DBX PROVOCAL
Physical Modelling Recording Channel
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Paul White
Recording front-ends vary in complexity from simple mic preamps to complex channel strips with full dynamics and equalisation. Most are essentially analogue devices, albeit with a digital output option sometimes available, and typically you have the choice of solid-state or tube circuitry. The Dbx ProVocal channel strip is somewhat different, because, although its mic preamp section is necessarily analogue, the rest of the signal chain is digital.
Another interesting feature is that, in addition to compression, de-essing, gating and parametric EQ, it also offers mic modelling and preamp modelling, as well as a range of digital effects covering all the usual chorus/flange/delay and reverb treatments. Its outputs are available both on balanced analogue connectors and as a 24-bit S/PDIF digital signal (at 44.1kHz or 48kHz) on the usual phono connector. A degree of MIDI control is also provided, via the single rear-panel MIDI In socket. While most of the elements of the unit are fairly familiar, the modelling side of the product deserves a little more attention.
Mic modelling is a contentious issue, as it purports to make one type of mic sound like another — clearly a process with inherent limitations. For example, while it might be reasonable to emulate the restricted high end of a dynamic microphone using a wide-bandwidth capacitor microphone as the source, trying to do the opposite is clearly a tall order. Furthermore, the characteristics of the source mic needs to be known reasonably accurately in order to derive an accurate transfer characteristic.
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So, while it may be possible to produce a very close approximation where the source mic has lower distortion and a wider bandwidth than the modelled mic, and where the recording is taking place in conditions where reflected sound doesn't contribute in any significant way to the result, any real-life application of this technology can produce at best an approximation. Having said all that, if you approach mic modelling as being there to give you a choice of tonal characters from which to choose, rather than being a means of faithfully recreating the sound of a top-flight studio mic from something you found at the bottom of your gig bag, it can be a very useful technology.
Amplifier modelling has a far better chance of success, as the relevant parameters are measurable and, in the main, constant. While the main task of an amplifier is simply to make signals larger, many designs introduce non-linearities or tonal changes that add to the character of the sound, especially those utilising tube circuitry. If these idiosyncrasies can be modelled fairly accurately, then there's a good chance that the sound of a classic mic preamp can also be emulated. However, doing this properly requires that the input stage loading of the original mic amp is also recreated, which means using a variable-impedance input stage. This is technically difficult to do and, as far as I'm aware, it has not been attempted here.
The Dbx ProVocal is a 1U, single-channel processor that can accept mic, line or instrument inputs and generates a stereo output (necessary because of the effects) available on balanced XLR, balanced jack or as S/PDIF. The maximum analogue output level is +18dBu. An electronically balanced mic preamp provides switchable phantom power and a 20dB pad, along with simple three-LED metering of level. Both XLR and high-impedance instrument jack (450k(omega)) inputs are to be found on the front panel with a Line switch that engages a rear-panel balanced line input jack instead of the front-panel instrument jack.
After that point, the signal is digitised using Dbx Type IV conversion, an ingenious proprietary system that provides linear conversion up to a level of 4dB from clipping, after which a nonlinear (logarithmic) conversion system is employed to implement a soft saturation characteristic that extends the dynamic range enormously and makes the converters virtually impossible to overload. I've used this system before and like it very much, as it allows you to work close to the full-scale level without ever having to worry about clipping and the unrecoverable loss of high-frequency information (from the top of the clipped waveform) that entails — not to mention the evil-sounding distortion!
As with any digital system, there are far more parameters to control than there is space for knobs, so a simple matrix system has been employed where five knobs address six rows of parameters, all clearly printed on the front panel. These are regular pots, not rotary encoders, so you need to turn them through the stored value position in order to get them to pick up the parameter for adjustment. A two-digit LED display shows the number of the current program (40 factory and 40 user) and also serves as a parameter display when editing. Two buttons control the patch scrolling or can be pushed simultaneously to bypass all effects, while another button steps through the vertical axis of the effects matrix. That leaves just a Store button, for saving programs, and a power switch.
Most of the parameter rows are logically grouped. The mic model type, amp model type, drive amount and gain are controlled by the first row, along with the low cut frequency. The second bank is given over to a three-band EQ with shelving extremes and a parametric middle section, while row three deals with the compressor, row four addresses the gate and de-esser and the remaining two rows look after the effects.
