![]() VCF (see VCF and Filter Envelope Sections for details).Mode Filters (see Mode Filters section for details).Resonators (see Resonators section for details).Mixing and matching these in the Master Gain mixer section, along with the unfiltered Direct signal, is one of the niftiest things to do with the original (and this is greatly enhanced by the Master Gain mixer's mod inputs). The downside of this arrangement it that the VCF cannot be self-resonating because once it started "ringing," there would be nothing to stop its sound - in a more conventional analog synth, playing the keyboard would open and close the VCA, thereby stopping sound from the ringing filter.įollowing the VCA/Amplitude Envelopes are three (!) separate filters, all fed in parallel. It also means that every single note has its own independent envelope generator. This is because the original featured individual VCA/Amplitude envelopes for every note on the aforementioned poly com cards beneath each key. This is unlike just about any other analog synthesizer - the VCA is almost always after the filter(s), not before. Here's a block diagram of the signal path:įollowing the oscillator "ranks" are the VCA/Amplitude Envelopes. Because of its separate note generation for each key, TOC-based synths can't have a "mono mode," thus keyboard glide can't be implemented. To get around this, the original instrument contained a small circuit board called a "poly card" beneath each key with a custom IC chip (the "Polycom IC") that converted the square wave to a ramp wave and allowed the pulse width of the square wave to be adjusted and modulated. ![]() Though it sounds convoluted, top-octave divide synthesis (referred to as "TOC") was common because it's easy and cheap from an electronics standpoint, but it can only product square waves, which severely limits the tonal palette. This is how just about every 60s and 70s transistor organ and 70s string synth works (as well as a number of 70s polyphonic sort-of synthesizers such as the ARP Quadra, Korg Delta, etc.). Another series of IC's halves the top-octave chromatic notes to create the remaining pitches for the entire length of the keyboard. slowed down) to the frequency of each chromatic note of the very top octave. The pitches of two fixed high-frequency square-wave oscillators are "divided-down" (i.e. The original Polymoog works like a string synthesizer (or a transistor organ) on steroids.
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