Put simply, digital designs allow for much greater versatility, as we shall now discover. Digital pedals do not employ discrete analogue components at their hearts, but instead use DSP (Digital Signal Processing) technology to 'physically model' the most popular units of yesteryear. But how does physical modelling work?
In order to 'model' the sound of a classic pedal, the first thing the design engineer needs is a complete understanding of the inner workings of the original unit. He'll take it to bits and examine every component within the circuit. He'll then spend hours taking detailed electronic and acoustic measurements and performing listening tests, until he has eventually built up a complete profile of the device and understands how it works, both electrically and mechanically.
Now it is his job to try to represent these characteristics using a mathematical model that can be stored in digital circuitry and used in real-time to 'flavour' your guitar signal, in exactly the same way as the original 'classic' wah would have done. This of course is easier said than done! With the sheer number of variables involved, and the huge amount of interaction that inevitably goes on between the various components, trying to come up with a workable global mathematical model like this is very difficult to say the least. So the model is usually built up from smaller pieces - each piece is a mathematical equation or 'algorithm' which describes the working of just one part of the original circuit's function - perhaps a representation of how a transistor works, or maybe the resistors and capacitors comprising the filter section.
Provided that each component has been modelled properly, and under all extremes of operation, the 'virtual' components can be assembled - just like building the real thing - into a 'composite object' which will then run like a computer program inside the host, mirroring the characteristics of the original wah-wah with all of its nuances and interactivity. As the pedal is effectively a small computer processor running a program, it is possible to imagine selecting from a list of alternative built-in programs using a simple control knob, calling up different models of well known wah-wah pedals, and this is in fact exactly what digital pedals offer. Note that digital units use ADC (Analogue to Digital Conversion) at the input and DAC (Digital to Analogue Conversion) at the output. When the unit is bypassed, even though the effect is disabled, the signal still goes through ADC and DAC, and some claim that they can hear this. With modern conversion techniques this is unlikely - a quick test with your own gear should put your mind at rest.
A good example of a digital pedal is the Boss PW-10 (V-wah), which offers faithful emulation of Crybaby, Vox, Morley, and many more. In addition, the pedal operation is ideally suited for controlling other effects, and so rotary speaker, Roto-Vibe and talkbox emulation (as made famous by Peter Frampton) are also provided. Some digital pedals also have the provision to store a selection of favourite sounds and settings in internal memory - three in the case of the PW-10, whose stored sounds can be recalled by using an innovative heel operated switch under the back of the pedal.