GS1 synth, photo from Yamaha via 120years.net
Yamaha's first commercially produced synth to use frequency modulation as its method of synthesis, preceding the much better known DX7 by about three years. Introduced in 1980, the GS1 had an internal architecture consisting of 16-voice polyphony, with eight operators per voice. The project was a spin-off from an in-house Yamaha project known as PAMS, for Programmable Algorithmic Music System. PAMS had the ability to perform multi-algorithm synthesis, but Yamaha decided that in order to keep the price (somewhat) down, and reduce the complexity to something that would be understandable to a wide audience in 1980, they limited the GS1's choice of synthesis methods to FM.
The 8-operator voice architecture was not as impressive as it sounds, since the synth gave the user no choice of algorithms; the only algorithm available divided the operators into four separate carrier-modulator pairs, meaning that the types of complex modulations which the DX7 could do with two fewer operators were not available. Nonetheless, the GS1 was considered a breakthrough at the time that it was released.
Eight envelope generator / voltage controlled amplifier pairs, one per operator, completed the voice architecture, along with a low frequency oscillator that could be applied to a few parameters. (The synth contained no voltage controlled filters.) A few onboard effects were also available. The 88-key, piano-weighted keyboard was both velocity and aftertouch sensitive. For reasons unknown, Yamaha chose to package the synth in a piano-like case, complete with piano-style pedals. Buttons and knobs along the fallboard allowed the user to select patches and tweak the few user-variable parameters.
The GS1 was essentially a preset synth from the performer's standpoint, as it contained no controls for patch editing. Yamaha constructed a computer with a specialized monitor (consisting of four 7" monitors in one box) and a panel of function-specific controls, which served as a programmer for the synth. Probably less than 10 of these were built; they were distributed to Yamaha national offices. None were sold to customers originally. Yamaha made patch libraries available on a magnetic-card medium, which the synth had a reader for.
The GS1 originally sold for nearly $10,000 US in 1981 dollars. Yamaha attempted to expand the market with a less expensive derivative, the GS2. This replaced the fancy wood case with a less expensive and more road-worthy plastic case, substituted a non-aftertouch 73-key keyboard, and omitted some other features. The GS2 used most of the same electronics and accepted the same patch cards.
Fewer than 100 GS1's were sold before the model was phased out in favor of the DX7 in 1983. It is unknown how many GS2's were sold.