Behringer MDX 4000 Mastercom

Multi‑Band Compressor

Behringer are well known for their signal processors, and this latest offering is a very worthy addition to their range. The Mastercom is a dual‑band stereo compressor with separate peak limiter system. It's well thought out, provides sensible control parameters, looks very elegant, and does the business as far as controlling dynamic range is concerned.

Operating Principles

The Mastercom is rather different to the majority of compressor systems currently on the market because of its dual‑band operating principle. The idea is not unique, of course — just expensive to implement, and most manufacturers do not perceive a need for such an elaborate device. However, dual‑band or multi‑band compression can be extremely effective, and often allows compression to be applied with much less obvious side effects.

One of the most common problems with a broad‑band compressor is when a heavy bass signal (such as bass guitar or kick drum) dominates the dynamic control of the compressor, effectively modulating the high‑frequency elements of the programme — an effect particularly obvious when compressing a complete mix. The dual‑band approach of the Mastercom goes some way to reducing this problem, by splitting the audio into two bands (at either 500Hz or 2kHz, 24dB per octave) and compressing each band according to its own requirements. Compression follows a custom soft‑knee curve, allowing very transparent effects with gentle settings, which then become harder as the compression reaches high levels.

Unusually for a compressor, the Mastercom has no dynamic controls, only the static ones for threshold, ratio and output gain. Attack and Release time constants are determined by the system itself and are optimised for each band, the user having no control over the settings. Even more surprising is that the otherwise excellent handbook makes no mention of this aspect of the compression process!

A further improvement over conventional compressors is the inclusion of a 'Leveller' system. This provides a long‑term level control which is common to both bands and ensures a consistent output level with very slow attack and release time constants. The long‑term aspects of this process do not interfere with the dynamic content of the programme — these are controlled purely by the main compression stage.

Finally, a separate limiter is employed in each band to control the peak output level. These limiters are almost as complex as the rest of the system and go to great lengths to ensure that even severe limiting remains as inaudible as possible. To achieve this level of signal processing, the Mastercom uses no fewer than six level detection stages and four VCAs!

The Back End

The Mastercom is housed in a standard 1U‑high 19‑inch rack unit, with a clear block diagram of the signal processing etched into the lid. The rear panel carries the IEC mains connector (with integral fuse holder and mains‑voltage selector) and two sets of input and output connectors. In both cases, there is an XLR in parallel with a standard jack socket, all providing electronic balancing (although transformer‑balanced outputs are available). The unit is capable of operating at either the professional +4dBu or the MI (Musical Instrument) standard ‑10dBV level without the need to switch between modes. The Threshold and Output level controls have sufficient range to allow operation in either type of system, but the Mastercom is actually more flexible and slightly easier to use within a +4dBu system, and the bargraph meters are calibrated for this type of use. The unit is bypassed through a relay system which maintains a direct signal path in the event of power failure.

Front Panel

The front panel is deceptively simple, with only five rotary controls and a few push buttons. All of the controls have a quality feel to them, the knobs having a nice weight and subtle detenting, while the buttons have built‑in LED indicators.

The audio controls start with the Threshold knob. This covers a range between ‑40 and +20dBu and simply determines the point where the signal will start to be compressed. The next knob, labelled Leveller, is rather unusual and controls what may be thought of as a slow AGC circuit. In other words, it controls a high‑ratio compressor which has very slow attack and release time constants. The effect of this process is to provide long‑term level control, much as you might gently ride the master fader during a mix to maintain a consistent overall level, but without altering the short‑term dynamic content of the audio signal. The control is calibrated from 0 (no effect) to 6 (maximum effect).

The third control knob determines the compression ratio, over a range of 1:1 up to 6:1 and it is accompanied by a pair of gain‑reduction bargraph meters (one for each band) which show up to 30dB of gain reduction. Between the meters is a push button which determines the frequency of the band splitting. It is normally set to 2kHz, which seemed to work well on solo instruments and voices, but may be reduced to 500Hz, which is sometimes more effective for complete mixes.

To the right of the gain‑reduction meters is an Output level control calibrated for a +/‑20dB swing. This control is not only used to make up for the natural loss in level associated with large amounts of low‑threshold compression, but also to tailor the signal level for ‑10dB operating level systems.

The separate peak limiter is controlled with a single knob labelled Level and calibrated from 0 to +20dBu. This is accompanied by two LEDs, which indicate the limiter's activity in each of the two signal‑processor bands. When operating at ‑10dB system levels, the flexibility of this control is reduced, since it really can only operate as a true peak limiter. In +4dB systems, it may be used at rather lower programme levels for very hard limiting effects.

Separating the Limiter section from the input/output metering is a bypass switch, which provides a true hard‑bypass by means of a relay. This connects the input sockets directly to the output sockets, and is automatically engaged if the Mastercom loses power. The metering follows the bypass switch, such that when the unit is in‑circuit, the metering shows the compressed output signal, and when the unit is bypassed, it shows the incoming signal level. The meters cover the range ‑30 to +15dB (zero equates to +4dBu) and are rather unusually arranged side by side, rather than one above the other, which would seem more appropriate for a stereo compressor.

In Use

The Mastercom works well, and even severe provocation failed to make it produce anything that could be described as sounding really nasty. The manual is excellent, but whether you bother to read the manual or not, this device is supremely easy to set up, with only the Threshold, Ratio and Output controls to tune for the desired amount of compression. The Leveller helps to keep programme levels consistent in a very subtle way, and the peak limiter guarantees that maximum programme levels stay within the given bounds, even with digital recordings set to peak to zero.

Although I felt a bit lost without a Release control to play with, I cannot fault the Mastercom's dynamic control capabilities in any way. This is an excellent compressor best suited to final mastering applications or overall output level control for a recording desk or live‑sound system. Dual‑band compression sounds different to normal broad‑band compression, usually being smoother and brighter, so that it often suggests a different approach to equalisation. With solo instruments, I found the Mastercom frequently removed the need for equalisation altogether.

It is possible to get some poor results from the system if it is used inappropriately — for example, compressing an acoustic guitar with the band‑splitting set to 500Hz made even a Lowden, which is a pretty up‑market instrument, sound cheap and boxy — and some settings of the Leveller control could, surprisingly, affect the perceived brightness of the signal. However, overall, when used sensibly, this box is an absolute cracker and I would be quite happy to strap it across the output of my desk for ever more!

Hugh Robjohns is a lecturer at the Centre for Broadcast Skills Training at BBC Wood Norton. The views expressed in this article are the author's own and are not necessarily those of his employer.