No information is given concerning the ten mic amp settings other than their number, with the first setting being flat and the remaining nine getting progressively more tube-like. The Drive control adjusts the input drive to the amp model to simulate tube distortion. More information is provided for the mic modelling, where ten options are available for both dynamic and capacitor sources, if you count the flat option at setting one. Two of the models are special effects for telephone and megaphone simulation, with the rest modelled after distinctive models including the Shure Green Bullet harmonica mic and the famous RCA ribbon. The large-diaphragm models are based on the characteristics of popular Shure, AKG and Audio Technica mics, while the small-diaphragm model is based on an Oktava pencil mic.
In order to keep operation of the unit simple, the effects have user parameter sets more akin to those found on effects pedals rather than studio rack processors. For example, the reverb can be selected from ten types, after which only the mix and decay time may be adjusted, in this case by a single control that brings in more reverb level as you lengthen the decay time. Similarly, the delay (1380ms maximum) offers only time, feedback and mix, while the modulation effects offer type, depth, speed and mix. Although delay and reverb may be used together, you have to choose between chorus and flanging. There's a MIDI control facility, where the user can select one of ten variable parameters within the patch to be controlled by an outside MIDI control source.
 In addition to the analogue audio connections, there is also a MIDI In socket for remote parameter control and an RCA phono socket for the 24-bit S/PDIF digital output. |
Operation
Once you start using the Dbx ProVocal, several shortcomings in the user interface very quickly become apparent, not least being the lack of any form of gain reduction display for the compressor or any meaningful display of parameter values such as ratio, attack and release. All parameters, including the EQ settings, are adapted to the two-digit read-out with notional values that bear no resemblance to their actual values, so all judgements have to be made by ear with no real visual confirmation that your settings are sensible. This is particularly confusing with the EQ, where the manual provides a table of 61 mid-band frequencies (400Hz to 8kHz) that correspond to values one to 61 in the display. The lack of visual feedback also extends to the de-esser and gate (which behaves more like an expander) where there are no obvious indicators that these functions are being triggered.
The basic sound of the unit is best evaluated with all the processing bypassed, in which case the result is very open and clean sounding with an impressively low noise floor. Running through the mic models reveals that the tonal changes are generally subtle (as they should be) until you get to the harmonica mic and special effects settings, so I have no issue with this section of the machine provided that it is used for creative tonal processing and not for trying to make a gold-sputtered purse out of a sow's ear!
Moving on to the tube amp modelling, the mildest of these settings sounds subjectively clean with the Drive control turned to a low setting and quite obviously distorted with Drive turned more than about 30 percent of the way up. This brings me to ask what the remaining amp models are for, as all offer varying degrees of what I can only describe as singing through a fuzz box. You might find use for some of these extreme settings when recording harmonica or perhaps when DI'ing guitar, but they are far too extreme for vocal use. The compressor has the classic Dbx sound that leaves you in no doubt that compression is taking place, while the de-esser can be very effective in softening sibilance provided you don't apply too much of it, otherwise you can hear it working.
The effects are simple, lively and sound as you'd expect a mid-price multi-effects unit to sound. However, many of the factory presets are perfect examples of how not to use the unit to its best advantage, as the amp, EQ and effect settings are far too extreme! Having a combined reverb decay and mix control is, I feel, taking simplicity one step too far and I have to confess that I wasn't over-impressed by the EQ, which either sounded as though it wasn't doing much at all or, when it was pushed slightly further, sounded too peaky.
Used with care, the Dbx ProVocal is capable of producing extremely good results, though you have to be a fan of the Dbx style of super-assertive compression, as that's all that's on offer. The use of Type IV conversion makes the device very tolerant of input levels and the inclusion of effects could be particularly useful in live situations or when recording to a computer workstation that is short on horsepower to drive decent effects plug-ins. Though the reverb adjustment is limited, the general quality of the effects is actually very good.
The weakest aspect of the Dbx ProVocal is its lack of serious visual displays for gain reduction and parameter values. And by the same token, it's time we finally abandoned the concept of calling up patches by number instead of by name. There's also the risk that inexperienced users will find it too easy to make something sound bad, as virtually all the amp models (bar the mildest) are overkill for vocal work, and you also need to go easy with the effects and the drive setting. But looking on the positive side, even if you never switch on any of the effects or amp models, you get a clean mic preamp with bullet-proof Type IV conversion, dynamics processing and EQ plus a digital output for a very competitive UK price. It's down to you to weigh the extra features against the somewhat disappointing metering and display functions, so that you can see if this product fits in with your requirements
