It’s a perplexing problem: the mystery of the disappearing audio. No, not a sound file lost to a corrupt memory card, or crashed hard drive, or accidental deletion. Instead, a recording is made in the field, played back, logged, edited, and perhaps even mixed into a final production with no indication of a problem.
Yet, when the production is played back, some of the sound files are missing, silent. Play the same mix back on a different system, and the mystery files are back.
Or perhaps when the recording is made, it sounds fine in headphones; the meters bounce normally. Yet when you play it back on the recorder’s built-in speaker, there’s nothing there. Even more mysteriously, the level meters are still bouncing as if there’s audio. Plug in the headphones, and the audio is back.
What in the World is Going On?
The problem is phase: in particular the phase relationship between two sound files that ostensibly are identical, but whose peaks and valleys are out of synch, in fact they may be exact opposites of one another. Technically, there are two different phenomena: Polarity and Phase, and signal problems resulting from wiring differences are more accurately the result of a polarity issue, not phase. The term “phase” is more properly used to refer to waveform interactions based on time and distance. However, both issues are commonly referred-to as “phase.” A switch that flips the polarity of a mixer or preamp input is usually called a phase button, even using the Greek symbol phi – Φ. Cable testing boxes will often have a red “phase” light that probably should be labeled “polarity.” Both polarity and phase manifest similar interference and cancellation issues, so for the sake of simplicity, we’ll use the more generic term “phase.”
The problem most commonly arises when the sound file was made from a mono source, probably a single interview microphone recorded to a stereo file, which is to say, a sound file with identical left and right channels. It’s not uncommon for audio recorders to work that way by default: some devices only record stereo files. Even if they’re capable of making truly single-channel mono recordings, it can be confusing or cumbersome to set the recorder to do that properly, while still feeding a monitor signal to both ears of your headphones.
Recording a stereo file from a mono source is NOT inherently problematic, it just means that the same audio signal should be sent to both the left and right channels. That’s a little inefficient: it’s doubling the space used on your memory card or disc drive — by recording redundant information on the left and right tracks. Similarly, it will take twice as long to FTP a stereo file compared to a mono file. But it’s often easier to just record the stereo file. Large memory cards are affordable these days, FTP speeds are fast enough, and when it comes time to assemble your production, you don’t have to keep track of which sound files are stereo or mono…if they’re all stereo.
When mixing an audio production, there’s no audible difference between a stereo file with the same audio on the left and right channels and a mono file panned to the center. However, the single mono track will show up as a few dB quieter in the stereo master track. That’s easy to compensate for — just give that mono track, or clip, about 3 dB of boost, compared to the stereo version.
But, there IS an audible difference when using a stereo track with the left and right channels “out of phase” (with reverse polarity.) The left and right channels interfere with each other when combined, which can create various effects. Played back in stereo, the out of phase audio can sound relatively normal, although it won’t seem to have a solid location in the stereo field. The sound from a single mono microphone, recorded in-phase on a stereo track (or a mono track panned to the center) will appear to be perfectly in the center of the stereo image, seeming to hover directly between the two stereo speakers. Out-of-phase audio has an indistinct stereo image, it will seem to come from out beyond the edges of the left and right speakers, with a weird, empty hole in the center of the stereo image. The sound isn’t tilted to the left or right; it’s both at the same time, and missing from the center.
More important, when monitored in mono, audio that has its left and right channels 180 degrees out of phase (with reverse polarity) will literally disappear. The left and right channels cancel each other out, leaving no trace of the original sound. Occasionally some ghostly shadows of the audio remain: perhaps there was some cross-talk between the channels, or a volume imbalance between the channels resulting in the left and right not canceling perfectly, but in either case the original sound will be significantly weakened.
Here’s what that sounds like:
If you listen to enough out-of-phase audio, you can hear that lack of center and quickly identify problematic audio. It’s a little harder to do in headphones than speakers, but still identifiable by ear.
The better solution is not to rely on your ears, instead to verify the phase coherency of your audio. The simplest way is to listen in mono. Some editing programs have a mono button on the stereo master track, which allows for quick and easy checking of mono compatibility (e.g. REAPER). Monitor controllers that allow switching between multiple speakers often include a mono button. All digital audio editors SHOULD have an option to temporarily switch to mono, but not many do.
Some plug-ins will allow you to change the stereo image of the audio, so using a plug-in to reduce the stereo spread to zero can reveal phase problems. Even better, inserting a phase scope on the master track (or a stereo track that you suspect might have a problem) can tell you a lot about you audio content. If your audio is mostly extending horizontally on the scope, and the phase meter is significantly less than zero, you have a phase problem. If the scope shows more of a vertical line, and the phase meter is above zero, at or near +1, your audio is in-phase.
Of course, complex stereo recordings won’t show up as simple, neat lines on a phase scope; some degree of phase difference is natural in a stereo mix. Instead you’ll see a crazy tangle of squiggles and shapes, but you want those squiggles to be more vertical than horizontal on the scope, and for the phase meter to stay mostly above zero.
