Whether you record and mix with hardware consoles or operate in the virtual world of the digital audio workstation (DAW), you need to know the various ways to route signals to effects processors in order to get optimal sound from your equipment. In this article, I will explain the advantages of using channel, subgroup, and master bus inserts; channel and subgroup aux sends; and master outputs for signal processing. I will also explore the reasons why using one signal route instead of another will yield superior results in specific applications. Advanced applications involving the use of mults, by which signals are split into two or more paths for separate processing, will also be discussed.
For simplicity's sake, I will address each topic from the perspective of hardware consoles, because DAW-based virtual mixers tend to emulate the routing capabilities of their real-world counterparts. However, CPU-processing limits occasionally require alternative methods of doing things on a DAW, and I will focus on those when appropriate.
ALL OR NOTHING
I'll begin by discussing ways to process individual channels rather than an entire mix. The first output you're likely to encounter in a mixer's audio path, the channel insert, provides one of the best ways to process audio. When you patch a signal processor to a channel insert, 100 percent of that channel's signal flows out of the insert's send connector, through the signal processor, and back into the same mixer channel via the insert return (see Fig. 1). For that reason, it is best to use a channel insert when you want to process the entire signal, not just a portion of it.
Equalizers, compressors, and reverb units work best when you process the entire signal, so they are commonly used with inserts. The reason for compressing a vocal track, for instance, is to limit its dynamic range. Unless all of the vocal's signal is sent through the compressor (through a channel insert, perhaps), a portion of the signal will retain its original dynamic range, thus preventing you from keeping a firm lid on loud passages.
Instead of using the channel insert, you can patch the compressor in-line with an outboard mic preamp while tracking the vocal or route the output of your recorder through it while mixing. But if you need a mixer's pad and input preamp to optimize the signal level before compressing it, the channel insert is the best place to patch the compressor into the mixer.
An outboard equalizer is another signal processor that can be used in a channel insert. If you have a bass-guitar track, for example, that is boomy in the 100 Hz range, patching an equalizer into the track's channel insert will let you reduce the desired frequency. Because 100 percent of the bass guitar's signal flows through the equalizer with that setup, none of the signal will escape the EQ's corrective influence. Other effects that work well in channel inserts include aural exciters, which generate harmonic distortion to make a track sound brighter and louder, and the BBE Sonic Maximizer, which corrects unwanted phase shift and enhances transients.
Using an insert requires an insert cable, which typically has a 3-conductor ¼-inch TRS plug at one end and a pair of 2-conductor ¼-inch TS plugs at the other (see Fig. 2). On most mixers, the ring of the TRS plug sends the signal to the processor, and the tip returns the signal to the mixer.
DO NOT INSERT
Although you can patch a time-based effects processor (such as a digital delay, a reverb unit, or a multi-effects processor) into a channel insert, there are several reasons not to do so. To begin with, you have to adjust the processor's onboard wet/dry mix control to set the level of effect you want. Many effects processors have an inversely proportionate wet/dry mix control. That means when you increase the dry output level, you simultaneously decrease the wet output level and vice versa. That type of control works fine with mono chorus and flanging effects, in which you generally want a fixed 50/50 mix of wet and dry signal. However, it's difficult to adjust the mix on delays or reverbs in a channel insert if your processor doesn't offer independent control of the wet and dry output levels.
Say, for example, you want to raise the reverb level on a vocal track during mixdown without lowering the amount of its dry signal in the mix. A processor's inversely proportionate mix control won't enable you to do that, because increasing the wet signal using that control will lower the dry signal at the same time, defeating your purpose. A few effects processors offer a discrete dry output, but patching the dry output to your mixer will use up another line input, which may not be available.
Furthermore, if your mixer offers fader automation, you'll be much happier recalling reverb levels on your mixer's aux-return faders than manually restoring wet/dry mix settings on your processor. Many effects processors do not store the wet/dry mix control's settings in RAM, though devices that control that parameter in software often do. Regardless, adjusting the wet/dry mix at the mixer using an aux send is much easier and faster.
LAST RESORT
Although the best way to process an individual track with reverb, echo effects, and stereo chorus and flanging is by using an aux send, sooner or later you will run out of available aux sends for that monster mix you're working on. If an extra effects processor is needed for one track only, don't hesitate to patch it in using the channel insert. A mono effect can be returned to the mixer channel's insert-return jack and the processor's wet/dry mix control used to set effects levels. Even though it's not the best way to add time-based effects to a track, sometimes it is the only available way. In that case, go for it. A good last resort is better than no resort.
