drivers and microphone are located, it can be just about anywhere.

Keep in mind that the phase trace is wrapped by default in Smaart since we are using it typically for the measurement of sound reinforcement systems, and we are more concerned about slope and phase differences between traces instead of their absolute phase angle. Because the wrapped display is used to show only 360 degrees worth of phase, anything beyond that is wrapped in the display. Typically I unwrap the phase display to aid in Subwoofer alignment to spot overall trends, but that's about it.