The Doepfer MS-404 is basically a TB-303 emulator. It's analog, monophonic, has a nice 24dB/oct VCF (filter), a simple envelope and the bonus of two LFOs. As with any 303 emulator, the MS-404 has the typical 303 controls you would expect for the filter and envelope. However the two added LFOs add a whole new dimension to the 303 sound. This is because they can modulate the VCOs! Other common 303 emulator features also found on the MS-404 include an external filter input, CV/GATE interfacing, MIDI implementation and a glide effect. All parameters are MIDI controllable. The slide and accent effects from the 303 are possible on the MS-404 from the glide control and MIDI velocity for accent. The MS-404 ranks high among the 303 emulators under $500, and it sounds pretty darn good too.
The sound generating section of the MS-404 is 100% analog (VCO, Noise, VCF, VCA, ADSR, 2 LFO's). Even the envelope and the LFO's are pure analog and not simulated by software. The immediate access to all sound parameters is realised by 15 knobs and 6 switches. The external audio input enables the subsequent sound modeling of other sound sources like samplers, digital synthesizers or expanders.
On top of that the MS-404 works as a monophonic MIDI-to-CV interface to control another analog synthesizer. The MS-404 is capable of controlling VCF frequency and VCA envelope amount (accent) via MIDI.