A voltage controlled filter on a chip, produced by Curtis Electromusic. It was widely used in early-to-mid-'80s polyphonic synths, including the Memorymoog and the Rev 3 versions of the Prophet-5. The 3320 implemented a four-pole filter, with voltage control over cutoff frequency and resonance. Each of the four poles was wired to its own I/O pins, and implementing multimode and complex responses was possible, but sadly this capability was rarely exploited; most of the synths that used the chip used it strictly as a low pass filter. The major exception was the Oberheim Matrix-12 and Xpander design, which used the 3320 to implement a variety of complex filter responses, and showed what the 3320 was capable of.