How to visualise signal flow in a Logic Pro project

You have opened a Logic Pro project you did not build — a client's session, a collaborator's, or your own from three years ago. Before you touch a fader you need to know one thing: what feeds what. Which tracks go straight to the output, which are grouped through a bus, what is being sent where in parallel, and whether anything is routed somewhere that no longer exists.

Logic Pro holds all of that. It just never draws it in one picture.

This guide is the method for assembling that picture by hand, in Logic Pro, with nothing else installed. It is worth doing at least once, because the shape of the answer is what you are really learning.

In this guide: what signal flow means here · 1 Open the Mixer · 2 Read the output slots · 3 Follow each bus to its aux · 4 Read the sends · 5 Put the routing strips where you can see them · 6 Write it down · when this method stops being enough · if you would rather see it drawn.

What "signal flow" means here

Every sound in the project travels the same way: out of a channel strip, into whatever that strip's output slot names, and onward until it reaches an output strip and leaves the machine.

There are two kinds of strip, and telling them apart is most of the job:

• Strips that make sound — audio tracks, software instruments. These have regions on them.
• Strips that only move sound — aux strips and output strips. Apple's own description is exact: "Signal flow channel strips, which include aux and output channel strips, do not contain regions, but serve to route the signals from other channel strips."

So the routing skeleton of any session is made of the second kind, and the second kind is invisible in the Tracks area until you ask for it. That is the single biggest reason a big session feels unreadable: the parts that explain the structure are the parts you cannot see.

There are also two ways a signal gets somewhere:

• Down the output slot — the whole signal, one destination, in series.
• Down a send — a parallel copy. Apple: "The channel strip's main signal continues its route to the chosen output. The other part of the signal is routed down a parallel path, via a bus to an aux channel strip."

Series is the spine. Sends are the branches. Read them in that order and a session stops being a wall of strips.

Step 1 — Open the Mixer and stop hiding things from yourself

Open the Mixer. Then switch it to the view that shows every strip, not only the ones that have tracks in the arrangement. Apple describes the control: the view buttons "switch between Single, Tracks, and All views, limiting the Mixer view to channel strips required for the task at hand."

You want the widest of those, because the aux and output strips are exactly what you are looking for, and a track-only view is precisely the view that hides them.

If the Mixer is still crowded, use the filter buttons — Apple: "Use to limit the channel strips displayed in the Mixer to specific types." Filtering down to aux strips alone, briefly, is the fastest way to see how many grouping stages this session actually has.

Step 2 — Read the output slot on every strip. This is the spine.

Work left to right along the bottom of each channel strip and read the output slot. You are not changing anything. You are building a list.

Most strips will say Stereo Out. Those are the simple ones — they go straight to the master. Apple: "By default, a project contains one output channel strip that takes the signal from all the individual tracks in the project."

The interesting ones are the strips whose output slot names a bus. Every one of those is a track that does not go straight out — it goes into a group first. Write down each bus number you see and how many strips feed it.

By the end of this step you have the session's grouping structure: Bus 1 ← 12 strips, Bus 2 ← 8 strips, and so on. That list is the thing Logic Pro never shows you in one place, and it is usually shorter and more surprising than people expect.

Step 3 — Follow each bus to its aux strip

Every bus you wrote down in step 2 arrives somewhere, and that somewhere is an aux strip.

Find the aux strip for each bus and read three things off it:

1. Its own output slot — does this group go to Stereo Out, or into another bus? Nested grouping is where sessions get genuinely confusing, and this is where you find out how deep it goes.
2. Its inserts — the plug-ins on the group, which hint at what the group is for. A compressor and an EQ suggest a processing group, such as drums or vocals; a reverb or delay alone usually means a send destination rather than a subgroup, and you will meet it again in step 4. Check the name and what feeds it before you decide.
3. Its name. If it has one. If it does not, that is worth knowing too, and worth fixing before you hand the session on.

Add each aux to your list under the bus that feeds it. You now have the spine, end to end.

