Curtis Electromusic Specialities (CEM) is a company founded by Doug Curtis in 1979 specifically to design and manufacture integrated circuits for music synthesizers. Although the company still maintains a Web page, it is no longer doing business. (At least not that they are willing to acknowledge: before his death in 2007, Curtis apparently helped Dave Smith Instruments secure a supply of certain CEM chips to use in their Evolver line of synths. Whether these chips are new production or just stock that Curtis had warehoused is unclear.) The company manufacturered a wide variety of ICs, many of which incorporated an entire synthesizer functional component (for example, the 3340, which contained a complete voltage controlled oscillator). By using chips from Curtis (and its competitor, SSM), synth manufacturers were able to considerably reduce the size of their circuitry. This (along with the advent of inexpensive microprocessor control) is what led to the wave of polyphonic analog synthesizers in the 1980s. By using the Curtis and SSM chips, manufacturers were able to shrink the size of their circuitry to where it all fit in a package no larger than what was necessary to contain the keyboard and control panel.
Later developments included more highly integrated circuits such as the 3389 signal processor and the 3396 synth-on-a-chip. The further miniaturaization led to the creation of rackmount synthesizers, played via MIDI from a master keyboard, and saving performers both money and space. However, Doug Curtis folded his interest in CEM into OnChip Systems in 1988, and further design work on music synthesis products ended. Curtis himself largely moved out of the music field, occasionally doing consulting work with synth manufacturers until his untimely death in 2007.
Just announced in June 2016, OnChip appears to now be making the CEM3340 VCO chips with an updated process.
Most CEM ICs have part numbers of the form "33nn", and synth designers and techs often speak these part numbers without further attribution. There also exist certain CEM ICs with part numbers of the form 53nn or 55nn.