Products > Digital Crossover

A crossover is a device that splits the frequency spectrum into two or more parts

Crossovers incorporate low pass and high pass elements. A low pass element allows low frequencies to pass but blocks high frequencies. Conversely a high pass element allows high frequencies to pass but blocks low frequencies. Crossovers are required because most speaker drivers cannot reproduce the full frequency spectrum of 20 Hz to 20 kHz with anything approaching equal amplitude.

An active crossover splits up the frequency spectrum before power amplification

Passive crossovers, the default in the vast majority of loudspeakers today, split up the signal after the power amplifier. Passive crossovers are created from combinations of resistors, capacitors and inductors that act as filters. Capacitors and inductors are, by their very nature, non-linear devices that can introduce distortion, noise and other forms of distortion into the audio signal. This is mainly because they have to operate at high voltage and current levels compared to a line level active crossover that operates at low voltages and low currents. Multi-amplification techniques, where one amplifier channel is used for each speaker driver, is typically combined with active crossovers and often labeled ‘going active’. Each amplifier can be reduced in size and complexity since each is directly connected to a driver’s terminal points, without any non-linear capacitors or inductors in the circuit path. It is therefore better able to control the motion of the drive unit and reduce disortions.

A digital active crossover uses the power of digital signal processing (DSP) to split up the frequency spectrum

The oldest type of active crossover is based on operational amplifiers (op-amps) that removed the need to include inductors in the circuit design. In recent years cheap and powerful DSP chips have become available that can filter frequencies in the digital domain. Digital crossovers offer great flexibility for loudspeaker development and replacement of existing passive crossovers, since they allow the user to rapidly experiment with different filter slopes and types, implementing changes in software rather than hardware. Processing a signal in the digital domain, if done to a high quality level at high bit (e.g. 24 bit) and word lengths (e.g. 96 kHz), can also reduce noise and distortion compared to an active op-amp based circuit.

Important functionality to look for in a digital crossover

• Digital input – any standalone hardware DSP crossover should have a digital input. A large majority of us use digital front ends such as CD players and music servers and for us to have to convert the signal to analog and then have another conversion from analog to digital at the input stage of the crossover is not a good way to preserve signal quality.
• Stability – whilst digital crossovers can be realized in software running on a PC or Mac stability is often a big concern. The last thing you want whilst playing your favorite music or watching a choice movie is for the blue screen of death to occur or for the sound to freeze up. There is also the very real possibility of fried drivers from crossover software crashes. Operating system problems such as the ‘blue screen of death’ or power outages can also cause large and damaging transients to occur whilst the hardware restarts.
• Ease of use – setting filter type, slope and frequency should be an easy exercise. In our experience this is best accomplished by having software control over all features and functions. Some digital crossovers are hardware control only, which makes setup and changes very difficult and time consuming. Some people have tried to create digital crossovers by using graphical pro-audio DSP plugins in a host program. We need to caution that these are generally not designed for precision crossover duty, rather they are designed for recording equalization. In any event it is almost always very difficult to route signals from your playback program to the equalizers and then to the soundcard. As in nearly every case in real life, it is better in our opinion to use a tool expressly designed from the outset to do a job rather than adapt a tool designed for another purpose.
• Volume control – a lot of digital crossovers do not provide any way to control volume after the digital to analog conversion. Because you need to feed in a full level digital signal to preserve the quality of DSP processing the output from the crossover will all be at full level. You will need to find a way to reduce the volume level before it goes into the power amplifier. Say you decided to use a digital crossover to replace the passive crossover in a three way loudspeaker. That means you now have 6 channels (3 per side) of analog out. The problem is that there are not many multi-channel volume controls on the market. So you will probably end up having to DIY one and that could be difficult – especially building one that has remote control functionality!

There are very, very few units that meet the requirements for high-end audio and home theater use. Most units available on the marketplace are pro-audio units that have some or all of the required functionality missing. DEQX offers the HDP-3 and HDP-Express which are perfect for the job.