Chapter 3 - Mixer Usage

Having spent some time working on prerequisites, we will now move into issues directly related to mixing.

The most important tool for mixing is the mixer. Most DAWs today include mixers as a built-in basic feature. These mixers are traditionally modeled after analog hardware mixers, and share a lot of the same principles of operation. This guide assumes that you are using a software-based DAW mixer.

A mixer consists of a series of channel strips. Each of these channel strips will correspond to one of the sounds in your mix: a virtual instrument, a drum kit, or a recorded vocal performance, for instance. Each channel strip contains a variety of tools to manipulate the sound going into it. The purpose of the mixer is to perform these manipulations, and then mix together the sounds coming from each channel strip, creating one audio signal that is the sum (both in the intuitive and mathematical sense) of all of the separate audio signals.

3.1 Leveling

Each channel strip will prominently feature a "level fader" which controls the volume of the sound going into it (usually calibrated in terms of dBFS). The level faders are the most basic tool for balancing mixes. The process of adjusting the level faders to achieve a satisfactory balance is called leveling.

This seems like a fairly easy thing to do, but it is surprisingly easy to get it wrong. Leveling is easy to get wrong partially because it's so easy to overthink it. The more you think about the levels, the more your perception becomes distorted, and the more likely you are to get things wrong. Leveling is really pretty easy if you approach it the right way. In general, if you have a good sound selection, then all of your sounds will be audible in any case, and tiny differences in level should not be of great importance. So leveling is just a matter of getting everything approximately right without losing perspective.

The main guiding principle of leveling is that you should make the most important parts of your music the loudest. If you're writing dance music, you probably want the drums and the basshne loudest, or whichever sounds are carrying the main groove. If you're writing pop music, you probably want the vocal line to be the loudest. If you're writing more left-of-field music, then you need to do some soul-searching and figure out which parts are the most important. Perhaps all of the parts are equally important, and you should level to achieve an even, unbiased presentation.

There are two general ways to approach leveling. The first approach is to just level as you go. This approach generally works fine in my experience, as long as you don't put too much thought into it. But if at any point you're not feeling satisfied with your levels, and you want to completely re-do them, there is a simple procedure for doing so.

To set your levels from scratch, start by dragging all of your faders down to zero. Then bring them up one by one, but put some thought into the order in which you bring them up. Generally speaking you should bring them up in order of importance, so that the most important (and loudest) parts come up first. This way you ensure a successful balance between the core elements of your track before considering the less important elements.

3.1.1 Input Gain

Many mixers offer an "input gain" control, which allows you to adjust the volume of the input to a channel strip before any other processing occurs. This input gain control is useful for getting sounds that are far too loud or far too quiet "in the ballpark," so to speak, so that the level faders aren't shoved off into the extreme ends of their ranges.

3.1.2 Headroom

One important topic that we have yet to address is that of headroom. It is important when you are mixing to leave a certain amount of "headroom;" in other words, to not allow the level of your mix to exceed a certain peak loudness. For instance, if your mix never goes louder than -5dBFS, you would say that you have 5dB of headroom. There are two reasons to leave headroom in this manner: first, to avoid digital clipping with levels greater than OdBFS, and second, to leave some space to perform mastering or finalizing processes (see Section 7.1).

How much headroom you need to leave is an open question, but in general, when working in 24-bit audio, it is better to err on the side of too much than on the side of too little. Anywhere between 3dB and 20dB of headroom should be fine. 6dB is a pretty good amount for music with a modest dynamic range, such as pop music or electronic dance music. For music with a wide dynamic range, you will want more headroom, to leave space for any unexpectedly large peaks.

In order to create a given amount of headroom, you will need to set your individual mixer tracks so that their levels are somewhat below the desired amount of headroom. If you want to leave 6dB of headroom, then you might set your loudest mixer tracks so that their levels do not exceed -9dBFS. Of course, this is only a starting point, and depending on the nature of the interactions between your mixer tracks, it may not work for your mix.

Naturally, your music will be quieter if it has a lot of headroom. Do not remove headroom because your music is too quiet; just turn up your monitoring volume. You will want to remove most or all of the headroom before you send your mix out into the world, but now is not the time to do that. You should only do so as one of the very last steps in the mixing process. See Section 7.1 for details.

3.1.3 Level Riding

One last thing to consider when leveling is the concept of "level riding." If you ride your levels, then what that means is that, rather than having your level faders always stay at a fixed position, they move up and down over the course of the track to shape the dynamics and the balance of the music. In my experience, level riding is very useful and important for music with a wide dynamic range. It is usually unnecessary with less dynamic music, such as electronic dance music.

3.2 Effects and Routing

You can go pretty far using a mixer just to combine your various channel strips at different levels, but mixers can do so much more.

