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Many input tools show phase analysis. Now, without going into what tool, at the base level, I think they do the same thing. They show a visual similar to

enter image description here

My question is about understanding this visual.

When looking at this graphic (animation) can you tell if the recording is in or out of phase. Let's assume I'm using 2 microphones on an audio source and the combined input produces the graphic shown. For something to be in phase, should the colours be narrow or wide? Or, is my understanding of phase analysis incorrect?

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migrated from Jan 24 '14 at 12:01

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It's much, much easier to just use your ears. – Todd Wilcox Dec 11 '15 at 4:59
up vote 1 down vote accepted

Being out of phase means that one microphone is picking up the low end of a waveform (not frequency, but the waveform itself) while the other is picking up a high portion of a waveform.

To see if they are in or out of phase (visually) you would need to compare the signal from one source with the signal from both. If the combined level shrinks rather than grows, then the microphones are out of phase (since the one signal is subtracting from the other.)

You want to look for the phase that has the most cumulative effect on the waveform (where the waveforms roughly align).

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My research suggests differently I'm afraid (based upon your first sentence). I thought it had nothing to do with low or high per-se (although it is more common on bass instruments) but about the microphones picking up the same frequency at different times (regardless of what the frequency is), if I'm wrong please do correct me. The rest makes perfect sense but can the phase analysis show this (so I can use it during recording, not playback)? – Dave Rook Apr 22 '13 at 13:54
@DaveRook - I'm not talking about high and low frequencies. I'm talking about the waveform itself. Imagine if you had a regular sine wave, I'm talking about the high and low peaks. If two sources are 180 degrees out of phase, the low points of the waveform match up with the high points on the waveform and the net result is no signal. This is how balanced inputs and active noise cancellation headphones work. Signals don't have to be exactly 180 degrees out of phase though. They can be partially out of phase, in which case you get a bump or beat pattern due to how the signals interact. – AJ Henderson Apr 22 '13 at 15:21
@DaveRook, Imagine you have two large diaphragm microphones. There is a front side and a back side to each. Imagine setting them up with one facing forward, and the other facing backwards. The source of the sound will reach the two microphones at exactly the same time (let's just say). The two diaphragms will move in perfect sync. However, upon reacting to the sound pressure waves, one diaphragm is moving forward through the magnetic field, and the other is moving backwards. You get identical waves recorded, just with opposite polarities. Those are exactly 180 degrees out of phase. – JoshP Apr 22 '13 at 16:03
@AJHenderson Your clarification here in comments make sense, I guess that when referring to "high end" and "low end" usually mean frequency wise. I think you should edit your initial answer to be more like your comment here. – Eugene S Apr 23 '13 at 1:13
@AJHenderson Thank you for making that clear. It makes more sense now! :) – Dave Rook Apr 23 '13 at 8:16

The specifics of different implementations of these visualisations vary a lot. This answer may therefore not accurately describe your tool, but should give you an idea of what's happening nonetheless.

These visualisations plot frequency against phase. An audio signal can be represented by the sum of several sine waves of different frequencies, amplitudes and phases. The phase analysis seperates a small section in time of the audio into these sine waves and compares the phase of each frequency component with that of the other channel. If they are in sync, a datapoint is added in the middle. If the left channel lags behind, a mark is drawn on the negative side and vice versa. If both channels oppose completely, i.e. the phase shift is 180°, the number is either -1 or 1. Those are of course equivalent, a sine shifted half a period to the right cannot be distinguished from one shifted left.

One of the applications of these analyses is merging channels, for instance when mixing to mono. Frequencies that are plotted in the center will increase in amplitude, whereas points far to the side of the plot will mostly cancel. If the distribution varies strongly between important frequencies, the resulting mix can sound very distorted. A narrow graph suggest both channels are very similar and there was very little stereo information to begin with.

A great way to experiment with these tools is to invert one of the channels and listen for changes in the mix and the visualisations.

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