EE Times Under the Hood - June 23, 2008 - (Page U44)

UTH0623COMP_pg42_55.qxd 6/11/08 4:31 PM Page 44 under the hood: w w w. e e t i m e s . c o m C OMPUT ERS & GAM I NG • w w w. t e c h o n l i n e . c o m Each line is isolated behind the special tailpiece. The traditionally allmetal 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. In case of a broken string, 12-V 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 the pegs is sent by what looks like a single-wire CAN bus implementation, indicated by the 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 focus mostly on power conversion to get from the 12-V rail to tuning-peg motor voltages and local electronics supplies. Most power management parts in the headstock electronics come from Linear. The MCU controls three 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 pegs can be pulled out for manual tuning or pushed in for autotuning. To exert large forces from a small dc motor, the rpms 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 advancing the state of the guitar. ■ and the cost that goes with them. At the core is a standard Gibson Les Paul electric guitar. A special bridge provides one of the two endpoints that set the string length; the other 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, 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 other Les Paul guitars. 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 knob still serves as a volume control, the action begins when the knob is popped out to go into “Robot mode.” 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. 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 12-bit analog/digital input. IN BRIEF Pitch was solid with care in tuneup strumming (the Robot needs a semi-continuous signal for frequency lock). Kudos to Tronical for achieving the ease-of-use and low-intrusion design.The preprogrammed alternative tunings seemed comprehensive. My primary reservations were around some rough workmanship on the guitar itself and a tendency for tuning to drift a bit faster than other comparably priced instruments. Gibson’s larger challenge is getting purist buyers warmed to the idea of electronic assist. Using a crystal-derived reference frequency, the C8051F120 can tell whether each string is running flat, sharp or on pitch. It then controls retuning to get everything in line. Other devices in the body electronics are mostly for power conversion. A 2.4V NiCd battery pack must be rejiggered for local power needs and stepped up to produce the 12-V power rail needed in the headstock for additional circuitry and motorized tuner pegs. In another clever step toward retaining as standard a guitar as possible, the Robot Guitar uses the guitar strings as six points of connection between body and headstock electronics. Component focus Outside of signal processing and motor drive systems, it’s worth highlighting the headstock tuners and traditional Les Paul electronics. Starting with the combination manual/auto pegs, a big surprise was the use of a belt drive for the first stage of motor-to-gear coupling.The drive and driven hubs achieve first-stage rpm reduction (perhaps 3:1), translating high speed to high torque. But why a belt? I’ve lost turntables, cassette decks and VHS players to stretched rubber over the years, and one have might have similar concerns here as time and tension wear on. Perhaps vibration isolation (for sonic purity during tuning?) and maybe just the need for a little play in the system is behind the choice. Either way, the little rubber belts sit at the gentler end of torque reduction and are a snap to replace if stretched.Tronical’s manual mode for the peg ensures a backup in a pinch.As to the balance of instrument electronics, it’s all decidedly old school.The guitar’s counter-wound “humbucker” pickups for common-mode rejection of noise have been in force for 50 years, and the tone controls are still nothing more complex than a potentiometer and capacitor to ground. 44 Electronic Engineering Times, TechOnline | June 23, 2008 http://www.eetimes.com http://www.techonline.com

Table of Contents Feed for the Digital Edition of Under the Hood - June 23, 2008

Under the Hood - June 23, 2008 Extreme Design: SuitSat Pushes Desigers' Limits Evolution of the Smart Phone Mature Devices get Rolly Rocking GPS: Garmin Nuvi 750 vs. HP iPaq 310 Inside the Sony OLED TV Multizone Dgital Audio Flip Ultra Camcorder - An Ode to Clean Design Robot Guitar Tunes Itself E-book is a Sight for Sore Eyes Scientific Calculator Boils Down to Two ICs $100 BOM Eludes First OLPC Laptop 45 nm: What Intel Didn't Tell You Next Step in NAND Flash Evolution Surveillance on a Shoestring Hot 3G Phone Owes Debt to Analog SecurID Fob: Single-Chip Safety Net

Under the Hood - June 23, 2008

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