Behringer Combinator

Multi-band Compressor

Published in SOS November 1994
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Reviews : Processor

Behringer claim that the Combinator can dramatically compress a signal without introducing any undesirable side-effects. PAUL WHITE checked it out on a mastering session.

 

While you might not think that what the world needs most is another compressor, the Behringer Combinator, a multi-band compressor designed for jobs like mastering or full-mix processing, is different enough to justify its launch onto the studio scene. But to fully appreciate what makes a multi-band compressor special, you first need to know what makes a conventional, full-band compressor less than perfect.

A full-band compressor is, as its name implies, a device which compresses the whole audio band in one go. In other words, it doesn't matter if it detects high levels in just the bass, middle or top end of the spectrum, it always responds by turning down the gain of the whole signal. Because most of the energy in a typical music track resides in the bass frequencies, you often get a situation where every time there's a bass drum beat or a loud thwang on the bass guitar, the overall gain is pulled down and any innocent, low energy/high frequency sounds (such as hi-hats) which happen to coincide with the offending bass sound are briefly dragged down in level, causing the mix to sound dull. One way around this is to set a longer compressor attack time, to allow transients to sneak through before being squashed, but this is only partially successful, and in situations where a lot of compression is needed, the tonal balance of the mix is likely to suffer. It's also possible that undesirably high peak transients will make it through to the output unchecked.

That's where the multi-band compressor has an advantage, because before applying compression, it splits the audio into several frequency bands and then compresses each band (more or less) independently. I say more or less because some clever linking is needed to prevent the ensuing changes in spectral balance from becoming audible. The advantages of such a system are apparent: a loud bass drum is no longer going to have any significant effect on how much compression is applied to mid-range sounds such as vocals, or high-frequency sounds such as cymbals so, in theory at least, it should be possible to apply more compression before any side effects, such as 'gain pumping' or 'breathing', become evident. Behringer's Combinator works on just such a multi-band principle.

The Combinator is a dual-channel processor working on four audio bands per channel, but it is more than a simple compressor. It is impossible to set up a simple compressor in such a way as to simultaneously maintain an even long-term level, control short-term fluctuations in dynamics, and catch very short peaks before they can overshoot and cause distortion. You could set up a standard compressor to make a fairly good stab at any one of these functions in isolation, but they all require widely different settings of parameters such as ratio, attack time, release time, threshold, and even whether the compressor should have soft or hard-knee characteristics.

Various companies have got around this problem by combining levellers, compressors and clippers in one box, one of the most famous being the Aphex Compellor which, although a full-band device, uses multiple side chains driving a single, high-quality VCA to implement levelling, compression and peak limiting all in one hit. In fact you could say it handles four functions -- since, because even a fast peak limiter isn't usually fast enough to catch the fastest transients before they overshoot, Aphex also included a clipping circuit to act as a brick wall beyond which the signal can never pass.

As you might expect, a signal driven into clipping sounds disgusting, but the trick is to design the limiter to bring the level down within a couple of milliseconds after a clip is detected, so that the period of clipping is too short to be audible. Other manufacturers, including Drawmer, use the same basic principle in some of their limiter designs, so it comes as no surprise that Behringer have adopted a similar system. In fact it might seem as though Behringer have produced the best of both worlds by combining multi-band operation with the Compellor's ability to simultaneously level, compress, clip and limit (see box for an explanation of levelling).

THE PACKAGE

Unusually for a non-valve compressor, the Combinator comes in a 2U package -- though when you consider that it contains no fewer than 24 VCAs (four in the audio path and eight as part of the side-chain control circuitry), it's hardly surprising that it needs a big box. Both the VCAs and the op-amps are Behringer's own, so it's difficult to comment on their quality other than in subjective terms. It's certainly nice to see that no external power supply is required -- the Combinator comes with a captive mains lead.

Because it may be used in professional or semi-professional environments, the Combinator is fitted with both balanced XLR connector and balanced jacks; the latter may be used unbalanced simply by plugging in an unbalanced lead. Either -10dBV or +4dBu operating levels may be selected by means of a switch on the rear of the unit.

