Mixing for Better Mastering

Mixing for Better Mastering

What you do during the mix can make a big difference ...

by Craig Anderton

Mastering your own recording projects at home has become a hot topic, and I’m often asked at seminars whether doing a great mix eliminates the need for mastering. Theoretically this is possible, but the analogy I’d use is putting dressing on a salad. You could put a certain amount of dressing on each piece of lettuce, tomato, etc., then when combined, you should have the same results as putting dressing on the entire salad. This would be like optimizing every track, and assuming that when put together, something would sound “mastered.” But in my experience, I’ve never heard a mix—no matter how good—that couldn’t benefit in some way by a little judicious mastering.

Nonetheless, there are techniques you can use while mixing to make the mastering process go more smoothly—here are some of my favorites.

MATCHING TIMBRES

If you use loops in your music, be aware of loops whose characteristics are wildly different from other loops. For example, suppose most of the loops were taken from a drum machine you use, but you also inserted a few commercially-available drum loops. It’s likely that the latter were already “pre-mastered,” perhaps with some compression or treble-boosting. As a result, they might sound brighter than the loops you created.

If you decide to boost the track’s overall brightness while mastering, the commercial loops will now seem “over the top” in terms of treble. I had this happen once when re-mastering a stereo track where everything needed a little extra brightness except for a high-hat loop. It took forever to use notch filtering to find just the high-hat frequencies and reduce those, while boosting everything else.

This kind of inconsistency can also happen if you use a lot of analog synths, which tend to have a darker sound, mixed with a few digital synths, which tend to be brighter. This will also give problems when mastering, because if you bring down the highs to tame the digital synths, the analog synths will sound much duller; if you bring up the highs, the digital synths may screech.

The solution is simple: to ensure that changes made during mastering will affect all sounds pretty much equally, before mixing, bring “minority” tracks into timbral alignment with the majority of the track’s timbres. However, don’t go overboard with this; some differences between tracks need to be respected (e.g., you might want a track to sound brighter or duller than others, regardless of any equalization done while mastering).

BRINGING PEAKS INTO PLACE

Another issue involves peak vs. average levels. A lot of engineers use mastering to increase a tune’s average level, thereby making it seem louder (regrettably, some engineers and artists take this to an extreme, essentially wiping out all of a song’s dynamics). To understand the difference between peak and average levels, consider a drum hit. There’s an initial huge burst of energy (the peak) followed by a quick decay and reduction in amplitude. You will need to set the recording level fairly low to make sure the peak doesn’t cause an overload, resulting in a relatively low average energy.

On the other hand, a sustained organ chord has a high average energy. There’s not much of a peak, so you can set the record level such that the sustain uses up the maximum available headroom.

Entire tunes also have moments of high peaks, and moments of high average energy. Suppose you’re using a hard disk recorder, and playing back a bunch of tracks. Of course, the stereo output meters will fluctuate, but you may notice that at some points, the meters briefly register much higher than for the rest of the tune. This can happen if, for example, several instruments with loud peaks hit at the same time, or if you’re using lots of filter resonance on a synth, and a note falls within that resonant peak. If you set levels to accommodate these peaks, then the rest of the song may sound too soft.

You can compensate for this while mastering by using limiting or compression, which brings the peaks down and raises the softer parts. However, if you instead reduce these peaks during the mixing process, you’ll end up with a more natural sound because you won’t need to use as much dynamics processing while mastering.

The easiest way to do this while mixing is to play through the song until you find a place where the meters peak at a significantly higher level than the rest of the tune. Loop the area around that peak, then one by one, mute individual tracks until you find the one that contributes the most amount of signal. For example, suppose a section peaks at 0 dB. You mute one track, and the peak goes to -2. You mute another track, and the section peaks at -1. You now mute a track and the peak hits –7. Aha! That’s the track that’s putting out the most amount of energy.

If you have “rubber band” mix automation, dive into waveform view for that track, and insert a small dip to bring the peak down by a few dB. Now play that section again, make sure it still sounds okay, and check the meters. In our example above, that 0 dB peak may now hit at, say, -3 dB. Proceed with this technique through the rest of the tune to bring down the biggest peaks. If peaks that were previously pushing the tune to 0 are brought down to –3 dB, you can now raise the tune’s overall level by 3 dB and still not go over 0. This creates a tune with an average level that’s 3 dB hotter, without having to use any kind of compression or limiting.

GETTING RID OF SUBSONICS

Unlike most analog recording, digital can—and sometimes does—produce energy well below 20Hz. This subsonic energy has two main sources: downward transposition/pitch-shifting, and extensive DSP operations that allow control signals, such as fades, to superimpose their spectra onto the audio spectrum.

I ran into this problem recently when doing a remix of a soundtrack tune. The client wanted a really loud, "squashed" mix, so I added a substantial amount of limiting to the finished mix. Yet in some sections, the level went way down—as if some hugely powerful signal was overloading the limiter’s control signal—but I couldn’t hear anything out of the ordinary.

Looking at the two-track mix showed something interesting: a massive DC offset (Fig. 1).

Fig. 1: The top graphic shows the original file prior to filtering out the subsonics. The lower graphic shows what it looked like after adding a sharp low-frequency cutoff at 30Hz. Normalizing it could raise the level significantly higher.

After a bit of research, I noticed that these dips (outlined in red for clarity) corresponded to places in the song where there was a long, rising tone. I had transposed the tone down by several octaves so it sounded like it was coming up from nowhere, but that transposition had moved it down so far into the subsonic region it created a DC offset. That’s the signal to which the limiter was responding.

So, I inserted EQ on this one track to create a super-sharp cutoff starting at 30Hz. When I redid the mix, the DC offset was gone.

Now that my curiosity was piqued, I called up a spectrum analyzer window and started looking at some of the files that had been subjected to multiple DSP operations. Sure enough, in a few cases there was significant energy below 20Hz. After a while this can add up, robbing available headroom and possibly causing intermodulation problems with audible frequencies. Since then, I’ve started using batch processing functions to run all files used in a project through a steep low-cut filter whose frequency is set just below the audible range. In some tunes this doesn’t make too much difference, but in others, I’ve noticed a definite, obvious improvement in headroom and overall clarity. You can also use a sharp low-cut filter with already mastered material to cut out subsonic frequencies, but it’s much better to do this type of processing before the files are mixed together, as this can lead to a cleaner mix.

Okay, now your tune is prepped for mastering. Hopefully, as a result of these techniques any processing required for mastering can be more subtle, so you’ll end up with a clearer, more natural sound – but one that still packs plenty of punch. -HC - 

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Craig Anderton is a Senior Contributing Editor at Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.