Or, simply zoom WAY in on the waveform display: zooming all the way in should reveal the situation. If the left and right waveforms rise and fall together, the audio is in-phase. If they seem to be mirror images of each other — if one channel has a peak and the other has a dip at the same spot — you’re out-of-phase.
Out in the field, there’s still one quick test you can do: if your recorder has a built-in speaker it’s almost certainly in mono, so just try playing back your recording over that speaker. If it sounds Okay, you’re probably fine.
Why Does the Sound Disappear in Mono?
Sound waves travel in waves of high and low pressure. We often visualize this as similar to an ocean wave, with peaks and troughs. That’s not exactly how sound waves work, but it’s a helpful analogy. Sound waves are drawn that way in our digital audio editors so we can visualize the positive and negative patterns of the sound waves.
If the left and right channels of an audio file are exactly out of phase, when the right channel audio has a positive peak, the left channel audio will have a negative peak of the same intensity. Each channel sounds fine on its own, but combine the two together and they will cancel each other out.
This particular problem arises most commonly from a cable wired incorrectly for its purpose. It’s especially common in cables that convert the XLR connectors of professional mics to the minijack inputs of small audio recorders or camcorders. There are several ways that the XLR-to-mini cable can be wired. The XLR jack has three pins (or sockets) and the TRS (stereo) mini has three connection points: the Tip, the Ring and the Sleeve. You’ll sometimes see the short hand “TRS” for that kind of connector, both on mini or quarter-inch connectors.
An XLR-to-stereo mini cable can be wired so that each pin of the XLR routes directly to one section of the stereo mini jack, but that will send audio from your mono mic to the stereo mini plug with the left and right channels out of phase. The left and right signals are, in effect, mirror images of one another: when one has a positive peak, the other has a negative peak, and vice versa.
An XLR-to-mono mini cable, where the mini connector does not have a Ring, just a Tip and Sleeve (a”TS” jack,) will only send audio to the left or right channel, with silence on the other. A stereo audio file with silence on one channel can be annoying to deal with when you’re mixing, and when recording in the field, it’s hard to accurately monitor your signal on headphones when there’s only sound in one of your ears.
In order to send the audio from a single mono mic to both the right and left channels of the recorder, the cable should be wired so that the tip and ring of the stereo mini connector are tied together and the same electrical signal is going to both connection points. The resulting stereo file will sound centered when you use it in the mix, and more practically, the signal will show up in both ears of your headphones.
It’s common to find XLR-to-mini cables that are wired so that the mono signal from the XLR appears on both the right and left channels of the stereo mini, but exactly 180 degrees out-of-phase. They’re not defective cables, they’re just created for a different purpose.
[update: I have been reminded that this wiring scheme is standard for a balanced mono XLR-to-mini cable. The confusion arises from the fact that the three-conductor TRS mini connector can be used for balanced mono connections, or unbalanced stereo connections. When connecting a mono mic to a stereo recorder input, you need to use an unbalanced stereo wiring scheme to send audio to both sides of the stereo input. XLR pin two is wired to the mini connector’s Tip and Ring, XLR pins 1 and 3 are wired to the mini connector’s sleeve. This unbalances the cable, making it more susceptible to picking up hums and buzzes, but also sends an in-phase audio signal to both the right and left channels. Check the comments for more details about XLR-to-mini wiring.]
What’s worse is that there are no consistent naming conventions that will reliably warn you how cables are wired. This EXACT problem arose recently: I was guilty of recommending a cable that claimed to be good for connecting a mono microphone to a stereo mini, but it turned out to have the left and right channels out of phase. I trusted the product description on the sales page, which was misleading… In this case it was the “Kopul XRSM-015 3-Pin XLR Female to 3.5mm RA Stereo Mini-Plug Cable.” Don’t buy that one for connecting a mic to an audio recorder!
Sadly, it’s also common to find XLR-to-mini cables that send signal to only one channel, which can be annoying: it’s hard to accurately monitor audio that’s in only one ear of your headphones. But at least it won’t accidentally create audio that disappears in mono.
The only useful terminology I can find is that sometimes the XLR-to-mini cables are called “Camcorder cables” which seems to indicate that they’re meant to connect a mono mic to the stereo mini input of a consumer or prosumer camcorder. But I’m not sure I’d even trust that… always plug those converter cables in and do a test recording, make sure they’re sending audio to both channels, and that it’s in-phase.
Unfortunately, we have not found a source for reliable XLR-to-mini cables that we fully trust. These cables from HOSA are okay — they are wired correctly and are available in convenient lengths, but we haven’t found them to be especially robust in the long run.
There are similar cables available from other manufacturers, but as mentioned above, we found that the cables from Kopul were wired incorrectly. A similar one from Sescom was wired right, but mine developed a short very quickly. Versions from Mogami or Sennheiser work great but are only available in an inconveniently short length.
There is a variation on this converter cable, sometimes called “Balun,” because it electronically compensates for the change from Balanced XLR to Unbalanced mini, but those tend to be expensive, and short, and sometimes add a heavy transformer that’s not completely necessary for most new recorders.
Cables, especially cables with mini connectors, seem to develop crackles or stop working altogether more often than we’d like, so you might want to get more than one, to have a back-up in case your main cable goes bad. If we find a reliable cable that we can wholeheartedly recommend, we’ll be sure to post about it here on Transom!