Step 4 — Now read the sends. These are the branches.

Go back to the top and read the Sends section of each strip. A send is a parallel copy, and it is the part of the routing most likely to be forgotten by whoever built the session.

For each send, read two things:

Where it goes. Apple: "You use the Send pop-up menu to change where in the channel strip signal the send occurs, and to set the destination for the parallel routing of the send." The destination is a bus, and that bus has an aux strip — you probably already met it in step 3, sitting there with a reverb on it and nothing feeding it that you could see. Now you can see it.

How much, and from where in the strip. The Send Level knob is the amount: Apple, "You use the Send Level knob to control the amount of signal that is sent via the bus." The tap point matters more than most people remember, and Apple states all three plainly:

• "Post Pan: The signal is sent after the fader and the Pan knob"
• "Post Fader: The signal is sent after the fader but before the Pan knob"
• "Pre Fader: The signal is sent before the fader and Pan knob"

A pre-fader send is the one that will surprise you hours later, because pulling the track's fader down does not pull its send down with it. If a reverb is still audible on a muted-sounding track, this is usually why. Note every pre-fader send you find. It is the single most valuable line in the document you are building.

One more thing to write down: how many strips share each send destination. Apple's reason for the whole mechanism is the giveaway — "This method allows multiple channel strips to be processed by one inserted effect, which saves vast amounts of processing power." A reverb fed by eighteen strips is a structural decision. A reverb fed by one may be a deliberate effect on a single sound, or something left behind — worth a look either way.

Step 5 — Put the routing strips where you can see them

Here is the step most people never take, and it is the one that turns the Mixer into something you can read over time.

Select the aux and output strips you care about, then choose Options > Create Tracks for Selected Channel Strips — or press Control-T. Apple's guide: "A new track is created in the Tracks area for each selected channel strip."

If you would rather keep them together, select them and choose Options > Create Track Stack for Selected Channel Strips, which makes "a new track stack containing all selected channel strips".

Why bother: the routing structure is now visible in the arrangement alongside the music, instead of living only at the far right of the Mixer. You can see the groups. You can automate them. And when you come back in a month, the session explains part of itself.

Step 6 — Write it down before you close the session

All that reading is worth nothing if it lives only in your head. Write it down as plain text, in the project folder — something like this (the names and counts are placeholders):

SPINE
  Bus 1  "Drums"      ← 14 strips   → Bus 5
  Bus 2  "Gtrs"       ←  9 strips   → Stereo Out
  Bus 5  "Mix Glue"   ←  2 busses   → Stereo Out     [compressor, EQ]

BRANCHES (sends)
  Bus 11 "Plate"      ← 18 strips   post-fader
  Bus 12 "Slap"       ←  3 strips   PRE-FADER  ← careful
  Bus 13 "?"          ←  1 strip    post-fader — check: one strip, nothing on the aux

ODD
  Bus 7  named on two strips, no aux strip found

That third section is the one that earns its keep. Anything you could not resolve — a bus with no aux at the other end, a send going nowhere useful, an aux strip fed by nothing — goes in it. Those are the things that bite during a mix, and they are almost always invisible until somebody goes looking. Some of them are deliberate — a strip sent to several places, an output feeding outboard gear — so mark them to investigate, not to delete.

Before you close the session:

• ☐ the Mixer on All
• ☐ every output slot read
• ☐ each bus followed to its aux
• ☐ every send read, pre-fader sends marked
• ☐ the routing strips made into tracks
• ☐ your notes saved in the project folder

When this method stops being enough

Everything above scales linearly with the number of strips, and your attention does not.

At thirty tracks it is a pleasant exercise. At a hundred it takes real time and you will make mistakes. At several hundred — an orchestral session, a film mix, a template somebody has been adding to for four years — reading output slots one at a time is not a method any more, it is a data-entry job, and the picture you are assembling will be out of date by the time you finish assembling it.

That is the point at which it is worth having the picture drawn for you.