DBX PROVOCAL
Physical Modelling Recording Channel
In
addition to the usual dynamics and EQ facilities, this voice channel
from Dbx offers physical modelling effects in the digital domain,
allowing it to simulate the performance of different types of mics and
mic amps.
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.
ProVocal Physical Modelling
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.
| Dbx ProVocal£400 |
| pros |
 | Lots of processing plus a digital output at a bargain price. |
| Creatively useful mic modelling and effects. |
| Good basic sound quality. |
| Type IV conversion avoids digital clipping. |
| cons |
| Inadequate metering of levels and gain reduction. |
| Inadequate parameter read-out and no patch naming. |
| Most of the amp models are too distorted to be useful. |
| summary |
| This potentially powerful processor is let down by poor displays and the tendency to sound heavy-handed. |
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In
the case of something like the Antares Microphone Modeller, the machine
incorporates a database of many popular microphone types, whereas the
Dbx ProVocal uses averaged data taken from several mics so that the user
only has to choose from capacitor or dynamic source microphone types.
Clearly any deviations of the source mic's actual frequency response
from this notional average will affect the algorithm's effectiveness
when it comes to predicting how the modelled mic would react to the same
input sound source. In addition to modelling frequency response, a
convincing emulation would also need to take into account phase shifts
and distortions in both the source mic and the mic being modelled, but
one aspect that can't be modelled is the off-axis response of a
microphone, because that is the result of the physical attributes of the
capsule, not the circuitry that follows it.
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.
Dynamics, EQ & Effects
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.
