Signal flow in a DAW mixer

What is signal flow?

Signal flow is the route audio takes through your setup, one stage after another. A vocal recording starts at the microphone, goes through a preamp, gets turned into numbers by your audio interface and lands on a track in your DAW. From there it passes through the track's plug-ins and fader, joins the other tracks at the master, and goes back out through the interface to your monitors or headphones.

The order is the whole point. Each stage only ever receives what the stage before it sends, so a change early in the path affects every stage after it, and a change late in the path leaves the earlier stages alone. Engineers describe this as upstream and downstream: the microphone is upstream of the preamp, a compressor is upstream of the fader that follows it, and the master is downstream of everything.

Once you know the order, a lot of confusing behaviour stops being confusing. Your fader doesn't change how hard the compressor works because the fader comes after it. A bus compressor reacts when you move a channel fader because the bus comes after the channel. A mix that distorts even though no channel looks too loud is usually clipping at the one place everything meets.

This page follows the path through a DAW mixer, which is where you'll make most of these decisions. Where an effect should sit on a channel, as an insert or on a send, has its own lesson in insert vs send effects, and the order of plug-ins inside one insert chain is covered in effect order.

Reading the lab

The lab is a small mixer with three channels: drums, bass and chords, all playing the same eight-second loop at 120 BPM. The big screen draws it the way the sound travels, left to right. The drums have a trim and a compressor in their insert slot; the bass and chords have empty insert slots. Every channel ends at its fader, and from there it goes either straight to the master or down to the bus, where the channels sent there are added together before a second compressor and the bus fader.

The amber number under each box is the peak coming out of that stage in dBFS. While the loop is stopped it's the loudest moment of the whole loop; while it plays it's the level right now. Lines get brighter as they carry more signal, and a compressor glows pink when it's switched on. Hover over any box for what it does and its exact level.

The knobs under the screens are the trim, the three channel faders, the bus fader and the master. Below them, the switches decide which channels go to the bus and turn the two compressors on and off. The panel marked Right now explains what your last change did, using numbers measured from the mix you're hearing, and its three readouts give the most gain reduction each compressor applies and the peak at the master.

A/B bypasses both compressors and leaves every fader and route where it is, with the bypassed version matched in loudness, so you hear what the compressors are doing to the mix and nothing else.

The order of a mixer channel

Every DAW draws its mixer differently, but the channels in Ableton Live, Logic Pro, Pro Tools, Cubase, FL Studio and Studio One all process sound in close to the same order. Reading a channel strip from top to bottom in Logic or Pro Tools, or a track's devices from left to right and then its mixer section in Ableton, follows the signal.

Clip gain and trim

The first control on a channel sets the level of the sound before any plug-in hears it. Most DAWs give you clip gain, which is a level setting on the audio region itself: the Gain slider in an Ableton clip, the Gain setting in Logic's region inspector, the clip gain fader on a Pro Tools clip. Some channels also have an input trim, and anywhere they don't, a gain plug-in in the first insert slot does the same job, such as Utility in Ableton or Gain in Logic.

Because it comes first, this is the control that changes how every plug-in on the channel behaves. A compressor's threshold and a saturator's drive both depend on the level going in, and the gain staging lesson shows that in detail.

Insert slots

Inserts are the plug-in slots on the channel. The whole signal goes through each one in turn, so a compressor below an EQ hears the EQ's result and swapping the two can change the sound. All of them come before the fader on almost every channel in almost every DAW. That's why moving the fader never changes what a plug-in does. Cubase lets you move insert slots after the fader, and the master fader track in Pro Tools is the other exception you're likely to meet, covered further down.

Fader and pan

The fader sets how loud the channel is in the mix, and the pan control places it between the speakers. Both are plain level changes. Turning a fader down 6 dB halves the amplitude of everything the channel sends on, and that's all it does. It can't make a compressor above it work any less, and it can't take the distortion out of a plug-in above it.

The lab shows this with real numbers. In step 2 the drum fader comes down 8 dB, the drums leave the channel 8 dB lower, and the drum compressor's gain reduction stays at exactly 4.5 dB. In step 3 the same 8 dB comes off the trim instead, which is upstream of the compressor, and its gain reduction drops to 0.4 dB. The drums end up at nearly the same level both times, but they sound different, because in one version the compressor is still squashing them and in the other it has almost stopped.

