Evolution of the DJ Crossfader
Evolution of the DJ Crossfader
Rick Jeffs, Rane Corporation
RaneNote 146, written 1999
- Pots - VCAs - Magnetics
The DJ mixer crossfader was originally developed as a control for implementing smooth fades from one program source to another, but where did the idea come from? Fading between two independent sources was first accomplished by DJs using two separate controls, often rotary controls. They would maintain constant acoustic energy (equal loudness) in the room while carefully fading from one program source to another. Some expertise was required to accomplish this effect accurately and consistently. It became obvious that if a way could be found to fade from one source to another with a single control, the task would be much easier and repeatable for the less experienced.
Panning circuits were already used in recording studios to move a single source from left-to-right while maintaining constant acoustic energy. While the requirements for a single source panning circuit were well defined, those for maintaining constant acoustic energy while fading from one source to another were not.
"Rosie" is the first known electronic DJ mixer, circa 1965. Courtesy EMP.
The exact origin of the first use of a crossfader in the DJ world has proven difficult to track down. It seems certain to have come out of the broadcast industry, where the term "fader" has been in use since at least the '50s (mentioned throughout the Radiotron Designer's Handbook, 4th ed., 1952) and the term "cross-fading" shows up in the Tremaine's Audio Cyclopedia in 1973. The earliest example documented so far was designed by Richard Wadman, one of the founders of the British company Citronic. It was called the model SMP101, made about 1977, and had a crossfader that doubled as a L/R balance control or a crossfade between two inputs.
1st Generation Curve Shape
Knowledgeable engineers noted that if two source signals of equal RMS amplitude were statistically random and incoherent, a slight modification to the standard panning circuit would allow constant energy fading between sources. The new control was called a crossfader and has achieved wide use and acceptance. Figure 1 shows the classic constant-power response.
Figure 1: Constant-Power Response
The curve shown in Figure 1 only yields constant-power fading when the original source signal assumptions are true.
Limitations: It wasn't long before the basic crossfader topology showed some limitations. Disco dance music with a dominant beat challenged the original assumption of random. As beat matching source signals gained popularity, the assumption of incoherence became invalid. Those who had mastered the skill of two-knob-fading scoffed at the idea of a crossfader control, and were now saying "we told you so." It was apparent the traditional crossfader lacked flexibility.
In addition to the fundamental, smooth crossfade response shown in Figure 1, DJs wanted to perform more complex mixing functions. They wanted to add one dance song to another without losing energy in either until fully mixed. They wanted to cut in a beat and then pump it up. They wanted to cut one program in and out without affecting the other. Figures 2-4 show some of the tapers required for various effects.
2nd Generation Curve Slope
It was soon clear that one crossfader response curve was not suitable for all applications. No matter how skilled the DJ, it was not possible to achieve all of the desired effects. At first, the applications were distinct enough that manufacturers could design special mixers by selecting one of the tapers shown in Figures 1-4 for specific applications. However, as DJ performances became more sophisticated and competitive, a fixed taper became inadequate. DJs wanted to mix it up. By now they were familiar with the results possible with the various tapers and wanted them all. For performing DJs, the days of the application specific crossfader were over.
Figure 2: "No Dip" Taper for Adding Sources
3rd Generation Curve Slope
The solution was to provide a second control that would allow the DJ to change the taper of the crossfader.
Limitations. At this point, most designers had lost track of the original constant-power crossfader taper. Implementations had become careless and undefined. Defined standards did not exist for the tapers shown in Figures 2-4. When crossfader taper control was added, it was not surprising that the range and shape of tapers was haphazard. Each implementation performed differently, causing confusion among performers.
Figure 3: Medium Taper for Cut In and Pump-It-Up
Figure 4: Sharp Taper for Cut and Scratch
The best passive controls could not meet the increasing demands on performance and usage. Passive controls are rated for a maximum number of operations, while maintaining given travel noise and force specifications. As the number of operations increases, travel noise goes up and travel force changes. Even high-quality controls with cycle life ratings as high as 100,000 to 300,000 require frequent service or replacement.
4th Generation Curve Slope
The high maintenance requirements of passive crossfader controls resulted in unacceptable service costs and down time. It was bothersome to disassemble a mixer just to clean and lubricate the controls. Replacement required costly factory service and could leave a DJ without income for weeks. The solution was to design mixers with field serviceable crossfaders. While doing nothing to resolve the reliability problems, the removable crossfader did help reduce service costs and down time.
5th Generation Curve Slope
To improve performance and extend service life, audio was removed from the crossfader control and processed in a voltage-controlled amplifier (VCA) or some other voltage/current controlled element. The crossfader control was only used to develop a DC control signal. However, this implementation was found only on expensive mixers. This practice greatly reduced travel noise and extended service life, but the performance of affordable VCAs was limited. In addition, crossfader tapers were still poorly defined as were the controls used to alter the tapers. Implementations were complex and consistency was poor.
Rane developed an Active Crossfader design, featuring high quality VCAs, low cost, and simplicity. The classic response of the design is shown in Figure 1. In addition to providing an accurate constant-power response, the circuit produced the optimum integration time for removing travel noise without noticeably affecting the reaction time of the control. The Active Crossfader topology created an excellent foundation for more sophisticated designs.
Musical Instrument Generation
A new art form emerged from hip-hop. Turntablist (scratch DJs) take small bits and pieces of music from different locations on vinyl records and create new compositions. A mixer and a couple of turntables become their instrument. This emerging art form again put demands on crossfaders that current state-of-the-art designs could not meet.
