Q: I have heard that the tremolo effect in different amp brands is quite different. Since they are all just a type of amplitude modulation, how can the effect be so audibly different?

A: Actually, there are a number of ways of producing tremolo effects and even within one manufacturer's line of amps, there can be some variation. (For example, Fender's Bandmaster amps had several different tremolo circuits in the 1960s.) A Fender tremolo is generally smoother than the same effect on a Vox amp, which tends to be deeper and somewhat choppier, but that's a real generalization when you take into consideration the many ways amp designers can produce the effect. The main sonic difference is heard when comparing a tremolo circuit that modulates the signal using a sine or triangle wave as opposed to a square wave. A sine waveform produces a smoother, more liquid tremolo effect, while a square wave produces a much edgier sound. Some modern tremolo pedals allow you to switch between the two waveforms. This is quite useful musically, since the variations can be quite dramatic; so a slow tremolo on a ballad using a sine wave sounds far less aggressive than a fast vibrato using a square wave would.