The ARP Avatar was one of the first guitar synthesizers. It used a hexaphonic pickup and patented pitch extraction circuit to sense the notes from a standard guitar, this was coupled with the tone generation circuits of an Arp Odyssey synthesizer.

This is ARP's description of the Avatar guitar synthesizer, from their own product brochure:

THE AVATAR works like any synthesizer except that it’s controlled by a guitar rather than a keyboard.

• HEXAPHONIC PICKUP The Hex pickup is actually six pickups in a single unit mounted between the bridge and treble pickup. There are six poles, each with its own focused magnetic field for better string isolation. To keep each string’s signal isolated, each pole is wired separately to a tiny gold contact in the connector. The Hex pickup comes in two sizes to accommodate a wide variety of guitars.
• MULTIPLEXER. The Multiplexer receives all six signals from the pickup and reads them, passing on the signal of the last string plucked to the pitch extractor. If two strings are plucked simultaneously, the Multiplexer selects the stronger signal and relays it to the pitch extractor.
• PITCH EXTRACTOR. Arp spent three years developing this Digital Pitch Extractor Circuit. The pitch extractor translates the notes played on the guitar into control signals that activate the synthesizer, telling it what to play and for how long. It follows the signal through every nuance of guitar playing style (finger vibrato,string bending, hammer-downs, etc.) until the signal becomes too weak to analyse.
• PROCESSING. Each string is connected to its own independent low noise pre-amp, separate gain control,and independent fuzz circuit. By processing each string separately, the Avatar lets you chord with an unusual“clean fuzz". The processed and straight guitar sounds can be mixed as you wish.
• TOUCH RESPONSE. The synthesizer triggering force of the Avatar is adjustable, so you can bring dynamics to your guitar synthesizer performance. On a high setting, light picking will trigger the Avatar. On a low setting, light picking will give you straight guitar and harder picking strokes will bring the synthesizer in. LED status lights on the console track the string being played and show the level of trigger sensitivity The Envelope Follower is also touch sensitive: the harder you pluck a string, the brighter the sound it produces.
• SYNTHESIZER FUNCTIONS. The Avatar is a variable synthesizer in the fine ARP tradition. It has two Voltage Controlled Oscillators that can be tuned over a IO-octave range and phase synchronized, a Voltage Controlled Filter, Voltage Controlled Amplifier, and Noise Generator. Other functions include Attack·Decay-Sustain-Release (ADSR) and Attack-Release (AR) Envelope Generators, High Pass Filter, Ring Modulator, Sample and Hold, Low Frequency Oscillator (LFO), and an Envelope Follower.
• JACK PANEL. Systems interface jacks allow you to expand the Avatar by adding on other Arps for greater synthesizer capabilities. There`s a mono output for single»amp use, and stereo outputs for use with two amps. In the stereo mode, the strings can be split: E, A, and D through one channel; G, B, and E through the other. The professional XLR connector can be switched to high or low level output for recording directly through studio equipment.
• HUMAN ENGINEERING. Like the other Arps, the Avatar has been Human Engineered to make it easier to play. The block diagrams and sliders are color-coded to give you a fast, clear reading of what sound you”resetting up or changing. The 48-page Owner’s Manual takes you through instructional experiments that teach you the basics of synthesizer theory, and a 35»minute cassette lets you hear exactly how each exercise should sound. There’s also a patch book with the control settings for 50 hot sounds so you can start playing your Avatar right away.
• FOOT CONTROLS. A sustain switch is available to give you foot control of the Avatar's infinite sustain capability. With this switch, you can latch on to a note at will and sustain it indefinitely. Filter pedal, Portamento, and Systems On/Off foot controls are also available.

References:

ARP Avatar patents: http://www.till.com/articles/arp/patents.html#US04300431

ARP Avatar brochure: http://retrosynthads.blogspot.com/2010/03/arp-avatar-brochure-and-sound-sheet.html

ARP Avatar review:Device magazine

ARP corporation history: http://www.rhodeschroma.com/?id=arp

ARP Picture: http://en.wikipedia.org/wiki/ARP_Avatar