Butterworth, Bessel, Linkwitz-Riley

Operating Manual - PROTEA SYSTEM II 4.24C Crossover / System Processor

slope for every application, so experiment to see which one sounds most pleasing in a specific system. Ashly factory default presets use all 24dB/octave Linkwitz-Riley filters in the crossover section, but of course they can be changed to suit the application.

Crossover illustration from Protea System Software, showing two way crossover at 1KHz.

In addition to the frequency and slope, crossover filters can be selected as having Butterworth, Bessel, or Linkwitz-Riley response. These refer to the shape of a filter's slope at the cut-off frequency, affecting the way two adjacent pass bands interact at the crossover point. 24db/octave Linkwitz-Riley filters produce a flat transition through the crossover region, assuming both overlapping filters are set to the same frequency, slope, and response type. 24dB/ oct Linkwitz-Riley filters are the industry standard, the easiest to use, and the filter type recommended by Ashly. Other filter types are available, but may require polarity switching or other adjustments for proper results. The follow- ing paragraphs offer a summary of the three filter types as used in the 4.24C crossovers.

Butterworth

Butterworth filters individually are always -3dB at the displayed crossover frequency and are used because they have a "maximally flat" passband and sharpest transition to the stopband. When a Butterworth HPF and LPF of the same crossover frequency are summed, the combined response is always +3dB. With 12dB per octave Butterworth crossover filters, one of the outputs must be inverted or else the combined response will result in a large notch at the crossover frequency.

Bessel

These filters, as implemented on the 4.24C, are always -3dB at the displayed crossover frequency. Bessel filters are used because they have a maximally flat group delay. Stated another way, Bessel filters have the most linear phase response. When a Bessel HPF and LPF of the same crossover frequency are summed, the combined response is +3dB for 12dB/oct, 18dB/oct, and 48dB/oct Bessel filters, and -2dB for 24dB/oct Bessel filters. One of the outputs must be inverted when using either 12dB/oct or 18dB/oct Bessel crossover filters or else the combined response will have a large notch.

Linkwitz-Riley

The 12 dB/oct, 24dB/oct and 48dB/oct Linkwitz-Riley filters individually are always -6dB at the displayed crossover frequency, however the 18dB/oct Linkwitz filters individually are always -3dB at the displayed crossover frequency. The reason for this is that Linkwitz-Riley filters are defined in terms of performance criterion on the summing of two adjacent crossover HPF and LPF filters, rather than defined in terms of the pole-zero characteristics of individual filters. The 18dB/oct Linkwitz-Riley individually are 18dB/oct Butterworth filters in that they have Butterworth pole-zero characteristics and also satisfy the criterion for Linkwitz-Riley filters.

When a Linkwitz-Riley HPF and LPF of the same displayed crossover frequency are summed, the combined response is always flat. With 12dB/oct Linkwitz-Riley crossover filters, one of the outputs must be inverted or else the combined response will have a large notch at the crossover frequency.

4.24C specifications

Ashly 4.24C is a versatile and powerful audio processor that has gained popularity among sound engineers and audio professionals for its robust features and innovative technology. Designed specifically for live sound reinforcement, installed sound systems, and broadcast applications, this digital signal processor stands out in a crowded market by combining flexibility, performance, and ease of use.

One of the key features of the Ashly 4.24C is its comprehensive digital signal processing capabilities. With 24 individual input channels and 24 outputs, the device supports extensive routing options, enabling audio engineers to manage complex setups with ease. The processor includes a 4-band parametric equalizer, compressor, limiter, and noise gate on every channel, allowing for precise sound tailoring. Additionally, the inclusion of 16 built-in presets makes it easy to switch between different audio settings in live sound environments.

The Ashly 4.24C utilizes cutting-edge technologies to achieve superior sound quality. D/A and A/D converters are of high fidelity, ensuring that the audio signals remain clear and detailed throughout the processing chain. The device also features 24-bit resolution at sample rates up to 96 kHz, ensuring that even the most demanding audio applications are handled with professional quality. This emphasis on audio fidelity is particularly important in live sound and broadcast applications, where every nuance of sound can significantly impact the overall experience.

Ease of use is another standout characteristic of the Ashly 4.24C. The unit comes equipped with a user-friendly interface that includes an intuitive touchscreen, which simplifies the navigation of its numerous controls and settings. This graphical interface allows for greater visual feedback, making it easier for sound engineers to make adjustments during performances or broadcasts without needing to memorize complex configurations.

In terms of connectivity, the Ashly 4.24C offers a variety of inputs and outputs, including analog XLR, digital AES/EBU, and even optional Dante networking capability for seamless integration into modern networked audio systems. This flexibility ensures that the processor can fit into various setups, from small venues to large concert halls.

Overall, the Ashly 4.24C combines advanced audio processing technology with user-centric design, making it an excellent choice for professionals seeking high-quality sound solutions. Its multitude of features and capabilities ensure it meets the demands of contemporary audio applications while maintaining an accessible and manageable interface for users.