Digital Crossovers
"The future of audio is digital. And the future is now."
We’ve all heard this cliché before, but what does it mean and is it really true?
Put another way, Digital Signal Processing (DSP) is available now in ever increasing use in the pro audio world. It is certainly employed in Home Theater receivers, but it is still less widespread in home audio-only systems.
So what’s the big deal?
What could DSP possibly bring to an otherwise high-performance audio/video system?
Here’s what:
  • DSP crossovers empower the loudspeaker designer to reach specific performance goals more directly and with fewer compromises.
  • Digital EQ empowers the audiophile to match subwoofer/monitor levels, correct for room response irregularities, and tailor tonal balance to preference.
Let’s look at these two benefits in closer scrutiny.
Before the loudspeaker ever gets to you, a crossover must be developed, voiced, evaluated, approved, fabricated, and then installed into the speaker. BESL is well known for doing this with advanced crossover designs. Our design methodology is meticulously refined; one that we honestly believe is second to none. While voicing is accomplished in-house, the process is not rushed. Final approval is judged ultimately by the owner, of course.
From the designer’s perspective, the crossover is a collection of filters that not only divides frequencies among the woofer, midrange, and tweeter, but they also reshape their individual responses until their combined output measures flat (or at least matches a custom target curve).
  • Traditional passive crossovers are comprised of resistors, capacitors, and inductors operating at high-level between the amplifier and speaker drivers.
  • Active, or electronic, crossovers operate at low-level located before multiple amplifiers, one channel for each speaker driver.
A passive crossover makes the speaker portable in that it can be connected to any amplifier and still sound as it was designed. This is because the crossover stays with (inside) the speaker that it was designed for. An active crossover should also be optimized to work with the particular speaker, but by this nature it must be assigned to that speaker (not portable), not to mention that two or three stereo amplifiers are required to power it.
It is generally accepted that active crossovers sound better than their passive counterparts. Essentially the reason for this boils down to the fact that high-power electronic components in passive networks exhibit greater non-linearity, and give rise to more distortion mechanisms than do electronics operating at line level. Without going into a long list of causes, a passive crossover can simultaneously sound more veiled, more distorted, and less dynamic than an equivalent active implementation. This is not to say that BESL passive crossovers are lacking; nothing could be further from the truth! However, to squeeze further performance out of an already exceptional design, one does not need to resort to spending a small fortune on exotic capacitors, inductors, and resistors. It is our firm belief that when standing at the passive/active crossroads, one is better off taking a hard look at the active route. The additional amplification needed—not the active crossover per se—is what costs more.
Are there any benefits at all with passive crossovers?
Certainly. The portability issue already mentioned from a simpler, lower cost system. The tweeter capacitor provides natural protection against a wiring error. And fewer wires! But convenience aside, the active crossover wins in all performance categories. So let’s have a look at that next.
We’ve already acknowledged the potential for the active crossover to deliver superior sound by way of less distortion, and greater detail and dynamics. What else is there? Well, with the aging analog active crossovers, there isn’t much more to be had. With the advent of DSP crossovers, there is much more!
As already stated at the beginning of this article, DSP also provides:
  • More accurate crossover filter alignment with greater EQ power to shape individual driver responses.
  • Additional EQ possibilities to combat room problems, speaker level matching, and skewing the tonal balance to individual preference.
A good example of the first capability is creating a second-order filter alignment for the BESL System 5 loudspeaker. This alignment is not practical in the passive network domain since the passive crossover complexity would get out of hand.
BESL sells and supports the Behringer DCX2496 DSP crossover, primarily for use with System 5. It has taken a reference grade speaker to new performance heights. Here is an example lineup of features that can be accomplished only with a 6-channel DSP crossover such as the DCX2496:
  • Second-order acoustic crossover alignment on S5-mtm monitors
  • EQ filters on separate low-mid-high outputs (as part of the crossover alignment)
  • Polarity inversion on the tweeters
  • Short time delay on tweeters allows for simpler filter function on the midrange
  • Long time delay on the left subwoofer only to improve left/right sub summation (because of asymmetry in the room)
  • Phase shift on the subwoofers to fine-tune the crossover summation with the monitors
  • Dynamic limiting of subwoofers protects them during high playback levels such as for use in home theater systems.
  • Different room mode EQ on left/right channels
  • Direct digital input from CD transport bypasses external DAC and preamp
  • Left/right analog inputs from preamp for all other sources
Below is a graph of the in-room response at the listening position, showing the result of all this power.
All controls are available graphically via PC interface remote patch cable. Make changes by ear, or with a measuring system (even a handheld SPL meter is helpful). Store the changes to your PC disk drive. Lock yourself out of the crossover page with a password. Given enough amplifier channels, you can effect a complete 5.1 HT system with 2-way monitors at L-C-R-surround locations, plus dual-mono subs, and all with individual EQ!
For complete Behringer DCX2496 technical specifications, click here.
Series 5 in-room response graph