Machine Design

Putting together a high-tech garage band

Somewhere, four computer-savvy kids obsessed with music are sure they will be the next big thing if they could only get the right gear. Here are the musical instruments they might want.

Senior Editor

The Variax 700 digitally models an array of 25 vintage rock-and-roll guitars using offtheshelf components and custom software.

The adjustable saddles on the bridge of the Variaz 700 hold the piezoelectric pickups (the shiny silver insets the strings cross over). They detect string movement and translate it into an electrical signal, which is then converted to a digital signal and manipulated.

The V-Synth from Roland Corp. features several innovative user interfaces for manipulating sounds and waveforms. The LCD in the center, for example, lets players change filters, sound source, and other functions. Players can also change two parameters, such as tempo and pitch, by moving their fingers on the Time Tip Pad.

The art of making musical instruments has long since gone beyond mere woodworking and other refined crafts. Digital signal processing and bandwidth are now king. And some companies are using the latest and greatest in hardware, software, and modeling techniques to faithfully replicate classic rock-and-roll instruments. But they're also coming up with new, otherworldly sounds. And there are still some craftsmen among the technophiles.

Every make and model of guitar differs slightly in its sound and tone. Some, like the '58 Gibson Les Paul Standard and '60 Fender Telecaster, have earned cult status as " classic" rock-and-roll instruments. As these instruments grow in popularity, the price tags on them grow as well, putting them beyond the reach of all but the wealthier musicians.

Engineers at Line 6, Agoura Hills, Calif., aware of the economics of vintage instruments, decided to build a guitar, the Variax 700, that would exactly replicate the sounds of two dozen or more famous guitars and switch between them quickly and effortlessly. The engineering team had a few years experience in modeling, but it was mainly in digitally mimicking tube-based amplifiers. So before they could build the Variax, they had to discover what it was in a guitar that contributed to its overall sound.

"We broke down the various elements responsible for a guitar's tone into basic components, then described them mathematically with algorithms," says Pete Celi, senior designer at Line 6 and responsible for the digital signal processing in the Variax. "The main components in an electric guitar are the electromagnetic pickups, at least one per string, and the body or physical structure of the guitar."

Pickups, the sensors that detect what the guitar strings are doing, are simply coils of wire wrapped around a magnet. They can be described by an inductor (the coil) and its magnetic losses, some parasitic capacitance, and the dc resistance of the wire. So the coil can be represented as a four-element circuit: two resistors, a capacitor, and an inductor.

"This lets us accurately describe a pickup's response by describing it with transfer functions," says Celi.

The body of each guitar was also carefully examined. "Each guitar is essentially a collection of spring-mass systems that reacts according to the composition of the wood, stiffness of the materials, and where the electrical components are mounted," says Celi. "We used spectrum analysis and some traditional vibration testing, such as an impact hammer and acceleration sensors, to get a signature of the guitar's physical characteristics. We modeled each guitar as a bank of parallel resonance, second-order systems."

Some guitars the team analyzed cost upward of $50,000, and desoldering a pickup to get a better look at it or any other minor destructive testing could lower the price by $1,000 or more. "We didn't want to destroy anything, so we came up with a noninvasive method that treats the guitar like a black box," says Eric Kirkland, Line 6's chief instrument designer. "We used it to generate characteristic values, then reverse calculated the data we needed."

Line 6 wanted the Variax to look and play like a normal guitar, but they had to make some compromises. The pickups, for example, are piezoelectric rather than magnetic. Each generates an ac time-variant signal that represents the changing tension in the string being struck. They use piezo pickups for their wide dynamic range and bandwidth to ensure they get all the information they can from each string.

"The analog signal from a pickup contains all the data needed to paint a full picture of the motion of a string, including pitch, vibrato, decay, and so on," says Celi. "The trick is figuring out what all those elements are once we convert the signal to digital."

Each string signal goes through a bipolar buffer amp before going through an AKM 4528 coder/decoder (codec) and being converted to digital. The 24-bit codecs sample at about 40 kHz on six strings, but because they are oversampling converters, the actual sampling rate is more like 10 MHz. Three codecs, each handling a pair of string signals, produce six digital signals. And each signal is handled separately by an off-the-shelf Motorola 56367 running at 150 MHz, giving the Variax hexaphonic processing. Handling each signal separately lets Variax precisely manipulate variables to replicate specific guitars. For example, detecting the pitch of the string, doing a subtle pitch shift, then remixing the shifted and original signal lets the sixstring Variax sound like a 12-string Martin guitar.

