The Studio Dominator
Is this the ultimate limiter?
Whenever Aphex tackle a design problem, you can bet that the finished item will contain certain innovations that nobody thought of using before. The Dominator is no exception.
The Dominator is strictly speaking a stereo limiter but one designed to overcome the problems associated with more conventional designs. So-called 'intelligent' circuitry is used so the setting up of the unit is relatively simple; the user just sets the Output Ceiling control to the maximum permitted output level and then sets the Drive control to give the desired amount of gain reduction.
The principle behind the Dominator is that the input is split into three separate frequency bands and each band processed separately. In itself this technique is not entirely original but it does form the basis of a high performance limiting system as it effectively prevents high level, low frequency sounds from unduly modulating the mid and high frequencies. Anyone who has made extensive use of compressors or limiters will be familiar with the effect whereby a heavily limited bass drum will cause the hi-hat or vocal track to dip out. The multi-band approach neatly sidesteps this problem and one of the other benefits is that highly compressed or limited mixes do not sound dull because important high frequency detail is not suppressed needlessly.
Sadly the problems do not end once a 3-band system has been decided upon because then decisions have to be made as to what are the best attack and release times for each band and should they all be the same or different? Of course all these variables could be brought out to the front panel on controls but that would lead to a very complex panel layout and leave the poor user with little or no chance of achieving optimum performance. Aphex have thus left fixed all the parameters that can be left fixed and put the other parameters under the control of the Drive control.
A further problem is that the exact output limiting threshold cannot be predicted because the output is the sum of the three frequency bands and so the output level depends on the relative energy levels in each band. Maximum output will only occur when all three bands are limiting at the same time (and in phase) and this does not occur often in normal use. Putting a broadband limiter at the end of the chain would only reintroduce the inherent disadvantages of such a system.
To circumvent this particular bugbear, the Aphex design team have devised what they call an ALT circuit or Automatic Limit Threshold. This exercises control over the output summing in such a way as not to introduce distortion whilst maintaining an accurate limiting level. Obviously Aphex are not going to tell us exactly how this works but the general outline is as follows; The output of the summing stage is monitored and when it falls short, the ALT automatically adjusts the limiter's threshold to re-establish the correct output level. Below the threshold, no gain modification takes place so small signal integrity is preserved.
In order to accomplish the necessary gain control with minimal noise and distortion, the Aphex 1537 VCA chip is used and this is widely regarded as one of the most sophisticated gain control elements currently available. As with most limiters in professional use, both channels may be ganged for stereo tracking operation. Also, extra plug-in circuit cards are available to tailor the unit's characteristics to broadcast use.
Because any circuit of this type has a finite response time, another line of defence is incorporated in the form of a clipper to curtail short duration overshoots. It has been proved that if a signal is clipped for very short periods, (less than 10mS, for example) the resulting distortion is totally inaudible and the effect of transients is preserved. Even so, heavy limiting does cause some degradation in transient response so this time, Aphex have drawn on their expertise in the field of psychoacoustic enhancers to build in a switchable transient enhancer which they call TEC. This manipulates the envelope of transients below the limiting threshold and is said to restore some of the missing feel.
So much then for the theory of operation; what control does the user have and how easy is it to use?
"...a busy professional studio could justify using a Dominator to limit vocal tracks, a job that it does beautifully, but sadly the home enthusiast will just have to dream."
As the photograph shows, this is a 1U rack mountable unit with the controls arranged along the front panel. This is stylishly fashioned from anodised aluminium and the construction throughout is exemplary.
The first control is Drive which as previously mentioned determines the amount of gain reduction by setting the input signal level relative to the internal limiting threshold. Next to this control is the Process In/Out button which bypasses the limiter for instant A/B comparisons but does not bypass the drive control or override any EQ settings. This switch in common with all the other front panel switches has LED status indication.
The three frequency bands into which the signal is split for processing are to some extent under user control in that both crossover points are switchable. The low crossover point may be either 80Hz or 160Hz and the upper one 1700Hz or 4500Hz. The low and high bands have gain modification controls which give an EQ range of ±6dB but the mid band level is fixed. These controls would normally be set flat but are included so that special processing effects or tonal changes may be realised.
