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Digital Dynamics Processor By Hugh Robjohns
Published August 1998


The DDP takes the five dynamics processes you're most likely to want, turns them digital, and squashes them into a 1U studio workhorse box for under £600.

Possibly the best known manufacturer of analogue dynamics processors — compressors, gates, de‑essers and so on — dbx have employed the all‑conquering binary bit in their first stand‑alone digital dynamics machine, the DDP. The name is a contraction of Digital Dynamics Processor, and that's exactly what the unit is, so why not?

Rather than merely producing a digital replacement for a conventional compressor or gate, what dbx have created in the DDP is an amazingly powerful and compact multi‑function processor, a machine which can perform up to five dynamics functions simultaneously on a stereo signal, or four processes on each of two independent mono signals.

The DDP offers a number of preset processing configurations: two stereo setups, six dual‑mono configurations, or one single‑channel mode with external side‑chain keying. The sequential order of the dynamics processes can not be altered, but the options (in the prescribed input‑to‑output order) are:

  • 3‑band parametric equaliser (inserted in the side‑chain in some configurations)
  • Expander/gate
  • Compressor
  • De‑esser
  • Limiter

There is also a sixth block, which uses channel 2's input to drive the side chain of channel 1, enabling external keying and ducking.


The DDP is a typical 1U rackmounting device with the traditional black paint finish and a very readable yellow back‑lit LCD window in the centre of the front panel. A large data‑entry knob and function select buttons cover the right half of the panel, while the left features input and output level controls with associated bar‑graph metering.

The rear panel is just as straightforward, with XLRs and TRS quarter‑inch jacks provided for the electronically balanced analogue inputs and outputs. There's also the obligatory pair of MIDI In and Out/Thru sockets. An optional digital I/O module is available (but was not fitted to the review model), providing AES‑EBU and S/PDIF interfaces at 24‑bit resolution and 44.1 or 48kHz sample rates.

Analogue inputs and outputs are converted at 24‑bit resolution with a quoted dynamic range of 105dB (A‑weighted). However, dbx have incorporated a sophisticated soft‑limiting function in the A‑D stage, and this is claimed to increase the effective dynamic range significantly. The so‑called Type IV Conversion System is supposed to prevent brief input overloads from transients turning nasty in that familiar way that digital equipment has, and, using this system with suitable transient‑rich material, the specs claim that the usable dynamic range is something more like 122dB (A‑weighted).

Unusually, there is a built‑in calibration routine to align the A‑D converters, although the manual offers no suggestions as to how often this needs to be done, or what audible degradations may be observed when the converters require re‑calibrating! However, during the time I had the unit I didn't notice any problems.

I'm pleased to report that the machine has an internal mains power supply with an IEC socket fitted at the rear and a front‑panel power switch. There is no provision to change the mains voltage, nor externally accessible fuses, and I was surprised at how hot the casing around the power supply area becomes after a short period of use. It certainly was too hot to rest a hand on, although the machine didn't seem to suffer! I would suggest that if the DDP is destined for rackmounting there should be at least 1U of space above and below, to allow air to circulate freely.

Dbx DDP rear panel connections.Dbx DDP rear panel connections.


The DDP's input and output gain controls are detented and are calibrated from minus infinity to +16dBu and +4dBu respectively. The 0dBu marks are at different physical positions on the scale, giving an odd visual impression — the input zero point is at 12 o'clock whereas the output zero is at 3 o'clock.

An illuminated push button between each pair of gain knobs assigns the relevant LED bar‑graph meter to showing either input or output levels. However, there are no markings to say which position of the button is which (for the record, the button illuminates bright green when the meter is showing the input level). The front‑panel meters show signals down to ‑24, with the usual traffic‑light arrangement of coloured LEDs, but there are also two pairs of bar‑graph meters on the left of the LCD screen, showing the input and output levels down to ‑50dBu. These meters have a further advantage, in that they indicate both peak and average levels simultaneously through the use of a fixed column with a floating bar.

All of the dynamics processes sound superb and work extremely well — even when pushed pretty hard.

The right‑hand side of the LCD is taken up with the 'curve window' — a graphical display showing either the composite input/output transfer curve (for all the dynamics processes simultaneously); a frequency response plot when the EQ section is recalled for adjustment; the frequency selectivity for the de‑esser; or the processing 'chain' structure, together with a set of threshold display boxes (more on this in a moment). To the immediate left of the curve window a pair of gain‑reduction meters show the total amount of gain reduction applied by all processes, and these remain active even when the unit is bypassed.