The 56367, the only processing element, has an operating system written in C and algorithms written in assembly language, according to Dave Fruehling, system architect. He says Line 6 plans to develop a box that will let Variax users manipulate algorithms and virtual components to generate entirely new guitar sounds.

After processing, the six signals travel back through the codec, are converted to analog, and mixed and merged to produce a monophonic output with a higher signal-to-noise ratio than each separate signal. This signal, like the output from any traditional electric guitar, can be sent to a standard amplifier. Variax can also output the six processed digital signals through a proprietary interface. And the guitar accepts midi data (a common format for exchanging musical data) to reprogram the Variax or sequence through a specific set of stored guitars.

To choose which guitar the users want to emulate, they simply twist a knob on the guitar. Once selected, the other knobs on the guitar replicate the functions of those on the original. On many guitars, or example, one knob selects which set or combination of pickups are used, the one near the bridge, those in the middle, or ones near the neck, all of which affects the tone and response.

The biggest challenge in this process, according to Kirkland, was deciding what guitar parameters they could easily capture that would let them mimic vintage guitars. The other half of the challenge was packaging all the needed signal processing and power (six AA batteries for 12 hr of playing time) into a guitar. The finished instrument, which costs about $1,400, lets musicians enjoy playing what sounds like high-priced vintage guitars, and gives them a broad palette of guitars to switch between even when playing live. It also gives those who own classic guitars a less-cumbersome and risky alternative to lugging all of them to every performance.

The V-Pro Series drum kit from Roland Corp. features (left to right) a sound module or controller, hi-hat cymbal with dual foot triggers, a crash cymbal, a PD-125 drum pad, two PDF-105 drum pads, a bass-drum type pad, a cymbal ride, and another crash cymbal and PD-125 drum pad.

Controlling the V drums is done from the TD-20 sound module. It lets the drummer tailor the tone and volume on each of his instruments.

The Series II bass from Alembic features a body carved from walnut burl over a maple core. Inlays on the neck are based on alchemy symbols.

A woodworker at Alembic carefully shapes the neck and head (or headstock) of a bass guitar. This particular guitar has a through-neck design, meaning the neck runs the entire length of the instrument.

Space for the bridge that will hold the strings is carefully routed in the body of an Alembic bass. The layers of different woods that make up the laminated body can be seen in the cross section.

The V-Synth, a $2,400 synthesizer from the Roland Corp., Los Angeles, might have a 61-note, black and white keyboard and be able to sound like a piano, but the similarity ends there. Inside, its all-digital processing and effects give keyboard players almost too many options for creating music.

At the heart of the V-Synth is a library of 324 waveforms, short musical samples that can be used as the basis for all keyboard sounds. There are also nine waveforms modeled after traditional analog keyboards, and players can use and modify signals arriving through analog inputs.

There are also dozens of methods for manipulating these waveforms:

  • Ring and frequency modulation are common methods for manipulating sounds. Ring modulators generate a signal containing the sum and difference of two input-signal frequencies, which usually creates dissonant, nonharmonic sounds. Frequency modulation changes the frequency of one signal based on the changing value of another signal.
  • Variphrase technology decouples a sample's pitch, tempo, and formant. (Formant is the harmonic content of a sound that determines its character.) "In the past, these three aspects of a sample were locked together," says Tara Callahan, a Roland spokesperson. "The only way to play a sample with a higher pitch was to increase the sample's tempo. And you couldn't change a sample's tempo without affecting the pitch. Variphrase lets users independently and simultaneously control all three in real time."
  • Composite object sound modeling (COSM) provides 16 user-programmable digital processors that replace the simple low, band, and high-pass filters in traditional synthesizers. They include wave shapers, comb and sideband filters, and digital models of analog filters from older, best-selling Roland keyboards.
  • The Arpeggiator generates rhythmic passages based on just a few notes played on the keyboard.
  • A bank of 10 reverb levels and 41 multieffect filters lets users shape sounds. Players can recreate the sound of small and large rooms, even garages, or add delay and nonlinear reverb. Effects include "radio tuning," "Hexa chorus," and "Auto wah." In all, the keyboard can produce 24 voices, but that varies with the complexity of the waveforms and filtering used.

Roland engineers have also taken great pains to make the user interface as simple and intuitive as possible. The keys, for example, are sensitive to how fast and hard they are being played and have been programmed to have aftertouch, a standard term for the technique of putting slight pressure on a key after the initial key strike. Most waveform and sample variables are controlled by a knob or slider, but for those without dedicated knobs, two extra dials can be assigned to control whatever the user chooses. A touchscreen front and center on the 28-lb instrument lets users manipulate any aspect of a waveform or sample in real time.

