You are here

Kurzweil FDFX

K2500 Series Expansion Board By Paul D. Lehrman
Published January 1999

Kurzweil FDFX

Sophisticated effects and audio processing have become pretty standard on professional synths and keyboard workstations. The Kurzweil K2500's capacities in these respects, however, were looking a little dated — until now. Paul Lehrman plugs in the new KDFX expansion board.

It's hard to find a synthesizer or sampler today that isn't billed as a "digital audio workstation" or a "studio in a box" — but how well do they really cover all the bases, when it comes to providing all of the functions a real studio offers? Sure, they've all got multitimbral sound engines, two or more sets of outputs, and at least some rudimentary effects, but is that all we expect from a 'studio'? Where are the audio (not just MIDI) faders, the balance and pan controls, the multiple effects with individual channel sends and returns? And how hard is it to get all those signals down on to a fixed medium like disk or multitrack tape without leaving the digital domain?

Well, in the competition to see how closely a stand‑alone box can approach the ideal of a real studio, the Kurzweil Musical Instruments division of Young Chang has just raised the bar with KDFX, their new add‑on for the K2500 series of sampler/synthesizer workstations.

Invisible Complexity

KDFX's outputs can be accessed individually — either analogue or digital — or can be combined, along with the K2500's internal processor, at the Mix output.KDFX's outputs can be accessed individually — either analogue or digital — or can be combined, along with the K2500's internal processor, at the Mix output.

KDFX, which stands for 'Kurzweil Digital Effects', lives on a daughterboard that fits inside either the keyboard or the rackmount version of the K2500, and needs to be installed by an authorised service centre. Some service centres (at least in the US) are including the installation in the price, but if yours doesn't, it shouldn't cost much. However, unless you're very familiar with the insides of digital synths, it's not something you should try yourself.

Along with the daughterboard, you also get a brand‑new operating system for the K2500 (currently version 4.04) on a couple of floppy disks, plus a tutorial disk. When you're installing a new operating system, Kurzweil recommends you follow it up with a 'hard reset', which wipes out all your old programs and keymaps, so it's a good idea to save them to an external medium before you start.

In the original K2500, the four 'program' output pairs from the synthesis engine are wired directly to four pairs of analogue output jacks on the back panel. With KDFX installed, a layer of 24‑bit digital processing is inserted between the program outputs and the physical outputs. Inside that processing layer are not one, not two, but five separate digital processors, all individually configurable and controllable, each one capable of a multitude of DSP functions, from reverb, flanging, and pitching, to dynamics control, sweeping resonant filters, wide‑field stereo enhancement, and beyond.

The Structure

The signal path of one FX Algorithm, consisting of a tone control, a tube amp simulator with distortion, a moving delay line, a cabinet simulator, and a chorus. This Algorithm uses three Processor Allocation Units (PAUs).The signal path of one FX Algorithm, consisting of a tone control, a tube amp simulator with distortion, a moving delay line, a cabinet simulator, and a chorus. This Algorithm uses three Processor Allocation Units (PAUs).

The overall gestalt of KDFX resides in a Studio. Only one Studio can be active at a time, but you can have as many in RAM or on disk as will fit, and you can bring them on line instantly. Studios can be linked to Programs or Setups (a Setup is Kurzweil's term for a multitimbral configuration), so that when you call up a Program or Setup, either locally or using a MIDI Program Change command, the Studio associated with it immediately becomes active and KDFX completely reconfigures itself. Alternatively, you can set KDFX to Master mode, in which case the Studio stays put unless you specifically change it.

The Studio contains the five processors, which are divided into two types. All of the processors have identical functionality, but the two types live in different places within the Studio. Four of them (we'll come to the fifth shortly) belong to a group known as Insert FX, and are generally used singly on individual output pairs, so that, for example, one output buss can have a reverb, another a delay, another a flanger, and another a compressor.

