MIXING TIPS

Audio mixing is used for sound recording, audio editing, and sound systems to balance the relative volume, frequency, and dynamic content of a number of sound sources.
Typically, these sound sources are the different musical instruments in a band or vocalists, the sections of an orchestra, announcers and journalists, crowd noises, and so on.

A recent trend is mixing to 5.1, which is "surround" audio. This requires 6 channels of audio: left, center, right, left rear, right rear, and low frequencies (subwoofer, or LFE). The demand for 5.1 in the audio and music domain was once small, but has recently increased dramatically, along with the introduction of 7.1 & 9.1 surround channel audio.

REFERENCING OTHER CDS AT THE MIX SESSION
Many times musicians and engineers will work for long hours on a mix only to find the next day that critical balances are off, i.e., vocals too loud / too low, certain instruments too loud / too low, the mix overall too bright / too dull, too much reverb / too little reverb, etc. The way they usually discover this is by playing them in a more familiar environment (their home or car), and comparing them to other CDs in their collection. Much better to bring those CDs to the studio and compare there, while mixing. Bring along some of your favorite CDs, and do an A/B test periodically. For example, if you find that you have more bass than the reference CDs, turn down your bass! Does your vocal seem low by comparison? Adjust it accordingly. If your mix doesn't pass this comparison test in the studio, it won't at home either.
If you are using a computer-based recording system (Pro Tools, etc.), try importing the reference songs right into your session file. Play the CD tracks through the same outputs as your song. Now you have a totally level playing field. Just remember to turn DOWN the CD tracks, as they will be much louder than your mix, due to the CDs having already been mastered. Also, if you have any plugins on your master fader, you will need to avoid your CD reference passing through them, so you may need to rout them to another set of outputs. But once you get this set up properly, consistency will be yours. There's no need to reinvent the wheel at every mix session.
Two things to be careful of:
1 - Don't try to match your mix to the volume of the reference CD. It's already been mastered. The mix that THEY used for mastering was not as loud as their final CD, and neither should yours.
2- Don't try to match their compression. Additional compression was likely added in mastering. If you squash your mix, you will be stuck with whatever you did.. Mastering is usually a better time and place for that.

CHECKING THE MIX IN MONO
Sometimes mixes fall apart in mono, for a variety of reasons. Why is this important? Who listens in mono anyway? We all do, all the time. If you are not sitting right in front of your speakers, you are hearing some form of mono. If are are listening from way off to the side, or from another room, it's now mono. Most club sound systems are run in mono. Unless you are right in front of the jukebox, you are listening in mono. The majority of TVs are mono. If you are listening to a boombox from more then a dozen feet away, it might as well be mono.
That said, here are the pitfalls to be careful of. Any instruments that have been recorded with more than one mic, such as drum overheads, acoustic piano or guitar, choirs, etc., AND panned wide apart, need to be checked in mono, as there may be phase cancellations that dramatically reduced their perceived level in mono. I have seen cymbals and pianos utterly DISAPPEAR in mono. This can lead to severe inconsistencies when your mix is played in different places.
There are several ways to watch out for this. The best place is when you are doing the initial recording. Some of the more expensive consoles have phase meters. If you are using Pro Tools, your can download the free Bomb Factory Phase Correlation Meter, which will do the same thing. Put it on the master fader, and solo your stereo track. The phase meter should swing to the right when there is sound. If it stays in the middle, or swings to the left, try moving the mics and checking again. You can also check with your ears by switching to mono for a moment, but the metering is much easier and more reliable. For "after-the-fact" phase canceling issues, simply narrowing the panning and raising the level a bit usually fixes it.

HOW RECORDING LEVELS EFFECT THE FINAL SOUND
Most people record at levels that are way too hot. Overly hot recording does audible damage on many levels, even if you don't hear distortion while you are doing it. Here they are, in order of appearance.But first, a quick primer on digital vs. analog metering.On a digital machine, the top of the meter is just that. Try to go above it, and you will distort the signal. But on an analog machine, it's not as easy to know where the top really is. There is a 0dB marking on the meter, and then there is another 3dB of markings beyond that, with the knowledge that one can maybe go even higher than that. How do these two different metering systems compare with each other? Depending on the individual unit's calibration, the "top" of the digital machine is 14-18dB higher than the 0dB marking on the analog machine. Does this mean you could (or should) using the apparent "extra" range? No, and this leads us to #1:

#1 - MIC PREAMP DISTORTION
If you are playing "pack the meter" on a digital machine (in other words, recording as close to the top as you can without going over), then you are running your mic preamps, compressors, and any other gear you are using PRIOR to the record inputs of your system, at maybe 10-15dB hotter than is was designed to do. You are certainly moving past it's comfort zone, which potentially impacts transients and adds distortion. It's like driving a car at ninety miles per hour all the time.

#2 - DIGITAL INTERFACE DISTORTION
Inputting overly hot signals will also assault the analog electronics in your convertor box, as well as the A-to-D convertors themselves. Some convertors do okay with this, but they tend to be the expensive ones.

#3 - PLUGIN DISTORTION
Now you're in the system. What happens next? Your hot signal will now likely go through some plugins. Some plugins may well show clips lights lighting up. Clip lights indicate overload (of course). Plugin designers DO NOT intend for you to do this. For example, you can hit an LA2A plugin hard and get some kind of emulation of "tube saturation". That is built into the modeling of the plugin itself. But pushing it until the clip light comes on IS NOT part of the design. Now your are past the available headroom of the plugin, and you are clipping in the digital realm. It's not emulated LA2A distortion anymore, it's a clipped digital signal path. Even if you think you can't hear it, it's adding to the global downfall of your mix sonics.To make matters worse, many plugins will ADD gain to your signal, will takes us to the MIX BUS.

#4 - MIX BUS DISTORTION
Here is where some serious damage gets done. Twenty or thirty channels of high gain tracks will kill the mix bus. Even if your individual channels show no clips, and your master fader shows no clips, the mix bus is still being slammed. Clipping in this place will rob your mix of transients, collapse your stereo image, and make for a small, ratty-sounding mix.
SOLUTIONS
1 - Don't record at levels higher than -10dB, and in most cases, -15dB would be better still.
2 - if your tracks have already been recorded hot, use a destructive gain plugin to permanently change the track to a lower level. In Pro Tools, the (Audiosuite) GAIN plugin works well for this. Carefully monitor the level of the track (not the fader level, but the ACTUAL level of the track itself) until you have it down to a reasonable level. In Pro Tools, you can monitor the actual level by switching the fader reading to show "peaks" instead of "fader level".
3 - Be careful to not add gain with plugins, and if you are using a plugin where that is unavoidable, then gain down the track an additional amount to compensate for it.
4 - Keep your individual faders low enough so that you don't get clips on the master fader. But you were already doing that, right?
5 - If you want to make a LOUD mix for a CD, put a limiter plugin on the master fader and go to town. That is the place to get your hot level. That place and ONLY that place. Record and process your tracks at a reasonable levels, and then crank it THERE. With your tracks clean and transients intact, it's not so hard to get it loud and still have it sound good.NOTE: If you are planning on doing ANY additional work on the final mix (like mastering), skip the limiter. Limiting must be used LAST, and if you mix is going to be mastered, then your mix is not the last processing step. Failure to do this will yield a noticeably inferior end product.

These mixing tips have been brought to you by computer music man
If you would like to suggest your own mixing tips feel free to leave a comment
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