Dbx Quantum

The Quantum from Dbx is the latest entrant into the growing market for all‑in‑one digital mastering boxes, offering competition to the likes of TC Electronic's Finalizer and Drawmer's Masterflow. Hugh Robjohns finds out how well it stands up in this illustrious company...

Mastering is supposed to be one of those 'black arts' of the audio industry — a process combining technology with art, and both at their highest levels, to magically turn a so‑so track into the next double‑platinum hit. The reality is that all that is actually required are a fresh pair of ears, an artistic appreciation of the material, a good technical understanding of audio systems, and some appropriate tools with which to perform the necessary processing.

The first three points are entirely defined by the attitudes and experience of the individual doing the mastering, but the last one is rather more easily addressed. Traditionally, mastering houses have contained an array of well‑engineered precision signal processors, carefully selected for their ability to affect the sound of a track, either in a technical or an artistic manner. Typically, level and stereo width adjustments, equalisation, compression, expansion, and limiting are the main processes used in mastering, but tape‑saturation emulators and other 'quirky' processes also crop up from time to time.

A few years ago, some of the leading 'hi‑tech' companies realised that the enormous power and popularity of digital signal processors could be used to provide a complete set of precision mastering tools within a single unit. Not all such machines became successful, of course, and the very fact that they conducted their processing entirely in the digital domain made the concept a non‑starter for some end users. However, some of these units grew to become industry standards — the most obvious being the original TC Electronic Finalizer. In recent years, many similar multi‑function mastering units have become available, most being intended for home‑studio users who want to apply mastering polish to their recordings without the expense of going to a professional mastering house. However, I must insert a caveat here to the effect that having the tools and knowing how to use them are completely separate issues — the art and craft of mastering should not be underestimated!

The Dbx Quantum is the latest 'mastering suite' processor to appear on the market, and faces stiff competition from the long‑established TC Electronic units as well as the other new boy on the block, Drawmer's Masterflow. In terms of specification, however, the Dbx unit offers impressive facilities which should enable it to take on this competition on a level playing field.

Structure

The Quantum's large LCD is used to good effect in setting up each of its processing sections.

The Quantum is capable of performing all of the key processes needed in a modern mastering application. Thus it offers four‑band compression, limiting and gating (with adjustable crossover frequencies); broad‑band compression, limiting and gating (with side‑chain equalisation); stereo balance and width adjustment; level normalisation; parametric equalisation; de‑essing; an ambience program (essentially compression with gain to bring out low‑level detail); and output formatting (selectable bit and sample rates, dither, and SCMS status). Also included in the package is Dbx's own 'Type IV' 24‑bit, 96kHz A‑D converter system, which is claimed to provide greater headroom than conventional designs, and incorporates a subtle 'Tape Saturation Emulation' mode. Another bespoke feature is a 'Transient Capture Mode' which enables the level‑sensing elements of both the multi‑band and broad‑band dynamics processes to get a sneak preview of the audio signal. The idea is to preserve the all‑important initial transient of the audio programme, by allowing the dynamics processes to act in advance, before the transient arrives — although care is needed in setting the TCM system up, as there are some technical and artistic pitfalls for the unwary! The Type IV converter, TSE and TCM systems have all been seen before on previous Dbx digital products.

The Quantum is housed in a slim 1U rackmounting black box with a brushed metal front panel. It is equipped with a full range of analogue and digital inputs and outputs, and balanced or unbalanced analogue signals can be hooked up to either the XLR or quarter‑inch TRS sockets on the rear panel, which can accommodate signal levels up to +24dBu. Digital inputs are catered for on both AES‑EBU (XLR) and S/PDIF (phono) connections, the appropriate input being selected from an on‑screen menu. Signals are output in the same formats, and are all available simultaneously. Also on the rear panel are a pair of BNC connectors for word clock in and out, including a Superclock mode for use with suitably equipped machines — indeed, the Quantum's digital synchronisation capabilities are comprehensive. The standard MIDI connectors are also present.

The internal structure of the machine is logical enough. The analogue inputs pass through an input buffer with a front‑panel level control (ranging from off to +16dB) and LED bargraph metering. The signal then reaches the 24‑bit, 96kHz A‑D converters with user‑selected Type IV characteristics and TSE mode — this last with a separate LED meter. Once the signal has been digitised it is available for selection alongside the digital inputs, and is routed on to the DSP for manipulation within a 48‑bit processing environment before being dispatched to a
sample‑rate converter and requantiser. The data can be requantised from 24 to 20, 16 or 8 bits with various dither and noise shaping options, and the SRC can transcode data between 96, 88.2, 48 and 44.1kHz sample rates. The data is then passed to the digital ports via an output formatter (which also sets SCMS codes in S/PDIF mode), and to a 24‑bit, 96kHz D‑A complete with separate analogue output level controls and LED bargraph meters.

