In the analog era, you couldn’t get much of anything done in the studio without a mastery of signal flow. Nowadays, those who’ve come up with DAWs may not have the benefit of being able to easily visualize the actual physical routings that are being made when they make i/o and bussing assignments from popup menus. So it’s not surprising that there may be situations where signal flow within the DAW may not be entirely clear, and mistakes can sometimes be made. Here are four things to look out for when it comes to signal flow in the virtual world.
Students and novice engineers often ask “what’s the correct order of plug-ins in a channel strip”? And they’re often given answers like “there are no real hard & fast rules” or “it depends”. Well, both of those statements are true, but there are some pretty solid recommendations for the order of processors and effects—at least when it comes the most basic processing.
Fig 1 A typical ordering of processers in a channel strip.
For example, processor(s) that are the most dependent on having a clean, unprocessed signal to work with should be first in the chain (at the top of the Inserts). Traditionally, this meant dynamics processors, but nowadays there’s also another type of processing that is likely to be most effective with the cleanest signal—pitch correction.
Now, I’m not talking about edit-based pitch processing—I mean real-time pitch processing, like Auto-Tune or others of that ilk. If a track is deemed to require correction, then this might be the one to place first in the chain. After that, if the track has leakage that needs to be addressed, then a Noise Gate would be the first dynamics processor to insert. The Gate requires the widest separation in level between desirable and undesirable sounds in the recording, and since compression will reduce that separation, and other effects may interfere, the Gate will need to precede everything else to be effective.
After that, you run into the classic chicken/egg debate—EQ or Compression first? Here, you really could go either way, but the conventional wisdom suggests dynamics first, so Compressor, followed by EQ, is the usual recommendation. The logic is that large changes in EQ are likely to affect the compressor’s Threshold and change its response, and you don’t want to keep having to readjust Threshold every time you make a big EQ tweak, just to preserve whatever compression effect you’ve already dialed up. It’s a good approach, although you could go the other way as well—subtle EQ tweaks (like with Mastering) shouldn’t affect the compression much (if at all), so then it’d be fine to have the EQ first. Alternatively, if large boosts you need to dial up with the EQ cause problems down the pike, then you might want to have compression follow the EQ, to keep any such unwanted side-effects in line. You could even have two compressors, pre- and post-EQ, to cover all the bases.
When it comes to delay-based effects, well, here again, it could go either way. I’d be inclined to add longer delay effects, like echoes, or delay effects that involve duplicating the track, like ADT, after the previous processors. And, of course, Reverb would usually come last, whether as an insert or via a Send & Return routing (see below). However, if something like a chorus or flanging effect would be an integral part of the sound, I might insert it after any pitch processing or gating, but prior to EQ and compression.
From teaching audio and recording I know that Send & Return routing is one of the more difficult signal flows for musicians, students and novices to get a handle on, probably since it kind of works backwards—you set the level of Reverb return (for an individual track) by adjusting its Send. But it’s something everyone needs to know, so here it is.
Reverb is undoubtedly the most common application for Send & Return wiring. You’ll often see novices opening up multiple copies of the same or very similar reverbs on various tracks—this is inefficient, both from a CPU standpoint, and from a workflow standpoint. The standard approach is to use one reverb, and feed different amounts of various tracks (the Sends) into that same plug-in—if hooked up correctly, you’ll get different amounts of the same reverb sound (for the different tracks) back in the mix (the Return).
To hook it up, instead of inserting reverb plug-ins on instruments and audio channel strips, you insert one on an Aux channel. The Aux’s output feeds the Master Stereo bus, to bring the reverb into the mix. The Aux’s Input is fed from a Bus (any unused Bus will do). To add that reverb, in varying amounts, to different tracks, you don’t feed the tracks themselves through it, you feed copies of the tracks. These copies are created by adding a Send to each track you want to add (that) reverb to, and assigning all these Sends to the same Bus you’d selected as the reverb Aux’s Input. The Sends routes copies of each track into the bus, where they’re all submixed together, and sent to the Aux Input and through the Reverb. The mix or wet/dry control within the reverb is set to 100% wet (100% reverb effect), so only the reverb effect is returned to the mix bus, alongside the original tracks’ signals, which are still routed to the mix normally.
