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Unwrap Robot Guitar's autotuning secret

Posted: 15 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:Robot Guitar? autotuning guitar? MCU?

On the heels of its Ethernet Out (digital) guitar, Gibson Guitar has released the Robot Guitar, which automatically tunes to a range of standard and alternative tunings at the touch of a button.

While Gibson is the public face behind the Robot Guitar, German company Tronical and guitarist and EE Chris Adams are the inventive forces. Tronical developed the automation as a guitar retrofit kit back in 2006, and Gibson licensed the technology to launch the original, limited-run Robot Guitar in December 2007. Other Gibson models are following that launch to market.

Electromechanics join with embedded processing to produce an electric guitar that takes over the often laborious task of tuning and retuning. Aside from simplifying life for tin ears and those who would rather play than tune, the design is useful for players who require both standard and "open" tunings. By supporting quick transitions among tunings, the Robot eliminates the need for multiple instruments (one for each tuning) and the cost that goes with them.

Core body
At the core is a standard Gibson Les Paul electric guitar. The departures from a stock guitar begin with a special bridge and tailpiece. The bridge provides one of the two endpoints that set the string length; the other string endpoint is the "nut" at the end of the fret board. The bridge has the usual individually adjustable "saddles" for each string, which adjust the intonation of the guitar. Unlike a standard bridge, however, the saddle pieces (normally metal) are replaced with isolated piezoelectric pickups to detect the string frequency. Audio output comes from a separate pair of wound pickups no different from any other Les Paul guitar, thus maintaining traditional sound.

The piezo pickups, which detect string frequency, feed signals for each of the six strings from the saddle, by way of a flex circuit, through the guitar body. The flex terminates at the body electronics module, mounted in the position normally occupied by the volume control knob. Although the body electronics knob still serves as a volume control, the action begins when the knob is popped out to go into "Robot mode."

Robot Guitar automatically tunes to a range of standard and alternative tunings at the touch of a button (Click to view teardown.)

Automated tuning
Pulled out, the knob becomes a multiposition rotary switch that allows players to select the desired tuning. LEDs in the special control knob blink to indicate that retuning is complete, as well as to communicate other information.

The body electronics thus provide not only the controlling user interface (by way of the knob) but also the brains of automated tuning. Using the six individual piezo signal inputs, the body electronics multiplex all string signals down to a single line that ties into a Silicon Labs C8051F120 MCU's 12bit analog/digital input. Using a crystal-derived reference frequency, the C8051F120 can tell whether each string is running flat, sharp or on pitch. The Silicon Labs part (originally from Cygnal Semiconductor, a late 2003 Silicon Labs acquisition) then controls retuning to get everything in line (more on that shortly).

Outside the special switch/volume-pot assembly, MCU and analog multiplexing, other devices in the body electronics are mostly in place to serve power conversion needs. A 2.4V NiCd battery pack must be rejiggered for local power needs and stepped up to produce the 12V power rail needed up in the headstock for additional circuitry and motorized tuner pegs.

To communicate with and power the headstock electronics, the Tronical solution takes another clever step toward retaining as standard a guitar as possible. A cable running through the guitar neck would, of course, work, but it would also be a major modification to the basic guitar design and nearly impossible to retrofit. Instead, the Robot guitar uses the guitar strings to provide six points of connection between body and headstock electronics.

Each line is isolated behind the special tailpiece. The traditionally all-metal part instead has ceramic insulating ferrules in each string feedthrough and a bottom-side circuit board to allow each string to connect from the body to the head. A flex circuit out of the body electronics and to the tailpiece distribution board wires things up, and a thin circuit board sits underneath each tuner peg to pick off and deliver power and communication signals to the headstock electronics assembly. To add redundancy in case of a broken string, 12V power and ground each appear to be delivered over two strings.

Signal communication lets the headstock electronics know which way to tune each peg. Redundancy is again seen, with what appear to be duplicate data signals over two remaining strings. Packet data that must deliver tuning instructions for all the pegs is sent by what looks like a single-wire CAN bus implementation, indicated by the presence of an NXP AU5790 CAN transceiver at each end of the signal path.

After tuning data is decoded and sorted out for each string in a second Silicon Labs C8051F120, the remaining headstock electronics are focused mostly on power conversion to get from the 12V rail to tuning-peg motor voltages and local electronics supplies. As in the body electronics, most of the power management parts in the headstock electronics come from Linear Technology. The MCU directly controls a trio of dual-channel motor drivers (for the six strings) via Rohm's BA6845FS motor driver.

The motorized tuning pegs contain a motor, reduction mechanism and peg worm gear. The tuning pegs can be pulled out for manual tuning or pushed in to allow the mechanized system to take over. To exert large forces from a small dc motor, the rpm are reduced by what I'd estimate to be a 500:1 ratio to go from the low-torque, high-rpm motor to the low-rpm, high-torque environment needed for string tensioning. Down-gearing comes from a mix of reduction cogs, a reduction pulley driven by a rubber belt (surprising!), and a concentric reduction axle, all of which hide in an oversized but still compact assembly that fits the existing tuning peg holes.

As an EE and hack player, I have to give Gibson and Tronical credit for creating an instrument that advances the state of the guitar.

- David Carey
President, Portelligent

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