The front panel of the Quantum is clearly laid out and pretty easy to use, although not all functions can be found intuitively. At the left‑hand end of the box are two pairs of analogue input and output level controls, with the relevant stereo bargraph meters above them. Between these meters is a four‑segment display which indicates the operation of the Tape Saturation Emulation. Next is a large backlit LCD panel and a large encoder knob. The LCD carries details such as the current program name and number, a block diagram of the active processing chain, the status of functions such as the Type IV converter and TSE, and an excellent multi‑band gain‑reduction meter for each channel. The control knob has a detented action when rotated in either direction, as well as a push switch function, and provides the means of selecting and adjusting algorithm parameters. An array of 15 illuminated push buttons provides dedicated access to most processing and utility elements of the machine, although some functions are hidden in layers behind some of the buttons (the 'Other' button being an obvious example). A small mains power rocker switch completes the operational controls.

Setting The Quantum

Hooking up the inputs and outputs is a simple business, but is very flexible and allows the machine to fit easily into any audio system. The facilities to adjust sample rate, bit resolution, dithering and SCMS codes, as well as the provision of comprehensive clocking sources (internal crystal, external word, Superclock, S/PDIF or AES‑EBU references), makes this a very versatile and professional machine. Digital inputs can be accepted at 96, 88.2, 48 or 44.1kHz rates and output at any of the same rates, although the only up‑sampling provided by the sample‑rate converter is from 44.1 to 48kHz — 88.2 and 96kHz data can not be generated from lower source sample rates. The internal crystal word clock is claimed to be highly accurate and stable, with very low jitter, thanks to some proprietary clock chips. Consequently, Dbx make several recommendations in the handbook about using the Quantum as a master clock source in a digital chain.

The signal processing within the Quantum is organised in three distinct chains depending on the nature of the starting program (stereo multi‑band, stereo broad‑band, and dual mono), but there are two subsets of each of these, thanks to the pre‑ and post‑dynamics equalisation options. The de‑esser is only available in chains which incorporate the broad‑band dynamics section, and the dual‑channel mono mode cannot access the multi‑band dynamics at all. In multi‑band stereo mode the complete processing chain consists of either:

Stereo Adjust > Ambience > Crossover > Multi‑band Dynamics > Normaliser > Clipper > Output formatter

or, alternatively:

Stereo Adjust > Ambience > Crossover > Multi‑band Dynamics > Parametric EQ > Normaliser > Clipper > Output formatter.

In the two broad‑band modes, the basic chain is the same, except that the de‑esser replaces the crossover and a side‑chain equaliser is made available for the dynamics section. The dual‑mono modes consist of either:

Parametric EQ > De‑esser > Dynamics > Clipper > Output formatter,

or:

De‑esser > Dynamics > Parametric EQ > Clipper > Output formatter.

Algorithms

The Quantum offers an impressive array of digital interfacing and clocking options, and is capable of 24‑bit/96kHz operation.

Going through the list of processes, the Stereo Adjust section allows the left‑right balance of stereo material to be altered along with its stereo width, from mono through to extra wide. This function does not have a dedicated access button, and is found by repeated pressing of the 'Other' button. The Ambience mode resides here too, and is basically a low‑level compressor arranged to increase the audibility of quiet signals. Controls are provided for ratio, width (range) and threshold, and the effect of the process is to make background noise more obvious — hence apparently increasing the 'ambience' of the recording.

A third function accessed through the 'Other' button is the TCM or Transient Capture Mode. Most conventional gates and compressors can't react until a signal transient has exceeded the threshold, and thus part of the wanted signal's transient is often either damaged by overload or missed by a tardy gate. The TCM delays the main signal path through the dynamics processor, so that the side‑chain processing benefits from a 'sneak preview' and can therefore react before the initial starting transient arrives. Thus, the gate opens, or the compressor dips the level, fractionally in advance of the leading transient. The system works very well, but care must be taken to ensure that the TCM time is less than the recovery time of the compressor, otherwise the gain will have dipped and recovered before the transient peak even arrives!

The de‑esser is yet another function hiding under the 'Other' button. Parameters are provided for switching the process on and off, setting a filter frequency between 800Hz and 8kHz, selecting the filter type between band‑pass and high‑pass, adjusting the width of the band‑pass filter, and determining the amount of de‑essing applied — all standard stuff.

