PipeLine®
Tremolo / harmonic vibrato
Amplitude modulation — tremolo — is arguably the first overt guitar effect. Amps with built-in tremolo appeared by the late 1940s and were standard equipment by the late 50s. Surf music made it its own: the Del-Tones, the Ventures and the Chantays combined tremolo with reverb to build the definitive surf guitar sound.
In 1961 Fender released a complex variation known commonly as harmonic vibrato. It isn't technically vibrato at all, but the effect is vaguely reminiscent of a rotating speaker. It was costly and complicated, so it was short lived, and nothing adequately replaced it.
Development started in January 2009 and finished that December. Rather than simply duplicate the original, PipeLine was built as a stereo version that also does standard tremolo, with a low-pass frequency trim and expression pedal control. In mono the primary output gives you standard trem; plug into the bottom jack and the signal splits for stereo, which produces an alternating effect a mono source simply cannot reproduce.
The toggle selects between normal tremolo and harmonic vibrato. The waveform knob shapes the LFO, from a smooth sine at the left to a clipped square at the right. Speed sets the LFO rate, Intensity sets how much effect is in the signal, and there's an internal trimmer for the low-pass cutoff. Connecting an expression pedal disables the Speed control and hands manual speed control to your foot.