Whetheryou record and mix with hardware consoles or operate in the virtualworld of the digital audio workstation (DAW), you need to know thevarious ways to route signals to effects processors in order to getoptimal sound from your equipment. In this article, I will explain theadvantages of using channel, subgroup, and master bus inserts; channeland subgroup aux sends; and master outputs for signal processing. Iwill also explore the reasons why using one signal route instead ofanother will yield superior results in specific applications. Advancedapplications involving the use of mults, by which signals are splitinto two or more paths for separate processing, will also bediscussed.
For simplicity's sake, I will address each topic from theperspective of hardware consoles, because DAW-based virtual mixers tendto emulate the routing capabilities of their real-world counterparts.However, CPU-processing limits occasionally require alternative methodsof doing things on a DAW, and I will focus on those whenappropriate.
ALL OR NOTHING
I'll begin by discussing ways to process individual channels ratherthan an entire mix. The first output you're likely to encounter in amixer's audio path, the channel insert, provides one of the best waysto process audio. When you patch a signal processor to a channelinsert, 100 percent of that channel's signal flows out of the insert'ssend connector, through the signal processor, and back into the samemixer channel via the insert return (see Fig. 1). For thatreason, it is best to use a channel insert when you want to process theentire signal, not just a portion of it.
Equalizers, compressors, and reverb units work best when you processthe entire signal, so they are commonly used with inserts. The reasonfor compressing a vocal track, for instance, is to limit its dynamicrange. Unless all of the vocal's signal is sent through the compressor(through a channel insert, perhaps), a portion of the signal willretain its original dynamic range, thus preventing you from keeping afirm lid on loud passages.
Instead of using the channel insert, you can patch the compressorin-line with an outboard mic preamp while tracking the vocal or routethe output of your recorder through it while mixing. But if you need amixer's pad and input preamp to optimize the signal level beforecompressing it, the channel insert is the best place to patch thecompressor into the mixer.
An outboard equalizer is another signal processor that can be usedin a channel insert. If you have a bass-guitar track, for example, thatis boomy in the 100 Hz range, patching an equalizer into the track'schannel insert will let you reduce the desired frequency. Because 100percent of the bass guitar's signal flows through the equalizer withthat setup, none of the signal will escape the EQ's correctiveinfluence. Other effects that work well in channel inserts includeaural exciters, which generate harmonic distortion to make a tracksound brighter and louder, and the BBE Sonic Maximizer, which correctsunwanted phase shift and enhances transients.
Using an insert requires an insert cable, which typically has a3-conductor ¼-inch TRS plug at one end and a pair of 2-conductor¼-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 tipreturns the signal to the mixer.
DO NOT INSERT
Although you can patch a time-based effects processor (such as adigital delay, a reverb unit, or a multi-effects processor) into achannel 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 setthe level of effect you want. Many effects processors have an inverselyproportionate wet/dry mix control. That means when you increase the dryoutput level, you simultaneously decrease the wet output level and viceversa. That type of control works fine with mono chorus and flangingeffects, in which you generally want a fixed 50/50 mix of wet and drysignal. However, it's difficult to adjust the mix on delays or reverbsin a channel insert if your processor doesn't offer independent controlof the wet and dry output levels.
Say, for example, you want to raise the reverb level on a vocaltrack during mixdown without lowering the amount of its dry signal inthe mix. A processor's inversely proportionate mix control won't enableyou to do that, because increasing the wet signal using that controlwill lower the dry signal at the same time, defeating your purpose. Afew effects processors offer a discrete dry output, but patching thedry output to your mixer will use up another line input, which may notbe available.
Furthermore, if your mixer offers fader automation, you'll be muchhappier recalling reverb levels on your mixer's aux-return faders thanmanually restoring wet/dry mix settings on your processor. Many effectsprocessors 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 ismuch easier and faster.
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 thatmonster mix you're working on. If an extra effects processor is neededfor one track only, don't hesitate to patch it in using the channelinsert. A mono effect can be returned to the mixer channel'sinsert-return jack and the processor's wet/dry mix control used to seteffects levels. Even though it's not the best way to add time-basedeffects to a track, sometimes it is the only available way. In thatcase, go for it. A good last resort is better than no resort.