One final process lurking under the 'Other' button is the Type IV converter setup, with its Tape Saturation Emulation. This facility can be turned on and off, the degree of processing adjusted between 1 and 4, and the 'Colour' selected from 'dark', 'warm', 'none', 'light' and 'bright' — all self‑explanatory I hope. As this process is implemented through the A‑D converter circuitry it is not available to the digital inputs, which seems a shame, since digital sources are often far more likely to benefit from such a facility. In use, the TSE effect is quite subtle, affecting only high‑level signals, imparting the kind of progressive saturation and softness associated with analogue tape recording in days of old.

Many Bands Make Light Work

The Crossover section, accessed through the EQ/Xover button when in multi‑band mode, defines four separate bands for the dynamics processing. The individual crossover points and slopes (8 or 18dB/octave) can be tailored to suit specific programme material. In broad‑band mode, a five‑band parametric equaliser is made available in the side‑chain of the dynamics section with adjustable frequency, Q (0.25 to 16) and level (in 0.5dB steps) for each section. There is a global control over the type of Q (constant or adaptive), and the top and bottom bands are shelving types. Constant Q is typically found on graphic and parametric equalisers for corrective applications, whereas adaptive Q is more common on mixers, and tends to be a more creative tool. The standard parametric equaliser section is switchable to pre‑ or post‑dynamics processing, but otherwise has exactly the same facilities as the side‑chain equaliser.

The dynamics section contains a gate, compressor and limiter, in either a multi‑band or broad‑band configuration depending on the selected program. In both cases all the parameters are identical — you simply get four parallel sections to adjust in the multi‑band mode! Three dedicated buttons — Compressor, Gate and Limiter — access the components of the dynamics system, and pressing the relevant button access the first parameter page, the second and third pages being revealed by pressing the Previous and Next page buttons. In the multi‑band mode, the first press of the function button accesses the control pages for global adjustment of all bands together, but subsequent presses provide access to the parameters for each of the four individual bands.

Taking the compressor section first, the first parameter page provides an on‑off switch and the classic Dbx 'OverEasy' soft‑knee feature which softens the transition into compression. The parameter is scaled from 1‑10, with 1 being a hard knee and 10 providing an extremely gentle transition. The usual Threshold (‑60 to 0dBFS), Ratio (0.75:1 to infinity:1), and make‑up Gain (‑20 to +20dB) controls define the static characteristics of the compressor, and are found on the second parameter page. The third page carries Attack (0.1 to 200mS), Hold (0 to 500mS) and Release (360 to 5dB/sec) settings to define the temporal properties. Although it is unusual to see release times given in terms of dB/sec, it is a technically accurate way of specifying the response, and one which Dbx have chosen to use on many of their processors. If the control was set to 360dB/sec and 20dB of gain reduction had been applied, it would take 20/360 = 55mS to recover. At 5dB/sec it would take 20/5 = 4 seconds — all quite logical really.

The hold control is unusual in a compressor, but it works as you might expect, delaying the onset of the recovery slope for a set period after the signal has fallen back below the threshold. It is useful in avoiding repetitive dips in transient‑rich material, and gives a very smooth sound compared to a more simple device. An optional Auto mode (found on the first parameter page) continuously adjuss the attack, release and hold times according to the dynamic characteristics of the audio signal. In the multi‑band compressor, all of these controls can either be set globally for all four bands, or adjusted separately for each one. Obviously, the last option is the most powerful and creative because the four compressors can be optimised precisely to their frequency ranges — slower release for the low frequencies and faster for the higher ones, for example.

Gain reduction for each channel is shown on the LCD panel as a descending vertical bargraph; for the multi‑band mode, four adjacent columns represent the four separate frequency bands per channel, so the user is always aware of exactly what is going on. Also, when editing the compressor characteristics, the LCD shows a transfer plot of input levels against output levels, and a +/‑ display to show where the signal peak is in relation to the threshold — both very helpful tools.

The gate and limiter are adjusted in much the same way as the compressor, and share the same kinds of display features. The first parameter page for the gate provides on/off, Threshold (‑75 to 0dBFS) and Ratio (1:1 to 1:15) controls, whilst the temporal parameters of Attack (0.1 — 200mS), Hold (0 — 500mS) and Release (360 to 5dB/Sec) are on page two. Page three only contains the Maximum Attenuation parameter (0 to infinite dB). The limiter boasts on/off and Threshold (‑60 to 0dB) controls on the first page, Attack (0.01 — 200mS), Hold (0 — 500mS) and Release (360 — 5dB/sec) on the second, and OverEasy (off — 10) and the Auto mode on the last.

