Acoustic guitar subtleties re-created by DSP

As a result, he said, "when we make a filter from our acoustic image and download it into the Blackfin, then drive that image with the raw guitar signal from the transducer, it is transformed into what the microphone would be hearing in a professional studio."

In practice, Fishman records each instrument with a half-dozen well-known studio-recording microphones, then offers those sounds as selections to the guitar user. Musicians who add Fishman's Aura preamp to their own guitar, as opposed to buying a guitar with a built-in Fishman preamp, can download as many as 16 acoustic images from a Fishman Web site. The site already offers more than 1,000 acoustic images for nearly every acoustic guitar in existence. "We have a continuous flow of instruments coming into our shop from around the world, and we update our list with new acoustic images weekly," said Fishman.

Users can select acoustic images by guitar body type, wood, microphone type and distance between microphone and instrument. "You don't need an exact match always, and there are some surprising combinations that just happen to sound good," Fishman said.

For serious guitarists, the company will create a custom acoustic image for $250. Musicians ship their guitars to Fishman's studio to get a professionally calibrated acoustic image they can load into their Aura preamp. "This really lets your guitar shine, especially onstage or in home recording situations, where you don't have a pristinely quiet recording environment or a closet with $250,000 worth of fancy microphones," Fishman said.

A major portion of Fishman Transducers' business is from guitar manufacturers that include a built-in Fishman pickup and preamp inside their acoustic guitars. Guitar makers send finished instruments to Fishman, which calibrates an acoustic image for that particular type of guitar and supplies a custom pickup and preamp to re-create the plush studio sound at the push of a button.

To meet the long-battery-lifetime requirements of in-guitar preamps, Fishman says the company evaluated the available DSPs with energy efficiency as the No. 1 concern. "We ran a competitive evaluation of all the other DSPs, and Blackfin was the clear choice for us, because its greater power efficiency gives us a much longer battery life than we could get with any other commercial DSP processor," said Fishman.

To get that longer battery life, Fishman runs the Blackfin processor at an energy-saving 169 MHz with a supply voltage of just 0.8 V.

"Equally important is that we don't need a separate microcontroller for the user interface," said Fishman. "Previous to Blackfin, we had to use a separate microcontroller to read the knobs and handle the user interface, but it was power-hungry and needed a different supply voltage from the DSP."

At the recent National Association of Music Merchants trade show, Fishman showed a new line of acoustic-effects pedals--reverb, delay and chorus--that use the same printed-circuit board as Aura, but program the Blackfin DSP to perform these more-traditional guitar effects.