Each of the four stereo program outputs of the K2500 can be sent to any two of these Insert FX busses, with separate level and stereo balance or pan controls on each send, as well as a two‑band fully parametric EQ. The stereo channels of the program outputs can in fact be split into two mono channels, and each of the mono channels sent to its own combination of Insert FX busses, with its own EQ, level, and balance/pan, so that you can have up to eight different signal paths going at once. There's no limit to how many different program outputs can be sent to a particular Insert FX buss.

Inside each of the Insert FX busses is an FXPreset, which handles the processing on that buss. Each FXPreset is based on an 'FX Algorithm', which defines the nature of the processing. KDFX comes with 108 different Algorithms, which include all of the functions you would normally associate with digital audio processing, as well as a lot of fancy combination processors, and a number of interesting surprises. Their scope is impressive, and since they all exist in software, Kurzweil engineers will be adding to the list over time (see box: page 198). All of the Algorithms, with a few exceptions, are stereo.

An FXPreset consists of the Algorithm plus the settings of the Algorithm's various parameters. While the Algorithms themselves are unchangeable by the user, what they actually do to the signal, as defined by the FXPresets, is of course editable. A single Algorithm, such as a reverb, can take on a large number of different identities, depending on how its parameters are set. You can create and store as many different FXPresets using the same Algorithm as you want. KDFX comes with over 250 FXPresets in ROM.

When a signal leaves an Insert FX buss, it can go to three different destinations, either individually or in any combination, with separate level and pan controls on each path. It can go straight to one of the K2500's four sets of physical outputs, or it can be combined with other buss outputs at the K2500's Mix output, or it can go through an auxilliary, or global effects buss, known as Aux FX.

The Aux FX buss is where the fifth processor lives. It is identical to, but completely independent from, the Insert FX busses, and provides the same high degree of flexibility. Often the Aux FX buss will contain an ambience or reverb program for use on multiple signals after they pass through their individual Insert FX processors, but it doesn't have to be set up that way. For example, a program output can go through an Insert FX buss that is set up to be a 'dummy', and go right to the Aux buss, where it is the only signal processed; or two Insert FX busses can carry the same signal, and be combined at the Mix output without going through the Aux buss. Hopefully, you're beginning to see how complex the combinations of signal paths can be.

If you're an experienced K2500 user who's become attached to the original onboard effects chip made by Digitech, you'll be happy to know that it behaves in just the way it always has, and the original effects can be used in conjunction with KDFX.

Analogue & Digital Outputs

One possible way that KDFX might be set up: here, four different processors are used in various combinations on four different instruments, and a global reverb follows all the other processing.One possible way that KDFX might be set up: here, four different processors are used in various combinations on four different instruments, and a global reverb follows all the other processing.

If you have a K2500 with the sampling option installed, then you already have an AES‑EBU digital output (in parallel with the Mix output); when you install KDFX, however, you get four more pairs of digital outputs, corresponding to the four pairs of individual analogue outs that have always been there. You can also now have all of the digital outputs operating at 20‑bit resolution, and if you don't have the sampling option, installing KDFX provides you with an AES‑EBU Mix output.

The individual output pairs are accessed through a proprietary KDS multi‑pin connector. While this is a non‑standard format, it can be converted to ADAT, DA88, or multiple‑socket AES‑EBU format using Kurzweil's optional eight‑channel Digital Multitrack Interface (DMTi) — though this is not cheap, retailing in the UK at £1073 for the DA88 and AES‑EBU versions and £1330 for the ADAT version. Thus you can record the K2500's outputs directly onto digital tape or disk while staying in the digital domain.

One possible problem is that when it is hooked up to other digital equipment, the K2500 likes to be the sample‑rate clock master, so if you are sending the AES‑EBU signal to a processor or mixer that doesn't accept external digital sync, you may have a problem remaining in the digital domain. The DMTi, however, is capable of re‑clocking the KDS output to match the sample rate of the receiving device, so this should take care of the issue. Personally, even if I had a DMTi, I might be tempted to go with the analogue outputs anyway — their sound is excellent, and I don't think I would be losing anything by using them, while at the same time I could avoid a lot of potential hassles.