When using the described routing method to add one last stereoeffect to a mix, the two effects-processor outputs can be patched to astereo aux return or to two available line inputs on your mixer.Because the processed sound is not returning to the board through thechannel insert, you can set the processor's wet/dry mix to 100 percentwet and control the effect's return levels at the mixer.
There's one catch, however: unless the track's dry signal returns toits channel insert, you will not hear anything on the original channel,because the mixer's insert send is typically internally normaled to itsinsert return. If the normal is broken by plugging a cable in to theinsert jack and the signal is not returned to the insert jack, thechannel's dry signal will not reappear on that channel fader. To solvethat problem, simply patch your insert send and return connections to ahalf-normaling patch bay (see Fig. 3). With the channel's insertsend half-normaled to its insert return, you can route a cable from aninsert send at the patch bay to an effects processor withoutinterrupting the patch bay's normal, thus preserving the signal flowback to the mixer's insert return.
Channel inserts can be used to expand the de facto number of effectssends your mixer provides. The major drawback is that each insertserves only one mixer channel. In most cases, the best way to addreverb, echo, and stereo chorus and flange effects is by using an auxsend.
Another thing to consider is that all signal processors degradesignal quality to some degree. Preventing a portion of the originalsignal from going through your outboard effects processor by using anaux send rather than an insert lets you retain some of the originalsignal purity. If you route the track to the effects processor using achannel insert, the entire signal passes through the processor'selectronics, arbitrarily degrading the signal to some (albeit small)degree. Depending on the quality of your processor, the degradation mayor may not be audible, but it's something to consider.
YOU SEND ME
Aux sends are great for situations in which you want to sendmultiple mixer channels to the same effects processor, such as areverb. When you use an aux send, route only a portion of the channel'sdry signal to the outboard processor; that way, the dry signal willstill show up on its channel fader, letting you control its level inthe mix. The dry signal does not noticeably decrease in level when youuse an aux send. Rather, an amplified mult of the channel's signal issent to the mixer's master aux send. From there the signal goes to youreffects processor, along with the signals from other tracks using thesame aux send.
The wet/dry mix on an effects processor is usually set to 100percent when patched to an aux send, because the level of the wetsignal is controlled by the mixer's effects-return faders or knobs. Thedry-signal level, on the other hand, is controlled with itsmixer-channel fader. That setup gives you independent control of wetand dry levels at the mixer.
The aux send's ability to apply the same effect to multiple trackssimultaneously has a couple of benefits. For example, fine-tuning theparameters of one effects processor that is used on a number ofchannels is more efficient than attempting to match settings on anumber of processors that are dedicated to individual channels.
In addition, using aux sends for plug-in effects on a DAW lets youuse CPU resources more efficiently than you could if you added effectsto each track. Plug-in effects, especially reverbs, can be huge CPUhogs. You'll quickly run out of CPU resources if you use the sameplug-in (and redundant parameter settings) on inserts for multiplevirtual mixer channels. A better way to add the same effect to multiplechannels is to bus each channel to a common aux track and apply theplug-in just once to the aux track's insert, thereby applying theplug-in's effect to all bused channels simultaneously.
In Mark of the Unicorn's (MOTU's) Digital Performer, forexample, you can use channel sends to route several background vocalsto a common aux track by way of bus 1 (see Fig. 4). The auxtrack's input is set to bus 1 so it can receive all of the send signalsand 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, thedrain on your CPU will be a fraction of what it would be if youinstantiated the plug-in for each mixer channel.
ACT OF PRESERVATION
Many hardware mixers and some DAWs let you route an aux-send signaleither pre- or postfader. When an aux send is configured postfader, theeffect's wet/dry ratio is preserved at the mixer as you raise and lowerchannel faders; lowering a channel's fader simultaneously lowers theaux-send signal level. That keeps you from having to adjusteffects-return fader levels at the mixer or wet/dry ratios at theeffects processor every time you make a level adjustment with a track'schannel fader. (In most cases, you'll want to keep the balance betweena dry track and its effects relatively constant throughout the mixdownprocess.) As a result, postfader effect sends are used more often thanprefader effect sends, although the latter configuration definitely hasits uses.
As the name implies, a prefader aux send is not influenced bychannel-fader moves, because the signal is sent to the processorthrough the aux send before it gets to the fader. Therefore, theprocessed signal level from a prefader aux send remains constant, nomatter how you move its corresponding channel fader.
