Digitally Controlled Analog Provides a New Methodology for Embedded Systems in Subwoofer Applications

Motion picture surround sound, home theatre systems and automotive audio have created a new market for sophisticated, active subwoofers. The challenge for these systems is designing them to deliver sound that is appropriate for the space or spaces in which they will be used.

For aesthetic and practical reasons, the bass driver speakers used in home and automotive environments are often housed in cabinets that are smaller than they should be, which leads to cabinet resonance and a need for custom compensation circuits. More recently, subwoofer designers have created active autotuning features inside the subwoofer that use sensor inputs and signal analysis to compensate for room acoustics. This also allows the user to optimize the compression characteristics for different types of music, as well as the various accompanying speaker systems.

As an alternative to discrete component and DSP solutions, these field programmable analog arrays (FPAAs) give audio designers the flexibility they need to develop subwoofer designs that deliver pleasing results whatever the contours of the environment in which they are used.

Where a classic discrete design is used, the two most commonly seen parts of any active subwoofer system are the low-pass crossover filter that attunes the subwoofer to other speakers in the system, balancing their respective frequency responses, and the speaker amplifier itself. Additional signal conditioning is frequently required, however. Subsonic filters are often used to remove near-dc signals, which can damage the driver coils. But one side effect of these filters is the attenuation of low-frequency tones (15 to 50 Hz), which are often highly desirable to give that special "punch" to the audio experience. Conversely, we see the accentuation of resonant frequencies of the speaker housing or environment, which need suppressing.

To address both of these issues, a popular solution is the Linkwitz Transform crossover, which elegantly provides low-frequency "boost" and resonant frequency "notch." Audio signal level compression is another desired feature, either to suppress unwanted large transients or to allow music with large dynamic range to be heard in smaller, enclosed environments, such as cars.

Analog array conditioners

The problem with discrete implementations of these functions is that they are difficult to design and fairly inflexible once in place. For this reason, the DSP approach has been tried, but the resulting sound quality varies widely and the cost of the processor can be prohibitive for all but the most high-end installations. A third approach, which shares the DSP advantage of programmability while maintaining an analog signal path from front to back, is the field programmable analog array implementation.

In FPAA implementations of subwoofer designs, the functions described here are programmed using software representations of analog functions, meaning that these can now be modified or tuned digitally. The designer builds a design from a library of circuit building blocks called Configurable Analog Modules (CAMs), which are pre-built and pretested analog functions that can be selected and interconnected within the development environment, using a drag-and-drop function to create the entire circuit.

Field programmable analog arrays can be used to implement flexible subwoofer conditioner designs such as those comprising a subsonic filter, audio compressor, Linkwitz Transform equalizer and crossover filter. Source: Anadigm

Each of the CAMs has function-specific parameters from which the design tools determine the underlying circuit structures and settings, and map these onto the configurable analog blocks inside the FPAA. The development tools then deliver configuration information that must be loaded into the FPAA at power-up, either directly from a PROM (for those designs that are configured at time of manufacture) or — more significantly — from a microcontroller that accompanies the FPAA. The latter case allows features of the design to change in the field under software control.

Dynamic and automatic tuning

To allow in situ tuning, the microcontroller that accompanies the FPAA is supplied with simple software instructions to modify any specific aspect of the design. These instructions are custom-generated according to the form of the data or C-code application programming interface that has been identified by the FPAA development tools in which the circuit is designed.

From the user's point of view, this means that an active subwoofer system can be adjusted in the field using some familiar mechanism — such as an infrared remote control — rather than requiring an audio technician to adjust a filter notch in the back of a speaker cabinet.

Taking this concept just one step further is the ability of these systems to self-tune. For example, the peak response of speakers within the low-frequency spectrum is always a function of the environment in which they are placed. With an FPAA implementation, the system microcontroller can instruct the FPAA to stimulate the speaker with a low-frequency signal and monitor ambient sound level from a microphone. As the microcontroller programs the FPAA to deliver a sweep of oscillator frequencies, it can capture the return characteristic to determine which frequencies need suppression.

Having performed this calibration sequence, the FPAA is assigned its primary role as a subwoofer conditioner circuit, and the newly determined frequency compensation characteristic can be applied to the Linkwitz Transform circuit.

Ian Macbeth is chief technical officer and Nathan John is vice president of marketing at Anadigm Inc. (Cheshire, England).