When using the described routing method to add one last stereo effect to a mix, the two effects-processor outputs can be patched to a stereo aux return or to two available line inputs on your mixer. Because the processed sound is not returning to the board through the channel insert, you can set the processor's wet/dry mix to 100 percent wet and control the effect's return levels at the mixer.
There's one catch, however: unless the track's dry signal returns to its channel insert, you will not hear anything on the original channel, because the mixer's insert send is typically internally normaled to its insert return. If the normal is broken by plugging a cable in to the insert jack and the signal is not returned to the insert jack, the channel's dry signal will not reappear on that channel fader. To solve that problem, simply patch your insert send and return connections to a half-normaling patch bay (see Fig. 3). With the channel's insert send half-normaled to its insert return, you can route a cable from an insert send at the patch bay to an effects processor without interrupting the patch bay's normal, thus preserving the signal flow back to the mixer's insert return.
Channel inserts can be used to expand the de facto number of effects sends your mixer provides. The major drawback is that each insert serves only one mixer channel. In most cases, the best way to add reverb, echo, and stereo chorus and flange effects is by using an aux send.
Another thing to consider is that all signal processors degrade signal quality to some degree. Preventing a portion of the original signal from going through your outboard effects processor by using an aux send rather than an insert lets you retain some of the original signal purity. If you route the track to the effects processor using a channel insert, the entire signal passes through the processor's electronics, arbitrarily degrading the signal to some (albeit small) degree. Depending on the quality of your processor, the degradation may or may not be audible, but it's something to consider.
YOU SEND ME
Aux sends are great for situations in which you want to send multiple mixer channels to the same effects processor, such as a reverb. When you use an aux send, route only a portion of the channel's dry signal to the outboard processor; that way, the dry signal will still show up on its channel fader, letting you control its level in the mix. The dry signal does not noticeably decrease in level when you use an aux send. Rather, an amplified mult of the channel's signal is sent to the mixer's master aux send. From there the signal goes to your effects processor, along with the signals from other tracks using the same aux send.
The wet/dry mix on an effects processor is usually set to 100 percent when patched to an aux send, because the level of the wet signal is controlled by the mixer's effects-return faders or knobs. The dry-signal level, on the other hand, is controlled with its mixer-channel fader. That setup gives you independent control of wet and dry levels at the mixer.
The aux send's ability to apply the same effect to multiple tracks simultaneously has a couple of benefits. For example, fine-tuning the parameters of one effects processor that is used on a number of channels is more efficient than attempting to match settings on a number of processors that are dedicated to individual channels.
In addition, using aux sends for plug-in effects on a DAW lets you use CPU resources more efficiently than you could if you added effects to each track. Plug-in effects, especially reverbs, can be huge CPU hogs. You'll quickly run out of CPU resources if you use the same plug-in (and redundant parameter settings) on inserts for multiple virtual mixer channels. A better way to add the same effect to multiple channels is to bus each channel to a common aux track and apply the plug-in just once to the aux track's insert, thereby applying the plug-in's effect to all bused channels simultaneously.
In Mark of the Unicorn's (MOTU's) Digital Performer, for example, you can use channel sends to route several background vocals to a common aux track by way of bus 1 (see Fig. 4). The aux track's input is set to bus 1 so it can receive all of the send signals and route their combined signals to a plug-in by way of its insert. Because you're using only one instance of the plug-in in this case, the drain on your CPU will be a fraction of what it would be if you instantiated the plug-in for each mixer channel.
ACT OF PRESERVATION
Many hardware mixers and some DAWs let you route an aux-send signal either pre- or postfader. When an aux send is configured postfader, the effect's wet/dry ratio is preserved at the mixer as you raise and lower channel faders; lowering a channel's fader simultaneously lowers the aux-send signal level. That keeps you from having to adjust effects-return fader levels at the mixer or wet/dry ratios at the effects processor every time you make a level adjustment with a track's channel fader. (In most cases, you'll want to keep the balance between a dry track and its effects relatively constant throughout the mixdown process.) As a result, postfader effect sends are used more often than prefader effect sends, although the latter configuration definitely has its uses.
As the name implies, a prefader aux send is not influenced by channel-fader moves, because the signal is sent to the processor through the aux send before it gets to the fader. Therefore, the processed signal level from a prefader aux send remains constant, no matter how you move its corresponding channel fader.
