Offered as an ambience recovery device, the Digital Domain DD-2 K-Stereo Processor ($3,500) is that and more. Practically a career's worth of customized tricks-of-the-mastering trade have been built into the device by its creator, mastering engineer Bob Katz. These are useful for enhancing the depth and dimension of stereo mixes (including spatially-challenged ones), credible stereo upmixing of mono sources, making mixes sound clearer or bigger, adjusting the front-to-depth ratio of a recording, and Mid/Side rebalancing. Added for good measure are high and low-shelving program equalization and POW-R word-length reduction.
Product Points
Applications: Mastering, post production

Key Features:Choice of processing algorithms; mono to stereo up-mix; program EQ; M/S processing; POW-R dithering; digital I/O; MIDI programmable

Price: $3,500

Contact: Digital Domain at 407-831-0233, Web Site.


+ Easy to use

+ Capable of improving a wide range of sources in several ways

+ Excellent mono compatibility

+ 96 kHz/24-bit resolution


- Digital I/O only

Operation of the DD-2 is simple and straightforward. The first step is to select the ambience recovery algorithm using the central rotary encoder. The choices are: wide/deep, wide, small/deep, and small. Follow this by setting the "K-level" rotary control for the desired amount of process and, for the most part, you are done. The K-level value can be thought of as the relative position of a stereo effects return fader blending the recovered ambience back into the main signal. The range of this control is

-90 dB to +6 dB. At its +0.0 position, the K-level is deemed to be at a viable starting point. This setting should not be confused with zero effect. Think of it as a return set to unity gain. At 0.0, the effect level returned to the mix is approximately 15 dB down from the source. The maximum K-level setting of +6.0, while not unusable, approaches the point of the tail wagging the dog. The right-hand rotary encoder is then used to switch the effect on and off for comparisons without affecting any other settings.

This basic operation of the device is supplemented with an assortment of filters and level controls. The effect, or recovered ambience, can be EQed with high-pass, low-pass and bell-shaped filters ranging from 28 Hz to 20 kHz; the bell filter permits ± 12dB and a Q of .4 to 8.0. The direct, preprocess input can also be equalized with high and low frequency shelving filters, again ranging from 28 Hz to 20 kHz. Individual level offsets can be set for the left, right, middle and side channels. The latter pair are very useful for tweaking mixes in which the phantom center is slightly out of balance with the sides. These parameters are selected with a bank of pushbuttons on the right side of the unit, and adjusted with the rotary encoders.

The device will store 99 user presets, with preset 0 reserved as default. An A and B register are provided for instant comparisons of two settings. To top things off, the DD-2 offers TPDF dithering of its output to 24 bits and 20 bits as well as the highly-regarded POW-R process for dithering to 16 bits. The device is digital I/O only, and will run at sample rates up to 96 kHz. The POW-R dithering function is not available at double-sampling rates, though I would be hard pressed to think of an instance where anyone would want to master at 96 kHz/16-bit. All functions of the device are MIDI-controllable, allowing the DD-2 to fit into automated scenarios. Upon powering up, the unit reverts to the last user settings. A nice touch is an indicator that blinks if any parameters are set away from their null positions.

In Use

Just what is an "ambience recovery" device and who needs one? I am not sure I can really answer the first question because the designers are keeping the specifics under wraps while a patent is pending. In the most general sense, the algorithm is an intricate elaboration on elements of spatialization theory such as Haas Effect and acoustic channel differences in the stereo field. The underlying proprietary techniques were created by grandmaster mastering engineer Bob Katz, culled from over 10 years of development at Digital Domain, and were implemented in silicon by Dr. Glen Zelniker of Z-Systems. What I found remarkable in my trials with the DD-2 is that the implementation did enhance a wide variety of sources, some quite significantly, while retaining very good mono-compatibility. For some tracks the unit literally saved the day, for others it was not really appropriate, but when it was right, it was really right.

My first application of the box was for a client who had just composed and recorded a score for an independent film which, due to budget restraints, was mixed from synthesizers in his project room. He specifically requested that I try to add some depth and to increase the size stereo image. It was ironic that the review model had arrived only a day before. This job was made to order for the DD-2 and we both ended up very pleased with the wide/deep setting at +3.0. The DD-2 mastered version went to the dub stage for use in the movie. Overall, the treatment gave the score a much more polished and impressive stature.

My next trial involved the remastering of a 1952 mono recording by legendary Columbia Records producer Goddard Lieberson of the operetta The Student Prince. The recording was done live in one of the great New York City rooms of the era. I have never been a fan of creating simulated stereo from a mono source, but the DD-2 did not disappoint. The process gave the mono source just enough spread to be credible. The orchestra and chorus took on a more involving image and there was a realistic sense of depth. The DD-2 effect created what seemed like specific room reflections on the pizzicato bass, but not unlike the kind of echoes discernable in a stereo recording of this type.

Every mastering engineer must deal with the less than stellar mix from time to time. With more and more material coming from rooms with compromised monitors and less-experienced engineers, the problems can sometimes be severe. The DD-2 excelled at resuscitating some mixes where there was too little definition, where too much information was crowded into the center, or where there was not much left-right differentiation. That is not to imply that the device is a panacea, but it did literally save the day on more than one occasion. I do not think some of these projects could have been saved another way short of a daunting amount of heroics and microsurgery. Of the dozen or so projects on which I tried the device, only in one instance did the producer prefer the original imaging, before processing.

I placed a call to Jonathan Wyner of M-Works in Boston to get his take on the DD-2 after he had made some trials at his mastering room. Jonathan said he found that although not every mix benefited, it did make some mixes sound "more like you thought they should have sounded."

Although the designers may not have envisioned this, the DD-2 can also be pushed to the extremes by applying an overly large amount of the onboard EQ to the recovered ambience. The EQ is of very high quality, similar to that of the venerable Z-Systems ZQ-2, and with it the device can be made to produce some fun, if not bizarre ambient effects.


Some mastering processors are the steak, and others, the sauce. Depending on the program, the DD-2 can be either, or neither. While not appropriate for some projects, the device seems virtually essential for others. The DD-2 offers some very clever tricks for enhancing stereo imaging and definition in a high-quality and easy to use implementation. The device is a valuable addition to the mastering or post production house that has the basics covered, and is now in need of the kind of special capabilities that distinguish the cream-of-the-crop facility.

Review Setup

SEK'D Sequoia DAW, LynxTWO sound card; Z-Systems digital router; Mytek DAC; EMM Labs Switchman input controller; Bryston 5BST amplifier; Dunlavy Aletha monitors

Katz on K-Stereo

Bob Katz, the inventor of the K-Stereo algorithm, says: "K-Stereo is a process that literally extracts the inherent ambience, space and depth in a recording and allows you to manipulate it. For example, if the original recording was made with a dry snare drum and a wet vocal, you can enhance the inherent reverberation in the voice without changing the mix of the snare! It's literally like having a handle on the reverberation returns after the mix has been made. I know this sounds implausible, but it can be done and there is a solid psychoacoustic (patent pending) basis for the principle. In mastering we often get recordings that are too small, and K-Stereo is one of the best cures for that disease because it does not affect the inherent mix, unlike typical M/S and other such 'widening' techniques. K-Stereo is totally natural and in addition, it improves the clarity and definition of the instruments and vocals as part of the psychoacoustics of the algorithm."

- Alan Silverman