Setting Audio Control LFOs in Reaper

Previously, we have looked at automation as a tool to control various parameters of an instrument or effect. It is incredibly useful if you don't want a fixed sound in your song, Perhaps you want the reverb to be more intense at the chorus, perhaps you want the delay time to gradually increase as the verse progresses, perhaps you want a low-pass filter to be opened and then shut again. Usually it is best to draw out the lines and add curves manually however, if you know that the sound you want is repeating at a fixed rate, e.g. the volume rising and falling, or the pan alternating, you can use an Audio Control LFO (low frequency oscillator) to control it. This saves you drawing the curve and essentially automates the automation.

 Here you can see the automation is controlling the volume of 
frequency point 2 on the ReaEQ plugin.

The fist step is to identify what you need to modulate. In this case, I have decided to modulate the volume by using a dedicated volume fader VST. So once we have identified this, we go to the automation button as shown below:

This will open up a list of every controlled parameter in the track, if you have lots of effects and a complex synth, you may be looking at hundreds of items so there is a handy search to easily find what you want.

Find the parameter and click "Mod" which will give you this:

Select LFO:

This is the cool bit. You will see the green bar move as the oscillator works, this is the rate of the modulation. The first thing you will need to do is decide on the wave shape. If you want a smooth oscillation try going for a sine or triangle wave. If you want it to alternate with no transition you want a square wave. 

You would usually want the oscillator sync'ed to tempo, all this does is change the speed fader's units from Hz to Beats.

Strength is an obvious one, it determines how strongly the signal modulated the parameter. The bigger the percent, the more extreme the change.

Phase controls what part of the cycle it starts at, for most of the uses for this it is of little consequence but if you are doing extremely long cycle, it may be of importance.

The direction is important. For a volume application, positive means it modulates upwards from the point it is currently at, centred means the faders position is central and the volume is modulated both above and below. Negative, which is what I used modulates the volume downwards from the fader level. This is good for a stutter effect.

It should be noted that the gif has rendered this to an irregular cycle. 
In reality, the fader would be precise and regular in timing.