As previously mentioned, channel strips have a variety of controls to manipulate the sounds going into them. These controls vary somewhat from mixer to mixer. You can be quite certain that you'll have a "pan" control (discussed in Section 6.1). You might also have a built-in equalizer; equalizers in general are discussed in Section 4.

3.2.1 Inserts

One universally available feature is that of inserts. An insert allows you to use an effect plugin to process the sound going through the channel strip. This opens up a world of possibilities, and the bulk of the remainder of this mixing guide is concerned with the usage of various insert effects. Popular insert effects include: equalizers (Section 4), compressors ( Section 5), limiters (Section 5.4.1), gates (Section 5.4.5), delays (Section 6.3), stereo effects (Section 6.2), and distortion, chorus, flangers, phasers, filters, ring modulators, vocoders, pitch shifters, exciters, harmonizers, auto-tuners, and FSU plugins (not discussed).^[1]

3.2.2 Auxiliary Sends

Inserts are not the only way to make use of effect plugins. There is another method, known as auxiliary sends, or aux sends, which is useful in a slightly different set of situations.

Insert effects are useful when you want to use an effect to process the sound of one channel. Aux sends are useful when you want to send several otherwise unrelated channels through an effect, or to blend a processed version of a channel with the normal, unprocessed version.

When you add an aux send to your project, every channel strip will have a volume control corresponding to that aux send. That volume control, if turned up, will allow you to send varying amounts of each channel to the aux send. The audio thus sent to the aux send will be processed through the effect and added to the mix.

Auxiliary sends are, in mixing, most often used for reverb (Section 6.4) and delays (Section 6.3). They are also useful for performing parallel compression (Section 5.4.3).

Most DAWs provide two kinds of aux send: pre-fader and post-fader. These two types differ in their relationship to the main level fader of the channel. A pre-fader send happens "before" the fader, and a post-fader send happens "after" the fader. The practical effect of this is that changes in the level fader will not affect the send level of a pre-fader send, but they will affect the send level of a post-fader send. There are a variety of reasons to choose either, and it's best to make this decision on a case by case basis.

3.2.3 Busses

Normally channel strips take their audio input from some source elsewhere in the DAW; a software synthesizer, a track of recorded audio, etc. But channel strips can also take their input from other channel strips. A channel whose input consists of multiple other channels is sometimes called a "bus" or a "group channel."

Busses are very useful. Essentially, what they allow you to do is to manip- ulate several channels as one. You can process them with the same effects, and you can control their levels as a unit, using the level fader on the bus.

A common use of busses is on drum kits. Suppose that you have a drum kit with a separate channel for each drum sound: kick, snare, three toms, and four cymbals. You could then make a bus called "drums," and route all of the drum sounds into that bus, so that they could be controlled as a unit.

You can also have hierarchies of bus groupings: channels that are grouped into busses, which are themselves grouped into busses. A refinement of the previous drum kit example would be to first create a "toms" bus and route of all of the toms to it, and then a "cymbals" bus to which all of the cymbals are routed. Then your drum kit would be described by four channels: kick, snare, the toms bus, and the cymbals bus. You could then route all four to one big "drums" bus as before.

3.2.4 Master Bus

There is one special bus which is present in every mix, called the "master bus." The master bus is the bus that everything else goes through: it's the final destination of all the audio. You can use the master bus to apply insert effects to the mix as a whole.

In general, you should leave the level fader on the master bus set to OdBFS. In the context of a normal mixdown, there is no good reason to adjust it. There are a number of reasons you might want to adjust it, but in all cases there are better ways to do the same thing:

You might turn it up or down to adjust your monitoring level. Instead, you should adjust the volume using a hardware or software volume control outside your DAW.
You might turn it up to remove headroom at the end of the mixing process. Instead, you should use a limiter; see Section 7.1.
You might turn it down to add headroom. Instead, you should turn down all of the tracks going to the master bus by an equal amount, or turn down the input gain on the master bus, because if you add headroom by adjusting the master level fader, then the headroom adjustment will occur after any insert effects on the master bus, which is not desirable.

3.2.5 Advanced Routing

Many DAWs allow even more sophisticated signal flow ("routing") possibilities than the ones described above. For instance, it is often possible to send the output of a channel strip to multiple other channel strips.^[2] Some DAWs have "anything to anywhere" routing, which means that you can send the output of any channel strip into any other channel strip with no restrictions, creating signal flow paths of arbitrary complexity.

[1] Most of the insert effects that are not discussed are not discussed because they are used to create dramatic changes in sound, rather than subtle sonic enhancements, and therefore fall somewhat outside the scope of a guide to mixing.

[2] This is useful for performing techniques such as parallel compression (Section 5.4.3.)

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