Sends

A send copies some of the channel's signal to another channel, usually a return with a reverb or delay on it, and leaves the channel itself alone. Where the copy is taken from is part of the signal flow. A post-fader send taps the channel after its fader, so the effect follows the fader up and down; a pre-fader send taps it before, so the fader doesn't touch what goes to the effect. The insert vs send lesson has its own lab for that difference.

The output

The last setting on a channel is where it goes. By default that's the master, but you can point it at a bus instead, and then the bus decides what happens next. In Ableton Live it's the Audio To menu at the bottom of the track's I/O section, in Logic Pro and Pro Tools it's the Output slot on the channel strip, and in FL Studio it's the routing between mixer tracks.

What is a bus?

A bus is a path that carries several channels to one place, where they're added together and handled as one sound. Route the kick, snare and hats to a bus and you get a drum bus: one fader that moves all the drums at once, and one set of insert slots where a compressor or saturator works on all the drums together. Depending on the DAW, the receiving channel is called a group or an aux, and it works the same way.

Routing alone doesn't change the sound. Adding the drums and bass together on a bus and then sending the bus to the master gives exactly the same sum as sending each one to the master directly, which is what step 4 of the lab shows: the master still peaks at −6.1 dBFS. The bus only makes a difference once you use it, either with its fader or with plug-ins on it.

The bus fader is the simple use. It sits after everything on the bus, so it turns the whole group up or down while keeping the balance inside it. In step 5 it comes down 6 dB and the drums and bass drop together, while the chords, which still go straight to the master, stay put. If you've got eight backing vocals that are right against each other and wrong against the lead, a bus means one fader move instead of eight.

A compressor on a bus hears the sum

Plug-ins on a bus process the channels after they've been added together, which no plug-in on a single channel can do. A bus compressor reacts to whatever is loudest in the group and turns all of it down. On the lab's bus the kick is the loudest thing, so each kick turns the bass down too, by up to 6 dB. That's where the glue of bus compression comes from, and pushed harder it's the pumping you hear on some dance records.

Now the channel fader does change what a compressor does. The drum fader comes after the drum compressor, but before the bus compressor. In step 7 the drum fader comes down 8 dB again, exactly as it did in step 2: the drum compressor still doesn't notice, but the bus compressor gets 8 dB less kick and its gain reduction falls from 6.0 dB to 3.4 dB. The bass pumps less because a drum fader moved.

So get the balance roughly right before you set a bus compressor, and check its gain reduction again whenever you make a big fader move on one of its channels. The drum bus lesson goes further into what to put on a bus and in what order.

Buses, VCAs and folders

Many DAWs have other ways of grouping channels that look like a bus and don't carry any audio. A VCA fader turns the channels assigned to it up or down by adding its own setting to each of their faders, usually without moving them on screen. Nothing is added together anywhere, so there's nothing for a plug-in to work on, and you can't put one on a VCA. Linked fader groups, such as a mix group in Pro Tools or a group in Logic Pro, also move levels without carrying audio. Folders vary: a folder stack in Logic Pro and a basic folder in Pro Tools only tidy the tracks on screen, while a summing stack in Logic Pro and a routing folder in Pro Tools are buses. Use a bus when you want to process a group as one sound, and a VCA or a linked group when you only want to move several faders at once.

The master: where everything meets

Every channel and every bus ends at the master, called the Main track in Ableton Live 12 (Master in earlier versions), the Stereo Out in Logic Pro and a master fader track in Pro Tools. Anything you put on it processes the whole mix, and its fader is the last level control before the sound leaves the DAW.

Inside a modern DAW, levels over 0 dBFS between stages aren't a disaster. The mixer works in floating point, so a channel that peaks at +4 dBFS still carries its waveform intact to the next stage, and turning something down later brings it back cleanly. The output is different: your interface's converter and an exported 16-bit or 24-bit file can't store anything past 0 dBFS, so whatever goes over at the very end gets cut off. That's clipping, and the master is where it usually happens.

Step 8 pushes the master fader to +12 dB. The mix peaks at +3.1 dBFS and the output cuts the tops off those peaks, which show in red where they cross the dashed line on the output screen. Bring the master back to 0 dB and the clipping goes, with the balance between the channels exactly as it was, because the master fader comes after everything else and moves it all by the same amount. Leaving some headroom on the master, a few decibels under 0 dBFS, gives you room to work before that happens.