If you're an experienced K2500 user who's become attached to the original onboard effects chip made by Digitech, you'll be happy to know that it behaves in just the way it always has, and the original effects can be used in conjunction with KDFX.

Controlling Kdfx

Every parameter of every bus and FXPreset can be set from the K2500's front panel and LCD screen, and some of the FXPresets have many controls indeed: the more complex delay and combination algorithms have over 40 adjustable parameters! Fortunately, along with the enormous library of FX Presets, you are provided with over 200 Programs and 200 Setups, each of which comes with its own Studio, which should give you a running start when it comes to setting up KDFX to do what you want.

But besides being able to control Studio settings from the front panel, you can also control them in real‑time with MIDI. And this is where KDFX starts to get really involved. Each Studio allows up to 18 real‑time control modulations (known as FX Mods) over any of its parameters: EQ, send and return levels, or processing parameters with the FXPresets. You assign a MIDI source, a destination within the Studio, and a starting value and range for the destination parameter.

The MIDI sources can be anything that the Kurzweil recognises in its normal VAST (Variable Architecture Synthesis Technology) synthesis mode, including not only standard MIDI controllers, but also note numbers, velocities, mono and poly aftertouch, attack state, several built‑in envelopes and LFOs (and KDFX has a number of additional dedicated LFOs and envelopes, independent from the VAST ones), and FUNs (ie. Functions), which combine two control sources in various arithmetic and algebraic ways.

Controlling parameters over MIDI in real time is just as smooth as doing it from the front panel. The Algorithms are designed to be modified on the fly — clicks and pops are kept to a bare minimum, whether the changes are subtle or huge, and parameter and even Algorithm changes are clean and quick. In the worst case, you may hear some momentary dropping‑out of the processed signal as a processor clears itself out and loads in new data, but it behaves no worse than any other dedicated processing units I've dealt with, and a lot better than most.

KDFX parameters can also respond to tempo, in two different ways. One of the modulation control sources is Tempo, which reads MIDI timing bytes coming from an external sequencer or the K2500's onboard sequencer. Another is Tap Tempo, which lets you set and change the tempo in real time from a key or foot pedal. These sources can be assigned to any number of parameters, and many of the Algorithms have parameters designed expressly for this feature: they are calibrated not in standard time units, but in bpm and 'beats'. This lets you set up, for example, a multiple delay line that follows MIDI tempo, in which one of the taps repeats twice per beat, another tap repeats every beat, another every other beat, and a fourth every three beats. But that's a simple example: how about a distortion unit that gets nastier as the tempo speeds up, or a gate whose release time shortens or lengthens with the music?

The Studios that come with KDFX are maximized for use with the K2500 keyboard, and the instrument's large number of onboard sliders, plug‑in pedals, and two ribbons are put to good use as faders, effects sends and returns, and dramatic parameter adjusters. If you have a rackmount K2500 (which I do), a dedicated programmable MIDI fader box like the JL Cooper FaderMaster or the Peavey PC1600 is a very good idea, so that you can take advantage of all the real‑time control that KDFX has to offer. All controller movements, of course, can be recorded into a sequencer, edited and played back, so it's easy to use your sequencer to automate an entire mix with level changes, pans, EQ, switching and flying effects, and other subtle or not‑so‑subtle changes.

The Limits Of Power

Before you sell off or simply throw away all your other gear, you do need to know that KDFX does have its limitations, and won't replace everything else in your studio.

For one thing, KDFX's processing power is not infinite. The DSP chips in KDFX are broken down into Processor Allocation Units, or PAUs. Different Algorithms, depending on their complexity, use different numbers of PAUs. A simple reverb, for example, requires 1 PAU, while a combination flanger/waveshaper takes 2 PAUs, and an Algorithm that combines a tube amp simulator, a mono distorter, a moving delay, and a chorus, configurable in any combination, uses 3 PAUs.