The following is a list of the new requirements:
- Music instrument quality and performance.
- Accuracy, reliability and repeatability for all functions.
- More than a 10-times increase in crossfader usage over previous applications.
- Crossfader with a taper range adjustable from constant-power to less than a .1 inch (2.5mm) pitch between full off and maximum level.
- Mechanically durable crossfader control with a knob that provided a fine music instrument feel.
- Crossfader taper control with smooth and predictable settings.
- Reverse operation of the crossfader.
In addition to the new crossfader demands, all of the same demands were now placed on the input (or program) faders.
None of the existing designs met all of the new demands. In addition, many manufacturers were timid about providing any product for fear of service liability problems. Available products were either very expensive with limited performance and feature-sets, or cheap throw-away toys with virtually no warranty.
Rane accepted the challenge and designed a performance mixer meeting all of the new demands, with music instrument quality and reliability. Because the combination of features was complex, and performance requirements very high, it was apparent that the new design would need new technology.
The challenge was to find an active or VCA topology that would provide the required performance without excessive cost or complexity. A single, low-cost, high performance, quad gain core that provided crossfader and input fader gain control for a stereo, two bus system was one answer. The actual audio signal path is very simple, yet the topology allows complex control.
This design isolates all audio from the control elements, greatly extending the life and performance of the controls.
The taper of the crossfader is adjustable from the gentle, constant-power curve shown in Figure 1, to the steep taper shown in Figure 4. Careful control of attack and decay rates yields low noise and smooth performance. In addition to predictable taper control, the design provides crossfader reversal.
Because the input faders use the same VCA design as the crossfader, these controls also have excellent control isolation, performance and reliability. As with the crossfader, implementation of accurate taper control (shown in Figure 5) and reversal functions, is possible without affecting audio quality.
Figure 5. Input Fader Taper Control
The new patented Rane magnetic fader is the fastest, most accurate, and longest lasting on the planet. The design uses non-contact technology previously reserved for the most demanding aerospace and industrial applications. No travel noise -- no bleed -- ever.
Unlike optical non-contact faders, Rane's magnetic fader is impervious to smoke, moisture, temperature, ambient light and aging. (You know, only the stuff you run into at every club.) The electrical performance of the new fader is -- literally -- unaffected by use. The mechanical life exceeds 10 million operations.
Rane's magnetic fader accurately translates hand motion into precise audio level control. This system provides more flexible curve selection and fader assignment options than ever before. And they are the first to offer true morphing between curves.
It is common for a crossfader to undergo millions of operations over the life of a mixer. High-quality potentiometers used with VCAs yield 100,000 to 500,000 operations. More exotic, doped-plastic-element potentiometers yield 2 million operations. In all cases, contact-based controls generate travel noise and exhibit electrical performance decline with use. Non-contact fader technology is the only way to fully address the needs of the professional performance DJ.
Non-contact controls sense position without the use of electrical contacts. This eliminates both travel noise and electrical performance decline as parts wear. Possible methods include optical (light and a shutter), sonic (sound and a receiver), inductive (coupling between coils of wire), capacitive (coupling between conductive plates) and magnetic (sensing the strength of a magnetic field). Each of these methods varies in complexity, accuracy and immunity to environmental changes.
Rane favors magnetic position sensing due to its unique combination of simplicity, accuracy and environmental immunity. How does it work? It's as simple as 1-2-3:
- Finite Element Analysis (FEA) is used to map the flux density field of a small, rare-earth magnet. [FEA is a computer-based modeling technique.]
- A stationary Hall effect sensor is used to measure the flux density of the moving magnet. [Hall effect, named after its discoverer E.H. Hall, describes a voltage that is generated by the effect of an external magnetic field acting perpendicularly to the direction of the current.]
- The location of the magnet (your hand) is determined by comparing the measured flux density value to a stored master flux density map.
[For more details on Hall effect sensors, Lorentz forces and other exciting things see the Pro Audio Reference. Meanwhile, if you can't wait, Rick Jeffs explains: "When a perpendicular magnetic field is present, a Lorentz force is exerted on the current flowing in a thin sheet of semiconductor material (Hall element). This force disturbs the current distribution, resulting in a potential difference (voltage) across the output. The stronger the magnetic field, the greater the voltage difference."]
Rane uses only the finest materials: nickel-plated neodymium-iron-boron magnets, space age plastic with embedded Teflon®, surface-passivated and polished stainless steel bearing rods and stainless steel handle. The result is a control offering unprecedented feel, control and years of accurate, carefree operation.
In the end, the crossfader has provided functionality far beyond what was originally envisioned. Advanced topologies allow mixing styles not possible with two knobs (can you imagine scratching on a UREI; however, we must concede that some traditional mixing tasks are best accomplished with the two knob method.)
While disco dance, hip-hop and scratch are well established genre, new mixing styles are evolving at a rapid pace: Trance - progressive trance - techno - deep tech - electro - jungle - breaks - funk/breaks - hip hop - scratch - house grooves - hard house - deep house - funky house - disco house - drum `n' bass - acid - live music.
What's next? As mixing styles continue to evolve, so will the performance mixer. There may soon be as many styles of performance mixers as there are guitars. The invention of non-contact fader controls has answered the need for reliable controls. The use of microprocessor based methods of translating hand motion into virtually any desired response answers the need for versatility. One thing is for sure; the evolution isn't over! Performers continue to demand new levels of performance and reliability, and designers continue to respond.
A condensed version ran in DJ Times, June 1999
"Evolution of the DJ Mixer Crossfader" This note in PDF.