Improving high-end active speaker performance using digital active crossover filters

-May 21, 2013

Consumer requirement for fewer wires connecting their home entertainment systems is driving up the demand for wireless active speakers. In order to achieve the best audio quality from high end active speakers, adoption of alternative technologies can improve performance; in this context, digital active crossovers can be shown to make a significant contribution.

Current wireless active speakers consist of four elements in the signal path before the drive unit; receiver, DAC, amplifier and crossover. The receiver may be Bluetooth running a high performance codec.  The amplifier could be a conventional analogue input class AB type to ensure a high audio quality with high a performance DAC at its input. The final element in the signal path is a passive crossover network.

Alternatively, utilising high performance Class D amplifiers, efficiency savings can make direct driving woofer and tweeter a reality. If the Class D amplifier features a digital input, the availability of DSP resources can facilitate the implementation of high performance digital crossovers which can offer substantial advantages over their passive counterparts.

Active speaker architecture
Figure 1 represents a conventional wireless active speaker architecture. The receiver is Bluetooth, potentially running a high performance codec such as aptXTM to ensure the optimum audio performance. To facilitate the change from digital to analogue domain, the system requires a high performance DAC before the amplifier input. Pre and power amps operate in the analogue domain, with the single power amplifier driving both woofer and tweeter.

Figure 1: Conventional wireless active speaker architecture

Delivering a high audio quality suggests a Class AB amplifier architecture. However, the significant power savings offered by an analogue input Class D amplifier can be attractive; today’s closed loop analogue input Class D amplifiers can offer good audio performance. This increased efficiency also means smaller power supplies.

With this architecture, passive crossover networks provide high and low pass filtering to split the audio signal into the appropriate frequency bands for the woofer and tweeter drivers.

The availability of very-high-performance digital-input Class D amplifiers makes an alternative architecture attractive; figure 2. Here the audio signal stays in the digital domain right up to the amplifier power stage output; this in itself offers audio performance advantages, eliminating conversion errors between digital and analogue domains by removing the need for a DAC.

Figure 2: Wireless active speaker using digital-input Class D technology

To achieve the best audio performance, a closed-loop digital amplifier needs to be selected. In this example CSR Direct Digital Feedback Amplifier (DDFATM) technology is used as the platform.

With this architecture, pre and power amplifier functions are achieved in a single circuit. Although an amplifier channel per driver is required, the power level of each is exactly scaled to match woofer and tweeter sensitivities.

The available signal processing capability offers major advantages with respect to the crossover. On chip DSP facilitates easy implementation of high performance active filters which can be configured to exactly match driver characteristics, eliminating the need for passive components.

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