Electronic balancing is used as standard, though a transformer-balanced version of the Combinator is available as an option. A nice touch is a 'hard' relay-operated bypass which links the inputs to the outputs when in bypass mode or in the event of a power supply failure, ensuring an uninterrupted signal flow. This also keeps the unit out of circuit during power-up to prevent switch-on thumps which could otherwise pose a serious problem in live situations.

Though the two channels may be used independently, they may also be linked for stereo use; to add to the creative flexibility of the Combinator, a separate level control is provided for each of the four frequency bands, allowing the four bands to be re-balanced if required. The limiter circuits come after these level controls and also after the master gain control so that the limiter threshold isn't affected by any resetting of the other controls. Separate Limit and Compress status LEDs are provided for each of the four channels to show exactly when and where gain reduction is taking place.

At first sight, the Combinator seems to have rather a lot of knobs, buttons and LEDs, but when you examine it more closely, it's actually very simple to set up and you don't even have to worry about setting attack and release times, because there aren't any! This might be considered a limitation on a compressor that you may want to use to create a specific effect, but the Combinator is optimised for unobtrusive gain reduction, so that criticism doesn't really apply.

The compressor section is controlled by just four knobs: Threshold, Process Balance, Ratio and Output. Threshold sets the level at which compression starts to take place and this may be set anywhere between -40 and +20dBu. Ratio may be adjusted between 1:1 and 6:1, and though this is far less than the range offered by general-purpose compressor/limiters, you have to remember that the Combinator has a separate limiter section. The Output level control provides additional gain to make up for that lost due to compression and this comes before the limiter, so setting a high Output level may drive the unit into premature limiting. Process Balance allows the user to set a balance between straightforward compression and levelling. Normally you'd want the unit to be doing a bit of both, so the control would be set somewhere near the middle. What you have to keep in mind is that though there is only one set of controls, each of the four frequency bands is being processed independently and the best way to keep a check on what's happening to each band is to watch the compress and limit action LEDs.

The Limiter controls are also very simple; there's an In/Out button, a High/Low Density button and a knob to set the Peak Ceiling which, in simple terms, is the level in dBu which signal peaks will be prevented from exceeding. Perhaps the only control that isn't self-explanatory is the Density switch which, as far as I can tell, simply selects between two preset limiter release times. The Low setting provides the slower of the two release times, making the recovery from limiting quite smooth, whereas High allows the limiter to track closely-spaced peaks more effectively, but might produce audible level modulation on some material.

The Multiband section is where things start to get exciting. Here we find a level control for each of the four frequency bands; these have characteristics similar to a four-way equaliser, in that the high and low ends have shelving characteristics and the two mid-range controls are band-pass types. The frequency ranges covered are: 10Hz-180Hz, 180Hz-750Hz, 750Hz-3.5kHz and 3.5kHz-20kHz. In addition to the level controls there are separate indicator LEDs for compressor and peak limiter action, In/Out buttons and Set In/Out buttons. The In/Out buttons can be used to disable compressor action in certain parts of the audio spectrum; for example, if you left only the high frequency band active, the Combinator could be used as a de-esser -- or you could leave only the lower band active, to help tame popping. The limiters still work, even when a band is switched to Off, so there's no risk of high-level signals sneaking through to the output unless the limiter itself is switched off.

The Set switches allow the gain reduction in each frequency band to be monitored on the main gain reduction meters. If no Set switches are active, the overall gain reduction is monitored, but if several Set switches are in at once, only the leftmost one counts.

The Gain Reduction meter is a high resolution, 30-LED affair with a range of 30dB, and this monitors the total gain reduction due to compression, levelling and limiting. As mentioned, gain reduction in individual frequency bands can be monitored using the Set switches.

A second pair of 30-LED meters monitor either the output level when the unit is operating or the input level when the unit is bypassed.

