You are here

Rocktron Xpression

Multi-effects Processor By Paul White
Published March 2004

Rocktron XpressionPhoto: Mike Cameron

This versatile effects processor has not only been designed for guitarists and bassists, but also for the home studio owner.

Rocktron have been designing and building high-quality effects for guitarists for longer than I can remember, and the Xpression continues that tradition. At the outset, though, it is important to understand that, although the Xpression is primarily a guitar/bass digital multi-effects processor that includes speaker modelling and mic position modelling, it doesn't include an amp modeller and so needs to be used in conjunction with a guitar preamp of some kind. The 1U rackmount Xpression can run up to 10 effects simultaneously and, as with some earlier Rocktron products, the clean portion of the signal patch may be kept in the analogue domain to avoid the tonal change that guitarists seem particularly averse to. However, to use the in-built noise reduction system on the dry sound, it's possible to select a digital clean signal path when needed.

Another very welcome feature, especially in live performance, is that the delay and reverb effects can continue to their natural conclusion when patches are changed, so there are no unwelcome abrupt changes as the reverb gets chopped off mid-flow. This is accomplished by separate Spillover settings for the delay and reverb within the patch parameters. However, it is important to note that, when this is selected, the reverb and/or delay settings for the initial track are carried over to the next patch when you change, though if the delay is switched out in the second patch it will still remain muted. Provided that you organise your patches so you switch from a patch featuring delay to one that doesn't (as opposed to switching between two patches with totally different delay settings and delay times) life should go smoothly. But now that RAM and DSP chips are so cheap, it shouldn't be too much to ask that serious processors, as this purports to be, should be able to handle completely seamless transitions quite regardless of the effect settings, without having to suffer effect truncation or limitations on the effect settings.

As the unit is intended for live use as well as studio applications, it has a real-time tap-tempo function and it also includes Rocktron's own Hush noise-reduction system to reduce background noise present in the input signal. This comes before the effects, so it won't affect reverb tails or repeating delays and may be applied to the dry signal if the digital dry path is selected in the Global menu.

Selectable High-pass Filter

Another very important feature for bass players in particular is a selectable high-pass filter section that works like a crossover to limit the low frequencies being fed to the effect section. This is important, because certain effects can really rob a bass of its punch, so being able to keep the bottom octave or two intact by limiting the processing to above a certain frequency is a big advantage. The low cutoff can be set to 80Hz, 160Hz or 240Hz, which should cover both bass and guitar requirements. Processing is courtesy of a 24-bit Motorola DSP chip.

The rear-panel connectivity of the unit comprises stereo inputs on quarter-inch jacks, though a mono source may be connected to the one marked Mono if needed. The outputs are, of course, in stereo to take full advantage of the stereo effects, and the output level may be set to -10dBV or +4dBu via one of the front-panel buttons as needed. There are also what appear to be two power input jacks, but in fact one is for the power and the other is to feed power to a Rocktron floor controller. The two sockets are identical, but are internally linked so that plugging into the wrong one shouldn't cause any nasty surprises. MIDI In and Out/Thru sockets are also fitted, so that a remote MIDI controller may be used and so that patches may be dumped to a suitable storage device via SysEx.

Other than the fact that power comes from an external adaptor, the Xpression looks extremely solid and professional, with a straightforward (if not exactly streamlined) operating system to match. The front panel sports just four detented programming knobs and four buttons, plus two level controls (Input and Output) in addition to a large, 16-character display window. Selecting one of the 128 factory presets is simply a matter of turning the preset knob until the desired preset shows up.

Rocktron Xpression rear panel.Photo: Mike Cameron

MIDI Control

Specific Xpression parameters can be assigned to MIDI Continuous Controllers so that any MIDI remote control device can be used to control the effects. However, Rocktron have their own series of floor controllers specifically designed for use with their products, which run from the power socket on the back of the Xpression and so need no additional power supply. Essentially you dial up the parameter you want to control from a menu of available parameters, then dial in the controller number you'd like to use to control it. You can also set limits on the control range of relevant parameters, and sets of controller assignments can be copied independently of other patch data for use in other patches.

