As far as we know, this was Yamaha's first Synthesizer. Before this, the only keyboard instruments we know of are organs and electric pianos. Like many synths that came out in the mid 70's (such as the Roland SH-2000 and the ARP Pro-Soloist), the SY-1 is a preset synthesizer. However it, as well as its more roadworthy brother, the SY-2, has a great deal of sliders and knobs which allow you to shape the sounds to your liking. It is monophonic and has a 37-note (C-C) pressure-sensitive keyboard, which is also a very nice feature.
Along the top panel, there are 29 color-coded switches and a volume knob.
The first 7 switches correspond to the 7 sliders on the control panel to the left of the keyboard. They allow you to switch between the preset levels of the patch and the levels indicated by the control panel sliders (more about these below). There are two switches for "Attack Bend" labeled "pitch" and "tone" (which we assume correspond to the "intensity" and "time" sliders on the control panel). There are also switches for Portamento, Pulse Width, Vibrato, Envelope, and Filter.
Next are 14 color-coded self-cancelling organ-like switches and one bank switch (labeled "Side 1" and "Side 2") used to select between the 28 preset. The first switch is black and it selects Flute/Clarinet. The next 5 switches are red and select Trombone/Bassoon, Trumpet/French Horn, Saxophone/Bass Clarinet, Oboe/English Horn, and Bow Violin/Pizzicato Violin. Then there are 5 yellow switches for Piano/Guitar, Harpsichord/Hawaiian Guitar, Contrabass/Pizzicato Bass, Tuba/Sousaphone, and Bass Guitar/Wah Guitar. The last three switches are green and they select Funny/Pulsar, Trumute/Growlpet, and Double/Reed.
Next, there are 4 transpose switches, an octave down, normal, one octave up, and two octaves up. The last 3 switches enabling the "Touch Control" to affect the Vibrato Depth, Wah Wah, and Volume.
The voice architecture seems to be two VCOs, one LFO, a high-pass VCF, a low-pass VCF, and two envelope generators (one for the VCA and one for the VCFs). Some patches seem to have the VCOs controlled by an envelope generator, most likely the VCF EG. One draw-back is that the VCF will not self-oscillate, a problem with many of Yamaha's early synths.
Far from being a strictly preset synth, the SY-1 allows for a significant amount of modification of its patches. To the left of the keyboard is a control panel with 5 knobs and 7 sliders. The knobs are for Tune, Touch Control Sensitivity, Portamento, Vibrato Speed and Vibrato Depth. The first two sliders are for "Attack Bend Tone" with one labeled "Intensity" and one labeled "Time". These are for the envelope generator that affects the VCO and/or the VCF, depending on the patch. "Intensity" adjusts the level of the attack portion of the envelope and "Time" adjusts the speed of the attack. The next slider adjusts Pulse Width of the VCOs. The next two adjust the attack time and sustain level for the main envelope (which one assumes controls the VCA). The final two sliders adjust the cut-off frequency and the resonance of the filter.
The back panel has an input for a foot controller, with knobs adjusting the amount of control over the VCF and the Volume (VCA). There is a 1/4-inch as well as two rca output jacks, one for high and one for low output. The 1/4-inch jack has a switch to select high and low output. One important item is missing from the back panel. There are no CV control inputs.
Along with the SY-1, Yamaha also came out with an organ, the CSY-1, which featured a monophonic synthesizer, basically the SY-1, built in. The very next year, in 1975, Yamaha came out with the SY-2 (and consequently the CSY-2) which were improved versions of the previous models fit into semi-flight cases.
[Information from The A-Z of Analogue Synthesizers, by Peter Forrest, published by Susurreal Publishing, Devon, England, copyright 1994 Peter Forrest]