Create an A-B Switch in Logic

Wednesday, March 16, 2016 20:11

Were you ever in the situation during production when you wanted to compare your mix to a reference track? In this tutorial, I show some elegant solutions how to configure your Logic Project to have a simple button for A-B comparison by (mis-)using the Side Chain feature in the Compressor Plugin.

A-B Comparison

Comparing two audio files, two mixes, or your mix against a reference audio file is very common in audio production. Here are a few examples:

  • Reference Track: You are mixing your song and want to compare it against a song from your favorite artist/producer/engineer to find out how they differ sonically.
  • Remixes: You are remixing a song and want to compare the new version you are mixing to the original mix.
  • Alternate Mixes: You can even configure Logic to have two mixes of the current song in your Mixer Window at the same time and switch between them with the click of a button.
  • Cloned Songs: In 2012, I re-recorded for BSX Records the unreleased Blade Runner soundtrack based on the unofficial bootleg CD. I had each track imported into Logic and reconstructed them note by note, matching the original track by constantly comparing my mix against the reference track.
  • Temp Track: If, as a film composer, you are presented with a temp track the director fell in love with, then while you are writing, you better compare your score against the temp to see if it will make the director happy while still keeping the copyright lawyer off your back.

The Concept

There are different ways how to configure Logic in order to switch between your mix and a specific reference track or multiple tracks. The easy way is to use the Mute or Solo buttons, but this often requires to press multiple buttons. In this tutorial, I want to show how to use the Side Chain feature in the Compressor Plugin to do the A-B switching with a single button that toggles between the two states.

I will later get into the different variation of this setup, but first, let''s start with a simple example to switch between your mix and a reference Track:

  • Mix Channel Strips: The first four Channel Strips ➊ represent your mix. They are routed  to the Output Channel Strip ➌.
  • Reference Channel Strip: An additional Audio Channel Strip in your Mixer window, "REF-TRACK" , contains the reference audio region. The output routing of this Channel Strip is set to "No Output"  and it has one Send  routed to Bus 1. Make sure that this bus is not routed to any Aux Channel Strip (which is usually created automatically).
  • Output Channel Strip: You add the Compressor Plugin  on the Output Channel Strip (after any other Plugins on that Channel Strip).


Here is a same configuration showing how the routing works:

  • Add Compressor: The Output Channel Strip has the Compressor Plugin added as an Audio FX Plugin.
  • Select Side Chain: In the Compressor Plugin, you select the Bus 1 that you send from the Reference Channel Strip  as the source for the Compressor''s Side Chain Input ➋.
  • Compressor On
    : You configure the Compressor (one button that I show in the next section) so the Compressor passes the Side Chain signal (Bus 1 = Reference Track signal) through to its output ➌. That means, you only will hear the Reference Track on the Output Channel Strip.
  • Compressor Off
    : If you bypass the Compressor (deselect its Power Button), then you will hear your regular mix  on the Output Channel Strip with all the Channel Strips that are routed to the Output Channel Strip ➎.

Compressor on/off

As you can see in the diagram, you accomplish the simple trick for the A-B switch just by turning the Compressor Plugin on or off. That means, you have only one button, the Compressor Plugin''s Power Button that acts as a toggle switch.

  • Compressor On
    : You listen to the Reference Track
  • Compressor Off
    : You listen to your Mix

Compressor Configuration

Here is how you set up the Compressor Plugin:

Side Chain

In case you don''t know what a "Side Chain" is. It is a term that describes a second input on an FX Plugin or Instrument Plugin, where you can feed an audio signal to control a specific parameter on that plugin. Even though this Side Chain signal will not be heard as an audio signal itself on the output of the plugin, the Compressor Plugin ➊ has a button that routes the Side Chain input directly to the output of the plugin so you can listen to it (usually for monitoring purposes).

First, you have to click on the "Side Chain" ➋ button in the upper-right corner of the Plugin window to make the Side Chain related controls visible on the right side. Now, when you click on the "Listen"  button in the Filter section, you switch the output of the Compressor from its regular input (that includes the processing) to the unprocessed signal that you selected in the Side Chain popup menu ➍.

Set the Frequency Knob to max and make sure not to select the "SUM" button, which would create a mono signal.

Side Chain Sources

The Side Chain popup menu displays all the available sources in your Project that you can choose from to feed into the Side Chain:

  • Audio Tracks ➎: This section lists all the current Audio Channel Strips in your Project.
  • Input Busses ➏: This section lists all the input channels of your currently selected audio device.
  • Auxiliary Busses : This section lists all the Auxiliary Busses that you currently use in your Project (please be aware of the difference between Auxiliary Busses and Aux Channel Strips).
  • Output Busses ➑: This section lists all the output channels of your currently selected audio device. That means, the sum of all Channel Strips routed to that Output Bus.

Remember that on the Channel Strip of the Reference Track, we routed its signal to Bus 1 using the Aux Send ➑. Now on the Side Chain Source popup menu, we select exactly that Bus 1  to feed the Reference Track signal to the Compressor''s Side Chain input.

You can also use any other source as the Side Chain Input to create different routing scenarios for the A-B switch. For example, select the Output 3-4 to create an alternate mix (Mix A routed to 1-2 and Mix B routed to 3-4), or you can use the switch to listen to the headphone mix that you might send to Output 3-4. Endless possibilities.

Signal Control

One important factor for any type of A-B comparison of audio signals is proper level matching. In our case, the level of our mix (set on the Output Channel Strip) would be the basic level that we don''t want to change (for the purpose of A-B switching). Instead, we need an easy way to adjust the level of the Reference Track independently.