OPERATION

What's going on inside this box is undoubtedly quite complex, but using the Combinator is really no more difficult than using any other compressor. If you set all four band Level controls and the Process Balance knob to their centre positions, the remaining controls operate and behave exactly like those on a standard full-band unit. Used in this way, the Combinator is capable of applying a significant amount of compression and levelling to a complete mix with none of the gain pumping or dulling that so often occurs with simple units. I wouldn't go as far as to say that the sound is exactly transparent, but then it doesn't sound obviously processed either. Even when the limiters are operating rather more frequently than might be considered normal, the overall sound isn't noticeably changed

More adventurous users can change the spectral balance of the mix by adjusting the four band Level controls; if you drop the mid-range level slightly, for example, the Combinator can produce results loosely similar to those of an enhancer. And, because the limiters come after these Level controls, it is possible to force some bands into limiting long before others (by turning up individual Level controls), which can occasionally create interesting effects.

Though the Combinator might be considered overkill for sampling applications, it could work extremely well in this area, where it is often necessary to maintain a high average signal level, again without destroying the impression of dynamic range. Keeping the average level consistent makes looping easier and helps keep the noise floor low -- especially important if you're using 12-bit machines such as Akai S900s and S950s. Certainly anyone interested in producing commercial samples but not having access to computer-based dynamic manipulation might benefit from a multi-band compressor of this type.

Non-digital users can also benefit from large amounts of 'invisible' dynamic range reduction, especially when mastering for cassette reproduction. Aside from the fact that cassettes need a high average signal level to keep the noise floor acceptably low, many are played in cars, where excessive dynamic range is a hindrance as the sound is always competing with engine noise.

SUMMARY

As an unobtrusive level-controlling device for use on complex stereo mixes, the Combinator is both effective and easy to use, though in all honesty, I don't think I'd consider it an essential purchase purely for general recording applications, where a good, single-band compressor can generally deliver the goods on a purely subjective level. However, the inclusion of limiting makes the Combinator especially well-suited to digital mastering, where any overload is unacceptable, and the multi-band approach seems to work well in minimising the audible side-effects for virtually any type of programme material on those occasions where particularly heavy-handed compression or limiting is needed. Particularly impressive is the Combinator's ability to impose quite high levels of dynamic range reduction while actually leaving the illusion of dynamic range intact. Though the Combinator is not the only device to offer 'invisible' gain reduction, it can't be denied that it does a good job at an affordable price. It's also a flexible piece of equipment that can be used for subtle (and not-so-subtle) spectral reshaping, making it a valuable creative tool.

 

THE LEVELLER

Compression and limiting are well-known to most SOS readers, but you may not have come across levelling. In essence, levelling ensures that the programme material maintains a similar average level over a long period of time without affecting short-term changes in dynamics. This might be advantageous when compiling radio programmes, because some records are mixed louder than others, which could create unwelcome changes in level. Similarly, it may be necessary to compile an album from recordings made at different times or in different studios, and again, levelling makes the different tracks 'sit' together more comfortably. Even within a single piece of music, it may be useful to use a leveller to subtly bring up the level of a quiet section without compromising the subjective dynamics of the piece.

 

PRE-EMPHASIS

Pre-emphasis is a 'top boost before processing/top cut after processing' system designed to act as a form of noise reduction, and is widely used in broadcast to overcome limitations in hardware and to make up for quality loss due to weak signals. Pre-emphasis is also used in Dolby encoded material. If a pre-emphasised signal is compressed using a normal compressor, and then de-emphasised after compression, the compressor will attempt to compress the emphasised frequencies more heavily, resulting in inaccuracy. To counteract this, the Combinator features switchable pre-emphasis and de-emphasis, accessible via a rear-panel switch. The three most common industrial standard -- 50uS, 75uS (mainly USA) and 25uS (Dolby) -- are supported, which means that signals in these formats can be treated correctly.

 

BEHRINGER
COMBINATOR
£1279

PROS
• Sensibly priced.
• Easy to use.
• Simultaneous levelling, compression and limiting.
• Allows high levels of compression to be added with minimal side effects.

CONS
• A built-in expander gate would have rounded off the package nicely -- but then again, at the price...

SUMMARY
This unit is particularly useful for digital mastering and mastering for cassette, where dynamic range needs to be restricted without significantly changing the subjective nature of the recording. Its very transparency may make it less useful for general compression applications, where side-effects are often used creatively.

 

info

£ Combinator £1279 inc VAT.

A Behringer UK Ltd, St Vincent House, 59 Woodbridge Road, Guildford, Surrey GU1 4RF.

T 0483 458877.

F 0483 458822.

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