Surprisingly, a lot of mileage can be obtained from even a 1‑PAU Algorithm: that category includes dual moving delays, resonant filters, hard‑ or soft‑knee compressors, stereo image generators, and even choruses in combination with four‑tap delays.

The problem is that the total number of PAUs available for use with the four Insert FX busses is four. So if you want processing on all four busses, you can't use any Algorithms that require more than one PAU. If one of the busses uses a two‑PAU Algorithm, one of the other busses will not be able to do anything at all; if two of the busses use two‑PAU Algorithms, then the other two busses will have to be left empty, and so on. In practice, however, it turns out that the occasions when you really need to use all four busses at the same time are not that frequent, and so this may not be as much of a problem as it might at first appear.

The Aux FX buss has its own set of three PAUs, so this isn't an issue on that buss. Unfortunately, you can't share PAUs between the Insert busses and the Aux buss. However, as I mentioned earlier, you can 'fake' the Aux buss into being an Insert buss, and that way have as many as three multi‑PAU Algorithms going on different busses simultaneously.

Nonetheless, it's important when you are designing KDFX Studios to make sure the system's resources are being used efficiently, lest you run out of processing power. The software is very careful to let you know what's going on, and always tells you how many PAUs are available in a Studio or on a particular buss.

Facing the world

Another reason why you shouldn't be throwing away your console and patch bay quite yet is the KDFX user interface. Don't get me wrong — it's very cleverly designed, considering the restrictions that the engineers had to work within. But getting all of the information you need to build Studios and set up FX Mods on to an 8‑line x 40‑character LCD screen is, as you might imagine, not an easy task. Of course, there are plenty of very complex signal processing boxes out there that try to make do with two‑line screens, so I suppose I shouldn't complain, but I can't help feeling that KDFX's front end would benefit greatly from a larger display, or a video output such as that found on Roland's high‑end samplers.

It is reasonable to say that, under the circumstances, Kurzweil's engineers have made the operating system as logical and clear as possible. They've been able to take advantage of the many interface tricks that the K2500 is well‑known for, such as the Jump and Previous page functions, the ability to edit many parameters directly without having to scroll to the page they are on, and "intuitive entry", which allows you to quickly assign a MIDI controller as a modulation control source by selecting the control source parameter, holding down the Enter key, and tweaking the controller in question. When you have as many MIDI mappings as KDFX allows, this last is a major time‑saver. They've even managed to include metering in many Algorithms, such as compressors, limiters, panners, and sweep filters, which, while they won't replace the plasma displays on your multitrack, are pretty useful.

Getting all of the information you need on to an 8‑line x 40‑character LCD screen is not an easy task. Under the circumstances, Kurzweil's engineers have made the operating system as logical and clear as possible.

The documentation comes in two books. The first contains an extensive tutorial that walks you through three different Studios, in increasing levels of complexity, and then defines all of the functions of KDFX in an operational way, rather than simply ticking off menu or screen items. The second, written mostly by the engineers who designed KDFX's software, goes into great detail about the Algorithms, with signal‑flow diagrams and complete lists of what the parameters mean and how to use them.

Do You Need This?

The short answer is: Yes! If you are a K2500 owner, no matter what you use it for, KDFX will add tremendously to your creativity and the general usefulness of your setup. In live performance, whether it's arena rock or dance music, the ability not only to control any and all processing parameters in real time, but also to change entire Studios in an instant, will prove extremely useful. In the studio, these five high‑powered, high‑resolution signal processors, boasting the most complete MIDI implementation imaginable, may not replace everything else in your rack, but they will certainly increase your options without taking up any more real estate.

If you don't already have an automated mixer, KDFX could conceivably make it unnecessary to get one, since you now have the ability to automate levels, pans, and EQs on as many as eight discrete channels, as well as five effects busses. And if you're of the experimental persuasion, Live Mode opens the doors to a potentially infinite sonic vocabulary.

