Expression Pedals – Way More Than Just a Wah

When you start getting into digital modelling of amps and effects, you will likely want to interact with them in the same way you would with conventional gear. That means stomp switches on the floor, knobs for tweaking parameters, and for many effects (such as wah, volume swells, etc.) you need an expression pedal.

In this blog post, I’ll cover how an expression pedal works with digital models, how to calibrate it perfectly for your hardware, and show you some expression pedal tricks that lets you get some studio-style effects with your gear.

Step One. Get a pedal.

Once you are ready to add an expression pedal to your digital modeling gear, your first idea may be to use a volume pedal you’ve already got, or modify some old scratchy wah pedal. Unfortunately, most volume and wah pedals use non-linear pots and while it might sort-of work, to get the best performance from an expression pedal it definitely should have a linear pot. A pot (short for potentiometer) is just a device that has changing resistance as its shaft is rotated. For an expression pedal, the shaft is rotated by the movement of the pedal but it’s really the same thing as using your fingers to turn knobs that are attached to pots.

A little bit on tapered pots

It’s worthwhile to have a good understanding of pot tapers. This knowledge comes in very handy when applying the concept to controlling digitally modelled effects. You’ll see why with some sound samples later in this post. The ‘taper’ on audio pots is often referred to as ‘logarithmic’. This is actually a misnomer, it’s an exponential taper, but due to the logarithmic nature of our ear, people always just refer to this taper as logarithmic and I’ll continue to do so here. Take a look at the figure below. The difference is just whether it increases slower (2) or faster (4) than normal (1).

Geofex wrote a great article on the secret life of pots but for now let’s just take a look his diagram for some different curves.

Your MIDI foot controller or digital modelling device is going to use a low-resolution analog-to-digital converter (ADC) to measure the voltage across the pot in your pedal. Since the ADC will always evenly sample the voltage range (linear) it’s best matched to a linear pot. A logarithmic pot will change too quickly at one extreme causing poor measurement of the pedals position.

The result is the pedal seems to do almost nothing until it’s nearly all the way to toe-down and may cause our virtual effects to be a little jumpy.

When do we actually want logarithmic controls?

Some things are meant to be changed logarithmically (such as volume) but others like delay are not. If we are using a linear expression pedal, how do we translate that to a logarithmic change in a parameter like volume?

Luckily, in most cases we don’t have to do anything different! For a control like volume, the digital model is mapping the MIDI control to the position of the volume knob, not the volume effect itself, so a linear mapping of the pedal position to the knob position works just fine, and the volume you hear changes as you would expect.

Expression pedals can do what real effects can not

Sometimes you specifically want nonlinear control of something. Digitally modelled wah effects are a great example of this. Since the expression pedal is controlling the position of a virtual wah pedal, the linear control means the wah behaves ‘normally’. But maybe you want to emphasize the lower frequencies (common on a bass wah) or the upper frequencies (more suitable for guitar wah). For bass, you want most of the pedal movement to be staying in the low frequencies, so you’d like a nonlinear curve that is flat near the start and steep near the end.

This is like customizing the taper on your wah pot to get just the right emphasis on the frequencies you want!

Most digital modelling hardware or software will let you alter the curve somehow to get what you want. It uses internal calculations to ensure the curve yields good precision even in the quickly changing nonlinear parts, something the ADC sampling of your expression pedal pot can not do.

For example, in Amplitube, you can set the curve shaping from 1 to 10 where 5 is linear. As you move from 5 to 10, it becomes more more ‘logarithmic’. As you move down from 5 to 1, it becomes more ‘exponential’.

For all these reasons, nearly all expression pedals are built with linear pots, and trying to use volume pedal with a non-linear pot results in sub-optimal results.

Okay, let’s try out some of these tricks. But before that you’ve got to make sure your expression pedal is calibrated with your hardware.

Gentleman, calibrate your pedals

As an example, I’ll calibrate a M-Audio EX-P with a Blackaddr Audio microMIDI, then use it to control some stuff in Amplitube. Refer to the manual for your hardware or software to see exactly how this is done on your particular gear.

First up, calibrate the pedal with the hardware. On the microMIDI, this is done from the Calibration submenu under the Setup menu.

It’s more than expression, it’s complete creative control

Okay, now we will add an effect to control in Amplitube, assign it to the expression pedal and set the curve. Let’s start simple with a wah pedal. Let’s play the same guitar lick twice with same effect, same amp. All we’re going to change is the curve controlling the Wah. This means the movement of our pedal is not 1:1 with the movement of the virtual wah pedal. As you listen to the demo, notice that the first curve 10 has more emphasis in the highs, making it have a little more twang. The second curve 1 has more emphasis in the lows, making it a little more mellow. The effect is subtle because I’m purposefully moving the pedal full range at the same speed. The effect would be magnified by keeping the pedal in the region where the effect changes slowly, giving you more control over those frequencies. Click ‘Watch on Youtube’ to get higher resolution than the embedded video.

Can you hear the difference in how the wah filter moves through the frequencies? The first time it moves quicker to the highs, the second time in lingers longer in the lows.

Okay, I got one more really cool thing to show you. In this trick you can use an expression pedal to glide between two completely different settings on an amp or a pedal. Here’s the process:

1. Setup MIDI control for every knob you want to be changed by the expression pedal. That’s rights, everything is controlled by the one pedal.
2. For each control, you can map the total movement of the pedal to a start value and an end value for the knob. For example the pedal could make a knob go from 5 to 9 instead of 0 to 10. You can even reverse the relationship so that as the pedal goes towards toe-down, the knobs turn down instead of up. In Amplitube it uses %, so the knob starting at halfway would be 50%.
3. For each control, choose an appropriate curve. Much of the time linear is fine, but not always. Maybe you want some knobs to move faster at the beginning or at the end. You do that by changing the curve.

In the studio, this is all done with something called automation where the studio engineer programs it into the computer. We’re basically doing the same thing, but a simpler version using the expression pedal.

Let’s try an example of this. I’ve got two settings on my amp that I like, one is a little bit crunchy, the other has heavy distortion. Instead of creating two different presets and changing them mid-song, I’m going to use the pedal to do it. Heel-down is light crunch, toe-down is heavy lead. I could just switch between the two by moving the pedal between it’s extremes, which is handy but the really cool part is I can glide from one setting to the other with the pedal! This is a great way to add creative dynamics to your intro, bridge or lead into a chorus.

You’ll notice as the gain goes up, the volume comes down to allow a subtle increase in volume rather than blowing your ears off. I also boost the bass and mids for the lighter setting, and roll them back as the gain goes up. Just for a little extra sauce I also throw in a compressor pedal that ramps up the compression as I transition to the heavy tone. Once again click ‘Watch on Youtube’ to get higher resolution than the embedded video.

Pretty cool, eh?

A downside of expression pedals is that many are seemingly overpriced, and the cheaper ones are prone to breaking the same way. In my next follow-up post, I’ll show you how to modify the inexpensive pedal shown in these videos to significantly increase its reliability and ruggedness so it’s ready for gigging.

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