For even more "hands-on" control, a pair of D Beams, which are actually two beams of infrared light, let players manipulate pitch and amplitude, and other pairs of waveform parameters, by moving their hand(s) through the IR light beams. A blue LED on the keyboard lights up to tell players when their hands are in range of the D beams. And the Time Tip Pad gives players an even faster method of shaping waves and sample. It is basically an X-Y touchscreen and where the player places his finger determines the sound. It can be set up to do Time Trip, playing samples faster and slower, forward or backward, depending on if the player's finger is moving fast or slow, clockwise or counterclockwise. It can even "freeze" a sound, sustaining a small portion of the waveform indefinitely.

There are also features for recording tracks, looping them, and playing them back. The V-Synth has a USB port for a midi connection and to import and export WAV/AIFF files (waveforms), a pair of stereo line outputs, a microphone/ line input, and a digital S/PDIF I/O. And for multimedia, Roland's V-link lets the keyboard trigger video clips and control playback speed.

As you might expect, there are also digital drums that give players literally thousands of different percussion sounds to choose from. The trick for drum manufacturers like Roland Corp. is making them easy to use and as much like real drums to play as possible. The V-Pro Series from Roland is said to be one of the best in both categories.

The V-Pro drum kit has a library of 560 drum and percussion samples, and 262 more backing instrument sounds for use in the sequencer. Some of the samples consist of several partials, which let the sound change depending on where and how hard a drum or cymbal pad is hit. And although drummers cannot sample and store sounds on the current V-Drum, future versions will probably allow it. Drummers can also design their own "virtual" drums, choosing between wood, brass, and steel shells, changing shell depth and diameter, adding different types of heads and playing with different drumsticks. They can also add various mufflers and cymbal effects, devices such as weighted rings or rivets and chains that sit on real drumheads and cymbals to alter the sound. Roland engineers even let drummers add and adjust snare-drum "buzz," resonances created on a snare drum when a nearby bass drum or tom tom is struck.

A six-part sequencer lets drummers record and playback six tracks at once, with 100 prerecorded sequences and memory space for another 100. Maximum length is 20,000 notes, with length dependent on the tempo chosen. Drummers also have a choice of sounding as if they are playing in different types of rooms, such as theaters, bathrooms, caves, and stadiums, for example. They can even choose the size and shape of the room, as well as what the walls are made of.

For realism from the drummer's perspective, the drum pads use a two-ply nylon mesh head with adjustable tension, just like traditional drums. But the nylon is very quiet when stuck, an important feature in electric drums. A triggering basket sits under the mesh head, providing even and accurate triggering wherever the head is hit. The newer pads also have dual triggers, letting players generate different sound when the pad is struck on the rim.

Cymbals, which are hard plastic with a rubber coating, give the feel of traditional metal cymbals. A piezo sensor near the bell or center of the cymbal turns vibrations into electric signals which trigger the cymbal sound. A flat ribbon FSR (force-sensing resistor) on the edge also senses pressure, giving the cymbals dual triggers and letting drummers "choke" it, i.e., putting a hand on it to abruptly bring the sound and ringing to a stop. The hihat cymbal pad replicates a pair of cymbals on a stand. It makes a range of sounds, depending on where it is hit, and whether the foot pedal has the virtual hi-hat open (apart) or closed (touching).

The $6,500 drum kit, which emulates 50 different drum kits, can generate 64 voices or sounds simultaneously. For recording, users have 10 analog outputs, so individual instruments can be placed in separate channels of mixing boards or multitrack recorders. This lets drummers mix the sound for live performances or recording.

At Alembic, Santa Rosa, Calif., digital hasn't caught on, and maybe never will. Instead, they're dedicated to building bass guitars, usually to order, that produce clean, pure bass notes and are suited to the player's style. They present customers with a laundry list of options and let them design their own instruments. For example, buyers can choose a bass with a through-body neck or a set neck joint. In the former, the neck runs the length of the guitar, isolating the neck and strings from the body, preventing energy the bassist puts into the strings from draining into the rest of the instrument. "This emphasizes sustain and brightness," says Mica Wickersham, General Manager at Alembic.

In a set neck joint, the neck is glued to the body, making the neck less isolated. This lets players choose materials for the body that will influence the sound. Maple bodies, for example, generate brighter sounds than mahogany bodies, which aren't as dense. The design also gives a significant midrange peak that musicians describe as "punchy."

