WTF is a VCA ?

Saturday, May 23, 2015 14:18

When the "VCA" feature was introduced in version 10.1 of Logic Pro X, it caused a lot of confusion. Experienced users who were familiar with that concept questioned how it was implemented in Logic, and newbies just asked the more fundamental question: "WFT is a VCA ?"

So let''s find out.

VCA Sightings

There are two areas where you already may have come into contact with the term "VCA".

You are a Keyboarder

If you ever worked with an analog synthesizer (based on subtractive synthesis), then you might be familiar with the knob that is labeled "VCA". There are usually three types of those terms, VCO - VCF - VCA, where "VC" stands for "voltage controlled" and the last letter represents the Oscillator, Filter, and Amplifier. These are the three main components of a traditional analog synth.

So the acronym "VCA" means "Voltage Controlled Amplifier".

You are a Sound Engineer

If your background is sound engineering and you had the pleasure to work on big mixing consoles, like an SSL, then you also may have encountered the term VCA. However, it has nothing to do with a built-in synthesizer in a mixing console. It refers to the concept of a VCA, like in synthesizers, but here, it applies to signal routing and Channel Strips. As we will see, this is close to the VCA feature we have now in Logic, so let''s have a look at the concept of a VCA.

VCA Concept

To better explain the mystery behind the functionality of a VCA, let me introduce an analogy. I often use that technique in my manuals to explain sometimes complicated concepts in an easy to understand way. In the case of a VCA, I want to compare "VCA" to a "Sprinkler System" in your garden.

The Sprinkler System

Here are four stages of a real life sprinkler scenario that could happen in your neighborhood:

Stage 1

Assume you have a water hose in your garden attached to a valve with a knob or a handle that lets you control the amount of water running through the hose to your sprinkler system. The important part is that the water itself flows through the "unit", the component you are using to control the water flow, the valve.

Stage 2

To upgrade your sprinkler system, you go to your local hardware store and buy yourself a programmable unit. The water is still controlled by a valve, but you don''t touch the valve directly anymore, it is hooked up to your programmable unit. Now, you have a controller (i.e. a keypad) that just sends a control signal to the unit, which sets the amount, how much to open the valve, for example, a range from 0 to 10.

Stage 3

With a manual valve, only one person at a time can control the water flow by turning the knob. A control unit, on the other hand, has the advantage that it could be designed to receive multiple control inputs (instructions on how much to open the valve). So in addition to the Main Control (the keypad on the unit itself), it might have a WiFi receiver that allows you to control the unit with a Remote Control (i.e. a wireless keypad placed in your living room). However, that Remote Control can only send an offset value. That means, it sends a control signal to raise or lower the value set by the Main Control. For example, if you are at the unit, you can set the water flow with the Main Control, i.e. to 5. Later, when you in the living room, you can use your Remote Control to turn it down 2 steps, which results in a value of 3.

Stage 4

Think about the following situation. Your two neighbors, who happen to have the same type of sprinkler system, are going on a vacation. They configure their units, so they can receive a control signal (an offset value) from your Remote Control. For example, Bob might have a bigger garden and sets his water flow higher). Now if it was raining a few days ago, you can use your remote control to reduce the water flow on all three systems at the same time, sending the same offset value. If you send a value of -2, then all three sprinkler units lower their setting by 2. If Bob''s system was set to 8, it would now be 6, and if your setting was at 5, it would now be at 3.

The VCA System

Now let''s see how that sprinkler system applies to the VCA concept:

Stage 1 - Standard Fader

A Standard Fader on an analog mixing console functions like that single valve. The audio signal runs through the Fader ➊ and by moving the Fader up or down, you affect how much of the audio signal goes through, and therefore, determining the audio level.

Stage 2 - VCA Fader

A VCA Fader uses a circuitry inside, the so-called VCA ➋ (Voltage Controlled Amplifier/Attenuator). This corresponds to the sprinkler unit we bought at the hardware store. The VCA component determines how much of the audio signal (the level) goes through, which means, how much to amplify or attenuate the audio signal passing through that VCA. Here is the important part: The Fader ➌ itself (similar to the keypad in the sprinkler analogy) is not part of the signal chain, it only sends out a control signal (a DC voltage) ➍, that sets the amount of amplification or attenuation for the VCA. So the term "VCA Fader" is based on the concept that the Fader is not directly controlling the audio signal, but instead, is controlling a VCA (which then controls the audio signal). Although all the Faders in the Logic Mixer are based on computer code (processing data), you can think of those Faders as VCA Faders.

Stage 3 - VCA Master Fader

So the Fader on each Channel Strip is technically a VCA Fader that sends its control signal to the VCA component on that Channel Strip to set the audio level. This represents the Main Control, the keypad on the sprinkler system unit. Now in addition to those VCA Faders (Main Controls), there are the so-called "VCA Master Faders", similar to the Remote Control of your sprinkler system in your living room. These VCA Master Faders are Channel Strips that contain only a VCA Fader ➎ that sends out a control signal ➏, an offset signal. There is no audio signal running through such a Channel Strip (VCA Channel Strip). It is a Channel Strip with just a Fader (plus Mute and Solo).

Be careful with the terminology, because a "VCA Master Fader" (VCA Channel Strip) is also called just a "VCA Fader".

Stage 4 - VCA Group

Now here is the important part. The VCA component on each Channel Strip (the element in the audio signal flow that sets the audio level) can receive two control signals. One control signal is coming from its own Fader ➐ on the same Channel Strip, and a second control signal ➑ can come from a VCA Master Fader ➒ sending an offset value. The actual result (how much the VCA on a Channel Strip changes the signal level) is the sum of both control signals (the main control signal from the Channel Fader, plus the offset control signal from the VCA Master Fader). All the Channel Strips that receive their additional control signal from the same VCA Master Channel "belong to that VCA Group". In our sprinkler system analogy, your sprinkler and your two neighbor''s sprinkler were assigned to the same "VCA Group", meaning they were controlled by the same Remote Control keypad in your living room (the same VCA Master Fader).

Of course, you can have multiple VCA Master Faders in your Logic Mixer (up to 256) controlling different groups of Channel Strips, but each Channel Strip can only be controlled by one VCA Master Fader ➑ (in addition to the signal coming from its own VCA Fader ➐).


In this article I only tried to introduce the concept of a VCA in a mixing environment, so you know what it is that you are using, when you are using the VCA feature in Logic.

The next step is the actual implementation in Logic. There are many details and functionality you have to be aware of. In my book, "Logic Pro X - The Details", I will dive into all that with lots of little tidbits that are not found in any other book, even the official Logic User Guide itself.

Here are some of the insights you will learn:

  • Learn all the user interface implementation and tricks when using VCA Groups
  • Find out why a Folder Stack is actually a VCA Group
  • Realize that the Master Channel Strip is nothing other than a VCA Master Fader
  • Weight the pros and cons of VCA Groups compared to other grouping techniques
  • Learn where and why VCA Channel Strips are displayed on the Mixer
  • Master the VCA Groups in a complex situation when using everything together and still staying on top of your signal flow.

After all that, it will be easy to decipher a mixed VCA/SummingStack/Master routing setup like in the following screenshot.

Graphically Enhanced Manuals (GEM)

Please check out the other books in my best-selling “Graphically Enhanced Manuals” (GEM) series, available as PDF files, interactive multi-touch iBooks on Apple’s iBooks Store, or as printed books on Amazon.


Thanks for your time and interest,

Edgar Rothermich