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Digital technology helps guitarists achieve pure analogue clarity

Producing a pure, organic and natural sound from an electric guitar has long been considered the Holy Grail for musicians. Traditionally, guitarists try to accomplish this by adjusting tone levels on the instrument and amplifier manually, or by applying EQ and compression via effects pedals.

However, problems such as frequency response and noise interference often make this a fruitless task. Now, guitar manufacturer Gibson claims to have overcome the problem by using digital technology to achieve analogue sound.

Unlike digital mathematical formulas – used in devices such as drum machines – analogue equipment creates richer textures and deeper nuances. With this in mind, Gibson developed a high performance multiprocessor called the Pure-Analog engine for its Dark Fire guitar. The result, says the manufacturer, is a guitar that provides an 'infinite' number of tonal variations, while achieving almost perfect sound.

Particular attention was paid to the Dark Fire's pickups. Pickups act as transducers that capture mechanical vibrations and convert them to an electrical signal which can then be boosted through an amplifier. Primarily mounted on the body of electric guitars, magnetic pickups consist of a permanent magnet wrapped with fine enamelled copper wire.

To produce a more natural sound, Gibson capped two of its electromagnetic pickups – the P90h and Burstbucker 3 – with new carbon fibre. These were combined with a piezo bridge pickup, consisting of six individual pickups – one for each string. Piezo pickups are more commonly used for acoustic or semi acoustic guitars and have the advantage of not picking up any other magnetic fields. By wiring it to the two electromagnetic pickups, the piezo pickup allowed each individual coil to be used in a switching matrix, enabling more than 20 combinations of tone.

But Gibson didn't stop there. For an even purer sound, a rotary potentiometer was installed as part of the pickup selector toggle switch. The potentiometer is a three terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. By adjusting the level of analogue signals and controlling the inputs for electronic circuits, it blends the guitar's piezo acoustic sound with minimal noise and signal loss, providing a significant change in tone.

And by standardising the sound processor engine form factor, the Pure-Analog engine can be used in different types of guitar, rapidly bringing down the cost.

The system has been designed to a new Gibson standard, allowing other companies to develop hardware and applications that can then be integrated. The guitar manufacturer is also setting up an online store where vendors can sell applications, once Gibson has verified that they are compliant. Product enhancements can be installed in a few minutes via a firmware update option in the accompanying Tone Editor software.

With such scalability, Gibson plans to roll out 'hundreds and thousands' of guitars in different price points. Naturally, Gibson has great confidence in the technology and describes the Dark Fireit as the 'most versatile tone monster ever made'. But it took a while to reach this point.

The road to success
When the Gibson Guitar Corporation introduced its first electric guitars in the 1950s, the company had already been established for nearly half a century. It was founded by Orville Gibson, a Michigan based mandolin maker, who invented archtop guitars by replicating the carved, arched tops found on violins. A legendary collaboration with electronics inventor and guitarist, Lester William Polsfuss – better known as Les Paul - came about in a bid to compete with rival, Fender, which had recently developed the highly successful Stratocaster and Telecaster.

A decade later, looking to create more radical body shapes, the company approached automotive designer Ray Dietrich, who styled the infamous Firebird, based on the shape of 1950s car tailfins. The Firebird went on sale in 1963 and, like the Les Paul, was used by top musical luminaries such as Brian Jones, Dave Grohl and Johnny Winter.

Nearly five decades on, Gibson has announced a major enhancement to the Firebird. Once again, at the heart of the limited edition Firebird X is the Pure-Analog sound processor, which enables it to produce a professional analogue sound as well as a range of effects.

While most manufacturers prefer to use digital mathematics to 'correct' pitch and tunings, Gibson has taken the decision to make these functions analogue based – the analogue signal is fed into a preamp, delivering a maximum dynamic range.

The whole process takes place within the guitar, so problems with noise from external sources or internally generated distortion have been addressed by using shielded cables and metal enclosures.

A/D converters transform the audio stream into a digital form on which the guitar's sound processor can act with virtually no latency. The technology maintains the signal stream with high internal precision so every operation has a high bit depth with minimal truncation errors. This preserves the analogue quality of the original signal and, using a d/a converter, the signal starts and leaves as pure analogue.

The Firebird X uses three mini humbuckers – a pickup device often used in jazz guitars – and a hexaphonic piezo bridge pickup with a low noise, high dynamic range preamplifier. By improving the signal, Gibson's designers have managed to get the dynamic analogue dynamic range to exceed 110dB.

Pedal power
In most signal paths, audio from a guitar goes to a pedal and is then daisy chained to the amplifier. However, to provide more control for the musician, Gibson wanted the processing to occur in the guitar. To incorporate Bluetooth wireless technology in the instrument, it turned to CSR, a specialist in designing and manufacturing single chip radio devices.

Jeremy Stark, CSR's eXtension Partner Program Manager, said: "A wired setup would mean connecting the guitar directly to the amplifier and also having some other wired connection to the pedals for control of the effects. This would probably be a step back from the end user perspective rather than an evolution because there would be two wires running from the guitar.

"CSR's technology has enabled the control of the audio processing in the guitar without the need to be tethered by a wire and a complicated setup." The other upside to this configuration is that the Firebird X guitar system now has visibility of the entire audio chain, all the way to the amplifier.

The Bluetooth radio technology is based on modified CSR software and provides a simple way to integrate Bluetooth functionality into the Firebird X system, which includes a host processor. Stark observed: "The CSR software provides a high level API to the host processor which hides much of the complexity of establishing and maintaining a Bluetooth connection from the host." This approach means that in depth knowledge of Bluetooth is not required by those using the API on the host.

CSR advised Gibson on wireless functionality and connection management, devising a solution that made the required input purely intuitive. "For example," explained Stark, "after the first connection is made, from that point on the connection is established automatically on power up, so no button pressing is required before the guitarist can use the pedal with the guitar."

Responsive connection
To get a responsive connection between a guitar and pedal, a number of issues have to be taken into consideration, such as network topology and power consumption. CSR engineered the system to support multiple Bluetooth connections to the guitar from peripherals such as the pedals.

"We need to avoid the scenario where the guitar is the master of a link to one pedal but a slave to another pedal – a scatternet," Stark said. "In such a scenario, the guitar can experience packet scheduling conflicts between the two networks, which leads to packet delay or loss and results in a poor experience for the guitarist.

"If the guitar is the master of both links and the pedals are the slaves, then the guitar has full control over how the bandwidth is used, as well as visibility of the traffic on both the links and hence the ability to schedule it successfully."

A trade off also has to be made between data transfer, latency and power consumption. "A radio link can be configured to use very little power," Stark noted, "but when this is done, so the guitar will communicate less frequently with devices to which it is connected.
This means there is inherent latency in the data transfer. In this application, too much latency will result in a poor user experience because it would be very difficult and frustrating for the guitarist to have to account for this latency when playing."

For the Firebird X guitar system, CSR wound down the time between data transmit opportunities to such an extent that the guitarist can get an immediate response to the press of the pedal controls, just as they would when using a wired connection.

Finally, all data transferred across a Bluetooth link is packetised. "To minimise latency in our Bluetooth stack and across the air, it is important, where possible, to match the data packet size to the packet size used over the air (thus avoiding fragmentation of data between packets) so that all the data that needs to be processed arrives at the same time," Stark concluded. "This negates the need to wait for more radio packets before processing of the data can commence."

In the 19th Century, Orville Gibson created an entirely new style of guitar from his workshop. His creations were deemed so innovative that he was granted a patent on his design. It seems that his company's spirit of innovation continues to this day.

Chris Shaw

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