The Release time is uncalibrated, but labelled Slow to Fast. This is not an oversight but specific values are omitted because the release time is continuously varied by the intelligent circuitry depending on the dynamic nature of the input. Adjacent to this control is the button for selecting Stereo Tracking.
Moving on, we come to the Output Ceiling control which sets the output level above which the signal is restrained from going. This is a rotary switch rather than a pot and covers the range -2dBu to +9dBu or +10dBu to +22dBu depending on the setting of the adjacent range switch. The output will not under any circumstance exceed the set level and so the Dominator is eminently suitable for use in radio transmission applications to control modulation depth. Likewise it is perfect for digital recording applications where even the slightest overload can ruin a recording.
The amount of limiting is continuously displayed on a ten LED display and to the right of this is the On/Off button for the TEC transient enhancement system.
The rear panel sports a cunning IEC mains socket with integral fuseholder designed for quick fuse changing. The fuse is covered by a perspex slide which can easily be slid back, but only when the power cord is removed. Both the input and output connectors are in the form of XLRs and the outputs may be switched for balanced or unbalanced operation. These XLRs are wired pin 3 hot rather than the more common (In Europe at any rate) pin 2 hot but as both inputs and outputs follow the same convention, this should cause no problems.
"One feature that I would like to have seen though is a true bypass switch so that the whole unit could be taken out of circuit for true A/B comparisons."
At first I thought that this was going to be a case of 'The King's New Clothes.' I switched the Dominator in and out and could barely tell the difference in sound. However, the meters told a different story, I was applying around 10dB of gain reduction and not even hearing it! As the Dominator is supposed to be an unobtrusive limiter rather than an effect, it follows that it does its job exceedingly well.
Using the EQ controls allows you to apply more gain reduction to the high end than the low end or vice versa and this is advantageous if your mix is well balanced but needs tweaking in the tonal department. Driving more signal into the high or low limiter gives an apparent increase in top end but of course the signal peak level does not increase.
The release time needs to be set up by ear but I tended to choose a fairly fast release time for most pop material and there were no noticeable breathing effects, at least, not that I could detect. When the TEC was switched in, the effect was very slight but nevertheless perceptible and was subjectively similar to adding a very small dose of Aural Excitement in that the top end was given just a hint of sparkle. This is worthwhile when a lot of limiting is being applied as it does indeed help to compensate for that slight dulling that heavy gain reduction always seems to cause.
In terms of ease of use, the Dominator must score full marks as the only control that requires any skill in setting up is the release time. It is also exceptionally quiet in operation though like any compressor, it will increase the level of any tape noise in direct proportion to the amount of gain reduction applied. Being a multi-band system probably makes it better than most other compressors in this respect but care still has to be exercised.
Despite the fact that the signal is split into three bands and then processed, the square wave response of the unit is little short of perfect, even when the input is significantly overdriven. When limiting due to overload does occur, it is progressive rather than straight clipping and so does not sound unduly unpleasant. Having said that, there is around 20dB of headroom so overloading should never be a problem.
This is a very well designed piece of equipment with significant advantages over the conventional type of limiter in common usage but, like the Aphex Compellor, it's expensive. It's not the sort of unit that you would buy for general purpose compression or limiting but it would be a worthwhile acquisition if you have a PCM digital system and indulge in direct to VCR recording of live events. I suppose that a busy professional studio could justify using a Dominator to limit vocal tracks, a job that it does beautifully, but sadly the home enthusiast will just have to dream.
Likewise, there is a case for using one of these machines when mastering from multitrack in order to tighten up a mix without incurring the side effects you would expect from ordinary limiters and even in high quality live sound reinforcement there is a good case for using a Dominator to keep a watchful ear on the stereo mix.
Other uses might include processing a master tape for cassette tape duplication in order to get the maximum level of signal onto tape and at the other end of the scale, the radio station or cutting room could benefit from the Dominator's unobtrusive capabilities. One feature that I would like to have seen though is a true bypass switch so that the whole unit could be taken out of circuit for true A/B comparisons. When reviewing an Aphex product, I always expect something special and once again I was not disappointed... but then this quality doesn't come cheaply.
The Dominator costs around £1250 including VAT.
Further details from Sound Technology, (Contact Details).
Review by Paul White
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