Clearly, with up to five different dynamics processes going on at the same time, it can be hard to figure out just which element is doing how much of what! The DDP provides a collection of small graphical boxes in the top‑left corner of the curve window, or on the main 'chain' menu page. The compressor is allocated a row of three of these little 'threshold meter' boxes, marked '‑', '0' and '+'. At any one time, one box is reversed in colour from the others, indicating whether the signal is below, at, or above the threshold. The gate and limiter have similar facilities, but are restricted to just '‑' and '+' boxes; the de‑esser has a lone '+' box. This novel system gives the user an at‑a‑glance idea of just what is going on, although I found it took a bit of getting used to.

Dynamics Processing

All of the DDP's pre‑programmed effects 'chains' include a compressor as the central element in the signal path, with variations over the surrounding dynamics modules. Most chains add a gate at the front of the signal path and a limiter at the back, while others introduce a de‑esser after the compressor, or an equaliser before the gate. In several of the modes the equaliser is moved to the side‑chain, so that a spectrally modified signal controls the dynamics processes. A point worth making here is that all the dynamics processes share a common side‑chain signal derived from the input. This means that adjusting one processor does not interfere with the performance of the others — something which can be advantageous over the more usual arrangement of daisy‑chaining separate analogue units.

Setting up the machine and navigating the menus is, frankly, tedious, frustrating, and not good for the band waiting in the studio! The basic problem with the DDP is that there are not enough real‑time controllers (knobs to you!) and the LCD only shows three settings at a time for each selected process. With the compressor, for example, that means three pages of menus to trawl through. This wouldn't be so bad if the really important parameters were gathered on the first page of each set of menus — but they're not!

In my opinion, the three most crucial parameters of a compressor (or most other dynamics processors, for that matter) are the ones which have to be adjusted all the time to get the best from the machine. These include the on/off function, the threshold, and the release or recovery time. However, with software v1.110 on the DDP, the proffered parameters on the first menu page are on/off, knee and auto mode on/off (automatic signal‑related attack and release times). The threshold control turns up on page two and the release time on page three, requiring two and four button presses respectively to allocate the data‑entry wheel to them.

In a busy recording session you need to be able to reach for the appropriate knob and tweak the setting immediately. By the time I had found the right parameter in the DDP, the singer had usually finished the verse, and although familiarity increases the speed of operation, this mode of parameter access is never going to be as quick or obvious as reaching for a dedicated knob. I would have preferred to see a couple of data wheels defaulting to threshold and recovery time, with a push‑button to bypass the dynamics module.

There's also a possible bug in the software concerning some inconsistency in how the front‑panel controls are handled. A slightly lingering touch on the Limiter or Expander/Gate processor buttons causes the Next Page button to illuminate and the menu pages to cycle around at break‑neck speed. Pressing and holding either of the Compressor or De‑Esser buttons did not have the same effect.

Dynamic Tool Boxes

The inclusion of 50 factory programs designed to lend the 'LA vocal' sound or a 'radio mix' feel to the audio signal is an interesting concept. The hardware makes it easy to do, of course, but its value is debatable. Providing a number of different module configurations is totally valid and worthwhile, but I remain doubtful about the practical use of many of the programmed settings, other than as a starting point. Small variations in input level can have enormous effects on the quality and character of the processed sound and I found I couldn't use factory presets in their raw state, but always had to tweak and tune virtually every parameter, to optimise them for specific audio material.

The various dynamics processors all have as comprehensive a range of controls and parameters as could be wished for. They can all be switched on and off individually, and have the usual threshold, ratio, attack and release settings. The last is interesting: dbx have chosen to calibrate the release time as the rate at which gain returns to normal, rather than the approximate time it takes. Although the former is, strictly, the correct way to calibrate a release time parameter, there are few other machines on the market scaled in this way, and I would suggest that few people will feel comfortable when confronted with a range of settings between 360 and 5dB/sec! In practice, I'd hope the ears would be the final arbiter of a suitable release time setting, but those who like to 'paint by numbers' may find this aspect of the DDP a challenge!

The basic problem with the DDP is that there are not enough real‑time controllers (knobs to you!).

