How Phase Meters Can Help Your Mixes
Monday April 15, 2019. 02:00 PM , from Sweetwater inSync
Of course level meters are helpful, but don’t overlook phase and correlation meters, which measure a signal’s stereo (or surround) qualities (see fig. 1 below). Both these meters answer a variety of questions about stereo imaging, phase, mono compatibility, and more.
Figure 1: Typical phase meters (outlined in orange) and correlation meters (outlined in red). Clockwise from upper left: Steinberg Multiscope, Samplitude Visualizer, Studio One’s project page metering, Sound Forge metering, and the Vectorscope module in Ozone 8.
The Correlation Meter (seen in the red boxes in figure 1 above) typically shows a bar, calibrated from -1 on the left (or bottom, if the bar is vertical), to 0 at the center, to +1 at the right (or top). Its main function is showing a stereo signal’s mono compatibility. The bar will vary among these numbers, depending on the audio material. So -1 means the right and left channels are 180 degrees out of phase, while +1 indicates the left and right channels are identical (i.e., mono audio). A reading of 0 means that the two channels are uncorrelated — in other words, the two signals have nothing in common. With audio that doesn’t have phase issues, the bar will typically fluctuate between 0 and +1. It’s okay if the bar goes negative on occasion, but you don’t want the bar spending too much time there, because the more time it’s negative, the worse the mono compatibility.
The Phase Meter (seen in the orange boxes in figure 1 above) shows how “stereo” a signal is, and where energy is distributed in the stereo field, by drawing a Lissajous pattern. (You don’t need to know what that means, I just said it to look smart. It’s the name for representing complex harmonic motion, and audio definitely falls under that category.) With a stereo signal, up/down indicates level, while right/left indicates stereo spread. The narrower the concentration of left/right energy, the more mono the sound. For example, figure 2 (as shown by Studio One’s phase meter plug-in) displays the stereo image of a typical well-mixed song. The Correlation meter indicates 0.47, which means that this momentary reading (a snapshot in time) shows the audio is distributed with roughly half its energy in the center, and half in its left and right sides.
Figure 2: Stereo imaging for a typical pop music mix.
Many DAWs include one, or both, types of metering. If not, plug-ins with phase metering are available from Waves (PAZ), iZotope (Ozone 8), and others.
Phase Meters for Mastering
Traditionally, phase meters are used for mastering and checking 2-channel mixes. Phase meters were crucial for mastering vinyl, because out-of-phase components in the left and right channels could cause an unhappy stylus to jump around (even out of the groove) and skip. With vinyl now representing a tiny percentage of how people listen to music, checking phase may seem quaint. However, mono compatibility still matters when listening over speakers, because the audio will mix in the air to some extent. Also, many laptop, TV, and smartphone speakers are mono. You don’t want parts of your music disappearing because of phase issues.
Phase Meters Meet Mixing
Phase meters are useful for mixing as well as mastering. I often insert a phase meter on the master bus and solo individual tracks to gain some insights into how they’re relating to stereo. Granted, there are other ways to do this — you can solo tracks, switch the output bus into mono, look over the pan pots, and so on. But for a quick peek into the state of the mix, examining each track with a phase meter can alert you to possible problems that may need further investigation. Here are some examples.
Left, Right, and Center
Mono mixer DAW channels use pan pots to place the mono signal in the stereo field. With a phase meter, it’s easy to confirm if the audio is mono, and if so, its position (fig. 3).
Figure 3: The left phase meter shows a mono signal panned 100% left, while the meter on the right shows a mono signal panned to center.
A mono signal creates a straight line in the phase meter. A vertical line means the signal is panned to center, and the Correlation meter shows 1.00 because the left and right channels are identical. I always check that the bass is centered and mono, assuming the music isn’t a dance mix with a synth bass. If the line is diagonal and slants 45 degrees to the left, then all the signal’s energy is in the left channel. If the line is diagonal and slants 45 degrees to the right, then all the signal’s energy is in the right channel.
In general, you don’t want signals panned to the extreme left or right (especially with a stereo input, because it will be converted into mono). The point of stereo is that the audio comes from two point sources, even if the level in one channel is way lower than the other one.
If you see a single horizontal line with a stereo signal, that’s not good — it means one of the channels is out of phase.
Digital Delay and Phase
Delaying the left and right channels by a different amount of time gives a mono signal the illusion of stereo, but the delayed sounds can sometimes fall out of phase with each other. The Phase meter makes it easy to adjust the delay time for a balance of wide signal and mono compatibility.
Figure 4: The left meter shows a stereo signal with delay times of 100ms in the left channel and 102.8ms in the right channel, which gives a bit of a stereo spread. With the delays set to 100ms and 97ms (center image), there’s more stereo spread, but the potential for less mono compatibility. The meter on the right, with 100ms and 105.9ms of delay, gives a highly uncorrelated sound, but without any phase cancellation.
Bear in mind that the phase difference in the middle isn’t huge, and you may decide you want a tiny bit more width in return for a tiny bit less mono compatibility. But if there are phase differences with lots of different tracks, you might want to dial back the width a bit on some of them to create a stronger mix.
The Wider Image
Now suppose you have a bus carrying a mix of massed, choir-like vocals. You want to widen the stereo image to fill out the sides more, as well as leave more room in the center for the lead vocal. But you want to do so without compromising mono compatibility, because the choir is an important part of the song. A good solution is mid-side processing (fig. 5).
Figure 5: The meter on the left shows the original choir imaging, which has a reasonably wide stereo image. After inserting Studio One’s Binaural Pan plug-in, the meter on the right shows very little correlation — the left and right channels are quite different, and there’s no phase cancellation.
But going for a wide sound can go too far. Figure 6 shows a signal from a guitar pedalboard that worked its way into a project. The vintage chorus effect sounds great live, but clearly, a phase inversion — as evidenced by the wide, low image — is causing an extreme out-of-phase condition.
Figure 6: The wide, low horizontal image gives a clue that phase problems are occurring.
After a little investigating, it turned out the chorus was one of those effects that created a stereo image by throwing one channel out of phase. Bypassing the pedalboard chorus, and using a chorus plug-in, solved the problem. This is also the kind of pattern you’d see with a program that nukes center-channel vocals for karaoke, or a stereo signal that uses XLR cables where one of the cables is mis-wired so that the polarity of one channel is reversed.
Turning Mono into Stereo
Now let’s see whether plug-ins can be effective at turning mono into stereo (fig. 7).
Figure 7: A mono signal, before and after inserting Waves CLA Guitars plug-in.
The left phase meter shows the pickup output from a Gibson J-45 acoustic guitar panned somewhat left of center. The phase meter on the right shows what happens after inserting the Waves CLA Guitars plug-in. Most of the energy remains panned to the left, but now the signal is wider, with about two-thirds of it uncorrelated. This essentially turns a mono, “point source” guitar into a wider image, but it still has a sense of being panned left of center.
Going Through a Phase
A phase meter isn’t something you’ll monitor all the time, by any means, but I like to check mixer channels periodically to see what’s happening with the phase and panning. For example, a pan pot can show the position of a signal in the stereo field, but it won’t show whether it’s a point source or more of a stereo image. And although you can always flip the main output bus interleave to mono and solo individual channels, it’s often quicker to glance at a phase meter than to spend time doing critical listening. If you haven’t given your phase and correlation meters some love lately, don’t let them feel neglected. To show their appreciation, they may even help you make better mixes.
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