MIDI may also be used for program changes, and there's the facility to create a program change map so that you don't have to move your presets around to make them match the patches on other MIDI devices you may have in your system. And of course you can use SysEx to dump or load any or all of the Xpression patches. The MIDI channel may be set to any number between one and 16, with a choice of Omni mode if you'd like the Xpression to respond to all incoming channels. Overwritten factory patches can be restored from the front panel, and this can be global or on a per-patch basis.

Editing Functions

Editing presets is almost as simple: turn the Function Select knob to locate the target Effect or Utility function, use the Parameter Select knob to find the parameter you wish to adjust, then change its value using the Parameter Adjust knob. Pressing Store saves the changes, and all 128 factory patches may be overwritten, modified and renamed where necessary. A Compare button is available to compare the difference in sound between the original patch and the edited version prior to saving. Tempo delays and modulation rates can be set via the front-panel Tap button, where a blue LED flashes at the current tempo/delay-time rate. On the review model, the detented Function control was somewhat stiff, but as the others were OK I'm assuming this was an isolated fault.

One of the effects is speaker modelling, with a choice of speaker sizes from eight to 10 inches, and also some miking options to provide a greater choice of tonal colour. The effects types (listed in the 'Xpression Effects Blocks' box) are mainly variations on the standard delay, modulation, reverb, and pitch themes we've all come to know and love, but don't let that put you off, as Rocktron deliver these effects with style. All ten effects sections can be active at once if you wish, and patch changing is fast and smooth with no embarrassing mutes or glitches.

Flexible Mixer Modes

You may have noticed that there's no wet/dry mix control on the front panel, and that's because the Xpression uses a separate mixer section to determine the effect levels and balances within the preset. Instead of a single mix parameter, there are separate left and right direct-level adjustments as well as an effect level adjustment, and it's in the mixer section where you decide whether the dry portion of the signal should be analogue or digital. In analogue mode, the input is passed directly to an analogue mixing stage at the output, where it is combined with the digital effects signal, rather like the aux send/return loop in a mixer. In fact, the main effects have their own input level controls within the mixer, which corresponds to having four different aux sends for feeding effects in a traditional mixer. The final parameter is volume, which allows the relative levels of presets to be adjusted.

Rocktron's Hush noise-reduction system seems to be based on an expander, and what's on offer here is a digital recreation of their original analogue Hush circuit. Essentially it provides gentle low-level gating to clean up noise, and it has a user-adjustable threshold just like a noise gate. It comes directly after the A-D converters so it can clean up the sound feeding the effects as well as the dry signal (where the digital path is selected). Like the other effects, it is always available in every patch.

Xpression Effects Blocks

The Xpression provides signal processing (in the form of noise-reduction, compression and EQ) as well as all the usual effects. These include a stereo delay with filtering in the feedback loop to help emulate the warmth of tape echo units, and a simple reverb section with no choice of models and adjustment only over level, decay time and high-frequency damping. The modulation section includes a tremolo with variable wave shape; a phaser that can emulate up to six stages of analogue phasing; a dual-voice flanger for stereo effects, where the modulation rate of one path can be set as a percentage of that used for the other channel; and a dual-voice chorus. There's nothing fancy about the pitch-shifter — it produces fixed offsets from -24 semitones to +12 semitones and has variable fine-tuning as well. The shifted signal may be panned in the mix, but is otherwise mono.

Speaker simulation is far simpler than on a typical guitar modelling preamp, and offers simply a choice of eight, 10, 12 or 15-inch speakers, with a mic placement function that moves the virtual mic position from the centre of the cone towards the edge of the speaker. A Reactance setting simulates the interaction between the output transformer of a guitar power amp and the speaker cabinet, where you can adjust the bass end from tight to thumpy. This works very convincingly.

Finally comes the rotary speaker simulation, and to use this you need to call up a special effects configuration. You can set different fast and slow rates, as well as determining the rate at which the speed changes when you switch from a fast to slow setting. It's also possible to change the balance between the upper and lower rotors.

Effects Routing Options

The effects themselves are arranged in specific configurations that differ depending on whether the rotary-speaker simulation is being used or not. In speaker simulator mode, the signal path appears to be split into two after the Hush, Compressor and EQ/Speaker Simulator blocks, with one arm feeding the rotary-speaker effect and the other the stereo delay. The signals are then recombined before passing through the reverb section. The crossover circuit that is normally used to prevent low frequencies from being effected is used in this configuration to split the signal between the upper and lower rotors of the rotary simulator, so it isn't available for any other purpose. Also, in this configuration, you can use either the EQ or the speaker cabinet simulation, but not both together.