One use for this type of configuration is to keep a vocal track's reverb level constant while you lower its dry level. Set the level of reverb you want the vocal to have in the mix by turning up its prefader aux send, which is routed to your reverb unit, until the effect sounds right. Then, slowly lower the vocal's mixer-channel fader. As you lower the fader, the vocal's dry level dips while the level of the processed signal remains constant. The result sounds as though the vocalist is walking away from you: the dry sound gets quieter, leaving just the reverberations of the room.
DOUBLE YOUR PLEASURE
A mono aux send works fine on lead vocals and other mono tracks but may not be exactly what you need to process a stereo pair of tracks that are routed to two mixer channels. Some mixers offer stereo sends for this purpose; other mixers, including many digital ones, let you pair two mono aux sends together so that they function as one stereo unit. Usually the odd-numbered sends (for example, aux 1, 3, and 5) serve as the left channels, and the even-numbered sends (aux 2, 4, and 6) serve as the right.
You must have a stereo effects processor to take advantage of stereo or paired aux sends (unless you are employing the aux sends to feed a headphone amp, which is beyond this article's scope). Simply patch your mixer's left and right aux sends to the left and right inputs, respectively, of your stereo effects processor (see Fig. 5).
When using stereo or paired aux sends, how you pan the mixer channels determines which aux sends — left, right, or a combination of both — they will be routed to. For example, if the left side of a stereo keyboard track is panned completely to the left, its stereo aux-send signal will be sent only out of the left (odd-numbered) aux sends. If you pan the keyboard channel dead center, it will be sent equally to both aux sends. Panning the two channels hard left and hard right, respectively, lets you keep the left- and right-channel effects processing discrete for each mixer channel.
GROUP STRATEGY
Discrete left and right processing offers a wider stereo field and avoids the phasing problems that occur when miked stereo tracks are combined into a mono aux send. Stereo aux sends can be used to process any stereo tracks, including acoustic guitar, background vocals, and drum tracks. Background vocals and drums are often assigned to the stereo aux sends that serve mixer subgroups. Here's how you can use a subgroup's aux sends and inserts for more efficient and creative mixing.
Say you want to add the same reverb to three or more background-vocal tracks. You might be tempted to set the aux-send level that feeds your reverb unit for each mixer channel separately. However, as the mix progresses and you begin to finesse the aux-send level for one of the channels, you'll likely need to rebalance the aux-send levels for all of the other background-vocal channels, as well. Fortunately, there's a more efficient way to do that.
You can work faster by busing all of the background-vocal channels to a common subgroup. To spread out the background vocals in the stereo field, bus their channels to two subgroup faders, pan the channels so that each one is where you want it to appear in the stereo field, and then hard-pan one subgroup channel fully left and the other fully right. Now you can use a single subgroup aux send to adjust the amount of reverb (or other effect) for all of the background vocals routed to that subgroup.
That strategy is beneficial only if you want to add the same amount of effect to all of the tracks in the subgroup, as is often the case. Subgrouping the background vocals also lets you control the levels for the background-vocal tracks using only one (in a mono subgrouping) or two (for stereo subgrouping) faders.
Some high-end mixers (Harrison consoles, for example) also offer subgroup inserts. They let you simultaneously and equally process subgrouped channels through outboard dynamics processors or equalizers. Many top-flight engineers will bus an entire drum kit into a stereo subgroup (or, alternatively, to a pair of multitrack bus outputs), out to a stereo compressor, and then return the compressed drums to a pair of line inputs on the console. That setup controls the drum kit's level as a whole while enhancing mic bleed to give the kit more of a live sound.
When you bus a DAW's virtual mixer channels to an aux channel, you are actually subgrouping. As noted previously, subgrouping a DAW's virtual mixer channels onto an aux channel enables you to conserve CPU resources by using only one instantiation of a plug-in effect for all of the subgrouped tracks.
It may surprise you to know that even the most inexpensive mixer offers subgrouping of sorts, though you might not think of it as such. The king of all subgroups is the stereo master bus, whose level is controlled by your master fader.
TAKE ME TO YOUR MASTER
When you assign all of your mixer's channels, subgroups, and effects returns to your mixer's master stereo outputs, you are subgrouping everything down to a pair of output channels. Most mixers offer you one more shot at processing the entire mix before it leaves the mixer by using master-bus inserts, which, like channel inserts, are commonly configured prefader.
Just like channel inserts, master-bus inserts are best used with gear that is meant to process 100 percent of the bus's signal. Therefore, master-bus inserts are commonly used for adding compression, limiting, and equalization to an entire mix. When using master-bus inserts, use the highest-quality gear you can get. Your entire mix is running through whatever you patch into those inserts, so the quality of the processor will have a profound impact on your mix's overall sound.
