Session Files: "A Recording No-No"

Phase, Bass, and Fiddling in the Mix

I've been working on another two-continent job where initially, the tracks were recorded elsewhere and brought to me and then my job was simply to overdub the vocals and guitars. The original spec called for me to record the overdubs to iso tracks with a stereo scratch mix played back in the headphones. When finished, I'd export the newly-recorded tracks and send them back to the home studio. To be honest, the job ended up being a lot bigger than that, really a fixer-upper. First off, the composer arrived with isolated multitrack files on a thumbdrive that would need a rough mix before we could even begin. We hadn't built time for that into the session so I had to rush. Then there wasn't a full drum kit in the thumb drive tracks so I had to build one while we were overdubbing vocals. Finally, the rhythm section and tension structure of the song were't working right either so I had to edit things to make the song work and build properly. Zip, zip, zip. Off I went.

But once we got into the groove of overdubiing, this turned out to be a tremendous chance to work with a couple of great singers who are really nice folks as well. It went so well in fact, that the producer asked me to take on the project as a co-producer and fix and mix it. At that point I came across one of those little "gotchas" as I began to approach the mix phase of the project: I kept feeling like I was being "steered" left. No matter what I panned right to overcome it I kept feeling "pulled." I took a copy of the stereo mix to another monitor system, this one with a centralized subwoofer, and immediately got an inkling of what was up: On the second system, the steering was nearly gone but it was obvious that I had panned too much other stuff to the right to compensate for it. I went back and checked the original mix on the console with several different metering systems and the red light went off in my head: I had a little more energy on the left than the right, deceptively so, but only during the whole-ensemble passages.

A suspicion began to dawn in my mind: Most of the stuff imported from the studio on the other continent came in as stereo tracks. Was the bass guitar recorded stereo as well? And if so, were there phase issues in the stereo bass track? Since the steering was only partially corrected by the centralized subwoofer, was there something else that was imbalanced as well? And why hadn't I noticed this and taken steps before?

Sleuth time. I pulled up the original project and started soloing tracks to see where their energy was. Nothing, nothing, nothing, until I got to the bass track. The bass recording was indeed stereo and there was indeed something funky happening between its two tracks that heavily pulled to the left without unbalancing the meters very far. I made up a mono track in the DAW and then dragged the bass clip over onto it. Voile'! That centered it, and the bass sound wasn't audibly damaged by being summed. Why hadn't I noticed it before? I was assembling the tracks under the time pressure of the initial session and had apparently filed it for later action that I never took. On to the next. Making the bass mono took care of most of the problem but there was still some pull to the left when I listened to the mix. I next soloed the strings and found it: We had a spread of violas on the left and violins on the right but the violas had a huge bulge of energy around 250hz. happening, and of course, that happened mostly in the left channel. I pulled up a stereo EQ and notched things out a little at 250hz. with wide 'Q' skirts. Voile'! again. With slight panning, the strings centered up and spread nicely, there was less competition with the vocals, and the string energy was no longer imbalanced.

This serves as a handy reminder of a big No-No in recording: Don't create out-of-phase or non-coherent energy in the bass frequencies. Why, you ask? For a couple of very good reasons:

1) You can get this amorphous "steering" phenomenon that I experienced here, and
2) If you end up with a mix in this condition and try to cut it to vinyl, the needle can be forcibly ejected from the groove by the difference in physical energy between the right and left channels and the difference in physical phases of the wave shapes!

A vinyl mastering engineer who s worth his salt will have to insert a crossover into each channel to isolate the bass and then either sum the bass channels and center the product or use just one and center it to keep mistracking from happening. He'll also probably peer down his nose at you thinking you are a cretin for not knowing this. Even if we set aside that embarassment, bass can pull interesting psychoacoustic tricks on you as it did in the case. While people say that low-frequency content is omni-directional, I haven't found that to be particularly true. I can often feel the tug of a subwoofer that is placed offset from center in the stereo field. And in this case I felt the out-of-phase nature of the bass in the form of the "steering" effect. Whatever the case, here was a classic case of a bass phase anomaly that niggled at me continually until I took the time to analyze and correct the problems. It was a wonderful chance to learn from someone else's mistake as well!