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A 1980s-era polyphonic analog synth from Korg. In Korg's lineup of polyphonic synths at the time, it was intended to be the low-cost unit, slotting in at the bottom of the price curve below the Poly-61.

In order to cut costs, the Poly-800 was based on a video game tune chip, the Oki Semiconductor MSM5232. This resulted in several peculiarities in the voice architecture. The MSM5232 implemented eight digitally controlled oscillators, or DCOs, each of which generated square waves at four octave-interval footages: 2', 4', 8', and 16'; it produced no other waveforms. The DCOs were divided into two groups of four, each of which was clocked by an external master oscillator. The chip provided the equivalent of a voltage controlled amplifier capability for each voice; the synth's patch parameter control system fed in a control signal which controlled the amplitude of each DCO, and could also turn individual DCOs on and off. Envelopes for the VCAs were generated in software. (The chip actually had onboard envelope generators, but the Poly-800 did not use them.)

An unfortunate characteristic of the chip was that it did not provide an individual output for each DCO. Rather, it combined the signals at a given footage for all eight DCOs and had an output for each footage. Since the MSM5232 did not have any onboard filters, this meant that the only way to implement a voltage controlled filter capability was to provide a single, paraphonic VCF for the combined output of the eight voices. This was a step backwards from the Poly-61 architecture, and it ended up being a disadvantage compared to competing products from Roland, Akai, SIEL, and others. The filter was a four-pole design based on an SSM circuit, and was in itself highly thought of.

Other controls included the ability to turn each of the footages off and on individually, or to select a pseudo-sawtooth waveform. The latter was implemented by mixing the four footages in a specific proportion, using a rough approximation of a Walsh function. A single low-frequency oscillator (generated in software) was provided, which oddly produced only a sine wave. However, the synth was generously provisioned with the software-generated envelopes, with six-segment envelopes for the DCO output levels, the VCF cutoff frequency, and the output level of the noise generator. It also contained an onboard digital sequencer, one of the first of the 1980s polyphonic synths to do so. 64 patch memory locations were provided, and the Poly-800 was the first Korg model which came equipped with MIDI from the start of the production run. Unfortunately, in the original MIDI implementation, the synth only operates in omni mode, which means that it cannot share a cable chain or interface with another synth. (Some available third-party mods fix this.)

Other cost-cutting measures were taken in the materials, which had the unfortunate effect of giving the synth a cheap feel. The casework was lightweight plastic, and the pushbutton switches of the one-knob patch editing interface were flimsy; most of them had to be replaced after a few years of use. The synth did have the advantage of being lightweight and easily portable; it could run off of batteries, and it had provisions to attach guitar strap buttons so it could be played as a keytar. It had a 49-key, C-to-C keyboard (some were made with reverse keys), and used the same pitch / mod joystick as the Poly-61.

The original version of the Poly-800 was introduced in 1984. The next year, this was followed by a Mark II model which added an onboard digital delay line, which could be configured to produce flanging, chorusing, and "slapback" echo / reverb effects. The Mark II also expanded the MIDI implementation, adding the ability to edit parameters and dump / load patches via sysex messages. The EX-800 was a tabletop version of the Mark I; no keyboardless version of the Mark II was produced.

Because of the synth's inexpensiveness and wide availability in the early 1990s, it began to attract attention from circuit benders and hardware / software experimenters and hackers. The result is that a huge number of third-party modifications have been developed, ranging from the "Moog Slayer" modification which allows manual control over the VCF (with the ability to push it to extreme resonance settings), to operating system upgrades which provide upgraded MIDI functions (some of which fix the omni mode problem), additional patch parameters, and improvements to the user interface. A notable mod package is the Hawk-800 hardware and software update, which is still available as of late 2015.

As of 2015, Poly-800s remain fairly widely available at a reasonable cost. However, many of them have been modified. A buyer looking to purchase one should have the seller describe all mods, and make sure they are understood and desired before committing to the purchase.

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