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DSP performs digital acoustic imaging

Posted: 25 Jan 2008     Print Version  Bookmark and Share

Keywords:acoustic guitar  DSP 

Guitarists usually associate DSP with electric, rather than acoustic, guitars, especially after early attempts to apply DSPs to acoustic instruments led to modeling effects that kept the guitars from performing as advertised.

Now, Fishman Transducers Inc. says it has repurposed the Analog Devices Inc. Blackfin DSP to perform "digital acoustic imaging" instead of modeling, making acoustic guitars with an inexpensive piezoelectric pickup sound as if they were in a pristine studio in front of an expensive condenser microphone.

"When we first considered DSP for guitars, we were just thinking modeling—making a Gibson sound like a Fender," said Larry Fishman, president of Fishman Transducers. "We tried that, but found that it's much too complicated a problem, with all the subtle complexities. Now what we do instead is make an acoustic guitar sound as good in your home recordings, or live onstage, as it does in a professional studio."

The idea, he said, "is to capture that great studio sound and bring it out to real world of performances."

Reinventing music
Fishman's digital acoustic imaging algorithm works by comparing the sound of a guitar under perfect conditions—in an ultraquiet studio with a variety of expensive condenser microphones placed at various distances in front of it—with the signal you get from a piezoelectric transducer or pickup, which Fishman places under the bridge saddle. The transducer senses the originating excitation of the strings, but is not sensitive to the sound hole resonances. And it doesn't hear the mix of phases in front of the instrument as various frequencies radiate differently off the top, sides and head of the guitar.

Fishman says it can capture and re-create all these subtle frequency and phase differences by running its algorithm on a Blackfin DSP from ADI, contouring the audio from the raw transducer so that it sounds as if were in the studio.

"First, we take a guitar into the studio and do a two-channel recording—one channel records from what the transducer hears and the other channel records what the microphone hears," said Fishman. "Then we run the algorithm we developed to make a very close comparison of the two signals in the frequency domain, subtract one from the other to get their difference, then convolve the two to get what we call a sound image."

Different flavours
From the sound image, Fishman Transducers creates a custom filter with more than 2,000 frequency taps for the Blackfin DSP. "We not only adjust the amplitude of each frequency, but also make critical-phase adjustments, which is where the magic comes in," Fishman said. "Without that phase information, we would just have a 2,000-band graphic equaliser. But by adjusting the phase information too, we get three-dimensionality in the sound."

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 Rs.9,869.60 ($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 Rs.98.70 lakh ($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.

Prime concern
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.

- R. Colin Johnson
EE Times

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