Pro Tools works differently here. On a master fader track, the inserts come after the fader, so a limiter on it sees every move you make with that fader. On the Main track in Ableton Live and the Stereo Out in Logic Pro, plug-ins come before the fader as they do on every other channel, so turning the master fader down after a limiter lowers the level coming out of the limiter without changing what the limiter does.

Signal flow when you record

Before a sound reaches the mixer it has already been through a chain of its own. A microphone sends a tiny signal to a preamp, the preamp's gain knob raises it to a level the converter can use, and the converter turns it into the numbers your DAW records. That converter has a hard ceiling at 0 dBFS, and anything that clips there is clipped in the file for good; no fader or plug-in later can put the tops back. Set the preamp gain so the loudest parts of the performance peak somewhere around −10 to −6 dBFS, and the recording has room to spare.

When you record, most DAWs store the sound as it arrives, before the track's plug-ins. A compressor or reverb on a record-armed track changes what you hear while you play, but the file on disk is dry, and the plug-ins process it again on playback. That's useful, because you can change your mind about them later, and it also means a plug-in on the track can't fix a recording that clipped at the converter.

Monitoring has two routes. Software monitoring sends the sound through the DAW and back out, so you hear the track's plug-ins, with a small delay that depends on your buffer size. Direct monitoring on the interface sends the input straight to your headphones before it reaches the computer, so there's no delay and no plug-ins. If you hear yourself twice, slightly apart, both are switched on at once.

How to trace a signal flow problem, step by step

When a track is silent, too quiet or distorted and you can't see why, follow the signal from the start and check each stage in order. The fault is at the first stage where the meter or the sound isn't what you expect.

1. Start at the sourcePlay the track and watch its meter. If nothing moves, check the region isn't muted, its clip gain isn't turned right down, and for an instrument track that the instrument is receiving MIDI.
2. Check mute and soloA muted channel is silent, and a solo on any other channel silences everything that isn't soloed. A forgotten solo on another track is a common cause.
3. Bypass the inserts one at a timeSwitch each plug-in off from the top of the chain down. A gate with its threshold set too high or a plug-in with its output turned right down shows up here.
4. Look at the fader and sendsCheck the fader isn't at −∞. If you only hear an effect, the dry fader is down and a pre-fader send is still feeding it.
5. Follow the outputSee where the channel is routed. A bus that no channel receives, or an aux whose own output goes nowhere, swallows everything sent to it.
6. Check the bus and the masterLook at the bus fader, the bus inserts and the master in the same way. If the master meter is moving but you hear nothing, the problem is after the DAW, so check the interface's output settings and its volume knob before the speakers.

Distortion follows the same path. Find the first meter that hits the top or the first plug-in that sounds wrong when you bypass the one after it. In a floating-point DAW that's usually the master output or a plug-in that clips inside itself, unless the recording clipped on the way in.

Common signal flow mistakes

Using the fader to make a plug-in work less

If a compressor is squashing a part too hard or a saturator is too crunchy, pulling the channel fader down only makes the same sound quieter. Turn the level down before the plug-in, with clip gain or a trim in the first slot, or change the plug-in's own threshold or drive.

Treating a red channel meter as distortion

A channel peaking over 0 dBFS in a floating-point mixer hasn't clipped yet, and turning it down later restores it. What you should worry about is clipping at the converter when you record, at the master output, and inside plug-ins that model analogue gear and distort when driven hard. Keep channel levels sensible anyway, because those plug-ins behave best when they get the level they were designed for.

Setting the bus compressor first

A bus compressor's response depends on every fader feeding it. If you set it up and then rebalance the drums by 6 dB, it's now working on a different mix. Get a rough balance first, then set the bus compressor, and look at its meter again after big changes.

Processing on the master that belongs on a bus

A compressor on the master that pumps every time the kick hits is pulling the vocal and the pads down with it. If the problem is the drums, put the processing on the drum bus, where it can only touch the drums.

Forgetting where the sends tap off

Automate a vocal fading out with its reverb on a pre-fader send and the reverb carries on after the vocal has gone. If an effect doesn't follow its track's fader, check whether the send is pre-fader.

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