QX-Over is an active digital crossover with Qudelix's proprietary DSP technology. Qudelix-5K automatically detects QX-Over Earphones, processes the stereo input, and outputs Stereo Low and High band independently to earphones units through 4-channel DAC/AMP.

Multi-Driver vs. Single-Driver
Earphones and Speakers can be designed with a single speaker unit or multiple speaker units. It depends on, but practically, a single driver unit can hardly reproduce the entire audio details precisely over the whole frequency range. Real-world audio is not like a single-tone but complex waves over the frequency range.

Frequency Response (FR)
We usually measure the frequency response of a unit with a sweep tone. The FR plot, measured with a sweep tone, can be a reference characterizing the tonal balance. However, the FR doesn't represent the overall performance of the unit. A sweep tone is still a series of a single frequency wave over time which is far different from real-world sound waves. Even if a single driver unit performs well for sine wave sweep over the frequency range, it doesn't represent that the driver unit would do the same for the complex audio waves.

Inter-Modulation Distortion (IMD)
We can measure the IMD with a multi-tones wave. Multi tones interfere and modulate each other, causing additional distortion and loss that couldn't be observed with a single tone test. Practically, real-world sound waves can be modeled with the combinations of a few thousands of multiple tones. It can explain why different units with a similar FR deliver different levels of sound quality.

Transient Response (TR)
The time-domain transient response is more crucial for the SQ. The TR represents how fast a unit reacts and how precise a unit reproduces the sonic wave details.

As described above, the FR doesn't define and represent the overall sound quality. There is something more concerned with SQ, like IMD and TR. That is why multi-driver Earphones and Speakers can provide a better sound quality than a single-driver system. The FR is something like salty, sweet, sour, or bitter. Those terms can define and characterize the taste but can't tell how delicious food is. Meanwhile, IMD and TR can help to describe how delicious food is.

The multi-driver earphones and speakers are mainly to enhance the IMD and the TR rather than the FR. We can get a specific FR as we want to have, using a single-driver unit. But physically, a single driver has many limitations in terms of IMD and TR.

What is a crossover?
In multi-driver audio systems, crossover means filtering and splitting the audio into the signals required for each driver unit. It's the key factor for any multi-driver audio system and defines the overall sound quality.

Passive Analog Crossover
In general, most audio systems are designed for stereo output and input. Every wired earphone gets the stereo analog audio from the DAC/AMP in the source devices. So, multi-driver earphones need to split the stereo analog signal into each frequency band for the multiple drivers by themselves. In this case, a passive analog filter is the only option. A passive analog filter is designed with L(Inductor), R(Resistor), and C(Capacitor).
The problem is that every passive L, R, C components have its error. Those errors make unwanted frequency band overlap, phase distortion, and SPL difference, resulting in poor sound quality over the benefits from the multi-driver. From a manufacturing viewpoint, it's very tricky that putting those filters into a limited space of earphones, and we need to make the filter circuit as simple as possible. Furthermore, the passive filter, located between Amplifier and Speaker unit, creates unwanted capacitances and resistances. Eventually, they make the driver movements less reactive to the amplifier.

Active Digital Crossover
Unlike passive analog crossover, active digital crossover needs multiple DAC/AMP and audio wires for every speaker or driver unit. For a 2-way crossover system, we need two speaker units per each L and R channel and a total of four-channel DAC/AMP accordingly. Of course, each speaker unit needs to be connected to each DAC/AMP directly and separately. We need to have the digital audio input, a DSP(Digital Signal Processor) process them, filtering them into each frequency band using digital filters. Yes, to make an active digital crossover system, we need DSP, multi-channel DAC/AMP, and a specific wiring interface. It's more accurate and more precise than a passive analog crossover. And there are no passive components, no unwanted capacitances or resistances between Amplifier and Speaker unit. Consequently, the drivers can react to the amplifier as best as they can. Lastly, band-limited digital audio input to a delta-sigma DAC can help DAC perform better in terms of mathematical viewpoint.

Summary
In short, at the additional cost of DAC/AMP and DSP, an active digital crossover can provide better sound quality than a passive analog crossover.

All these reasons are why we started developing Qx-Over Earphones. Qudelix-5K has built-in DSP and 4-channel DAC/AMP, which is already the best fit for making an Active Digital Crossover system. As long as the source device, Qudelix-5K, fulfills the system requirement, building an active crossover earphone is very easy and costs less. We're trying to deliver more with less.