Producers and recording engineers recognize that only a handful of digital reverbs are worthy of the title: main reverb. These are all great-sounding units that can adapt to any source, from strings and vocals to hard-sounding drums and percussion. Despite the TC Electronic Reverb 4000's diminutive physical size (its aluminum and steel chassis is just 1U and 8-1/2-inches deep), it belongs in that rarified class because it is a single-DSP engine version of TC's flagship System 6000. After using the Reverb 4000 during a month-long album-mixing project, I found it consistently proved itself on every song as a first-class, no-compromise main reverb.

Strictly a digital reverb (no phasers, flangers, chorus, delays, pitch-related effects, EQ or dynamics), the 4000 comes with 150 stereo presets culled from TC's 6000, M5000 and M3000 units, along with classic reverb emulations such as EMT 140 plates, EMT 250 and AMS NonLin. There are 100 additional memory locations for user-modified presets and another 100 can be held on a PCMCIA card (front-panel slot). The unit comes with editor software on a CD-ROM that, via a USB connection, turns any PC (Mac in late 2003) into a virtual version of the TC ICON remote controller: a hardware unit controlling the System 6000 and DB-8. Although using the 4000 with the ICON in the studio is tré sexy with every section and parameter quickly accessible on beautifully designed pages, the Reverb 4000 functions the same as a stand-alone unit, with all salient parameters available on the easy-to-learn and use front-panel interface.

The user interface design is very intuitive and immediate; in general, push a button, enter a short menu screen and adjust parameters with the large encoder wheel. The front-panel display is tiny and has LEDs for L/R input-level meters, overload, sample rate, MIDI programming and Preset Editing mode. The 56 X 128-dot graphic LCD shows which preset is running, as well as submenus for Recall, Store, Utility and Wizard functions.

The Reverb 4000 uses 24-bit AD/DA converters running at 44.1 or 48 kHz, 88.2 or 96 kHz in double-rate mode. Both A/D and D/A converters run at 6.144 MHz using delta-sigma conversion at 48/96 kHz. In addition to balanced XLR inputs and outputs, there are AES/EBU and S/PDIF digital I/O connections, as well as Tos-link/ADAT Lightpipe sockets and a Word Clock/Sync Input RCA connector.

The I/O menu lets you select between digital or analog I/O, clock source, analog and digital level trim (24-bit transparent at 0 dB), ADAT routing and output dither. Peak analog levels default to -18 dBU (12 dB of headroom) and are adjustable from -11 dBU to -21 dBU. There are no hardware front-panel input/output level or wet/dry mix pots. Wet/dry mixing takes place internally, and the Reverb 4000's internal signal path uses 48-bit double-precision processing. I tried the unit using both the analog and digital I/O and could not tell any sonic difference in the reverb sound or the dry-source audio.

Live sound mixers will like the Kill Dry feature: When Bypass is pushed, you have the choice (in the I/O menu) of hearing the reverb tail continue or not. Finally, the Utility menu is for MIDI parameter setup, memory copying and card formatting.

The Reverb 4000 uses only algorithms developed by TC Electronic, and each preset's unique set of parameters is fully adjustable. The manual provides good explanations, so you can edit with confidence and purpose. Algorithms include VSS-4, a true stereo reverb that, according to TC, is based on "source-related reflections from multiple angles--comparable to real-world mono or stereo sources positioned in an authentic or virtual space"; VSS-3, a multipurpose algorithm from the M3000 with many adjustable parameters in the early reflection, reverb diffusion and modulation sections; NonLin-2, an effect reverb with adjustable attack, hold and release; DVR-2 (the EMT 250 emulator), a generic reverb; Reverb 4, a very adjustable room ambience; VSS-4TS, another true-stereo reverb that utilizes two linked reverbs; and Ambiator, which provides many simulated acoustic environments. The latter, along with NonLin-2, are mostly used in the Effects bank presets.