Where the rotary-speaker simulation isn't being used, the effects configuration has the same chain of noise-reduction, compression, EQ and speaker simulator, but without the restriction on using the EQ and speaker simulator together. The direct portion of the signal is taken off after the speaker simulator, following which the switchable high-pass filter is used to remove the bottom end (where required) from the signal feeding the modulation, delay and reverb stages. The contribution of each effect block (Pitch, Phaser, Chorus/Flange and Reverb) is set using the mixer send level controls discussed earlier, and the tremolo effect may be switched before or after the reverb. Chorus and flanging can't be used together, so when one is switched on, the other switches off.

The Global menu enables you to select either guitar or bass operation, where the former bypasses the high-pass filter feeding the effects section. The outputs can be set to stereo or dual mono, and there's also a mute parameter that can be assigned to a MIDI controller for live use, which avoids noises when changing guitars and so forth. Although Hush is individually adjustable per preset, there's also a master offset parameter in the Global menu which affects all the presets, and I would imagine this would be very useful if you were playing live and turned up at a venue where the background interference level was higher than normal. You can also kill the dry signal in the Global menu to allow the Xpression to be used in the send/return loop of a mixer or guitar amplifier that has its own dry/effects mix system.

Xquisite Or Xecrable?

I have no problem with any of the effects provided except for the pitch-shifting, which has a noticeable delay of what sounds like 100mS or more (though this can be reduced at the expense of smoothness) and also has the characteristic modulation or shimmer that makes it sound slightly out of tune. However, it's absolutely fine for mild detuning effects. The delays offer a number of edit options that go some way towards capturing the warmth of tape delays (though there are no serious multi-tap variants for emulating multi-head echo), the reverb is dense and warm, and the choruses are quite excellent. The rotary speaker emulations are also rather better than I expected and include user-programmable acceleration and deceleration, as well as having the simulated high-frequency horn change speed faster than the low-frequency rotor.

In addition to the normal effects, we also have the familiar guitar processors in the form of compression and amp-style EQ with bass, middle, treble and presence, but with the addition of parametric controls. There's also the speaker simulator, but I found this to be quite disappointing, as the smaller speakers tended to sound a little edgy, while the larger ones (12 inches and above) sounded distinctly dull to my ears. Judicious use of EQ can coax some very usable sounds from the speaker simulator, but it doesn't have a naturally 'right' feel about it.

Although the Xpression delivers mainly stock effects, it does so extremely cleanly and with a very professional polish. Its only sonic shortcomings are the pitch-shifter (which is almost impossible to get right in a budget processor) and the rather bland speaker simulator, but other than that the effects are all extremely rich and musical, with the rotary speaker simulation worthy of special mention. The designers have included some useful functions for the live performer, specifically the low-cut filtering on the effects feed, for use with bass guitars, and the Spillover feature — although, as I mentioned earlier, this comes with certain restrictions, so it isn't a total solution to the problem of creating seamless effects changes.

So, who needs it? Well, if you already have an amp simulator such as a Line 6 Pod XT or one of its competitors, then there's probably not a lot to be gained by adding an Xpression, because virtually all the modelling preamps provide a decent enough repertoire of delay, reverb and modulation effects. However, if you record or perform live using a guitar (or bass) amplifier and can use an effects processor in its effects loop, the Xpression will give good results there, and of course it works fine as a regular studio rack effect in conjunction with a mixer. While the Xpression might initially seem a bit low on excitement value, it delivers the kinds of effects that most players and engineers use most of the time, and it does so quietly and with a lot of class.


  • Clean and warm effects.
  • Straightforward operating system.
  • Large display for live use.
  • Switchable pre-effects low-cut filter for bass use.


  • Disappointing speaker simulation.
  • Only one reverb algorithm.


The Rocktron Xpression doesn't offer anything new, but it does an excellent job at handling all the old standards in a guitar- and bass-friendly format.


Xpression £249; MIDI Mate foot controller £199.99. Prices include VAT.