A Plug In Primer


Pimp My QLab

[revised for QLab 4.1 Dec 2017]

Before getting into a detailed forensic examination of the use of plug-ins in QLab, and some fairly sticky problems, let us start with a few fireworks, and show what is readily achievable, with a large number of complex plug-ins, used in an optimum configuration. Here is a screen recording of a fully pimped QLab 3 Installation with probably more cue output and device output plug-ins than are sensible!  View full screen!

With version 3, QLab opened itself up to the world of third-party AU plug-ins. For smaller shows, this can be immensely useful, as it may mean that for a few simple effects on microphones, or audio cues, or some simple time alignment of loudspeaker systems, that you can save some of your budget by using a simple analog console, or no console at all.

The downside of allowing any third-party software to directly interface with QLab, through the plug-in architecture, is that some plug-ins will work better than others, and some will not work at all. It should also be noted that the way QLab uses plugins as effects on cues causes far more complexity than when plug-ins are used in a DAW. In the DAW they are generally loaded when a session is opened, either as channel inserts or on aux busses, and stay loaded. With QLab, when used within cues, they are loaded and unloaded with the cues, which can cause problems.

A word of warning about plug-ins particularly cheap or free ones.  Some are occasionally unstable in use with QLab. This instability can result in short loud bursts of noise under certain circumstances when cues are loading or playing.  Always exercise caution when testing any plugins for the first time. Only use them in show critical workspaces when you are sure they are reliable. Be careful with speaker levels and never test with headphones at anything more than minimum volume.

The best policy is to use the simplest plug-in that will do the job you want, at the quality you require. For EQ, Apple AU Filter gives you a 5 band parametric with variable Q and 20dB cut and boost, will work with any number of input channels and will be installed as part of the system on any Mac that can run QLab. Some plugins require very high CPU loads which can affect the performance of your workspaces. Some will cause very nasty clicks and bangs on output. Exercise caution when using any plugins you haven’t tried before, and test really thoroughly before using them in critical show applications.

This example workspace demonstrates some things that work, and some that don’t, things to watch out for to determine compatibility,  and where to place the plug-in in the signal chain. There are a lot of quite repetitive examples, but if you work through them you will potentially save yourself hours of frustration in trying to work out why plug-ins are not working as you expect.


The examples, where possible, use the basic apple AU plug-ins supplied with your system. In situations where the Apple plug-ins don’t work free or demo versions of third-party plug-ins are used with a URL to download them.

Audio plug-ins in cues have no metering capabilities regardless of what meters are built into the plug-in. If you need meters, then insert the plug-ins in cue or device outputs (as explained in Example 10 below.)

WAVES PLUGINS:  QLab is not a supported DAW for Waves Audio products. What this means is that Waves cannot guarantee their plug-ins will function correctly in QLab, and if they don’t, or cease working,  will not provide  support. (This is not unique to Waves. Other Plug-in manufacturers have similar lists of supported DAWs).

If you intend using Waves Plug-ins (and many users of QLab do) then it is recommended that you authorise to a USB key. If you are authorised to a USB (as opposed to a computer) then, according to Waves Tech Support, your computer can be offline, as the plugin will only seek the license on the local USB key.

Waves AU plug-ins were incompatible with all versions of Qlab prior to 3.1.23 . If you inserted a Waves plug-in you were not able to scrub in any numeric field until you quit QLab.

DO NOT LEAVE PLUG-IN WINDOWS OPEN AFTER EDITiNG:  If you don’t close a plug-in window after editing it, it will reopen the next time the cue is selected (even if not set to autoload). More often than not this will result in this window stealing keyboard focus, meaning the keyboard will no longer operate QLab. Clicking twice on the selected cue will give keyboard focus back to QLab.

In the screen recordings in the examples on the following pages, some vital details are quite small. It’s worth viewing them full screen by clicking on the expand arrows to the right of the video controller bar.

EXAMPLE 1: Apple AUMatrixReverb With a Stereo Audio Cue.

AU Matrix Reverb is the reverb effect included with core audio. It is a good general purpose reverb. Here it is in QLab applied as an audio effect on an audio cue:

The first thing to note is what happens if you stop the cue by panicking the workspace by pressing ESC. Look for the little hollow grey triangle to the left of the cue. This indicates that the plug-in is reporting to QLab that it is still playing the tail of the effect. While it is present you can’t play the cue again, in the same way that you can’t play any cue that is already running. When working with plug-ins you may have to press ESC twice in rapid succession to stop audio cues with effects applied.

In QLab 4.1, when an audio cue ends, a reverb tail indicator, which looks like a graph appears in the left hand column of the workspace. QLab 4.1 allows re-trigerring of cues during tails when this indicator is present.

The next screen recording shows what happens when this cue is played to the end:

At the end of the audio, the cue stays active, with the grey triangle present for 10 seconds or so, even though the reverb tail is only about 2s. This could be problematic. Some effects never report their tails as finished or take minutes to do so, and in these cases a programmed intervention is beneficial. In the next example  an OSC command to hard stop the cue after the real tail time has been added:


Altiverb is a very popular top end reverb. It has a feature-laden interface and is relatively resource intensive in use. Because of this, there are a few issues when applied to QLab cues. On lower spec machines, the initial load time when applying the plug-in may be very long. Complex plug-ins may cause some operations to become less responsive.

Here it is in action:

You will notice that when the cue is played it takes a few hundred milliseconds for the effect to be heard, while it t is loading. The second time the cue is played in the video it is pre-loaded, and there is no delay. When the cue is halted with a stop cue, the grey triangle tail indicator remains on. Altiverb will keep this on for minutes, so any cue using it has to be hard stopped, which the example shows with an OSC cue as before.

On the next pages, we will look at other plug-in examples.


Chapter Author: Mic Pool

Chapter Graphic: Original photograph By Lionel Allorge via Wikimedia Commons.
Licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.
Modified image  distributed under the same license.
Creative Commons License

Music in Demo Video: “Protofunk”
Kevin MacLeod (incompetech.com)
Licensed under a Creative Commons Attribution 3.0 Unported License.
 Creative Commons License