Bassists can also choose the neck wood as well as length. Dense woods produce the broadest range of frequencies.

But less-dense woods, such as mahogany, downplay or filter out higher frequencies. And while most woods let fundamental harmonics on sustainednotes die out, leaving second harmonics to ring, ebony lets the fundamental and second harmonics decay at the same rate. "This makes for an exceptional and unexpected sound on sustained notes," says Mica.

"As to length, the longer a string is, the more tension it has for a given diameter," notes Mica. "For larger strings, like the lowest bass string, some players notice a 'floppy' feel with standard

length necks, those 34 in. or shorter. So we let them choose from 35 and 36-in. necks. It means players have to reach farther to play the same notes, but they like the mechanical feedback from longer necks."

Alembic engineers rely on two sets of single-coil pickups feeding signals directly into a preamp. Output is two analog signals, one from each pickup. Pickups are mounted on the body with two supporting and two clamping screws. "You will get an output from moving the strings or moving the pickup," says Ron Wickersham, Alembic founder and chief engineer. Using four screws gives more accurate results than if they were mounted on springs or foams as they are in other basses."

Coil windings are immobilized with cyanoacrylate so vibrations in the guitar won't move the turns of wire and induce unwanted output. "The coils are also fully shielded against electrostatic fields with copper laminate on the top and bottom, and expanded copper metal on the sides," says Ron.

The coils use flat, ceramic magnets that are close to air coils in terms of low inductance. More conventional ferrous pole coils have much higher inductance, giving them self-resonant frequencies around 1.2 kHz. Alembic's coils also use windings that are spaced to cover or detect movement from two points on a string. Pole coils only sense movements at a single point. "We call the distance between sample points the aperture length and offer wide or standard, which is about an inch, and narrow apertures," says Ron.

On a molecular level, the metal in pole coils doesn't all switch or activate at the same field strength. "So as a bass note fades, the number of domains being switched decreases, and eventually individual domain switches can be heard, making the sound grainy, somewhat like the sound of driving over a gravel road," he says. Alembic's coils don't have this problem, called Barkhausen distortion.

With coils designed to detect the least magnetic nuances, they are also well built for picking up magnetic interference. So Alembic installs a third pickup wound in the same direction as the active ones but with an inert core rather than a magnet. "This dummy 'hum' canceller has its own preamp, just like the active ones. If there's interference, the dummy pickup detects it, and we subtract it from active pickup's preamp to get a clearer signal."

To protect the electronics inside the guitar from electrostatic fields, Alembic coats the cavity with silver paint, which lasts 30 years or more. Signals from the pickups travel via a shielded cable to the electronics cavity. This means the guitar strings don't have to be grounded and the bassist is somewhat protected against electrical shock because his hands are not grounded by the strings.

Basses can cost $2,600 to $13,000 and up. And while the less-expensive models have brass hardware machined in-house at Alembic, the top end has similar brass hardware, but it's been sent to a gold plater. "It's purely cosmetic and doesn't make the bass sound any better," says Ron. "But it sure looks nice."

Which guitar did you bring?

The Variax 700 guitar digitally mimics several vintage rock-and-roll guitars, as well as a dobro, sitar, banjo, and "tricone:"

  • 1960 Fender Telecaster Custom
  • 1968 Fender Telecaster
  • 1968 Fender Telecaster Thinline
  • 1959 Fender Stratocaster
  • 1958 Gibson Les Paul Standard
  • 1952 Gibson Les Paul "Goldtop"
  • 1961 Gibson Les Paul Custom (3 pick ups)
  • 1956 Gibson Les Paul Junior
  • 1976 Gibson Firebird V
  • 1955 Gibson Les Paul Special
  • 1959 Gretsch 6120
  • 1956 Gretsch Silver Jet
  • 1968 Rickenbacker 360
  • 1966 Rickenbacker 360-12
  • 1961 Gibson ES-335
  • 1967 Epiphone Casino
  • 1957 Gibson ES-175
  • 1953 Gibson Super 400
  • 1959 Martin D-28
  • 1970 Martin D 12-28
  • 1967 Martin O-18
  • 1966 Guild F212
  • 1995 Gibson J-200
  • 1935 Dobro Alumilite
  • Danelectro 3021
  • Coral/Dano Electric Sitar
  • Gibson Mastertone Banjo
  • 1928 National Style 2 "Tricone"

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