The new architecture eschews the use of digital signal processors to emulate tubes. "We considered modeling the sound of tube amps with a DSP; theoretically it could be done, given sufficient processing power and vacuum tube know-how," said Dale Curtis, vice president of research and development at Fender. But ultimately Fender believed that "modeling tubes using today's DSP technology could not be done well enough to meet our standards."

"Modeling amps" attempt to skirt the tricky aspects of analog design by using DSPs to emulate an entire amplifier. But they use only a few operating points per model for their DSP algorithms or may slow the DSP's sample-processing frequency to 30 kHz or less from the industry-standard 44.1 kHz.

At Fender, both of those options were "unacceptable solutions," Curtis said. "We wanted our solution to sound as good as or better than our vintage tube amps; otherwise the project wasn't worth doing. So instead of modeling the sound of different tube circuits, we chose to reconfigure the circuitry around real tubes. Then all the DSP had to do was implement our virtual tone stacks and run the effects."

The guitar signal enters the CyberTwin from the input jack to a very high-impedance variable-gain amplifier with a "trim" control potentiometer to match the level to an analog-to-digital converter. That signal is then sent to the DSP and, depending on the type of amp selected by the guitarist, is preprocessed before it is converted back into the analog signal that drives the tube preamp. The preamp uses two classic 12AX7s tubes.

The amount of processing the DSP does before the tube stage depends on the amplifier selected by the guitarist. For instance, if a vintage 1959 Fender Bassman amp is selected, then all the tone control processing by the DSP is reconfigured to occur before the tube stage, since in the Bassman, even reverb circuits go before the tubes. However, if a Fender Hot Rod Deluxe amplifier is selected, then the CyberTwin reconfigures itself to put the tone controls and reverb stages after the solid-state- and tube-preamp stage.

Relays and FETs

In all, 16 reconfigurable combinations are served by a combination of double-pole single-throw (DPST) relays, for routing signals, and analog switches (FETs), for switching cathode resistor bypass capacitors. The CyberTwin also uses optoisolators to keep control signals that perform signal path switching.

The CyberTwin DSP uses a library containing hundreds of reconfigurable scenarios stored in a read-only memory. The appropriate modules are then loaded and "patched" into the signal path along with the appropriate tube configuration, allowing the re-creation of nearly any guitar amplifier ever made.

The DSP processing, combined with the ability to rewire the analog tube stages on the fly, allowed Fender to re-create rather than emulate the architecture of any existing amp. Users can also build their own configurations, essentially creating completely new amplifier designs without ever touching a soldering iron.

"Since we wanted real tubes, plus the ability to reconfigure them into all the possible circuits you can make with tubes, we actually needed two A/D converters — one for everything that might be assigned ahead of the tubes and the other for everything after the tube stage," said Chuck Adams, director of external project management in Fender's research and development group.

Curtis described CyberTwin's virtual tone interpolation (VTI) circuitry. Instead of using dedicated analog switches to reconfigure the tone circuitry, as was done with the tube circuitry, the tone control circuits are implemented with a Motorola 24-bit DSP56362 running at 120 MHz. Tube preamps only have six possible manners in which they can be rewired, making analog switches an economical method of reconfiguring real tube circuitry. But tone control circuits have much more variation, making it uneconomical to reconfigure them directly.

Tone controls

"The tone controls on a guitar amplifier are very different from the tone controls of a stereo system. In fact, the tone control circuits in a guitar amp — what we call its tone stack — are one of the most important elements determining the overall sound of an amplifier," said Curtis.

The tone stack has a large effect on signal equalization, even when the tone controls are set in their middle positions.

"In a guitar amp you have bass, middle and treble knobs, and the equalization curves for them are never flat," said Curtis. " Plus, over the years, several different variations of the tone stack's topology have been used. Each has its own characteristic sound and very different circuitry, making it a very complicated engineering task to duplicate their sound with a DSP."

Further, Curtis said, "the tone controls interacted severely, but in a good way, so we wanted to make sure we accurately represented them." Curtis came up with an interpolation technique that measured the frequency response of hundreds of tone control settings, on 34 different real guitar amps, and stored the resulting DSP coefficients in a lookup table. Then all the DSP had to do was interpolate between lookup-table values when the user applied a tone-control setting that was between those that had been calculated ahead of time.

The resultant DSP filters could produce, given the correct filter coefficients, all of the equalization curves needed to reproduce the various tone stacks. The problem then became delivering the coefficients to the DSP in real-time.

"I came up with the idea of using interpolation in three dimensions, for the bass, middle and treble tone controls. The values for each coefficient could be stored in a 3-D lookup table at various operating points; then 3-D interpolation could be used to fill in the gaps between the stored settings," said Curtis.

The DSP algorithm thus became a pure mathematical-problem solver rather than an equalization curve generator. An 80C251 microprocessor interpolates between coefficients stored in the ROM before delivering them to the DSP.

Fender DSP engineering expert Keith Chapman designed a mathematically efficient algorithm to do the interpolation in real-time.