Here’s the backward part—to get more reverb back for a particular track, you don’t touch the Reverb Return level (the Aux channel fader—that should be left at Unity Gain). Instead, you tweak the levels of the Reverb Sends for each track—the more you Send to the reverb from a particular track, the more reverb you’ll hear on the track—simple as that! Well ok, maybe not that simple, but once you get the hang of it, it’s easy and efficient. And you can still have multiple reverbs active in a mix, for different reverb sounds, but if each is set up with its own independent Send & Return routing, then you’ll be able to enjoy both efficiency and maximum flexibility.
While we’re on the subject of Send & Return, if you set it up as described above, you’ll undoubtedly notice some options for each of the individual tracks’ Sends. There may be a button labeled Pre/Post, or Pre, or just P. I sometimes come across someone’s mix where these are randomly assigned in or out—the person didn’t know what it did, and he didn’t hear a difference, so he just left them wherever. But while you may not always hear a difference, this option does make the Send(s) function differently, and, again, everyone needs to know it works.
Fig 3 The Pre-/Post-Fader option in a Send.
Remember, I pointed out that a Send is a copy of the signal in that track, routed through a bus to another destination in the mixer (so far the only destination I’ve described is a Reverb Return). That copy can be split off either before the signal runs through the track’s Channel Fader, or after it’s passed through, and been level-adjusted by, the Channel Fader. This is what Pre or Post refers to—Pre- or Post-Channel Fader. A Post-Fader Send is what you’d want for reverb applications. When you adjust a track’s level with the Channel Fader, the level feeding the Post-Fader Send will follow that level change, preserving the balance between dry signal level and reverb level for that track—exactly the behavior you want.
But if those Sends were instead feeding a musician’s headphones, for an independent musician’s monitor mix while recording, then you wouldn’t want any level tweaks made to the Channel Faders during recording to distract the musicians, so the use of Pre-Fader Sends would be in order. These would tap off the tracks’ signals prior to the Channel faders, so the levels of the tracks in the musician’s headphone mix wouldn’t be affected by any adjustment made to those faders—the engineer could twiddle away to his heart’s content in the control room, without distracting the musicians in performance.
In mixing, a classic mistake would be to use Pre-Fader Sends for a Reverb Send & Return—as the Channel Faders are tweaked during the mix, the level of the Reverb won’t follow those changes, due to the Pre-Fader Send levels feeding it, so the reverb level inadvertently goes up & down for different tracks, confounding the mixer’s attempts to get everything in a good, consistent balance. But if you simply make sure the Pre-Post selector is correctly set on all Sends, based on the application, then you’re golden.
Finally, there may come a time when, despite the wealth of virtual processing available, you (or the artist or producer you’re working with) wants to utilize a real, physical hardware processor in the mix (maybe a favorite vintage EQ or compressor with a pedigree). Any professional DAW can handle this, if you know how. Now obviously, you’d need to be working with an interface with more than just 2 ins and 2 outs. You’ll need to route the signal to be processed out a spare Output, into the hardware box, then out of the hardware and back into the DAW via a spare Input. There are a couple of ways to do this.
Just like setting up a musician’s headphone mix, you could use a Send for the track you want to pull out, routing that Send not to a Bus this time, but directly to a spare hardware (interface) Output. Then you could bring the processed signal back in on another channel, with the appropriate spare hardware (interface) Input selected as the input to that new track.
However, many DAWs offer a more straightforward way—an i/o Insert plug-in. If available, this negates the need for Sends and a separate channel to receive the processed signal. The audio is pulled out of the DAW right there in the i/o insert slot—the i/o plug-in simply lets you select a hardware output and a hardware input, to route the signal out, through the external processor, and back into the DAW, right there, in-line, in the original channel strip.
There will usually be output and input gain settings, to insure that the level going out to and coming back in from the analog world is properly matched in level to the level the hardware will require to perform up to snuff. If this feature is available, then it’s by far the easiest and most efficient way to add outside-the-box analog processing to an otherwise virtual mix.
Of course, there are more signal flow scenarios I could get into, but that’s about all the room I have, so I’ll leave off here. Even though much of the time you can work—record, mix, process—inside the DAW without really giving too much thought to the actual routings of the audio zipping around in the virtual mixer, inevitably you’ll have to come to terms with signal flow, and, once you have, it’ll open up a lot more options, both technical and creative, that you can bring to bear in the course of production.