The Many Effects Of Kdfx

Here are all of the various effects Algorithms available in KDFX, by group. This list may expand over time as Kurzweil develops new Algorithms. Keep in mind as you read this that all of the parameters within these Algorithms are adjustable in real time over MIDI!

  • Reverbs (rooms, plates, halls, booths, chambers, reverse, gated, and more).
  • Choruses.
  • Flangers.
  • Phasers.
  • Multi‑tap delays (with and without tunable resonant taps).
  • Resonant filters (LP, HP, BP, notch, all with 50dB of resonance).
  • Triggered envelopes.
  • Envelope followers.
  • Morphing filters — two sets of filters (with 48dB of resonance) that you can slide between automatically or manually.
  • Compressors and limiters (both hard‑ and soft‑knee, with metering and 'look‑ahead' functions for peak limiting).
  • Expanders (with metering).
  • EQ (parametric, graphic, low/hi shelf, low/hi pass with 6dB/12dB filters).
  • Enhancers.
  • Panners.
  • Tremolos.
  • Pitchers (creates pitch from a pitched or non‑pitched signal).
  • Stereo image enhancers.
  • Mono to stereo simulators.
  • Multi‑peak resonant comb filters.
  • Rotary speaker emulation.
  • Tonewheel vibrato/chorus simulation.
  • Distortion.
  • Tube emulation.
  • Guitar cabinet simulators.
  • Ring modulators.
  • Wave shapers.
  • Quantiser plus flanger.
  • "Lazerverb" (a unique speciality reverb that creates buzzy pitch effects).
  • Many combination Algorithms(chorus+delay, chorus+reverb, flange+delay, etc.) in fixed or variable configurations.
  • Stereo analyser and FX modulation diagnostic.

Going Live

The version 4 software that comes with KDFX also offers another cool feature (which does not require KDFX necessarily): Live Mode. Do you remember the Korg Wavestation AD, which let you use some of that device's internal signal processing on live input signals? This sort of feature has made a comeback in recent years on synths like Roland's JV1080 and JP8080, Quasimidi's Sirius, Novation's Supernova and Waldorf's Microwave XT. Where most of these implementations allow you only to route audio through the synth's filters and effects, however, Live Mode allows more sophisticated processing using the K2500's synthesis engine, keyboard and sliders, pedals and wheels, arpeggiator, sequencer, the KDFX's five independent digital audio signal processors, and a 16‑bit stereo digital input.

In Live Mode, signals appearing at the K2500's sampling inputs can be accessed in real time just as if they were samples residing in RAM. This means that any signals you throw at the K2500 can be processed in real time, using all of the VAST synthesis functions, including key‑based pitch changing, filters, envelopes, LFOs, waveshaping, multiple layers, and even reversing (!), plus all of KDFX's DSP functions. You must have Kurzweil's sampling option installed on the K2500 to use this feature, but if you don't, this is a reason to get it! Either the digital or analogue inputs can be used for Live Mode, and the two input channels can be treated as a stereo pair or dealt with individually, each with its own VAST and KDFX processing.

The quality of the Live Mode feature is sufficient that you can use the K2500 just to do live signal processing and A‑to‑D conversion, and you could even conceivably use it to do CD mastering, taking advantage of the onboard EQs, compressors, limiters, and so on. For those of us long‑time synthesists who miss the input jacks on old analogue synths, which allowed us to do so many fun things with external sound sources, Live Mode is very exciting.


  • Very powerful and flexible DSP.
  • Excellent MIDI implementation.
  • Live Mode lets you use all of the K2500's processing features with external signals.
  • Hundreds of presets to fool around with.
  • Good user interface considering the small screen.


  • Very complex structure, not easy to grasp right away.
  • User interface confined to small screen.
  • Needs to be the clock master when interfacing with other digital gear.
  • Individual digital outputs in Kurzweil proprietary format.


KDFX adds an impressive amount of DSP power to the K2500, providing just about any kind of signal processing you can imagine, and then some, in a highly flexible environment. It may not replace your whole studio, but it will add an awful lot to it.