Here are three options:

  • Send Knob: Use the Send Knob ➊ on the Reference Track''s Channel Strip to adjust the level you are sending to the Compressor. Set the Send to "Pre Fader" to be independent from the Volume Fader.
  • Fader: Use the Fader  on the Reference Track''s Channels Strip to adjust the level you are sending to the Compressor. The Send has be set to "Post Fader" or "Post Pan".
  • Compressor Output: Click on the Output Button ➌ in the upper-right corner of the Compressor Plugin to switch that section to display the output controls. The Output Gain Knob ➍ adjusts, in our configuration, the level of the Side Chain signal running through the Compressor. BTW, make sure to leave the Mix Knob  all the way to the right (at Output).

Switch Option 1: Plugin

Now that we have configured the Compressor Plugin and set the level, let''s look at the different options how to actually switch between the Mix and the Reference Track.

The first option would be to just use any of the buttons that are already there in your Logic Project. There are three places:

  • Channel Strip: Move the mouse cursor over the Plugin Slot (the Power Button appears ➊) and click on the Power Button to toggle the plugin on ➋ or off ➌. Alternatively, you can also opt+click on the button.
  • Plugin Window: If you have the Compressor Plugin Window open, click on its Power Button ➍ in the upper-left corner.
  • Smart Controls: This is a more hidden switch. If you make the Output Channel Strip visible as a Track (sh+cmd+M), then you can select it and the Smart Controls Window (cmd+3) might have a separate switch  to turn the Compressor on/off.

Switch Option 2: Environment Button

The problem with the previous option is that you have to have those elements, the Channel Strip or the Plugin Window visible to have access to the button. With a few clicks in the Environment however, you can create your own button that functions as the A-B Switch that you can place as a floating window anywhere on your screen, so it is always accessible. The following steps require some understanding of the Environment. If you need some help to dive into the magic world of Logic Environment, I cover all that on a 90 page chapter in my book "Logic Pro X - The Details".

Create Button

First, you have to create a button and configure its functionality:

  • Create a Monitor Object ➊ and cable the output of the Output Channel Strip ➋ into it. Now toggle the Compressor Button  and see what MIDI Events are represented by that action. In this case, it is "Event Type = Fader, MIDI Channel = 1, Value 1 = 56, Value 2 = 0 (off) and 1 (on).
  • Create a Fader Object ➍ (i.e. Button 6), cable that into another Monitor Object ➎, and cable that one into the Output Channel Strip ➌.
  • Select the Fader Object ➍ to make its parameters visible in the Inspector on the left. Now, set the parameters so the Fader Object creates exactly the same Events that you''ve looked up previously. Output = "Fader" , Channel = "1" , -1- = "56" .
  • Clicking on the button now generates those MIDI Events that toggle the Plugin Button ➌. You can leave the Range parameter  at "0 127" or set it to "0 1", either one will work.

Create Floating Window

In the next step, you place the button (Fader Object) in its own floating window:

  • Select the Fader Object ➍ (A-B Switch) and copy it (cmd+C).
  • Create a New Layer, call it "Switch" and copy the Fader Object to it ➓ (cmd+V).
  • From the local View Menu deselect "Cables" to show only the object and select "Frameless Floating Window".
  • Resize the window so it only shows the button ➓. You can move the window anywhere on your screen and it will always be visible, always accessible (unless it is covered by another floating window of course).

Switch Option 3: External MIDI Controller

Although the previous option provides a dedicated, always visible, A-B Switch button, you still have to move your mouse over the button and click on it. For a power user, that is already too much work and it would be easier to use a dedicated button on an External MIDI Controller as an A-B Switch. Because we are using Logic and its omni-potent Environment, creating such a solution is a piece of cake:

  • In the Environment, move the Fader Object  you use for the switch to the "Click & Ports" Layer ➋.
  • From the Physical Input Object, cable the external MIDI Object that has the physical button (in this example USB 02 ➌) to a Monitor Object ➍, and cable that one to our Fader Object ➊.
  • Press the button on the external MIDI Controller that you want to use as a switch and read the MIDI Event that it generates in the Monitor Object ➍. In this example, Event Type = "Continuous Controller", MIDI Channel = "1", Value 1 = "18", Value 2 = "0 and 1".
  • Now, select the Fader Object ➊ so its parameters are displayed in the Inspector on the left and enter those parameters to externally control that Fader Object: Input = "Control" ➎, Channel = "1" ➏, -1- = "18" ➐, Range = "0 127" ➑. You can keep the Range value or set them to "0 1".

Now you can use that button on the External MIDI Controller to conveniently switch between your Mix and the Reference Track.

Switch Option 4: Logic Remote

Here is on more option that doesn''t require the Environment. Instead, the A-B Switch will be on your iPad running the Logic Remote app that doesn''t require any configuration.

  • This is how the previous Project I used looks like in Logic Remote ➊. Tap the button "Audio FX 1-8"  to change the view showing the Audio FX Slots.
  • In this view, double-tap the Compressor Button ➌ to change the view showing the parameters of the Compressor Plugin.
  • The Plugin View has a Power Button ➍ in the display on top. This is your A-B Switch that lets you conveniently toggle the Compressor on/off.

Graphically Enhanced Manuals

I hope you found this tutorial useful. If you are interested in learning more about Logic Pro X, check out my books "Logic Pro X - How it Works", "Logic Pro X - The Details", "Logic Pro X - Tips, Tricks, Secrets #1", and "Logic Pro X - Tips, Tricks, Secrets #2". They are available as pdf, printed books on Amazon and interactive multi-touch iBooks on Apple''s iBooks Store.

For a list of all my books in my "Graphically Enhanced Manuals (GEM)" series and all the links, go to my website.


Thanks for your time and interest,

Edgar Rothermich