The gate module adds to the basic selection of parameters with a hold facility (setting the time for which the gate remains open after the signal has fallen below the threshold), and a 'Transient Capture Mode'. This TCM system inserts a user‑selectable delay (up to 3ms) in the signal path, allowing the side‑chain to take a sneak preview of the audio signal. The idea is to allow the gate to be opened fractionally in advance of an arriving transient leading the wanted signal. Most conventional gates can't open until the transient has exceeded the threshold, and thus part of the wanted signal's transient is often missed. I found the TCM system worked extremely well; the quality of a heavily gated signal was remarkably good, and far better than many popular analogue gates.

The compressor processor supplements the basic collection of operational parameters with an automatic signal‑related attack and recovery time setting, make‑up gain (of +/‑20dB), hold, and the inevitable (since this is a dbx product) OverEasy soft‑knee facility, complete with 10 different transition slopes.

The de‑esser has only three parameters — on/off, frequency selection, and de‑essing amount — while the equaliser is a full‑facility, 3‑band parametric design. Centre frequencies of all three bell‑shaped sections can range between 25Hz and 20kHz, with Q values of 0.25 (wide) to 16 (very narrow), and cut or boost of up to 12dB. If the equaliser is used in a program which places it in the side‑chain, a monitor mode option becomes available, allowing the side‑chain signal to be auditioned while the EQ is adjusted.

Squeezing The Pips

The DDP does exactly what it is supposed to do, in every sense. All of the dynamics processes sound superb and work extremely well — even when pushed pretty hard. For example, if correctly set up, the DDP's compressor causes none of the usual dulling which tends to beset analogue compressors when they are pushed hard. The sets of operational controls provided for each process are exactly what are required, allowing precise adjustment of every aspect of the machine and its treatment of audio signals. However, there are a number of other aspects of the DDP which do not impress as much. For example, I feel that using the relatively uncommon dB/second scaling for release time constants is a big mistake. Virtually every other compressor and gate on the market specifies release time as exactly that — a time for the gain reduction to return to unity — and the DDP's settings caused considerable confusion and debate amongst other engineers.

A rather more serious issue is that adjusting thresholds and release times for the various processes quickly becomes extremely tedious, frustrating and just plain slow! Having identified a process which requires adjustment, you have to select it, scroll around the various menu pages to find the one containing the desired parameter, assign the parameter to the data wheel and, finally, make the adjustment. Then you have to repeat the procedure all over again for the next process! The same rigmarole applies should you wish to bypass a single dynamics process to check on its contribution.

In a multi‑function machine, with a limited number of controls, this kind of ergonomic and operational problem is to be expected, I'm afraid. However, some manufacturers find practical systems which can be used effectively in the heat of a real studio situation rather than the R&D lab. Given the choice, I would still prefer to stick with a rack of separate outboard processors, simply for the benefits of dedicated control knobs.

I really feel that the DDP should have had at least a couple of knobs which defaulted to the threshold and release times of any selected process, since these are the two most critical controls for any dynamics unit. In conjunction with the dedicated buttons to select a wanted process (perhaps with a double‑press facility to bypass individual sections) this would make the unit a whole lot more usable in a real working environment. I know the provision of more knobs would add to the cost of the machine, but its excellent sound quality easily justifies a little more than its very modest current price tag anyway.

Although the DDP packs an awful lot of dynamics processing power into a compact box, and is capable of stunningly good results, setting it up is certainly not a trivial business, despite the 50 preset programs. Rather like a child prodigy, the dbx DDP is gifted, but difficult to deal with.

A Bit Of TSE

One important function associated with the DDP's equaliser is 'Tape Saturation Emulation'. There are five TSE settings: dark, warm, none, light and bright. This feature is implemented through the A‑D converter circuitry and is therefore not available to digital inputs — a shame, since digital sources are more likely to benefit from such a process. The TSE effect is quite subtle and really only affects high‑level signals, imparting the kind of smoothness associated with analogue tape saturation in degrees described very well by the names of the settings.


  • Powerful, with excellent dynamics algorithms.
  • Integrated side‑chain access modes for keying and ducking.
  • Sound quality is superb.
  • Lots of processing for the money.


  • Too few operational controls for the wealth of functionality.
  • Busy, sometimes confusing LCD.
  • Fiddly, multi‑page setup procedures.
  • Potential bug in control software.


This is a powerful multi‑function processor, with gating, compression and limiting that sound stunning in subtle enhancement or brute‑force control applications. EQ and de‑essing add even more value to the package. However, it's fiddly to use, and although there are factory presets, most parameters require adjustment to match the program to the audio material.


DDP £599.95; DDP DIO (digital I/O expansion board) £199.95; DDP with pre‑fitted DIO £799.95. All prices include VAT.