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Boost high-end active speaker performance

04 Jun 2013  | Dave Brotton

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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.

Passive versus active crossovers
Figure 3 shows a typical passive crossover implementation; this example is considered in detail later.

Figure 3: Passive crossover implementation.

This design uses a conventional 2nd order filter for each driver with a crossover frequency of approximately 2.2kHz. The woofer impedance is 3.5Ω, the tweeter 3.2Ω.

The circuit is constructed with simple inductors and capacitors which, being in the power path, need to be relatively large. Efficiency losses are likely, resulting in heat dissipation and performance drift. These effects get worse as power levels increase, higher levels of distortion can result.

Although the circuit design appears simple, component interactions are complex making it difficult to fully isolate one driver from another. The response of the filters is also directly influenced by the drivers whose characteristics change with frequency, power and temperature.

The crossover accommodates the different driver sensitivities by adding padding resistors, resulting in further dissipation. It's important that this is done correctly, an overpowered inductor can saturate and cause distortion at high power or even failure resulting in blown tweeters.

A digital active crossover approach can address these issues resulting in a better performing, simpler product.

The active crossover is placed in the low-level digital signal path of the system, eliminating efficiency losses and thermal effects experienced with the passive design. Here, the filters are isolated from the load and from each other, eliminating performance degradation due to interactions between the two. Digital gain control is simple to implement, accommodating different driver sensitivities without losses and eliminating the need for the padding resistors.

Digital filters are not affected by signal levels so results are far more precise, linear and repeatable with distortion maintained at a constant low level. Elegant control of limiting can be introduced eliminating any overload issues. Delay is also readily available, enabling optimum driver time alignment.

DSP resources in digital amplifiers take up very little overhead allowing significant processing capacity to be made available. This means that higher order, more complex filters can be implemented allowing better performance levels to be achieved at no additional cost.

More complex filter designs can offer far better matching to speaker enclosure and driver characteristics. Furthermore, filter characteristic sets can be developed to allow choices in performance, for example, to compensate for room conditions or music styles.

DSP filter performance
With an active crossover implementation, attention must be paid to the filter specification in order to maintain optimum audio performance. For example, resolution errors can produce increased noise levels unless the appropriate architecture is applied.

In digital amplifiers, filters are created by a combination of biquad stages, each providing a 2nd order characteristic, with the type defined by a number of coefficients; in this example, 5 24 bit coefficients configure each stage.

Ensuring the system is capable of resolving and processing all expected input signals, requires consideration of computational resolution as well as coefficient bit width. For example, with an amplifier dynamic range target of 116dB, a computational resolution of 35 bits ensures filtering without compromising noise or distortion with a coefficient resolution greater than 20 bits.

Digital filter implementation
To illustrate the performance differences between passive and active filter implementations, an existing speaker design is used. The passive crossover (figure 3) is characterised whilst attached to the woofer and tweeter drivers (figure 4).

Figure 4: Passive crossover characteristic with attached drivers.

With a higher efficiency, the inclusion of the padding resistor is the reason for the amplitude reduction of the tweeter channel. With the digital crossover, this difference is easily accommodated with a simple gain adjustment during amplifier and filter configuration. This adjustment also improves tweeter channel SNR; it's advantageous to keep tweeter noise level low given its higher sensitivity.

The digital implementation for the tweeter is replicated using a single biquad filter stage with coefficients adjusted to match the passive crossover and driver combination (table 1).

Table 1: Tweeter biquad filter setting.

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