The D-550 is the rackmount equivalent of the Roland D-50, Roland's first fully digital synthesizer. Its synthesis is touted as Linear Arithmetic, which consists of layering periodic waveforms with sampled attack transients. Editing on the D-550 is not as convenient as on the D-50, due to the lack of vector joystick, soft buttons, and data entry slider, although the void is easily filled with the PG-1000 programmer. The D-550 has sixteen note polyphony, although most patches are layers which limits polyphony to 8 voices. Two oscillators are available per voice, consisting of sampled attack transients or square or sawtooth periodic waves. The periodic waveforms' cycle can be modulated: PWM for the square wave and slope modulation for the sawtooth. Digital low pass filtering is only available for the periodic waves and resonance does not track the cutoff frequency. Seperate amplitude envelopes and modulation are available for the two oscillator sections, allowing a sampled attack transient to fade into a cylic waveform. Ring modulation between two cyclic waveforms is also supported. Three envelopes are available but are hardwired to PWM, amplitude, and filter cut-off, however the 3 LFOs have a multitude of destinations. Two band EQ, with variable frequency and Q for the highs, and chorus/flanger are available for each voice. Reverb and delay are global and while I'm sure they were par for the late '80s, they now pale in comparison to today's inexpensive outboard. The D-550 is often written off as sounding dated, although this is due more to the generic patches than its true capability. I've been able to achieve some truely astounding patches, some which cannot be duplicated on my newer synthesizers or samplers. If you like digital grunge, looped waveforms, shimmering highs, and dare to be different, look into a Roland D-550. The PG-1000 is the dedicated programmer for the D-50 and D-550. Unlike several programmers for MKS series synths, the PG-1000 communicates with the D-550 via MIDI system exclusive and is powered by an external 9V power supply. The unit has four MIDI connectors: one for MIDI in, a MIDI thru, a MIDI out which transmits PG-1000 data merged with the incoming MIDI stream, and a second MIDI in used for displaying parameter values of the currently edited patch, which are transmitted by the MIDI out of the D-50 or D-550. The PG-1000 sports an impressive number of faders, but many serve multiple purposes for each patch's four partials, two tones, and common parameter block. Still, programming is fairly straight forward with the addition of a backlit LCD and dedicated partial and tone select buttons.