Utilities And Output Formatting

A button labelled Norm/Output initially provides access to the level normalising function. The user has to define a peak‑level section of the incoming audio by pressing the 'Edit' button at the start and the 'All Band' button at the end. The system then optimises the gain such that the highest level in that section reaches 0dBFS. Facilities are also provided to manually adjust the digital gain through the system over a +/‑12dB range, and to select a hard or soft‑knee mode for the final output limiter.

A second press of the Norm/Output button accesses the output‑formatting features. Here, the bit resolution, dither type and noise‑shaping parameters can be adjusted. A second page allows the requantised output to be dispatched to the digital ports, while the full 24‑bit signal drives the onboard D‑A. Three types of dither are provided: triangular, high‑pass triangular and a Dbx proprietary system called SNR2. There are also two noise‑shaping algorithms.

Whereas most of the menu screens consist of only a few pages (three for most and five for the equalisers), there are no less than 10 pages in the utility menu! However, most of these are 'set and forget' pages, although all are crucial to the correct operation of the Quantum. The first page concerns the input stage and selects the signal input source, the clock source and the operation of the sample‑rate converter. Clock sources can be the internal crystal at 96, 88.2, 48, or 44.1kHz, an external 256x Superclock at 48 or 44.1kHz, or an external reference signal on the word clock, AES‑EBU or S/PDIF inputs. The second page, logically enough, concerns the output stage, and determines whether the data format is S/PDIF or AES‑EBU, and whether the BNC output socket carries normal word clock or a 256x Superclock.

Page three returns to the input section again, and provides controls to adjust the levels of the two digital inputs and to insert a high‑pass filter (turning over at 1.75Hz) to remove DC offsets. The next five pages concern MIDI functions, and are extremely comprehensive. For example, three MIDI CC maps are included so that each fundamental type of processing chain (stereo multi‑band, stereo broad‑band and dual‑mono) can be controlled via external MIDI data. The usual MIDI channel, SysEx channel and MIDI Merge facilities are present, as are Program Change assignments and SysEx dumps.

The ninth menu page, unusually, provides access to a built‑in calibration routine to align the A‑D converters. However, the handbook offers no suggestions as to how often this needs to be performed, or what audible degradations might indicate a need for re‑calibration! The last page allows the LCD contrast to be adjusted, but fortunately you don't have to scroll through the preceding nine pages to reach it. Sensibly, Dbx have allowed the pages to be reached in reverse order by using the Previous Page button.

I can't fault the sound quality of the machine at all. The converters are excellent, and the ability to sample‑rate convert from a 24/96 analogue input to a 16/44.1 output, for example, is very useful indeed.

Impressions

The Quantum is a very impressive machine. The sophistication and controllability of its signal processing is exemplary, always allowing whatever effect was required to be found with relative ease. If you want to adjust every parameter manually you can, although it does become pretty tedious, with an awful lot of button pushing and knob twirling. Fortunately, the factory presets are well thought‑out, and offer excellent starting points. It is a shame the handbook doesn't list each of these programs' parameter settings, though, as that would make finding a suitable preset a lot easier than the random trial and error process I had to go through.

I can't fault the sound quality of the machine at all. The converters are excellent, and the ability to sample‑rate convert from a 24/96 analogue input to a 16/44.1 output, for example, is very useful indeed. Allowing the chosen output format to be optimised to the material with different noise‑shaping and dither options is also very good, and will certainly appeal to the professional mastering engineer.

Multi‑band compression is a supremely effective tool when it comes to really bringing a mix together and finding the sheer volume which radio airplay demands. The four‑band system on offer here is flexible but manageable, and is certainly highly effective. Combining simultaneous compression, gating and limiting allows a very tight and clean dynamic control without interaction, and with the side‑chain parametric equaliser available in the broad‑band mode, even this more traditional system works stunningly well.

The 'lesser' processes are equally important in determining the final sound quality, and I found the stereo width and ambience sections to be highly effective. The de‑esser was very capable, although I often found the multi‑band compressor and limiter to work as well, if carefully set up. The parametric equaliser is superb, both in terms of its sound quality and its controllability, and I really did find the whole machine to be an excellent synergy of related processes which allowed me to produce some very fine results indeed.

I'm not normally one to rave about a digital signal processor, but this one has impressed me a lot. It is well designed, has extraordinarily flexible I/O, creative factory presets, a reasonably friendly user interface, all the tracking, mixing and mastering tools you are ever going to need, sounds fantastic and, although expensive, is not ridiculously so. Although clearly aimed at the mastering market, the machine is flexible enough to find a worthwhile role in tracking and mixing applications too. Its combination of signal processing can be applied to good effect in almost any recording, PA or broadcast situation. One for the Christmas wish list, methinks...