Electronic Design

DSP Pumps Up The Performance In High-End Stereo Speakers

Even in this age of tiny earbud music players, audiophiles want high-end stereo systems that provide perfect acoustics and fidelity— and technology is making these systems less expensive than ever. In fact, I recently received a letter from a reader asking about some of these developments:

“I heard that Emerald Physics has a set of $3500 speakers that outperforms more expensive, $20k to $50k systems using a DSP. Is this possible?” asked S. Brown.

To get the answer, I talked with Clayton Shaw, president of Emerald Physics. His company’s $3500 CS2 system includes a set of open-baffle speakers combined with a Behringer DCX2496 Ultradrive Pro that sits between your preamp and a set of four amplifiers that drive the speakers (see the figure).

The DCX2496’s 32-bit Sharc DSP from Analog Devices implements an eighth-order digital filter network that Emerald Physics uses for active crossover support in addition to adjusting frequency, phase and time linearization, and lowfrequency (LF) equalization to deliver optimum audio performance from 20 Hz to 22 kHz.

The DSP performs these tasks to make up for the differences in performance between all the components in the system. Each speaker has a pair of 4-O, 380-mm, low- to mid-frequency woofer drivers operating below 1 kHz to handle vocals and large instruments and an 8-O, 25-mm exit compression driver. The speakers are placed 3 to 7 ft from the wall because reflectivity plays an important part in audio quality.

The DCX2496’s algorithms account for the position information, the amplifier characteristics, and the speaker characteristics. This is more difficult than it sounds because different amplifier pairs may be used with the system, and each type of speaker has different response characteristics.

The non-linear characteristics and distortion that can be added would result in a lower-quality playback. Signals routed to the low-frequency and high-frequency speakers normally go through a crossover system. The crossover point is where the differences of the speakers and amplifiers have the most effect.

The active crossover is set at 1 kHz. The system employs a Linkwitz-Riley filter also known as a Butterworth squared filter. Presets allow easy configuration by even non-technical audiophiles, though techies are free to tweak parameters.

The audio stream passing through the DCX2496’s 24-bit analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) is then modified so resulting wavefronts are in synch for the listener. This controlled directivity is harder to accomplish than you might think because of the nonlinearity of the components across the spectrum, including changes in dispersion patterns. For example, low-range output tends to be more omnidirectional while compression drivers and waveguides handling the upper end are more directional.

The precise reproduction of the original sound is why audiophiles will pay so much for speakers and amplifiers that can deliver this level of quality. It allows us to hear subtle nuances that would otherwise become background noise.

When it comes to manipulating the audio stream in the time domain, it’s easier to use the DSP than a passive crossover system. The DSP also allows a range of techniques to be applied to the stream so adjustments are made in a number of areas, delivering controlled directivity.

The CS2’s patent-pending controlled directivity applies to a wider range of speaker architectures. Its big advantage is its ability to provide consistent and realistic output regardless of room configuration. It also can deliver a binaural effect if the speakers are angled toward the center, though this can only be done with a very accurate tuned waveguide system.

In the future, a move to all-digital operation will eliminate the need for the DCX2496’s ADCs and DACs. However, systems will still require Emerald Physics’ compensation support because of the remaining components within the system. In any case, analog rules the high end where amplifiers use vacuum tubes because of distortion and stability issues.

ANALOG DEVICESwww.analog.com

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