One use for this type of configuration is to keep a vocal track'sreverb level constant while you lower its dry level. Set the level ofreverb you want the vocal to have in the mix by turning up its prefaderaux send, which is routed to your reverb unit, until the effect soundsright. Then, slowly lower the vocal's mixer-channel fader. As you lowerthe fader, the vocal's dry level dips while the level of the processedsignal remains constant. The result sounds as though the vocalist iswalking away from you: the dry sound gets quieter, leaving just thereverberations of the room.
DOUBLE YOUR PLEASURE
A mono aux send works fine on lead vocals and other mono tracks butmay not be exactly what you need to process a stereo pair of tracksthat are routed to two mixer channels. Some mixers offer stereo sendsfor this purpose; other mixers, including many digital ones, let youpair two mono aux sends together so that they function as one stereounit. 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, and6) serve as the right.
You must have a stereo effects processor to take advantage of stereoor paired aux sends (unless you are employing the aux sends to feed aheadphone amp, which is beyond this article's scope). Simply patch yourmixer'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 mixerchannels determines which aux sends — left, right, or acombination of both — they will be routed to. For example, if theleft 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 channelshard left and hard right, respectively, lets you keep the left- andright-channel effects processing discrete for each mixer channel.
Discrete left and right processing offers a wider stereo field andavoids the phasing problems that occur when miked stereo tracks arecombined into a mono aux send. Stereo aux sends can be used to processany stereo tracks, including acoustic guitar, background vocals, anddrum tracks. Background vocals and drums are often assigned to thestereo aux sends that serve mixer subgroups. Here's how you can use asubgroup's aux sends and inserts for more efficient and creativemixing.
Say you want to add the same reverb to three or morebackground-vocal tracks. You might be tempted to set the aux-send levelthat feeds your reverb unit for each mixer channel separately. However,as the mix progresses and you begin to finesse the aux-send level forone of the channels, you'll likely need to rebalance the aux-sendlevels 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 channelsto a common subgroup. To spread out the background vocals in the stereofield, bus their channels to two subgroup faders, pan the channels sothat each one is where you want it to appear in the stereo field, andthen hard-pan one subgroup channel fully left and the other fullyright. Now you can use a single subgroup aux send to adjust the amountof reverb (or other effect) for all of the background vocals routed tothat subgroup.
That strategy is beneficial only if you want to add the same amountof effect to all of the tracks in the subgroup, as is often the case.Subgrouping the background vocals also lets you control the levels forthe background-vocal tracks using only one (in a mono subgrouping) ortwo (for stereo subgrouping) faders.
Some high-end mixers (Harrison consoles, for example) also offersubgroup inserts. They let you simultaneously and equally processsubgrouped channels through outboard dynamics processors or equalizers.Many top-flight engineers will bus an entire drum kit into a stereosubgroup (or, alternatively, to a pair of multitrack bus outputs), outto a stereo compressor, and then return the compressed drums to a pairof line inputs on the console. That setup controls the drum kit's levelas a whole while enhancing mic bleed to give the kit more of a livesound.
When you bus a DAW's virtual mixer channels to an aux channel, youare actually subgrouping. As noted previously, subgrouping a DAW'svirtual mixer channels onto an aux channel enables you to conserve CPUresources by using only one instantiation of a plug-in effect for allof the subgrouped tracks.
It may surprise you to know that even the most inexpensive mixeroffers subgrouping of sorts, though you might not think of it as such.The king of all subgroups is the stereo master bus, whose level iscontrolled by your master fader.
TAKE ME TO YOUR MASTER
When you assign all of your mixer's channels, subgroups, and effectsreturns to your mixer's master stereo outputs, you are subgroupingeverything down to a pair of output channels. Most mixers offer you onemore shot at processing the entire mix before it leaves the mixer byusing master-bus inserts, which, like channel inserts, are commonlyconfigured prefader.
Just like channel inserts, master-bus inserts are best used withgear 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-businserts, use the highest-quality gear you can get. Your entire mix isrunning through whatever you patch into those inserts, so the qualityof the processor will have a profound impact on your mix's overallsound.
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 masterbus using the mixer's software interface. If your digital mixer doesoffer master-bus inserts and they are not digital, beware theadditional signal-degrading D/A/D conversions required to send your mixto an analog processor and return it to the mixer. Some high-enddigital consoles offer digital master-bus inserts that let you patch ina digital processor and thereby remain in the digital domain. There areseveral other ways to interface digital outboard processors withdigital mixers, but they involve in-depth discussions about digitalsync distribution and data formats — subjects that are beyond thescope of this article.