Memory locations are arranged in four contiguous banks: halls, rooms, plates and effects. Without using the ICON software, scrolling through presets is a little tedious with the wheel, as there is no direct-access keypad to access a specific preset. You can skip through banks with the Up/Down buttons and then scroll to the preset. Thoughtfully, TC has added the Recall Wizard feature, a kind of search engine or filter to find factory-recommended reverb presets. Search engine results are determined by the preset's origin (where it originally came from), size (micro, small, medium, large or extra-large acoustical spaces) and source (what the reverb is going to be on). In addition to new users of the 4000, this is the perfect feature for anyone who wants some type of ambience treatment but doesn't know which preset to start with (or just wants to explore). Search results can be selected with the wheel or listed on the ICON's main screen.

Modifying presets is very simple: Three dedicated front-panel knob controllers adjust the three most important parameters of any preset, usually (except for NonLin-2 and Ambiator) predelay time, decay time and a high-frequency parameter, such as roll-off, coloration or decay. Once a preset is recalled, the Up/Down keys move you through successive and lesser-important trios of parameters that are also adjustable by the same three knobs. Along with the selected preset, these three parameters remain current and running; they are always visible at the bottom of the display even when you browse to have a look at other presets. There is a helpful flashing Recall/Enter reminder in the display when you are not looking at the currently running reverb's parameter menu. If you get lost in the wilderness of parameters, push the Home button and you are automatically backed out to the root page; i.e., the initial page that comes up when you first selected the preset. Using the ICON after selecting a preset, you'll get a row of six virtual-parameter sliders: the previously mentioned first three and the next lower level's three.

Once you've selected and tweaked a preset to your needs, you can rename it and store it. I liked that the 4000 defaults to the next free user-memory location and stores it using the original preset's name if you don't rename it yourself. You can store at any time if you want to try different parameter settings of the same preset and A/B between them. Toggling between presets has a momentary lapse of one to two seconds when loading a new preset. I could not find a way to compare my parameter changes to the original factory settings other than to store both versions in two user-memory locations and switch between them.

I installed the included ICON software editor in my 800MHz Celeron PC. Installation went fine, although at a later date, I had to reinstall the software for some strange Microsoft XP Pro reason, I think. When you connect the USB cable to the 4000, the software detects it and loads all presets and setup data. The ICON pages filled up (vertically) my 768 X 1028 flat-panel computer monitor with the virtual sliders and buttons, which are visible from across the room under any lighting. ICON can control up to eight TC Reverb 4000 units on the USB bus.

When mixing inside of Pro Tools, using an external reverb saves loads of DSP resources. I connected the 4000 to my Apogee AD8000 interface using both stereo send and return paths. To take full advantage of the 4000's true-stereo reverbs, it is important to send in stereo, even from mono sources. My biggest problem was deciding which preset to use. I found the VSS-4TS true-stereo reverb algorithm wonderful for a realistic drum room sound, and the AMS NonLin-2 effect put the lead vocal track in a 'sonic picture frame' on a faster-paced song. The EMT 140 Long preset was a good track reverb for an old-school rock ballad, while Chorus Hall and Crystal Hall presets worked well as an overall main diffuse reverb on vocals and acoustic guitars. My favorite preset, Dual Backyard, is a true-stereo preset that uses two reverbs linked together (each reverb's parameters can be locked together) and works best if you use a pan pot on your stereo-effect send. I would match that pan pot to the track fader's pan position. I used this preset on an entire track with each instrument placed across on a virtual stage.

The TC Reverb 4000 sells for $2,999 MSRP and belongs in the pantheon of main, professional digital reverbs where smooth, lush and highly detailed reverberation is a must. I found that the 4000's reverbs were very wide stereophonically, noise-free, transparent and easily heard in the mix. The unit's small LCD and lack of a direct-entry keypad make the ICON software editor necessary for serious programming and preset management. I would suggest a less-expensive, blank front-panel version (ICON software required) as a more cost-effective way to enjoy the 4000's superb sound.

TC Electronic, 805/373-1828, www.tcelectronic.com.