What If You Have Already Recorded Out-of-phase Audio?
It’s easy to miss: when you encounter a stereo sound with the left and right out-of-phase, each channel on its own sounds perfectly fine: our ears are detecting the shapes and patterns of positive and negative peaks in an audio waveform, not the absolute values of positive or negative polarity at any given instant. Even listening to both of the out-of-phase waveforms at the same time can sound (relatively) normal — IF — you’re listening in stereo. In particular, on headphones it can sound fairly normal — each ear is hearing a perfectly valid audio signal.
If you don’t discover the problem until you’ve already assembled and mixed your program, you can’t solve the problem simply by mixing, or bouncing, to mono — the conversion to mono is actually when the out-of-phase audio disappears. In a pinch, use either the right or left channel of your production, not both, and mix to mono. But that might not sound like you want it to: if you have stereo material in your program, simply using the right or left channel might sound unbalanced. You really want to be able to combine the left and right channels together, but that’s where you’ll have a problem if you have phase issues.
The deeper, better fix, is to go back to your multitrack project and find the offending audio elements — the ones that disappear in mono.
You can do two things. The first is to split the stereo sound file into two mono sound files. This can be done in different ways depending on your audio editor. Some editors, such as Pro Tools or Audacity, do it at the track level. You should put all the problematic audio on its own (stereo) track — you can maintain its position on the timeline. Then highlight that track and select “Split into Mono” from the Track menu.
It works similarly in Audacity: go to the Audio Track drop-down at the left of each track — select “Split Stereo to Mono” then set the resulting mono tracks.
There are similar functions in most audio editors: Reaper puts it under “Item Functions” where you can select the dramatic command: “Explode Multichannel Audio to New One-Channel Items.” In recent updates Adobe Audition added an Edit Menu command called “Extract Channels to Mono Files.”
That will create two identical mono tracks from the formerly interleaved stereo sound file. The easiest next step is to simply delete one of them (it usually doesn’t matter which one, they should sound identical, but it’s a good idea to listen to each in case if one track sounds better than the other.) Then pan the remaining track to the center. You will probably need to boost the level of that track about 3dB to maintain the same volume you had. But that’s not always true — the out-of-phase audio might have caused you to mix it differently — so use your ears, and meters.
The second thing you could do is to keep both tracks and flip the phase on one of them (it doesn’t matter which one) using a plug-in. Some editing programs include a phase button on each track. But that doesn’t really get you anything: after fixing the phase, you end up with two identical sound files, which just creates the opportunity for a problem, like one of them slipping out of synch with the other, etc. You’re better off keeping only one of the channels, and using it in mono, panned to the center.
It’s safest to avoid the problem in the first place by getting a properly wired XLR-to-mini cable. But if the only reliable one you have creates out-of-phase audio, you can still use it, as long as you fix the resulting sound file as soon as possible after recording. Either the left or right channels of the recording is perfectly useable, just not both of them together. At your first opportunity, import the stereo file into your editing program, split it into two mono files, delete one of the mono files, and save the remaining one as the new master recording.
Other Phase Problems
A different kind of phase incompatibility can result from using two mics to capture stereo. If they are spaced improperly relative to the source, you can end up with a similar problem: the waveforms’ peaks and valleys interfere with one another. But that kind of phase problem rarely results in complete cancellation of the audio when monitored in mono. Instead you might just get a weak sound, or unwanted emphasis or reduction of specific frequencies.
When using two mics to create a stereo recording, the whole point is to pick up slightly different signals with each mic. If the left and right waveforms were exactly the same, it would effectively be mono… But you don’t want the left and right signals to be majorly out-of-phase either.
Multi-mic phase problems can be avoided by careful mic positioning. One of the main reasons that you often see two microphone capsules pointing in different directions, but crossed over one another is to avoid phase issues. (It’s called an X-Y stereo mic pattern — as you see on the built-in mics on many small portable recorders.) With the mic capsules right on top of each other, the waveforms arrive at the same time, preventing any major phase issues.
You can encounter phase problems in other ways. Other cables can be wired out of phase, not just XLR-to-minis: it’s possible for a tech to accidentally switch which wires are soldered to which terminals when a cable is made, resulting in a phase-swap. Or a sound file that you’ve imported into your project could have been recorded with a phase problem. I’ve encountered commercially-released music, even on major labels, that has significant phase incompatibility. It might not disappear completely in mono, but it changes character dramatically, and the balance of instruments is thrown off in an unpleasant way.
Who Cares About Mono?
It seems weird to worry about mono compatibility in this modern day, but many podcasts are mixed to mono in order to be more efficient when stored and distributed. Some radio station internet streams are in mono. Even if your production is being aired on a stereo FM station, or distributed over the internet as a stereo file, listeners at home might be listening on a mono clock radio, or a mono bluetooth speaker. So it’s important to make certain that your productions sound okay in mono, even if you think they’re only being used in stereo.
Verify that your productions are mono compatible. Switch your master output to mono and listen. Or check your mix on a mono speaker. Or use a phase meter plug-in. Do it for your listeners.