THE END OF THE LINE
If you use an analog console, it is almost always preferable to useits master-bus inserts instead of its master-bus outputs for routingthe entire mix through analog compressors and equalizers. But those whoown a low- or midpriced digital console that does not offer master-businserts must often use the mixer's master analog-stereo outputs forfurther analog signal processing of the mix. For instance, to addanalog EQ to your entire mix using this method, just route your digitalmixer's master analog-stereo outputs to an analog stereo equalizer'sleft and right inputs and patch the EQ's left and right outputs to yourmastering 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 yourdigital mixer's master analog-stereo outputs and your mastering deck orDAW. Nevertheless, there is one drawback to compressing your mix usingthat setup: your mix will become uncompressed once its level fallsbelow the compressor's threshold during the fade-out. The change indynamics is usually not too noticeable on the low-level materialcontained in the fade.
If the change in dynamics bothers you, however, there is awork-around: you can bus all mixer channels to two analog bus outputs,route those bus outputs to a stereo compressor, and return thecompressor's outs to spare mixer line inputs that are, in turn, routedto the master stereo bus. As long as you stay in the digital domainpostfader on the master faders, that is a good strategy. You don't wantto use that routing if you're printing your mix to DAW or DAT by way ofthe mixer's master analog stereo bus outputs, because that wouldrequire a second signal-degrading round-trip through the analog anddigital converters.
So far I've written about routing tracks to signal processors aftertheir signals are already inside the mixer. I'll now detail an advancedapplication for routing individual tracks to signal processors beforethey've entered the mixer, using the versatile and mysterious mult.
The term mult is commonly used in two ways: as a noun todenote a multiple (a signal split off from the original signal), and asa verb to denote the act of multiplying or splitting a signal. Multsare often used to split a signal into two or more paths that can thenbe processed differently.
For example, you might mult a lead-vocal track so that you can giveeach of the two or three resulting signals different EQ treatments: onesuitable for verses, another for choruses, and perhaps another for thebridge. A patch bay that specifically offers a parallel routingconfiguration is handy for multing a signal into three parts (seeFig. 6). With a parallel configuration, a signal that is patchedfrom your MDM, for example, into the top-rear patch bay jack is splitinto three identical signals. Those signals leave the patch bay at itstop-front, bottom-front, and bottom-rear jacks. You can then route thethree signals into separate mixer channels, where they can be processedseparately. As each section of the song comes up during mixdown, yousimply unmute the lead-vocal channel that has the desired EQ settingsfor that section of the song while muting the other two lead-vocalmults.
You can use the same multing method to create a killer snare-drumtrack. Split the snare track into two parts using half-normaled orparallel patch bay jacks (see Fig. 7). Send one signal to themixer unchanged. Route the other signal to a compressor, followed by agate. Set the compressor's ratio, threshold, attack, and releasecontrols so that the attack of the snare drum's stick hit is greatlyaccentuated. Then, gate that sound so that all that gets through thegate is a short burst of the accentuated attack you created with thecompressor. Route the gate's output to a separate mixer channel and mixit with the original snare for a rocking power-pop sound.
If you work with a digital mixer and MDM, you don't even need apatch bay to set up the aforementioned snare-drum mult (see Fig.8). Route the unprocessed snare track to your mixer through adigital input as you would normally do. At the same time, route yourMDM's analog output for that snare track to the compressor and gate fortreatment and send the processed signal from the gate's output to aseparate line input of the mixer. That will let you mix the processedsignal with the unprocessed digital input. For the method to work,however, your MDM must offer hot analog and digital outputssimultaneously.
THE CLEAREST PATH
Ultimately, the best route to use for signal processing depends onwhat you want to accomplish. Understanding the uses and limitations ofinserts, sends, buses, groups, and mults is the first step towardcreative mixing and your ticket to mastering advanced applications.
Once you have a firm grip on the basics, don't be afraid to try wildrouting variations with different processors. The next new sound to hitthe airwaves might be your own.
Michael Cooper is the owner of Michael Cooper Recording,located outside the beautiful resort town of Sisters at the base of theOregon Cascades.