The CS-40m is a duophonic (2-voice) analog synthesizer that was introduced in 1979. Although Yamaha had already conquered polyphony with their legendary 8-voice CS-80, the CS-40m offered a more affordable approach to something other than a mono-synth. It has a semi-compact 44 note (3 1/2 octave) keyboard, 36 knobs, 12 sliders and over a dozen buttons for easy hands-on performance and editing. There are also 20 patches of memory which can be off-loaded to cassette tape.

The CS-40m certainly isn't as phatt as the CS-80. In fact, the CS-40m is more like a 2-voice version of the CS-20m. Like most CS-series synths the CS-40m has two analog oscillators per voice. Triangle, sawtooth, and square (pulse) waveforms are available on each oscillator, plus there is a noise generator. There is also a simple AR (attack/release) envelope generator for each VCO.

The CS-40m has a multi-mode VCF (filter) switchable between lowpass, bandpass, and highpass filtering. Basic filter cutoff and resonance controls are here, but the filter's resonance can't be driven to self-oscillate. It does, however, have its own ADSR envelope generator for nice sweeping filter effects. There is a basic LFO (Low Frequency Modulator) for modulating the VCO, VCF or VCA. All these classic controls and features and sounds... you would expect wood panels... and there are! The CS-40m has been used by Electronic Dream Plant, Sneaker Pimps, Ultravox, Duran Duran, and Vangelis.