Most low-cost digital mixers don't offer master-bus inserts per se, but you can usually add digital dynamics processing or EQ to the master bus using the mixer's software interface. If your digital mixer does offer master-bus inserts and they are not digital, beware the additional signal-degrading D/A/D conversions required to send your mix to an analog processor and return it to the mixer. Some high-end digital consoles offer digital master-bus inserts that let you patch in a digital processor and thereby remain in the digital domain. There are several other ways to interface digital outboard processors with digital mixers, but they involve in-depth discussions about digital sync distribution and data formats — subjects that are beyond the scope of this article.
THE END OF THE LINE
If you use an analog console, it is almost always preferable to use its master-bus inserts instead of its master-bus outputs for routing the entire mix through analog compressors and equalizers. But those who own a low- or midpriced digital console that does not offer master-bus inserts must often use the mixer's master analog-stereo outputs for further analog signal processing of the mix. For instance, to add analog EQ to your entire mix using this method, just route your digital mixer's master analog-stereo outputs to an analog stereo equalizer's left and right inputs and patch the EQ's left and right outputs to your mastering deck or DAW to record your mix.
You can use the same setup for inserting an analog stereo compressor (or an analog stereo-linked, dual-channel compressor) in between your digital mixer's master analog-stereo outputs and your mastering deck or DAW. Nevertheless, there is one drawback to compressing your mix using that setup: your mix will become uncompressed once its level falls below the compressor's threshold during the fade-out. The change in dynamics is usually not too noticeable on the low-level material contained in the fade.
If the change in dynamics bothers you, however, there is a work-around: you can bus all mixer channels to two analog bus outputs, route those bus outputs to a stereo compressor, and return the compressor's outs to spare mixer line inputs that are, in turn, routed to the master stereo bus. As long as you stay in the digital domain postfader on the master faders, that is a good strategy. You don't want to use that routing if you're printing your mix to DAW or DAT by way of the mixer's master analog stereo bus outputs, because that would require a second signal-degrading round-trip through the analog and digital converters.
So far I've written about routing tracks to signal processors after their signals are already inside the mixer. I'll now detail an advanced application for routing individual tracks to signal processors before they've entered the mixer, using the versatile and mysterious mult.
I'M MULTING!
The term mult is commonly used in two ways: as a noun to denote a multiple (a signal split off from the original signal), and as a verb to denote the act of multiplying or splitting a signal. Mults are often used to split a signal into two or more paths that can then be processed differently.
For example, you might mult a lead-vocal track so that you can give each of the two or three resulting signals different EQ treatments: one suitable for verses, another for choruses, and perhaps another for the bridge. A patch bay that specifically offers a parallel routing configuration is handy for multing a signal into three parts (see Fig. 6). With a parallel configuration, a signal that is patched from your MDM, for example, into the top-rear patch bay jack is split into three identical signals. Those signals leave the patch bay at its top-front, bottom-front, and bottom-rear jacks. You can then route the three signals into separate mixer channels, where they can be processed separately. As each section of the song comes up during mixdown, you simply unmute the lead-vocal channel that has the desired EQ settings for that section of the song while muting the other two lead-vocal mults.
You can use the same multing method to create a killer snare-drum track. Split the snare track into two parts using half-normaled or parallel patch bay jacks (see Fig. 7). Send one signal to the mixer unchanged. Route the other signal to a compressor, followed by a gate. Set the compressor's ratio, threshold, attack, and release controls so that the attack of the snare drum's stick hit is greatly accentuated. Then, gate that sound so that all that gets through the gate is a short burst of the accentuated attack you created with the compressor. Route the gate's output to a separate mixer channel and mix it with the original snare for a rocking power-pop sound.
If you work with a digital mixer and MDM, you don't even need a patch bay to set up the aforementioned snare-drum mult (see Fig. 8). Route the unprocessed snare track to your mixer through a digital input as you would normally do. At the same time, route your MDM's analog output for that snare track to the compressor and gate for treatment and send the processed signal from the gate's output to a separate line input of the mixer. That will let you mix the processed signal with the unprocessed digital input. For the method to work, however, your MDM must offer hot analog and digital outputs simultaneously.
THE CLEAREST PATH
Ultimately, the best route to use for signal processing depends on what you want to accomplish. Understanding the uses and limitations of inserts, sends, buses, groups, and mults is the first step toward creative mixing and your ticket to mastering advanced applications.
Once you have a firm grip on the basics, don't be afraid to try wild routing variations with different processors. The next new sound to hit the airwaves might be your own.
