Practical Multi-band Compression
Technique : Effects / Processing
Multi-band compression is one of the most powerful processing techniques available to the recording engineer. However, if you don't know how to apply it effectively it can just as easily ruin the sound of your entire production.
Paul White & Hugh Robjohns
Most of the compressors we use in music production operate on what is known as the full-band principle, where the entire audio signal is processed via a single gain-control element. When gain reduction occurs, the whole signal level is reduced, just like turning down a volume control. In other words, whenever a loud peak occurs (whatever its frequency content) that forces the compressor to act and the level of the whole signal is reduced until the loud event has passed. A common problem arising from this method of compression is that a loud kick drum (which produces mainly low frequencies), will trigger the compressor and consequently pull down the gain of everything else that happens to be passing through the compressor at the time, even though those other sounds might not need compressing. If the compressor is acting on a solo kick drum track, then clearly this isn't a problem, but if the whole drum kit is being compressed, the high-frequency sounds, such as hi-hats and cymbals (which carry relatively little acoustic energy), will be compressed along with the kick drum, the outcome of which is often a subjective dulling of the sound during times of heavy compression.
The situation worsens when you're compressing a whole mix, because the low-frequency sounds in the mix determine the compression applied to everything else, so what tends to happen is that the kick drum and bass line dictate how the mix will be compressed. One way to disguise this aspect of full-band compression is to set a slightly longer attack time so that transients can pass through cleanly before the gain reduction takes place, but clearly this does little to control peak levels, which is important when you're working with digital systems that can't tolerate overloads, however brief.
In some situations these side effects can be musically desirable, and in some forms of rock and pop music a hint of gain pumping (audible compression due to rapid gain changes) can add energy and excitement, providing it isn't overdone. However, for mastering and other critical applications, multi-band compressors provide far greater flexibility.
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The Principles Of Multi-band Compression
In essence, a multi-band compressor comprises a set of filters (similar to a PA system's active crossover) that splits the audio signal into two or more frequency bands. Three- or four-band compressors are perhaps the best compromise between versatility and ease of setting up. After passing through the filters, each frequency band is fed into its own compressor, after which the signals are recombined. The main advantage of multi-band compression is that a loud event in one frequency band won't trigger gain reduction in the other bands, so when that loud kick drum comes along, instead of pulling the whole mix down with it, only the low-frequency sounds (kick and bass instrument) will be compressed, leaving the mid-range and high frequencies untroubled. Similarly, a loud event in the mid-range won't affect the extreme high or low frequencies for the same reason. There are however, other advantages.
Setting the correct crossover points is vital if you are to get the best results from your multi-band compressor. |
With such a system, it is quite possible to fully control the signal in each band but still end up with a signal that is too high in level when the bands are combined. One solution is to meter the combined level so that the user has a chance to reduce the overall gain if problems occur, but a good belt-and-braces approach is to follow the compressor with a fast-acting peak limiter to prevent overshoots or excess levels — again very important in digital systems. Another consideration, which admittedly affects the designer more than the user, is that any form of filter circuit introduces phase shifts, so great care has to be taken to ensure that, when the three bands are recombined, the optimal phase balance is restored. Failure to do this can result in loss of clarity, in much the same way as you might experience when using a very cheap equaliser.
Which Crossover Frequencies Are Best?
Commercial multi-band compressors often allow the user to adjust the crossover points, so if we take a three-band compressor as an example, where is the best place to set them? The answer is that it depends on the type of material being processed and on what adjustments are needed. OK, I know that doesn't seem very helpful, so let's narrow it down a bit and assume we're processing a full mix of a typical pop song. Setting a crossover point in the middle of the vocal range can mess up the vocal sound, especially if you use radically different compression settings on each side of the crossover frequency.
In my experience, it's best to set the low crossover point below the vocal frequency range and set the high crossover point no lower than 2.5kHz. A sensible low-frequency setting might be 120Hz, as this is below the vocal range, but above most of the deep bass and kick drum elements going on in the rhythm section. These are only suggested starting points and, as a rule, I'd suggest moving the high crossover point higher than this if you plan to do more than very subtle high-end processing, as it's easy to over-emphasise the 2-4kHz area of the spectrum, which is where things can start to sound harsh. By contrast, if you restrict heavy processing to above 6kHz, you can significantly enhance the sense of detail and air, without affecting the crucial mid-range and disturbing the natural reproduction of vocals and many acoustic instruments.
Setting Up The Compression Bands
The next decision is how to set up the compressor parameters in each of the frequency bands.
There are two very different ways of using compressors — and, of course, all the settings in between! Very often we use compressors to control only those signals that are too loud, in which case we set the threshold at the 'enough already!' level, then set a compression ratio so that gain reduction is applied to anything exceeding this level. The compression ratio is defined as the number of decibels the input level must rise (once the threshold has been reached) for the output to rise by just 1dB, so for a compression ratio of 5:1, a 5dB increase at the input would be needed to cause the output to increase by 1dB. Below the threshold, no gain reduction takes place, so the signal passes through unchanged. If you need to tame vocal peaks or other occasional excesses, this is the usual way to set the compressor.
A Schematic diagram of a simple multi-band compressor. |
These two strategies, and any settings between, can be applied to setting up a multi-band compressor, but what makes this approach so flexible is that you can use different compression settings for each band. For example, if I'm working on a track where I feel the bass end is lacking in weight, I may use a higher threshold and a higher ratio to bring up the average bass level, but still use subtle low-ratio settings for the mid-range and high end. The make-up gain (which is usually available independently for each band), may then be used to restore any spectral imbalance caused by the difference in compression settings. Similarly, by applying heavier compression to frequencies above, say, 6kHz it is possible to increase the high-frequency density to create an effect similar to that of using an enhancer.
Do bear in mind, though, that the more make-up gain you apply, the more you will be bringing up the background noise — tape hiss, spill, mechanical playing noises and so forth. Again, careful selection of crossover points and ratio/threshold settings can help to mitigate any adverse effects, relying on noise-masking in the different frequency bands in much the same way that Ray Dolby discovered with his noise-reduction systems.
That leaves the mid-range, and in a mastering situation that's the area I tend to process least. Often a ratio of 1.1:1 and a threshold of -35dB is enough to knit everything together, and when this is combined with the other two bands (which may be more heavily processed if they need it), the whole mix sounds more vibrant, clear and punchy, but without sounding obviously processed. The 'louder at all costs' brigade may then want to process the finished mix via a peak limiter set to trim around 6dB off the loudest peaks so that the overall level can be increased by 6dB without clipping.
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Dealing With The Time Constants
So much for ratios and thresholds, but what of the attack and release settings, where these are independently adjustable? Here I tend to judge each band on its merits, where I might use a moderately fast attack time on the bass end, because very low frequencies have no fast transients to compromise. This brings the level under control reasonably quickly, and I'll set the release time as short as I can get it without any audible gain pumping being evident. As a rule, the busier the music, the faster the release time you need to ensure the compressor's gain resets itself between notes, but low frequencies tend to hang on longer than high frequencies, so you may need a longer release time at the bass end than in the mid-range. As a starting point, setting twice the release time as for the mid-band might be reasonable, though if you prefer to keep it simple, you'll probably get reasonable results by starting off with the attack and release times for all three bands set the same.
The mid-band covers the majority of what we perceive in a musical mix, so this can be set up much as you'd set a full-range compressor. In other words, ease up the attack time if you need to enhance transients, and use a fairly short release time, but not so short that you hear pumping.
What you do in the high band depends on the result you're after. If the plan is to increase the density of the high end, use a higher ratio setting to push up the energy and possibly a faster attack if you need to prevent the transients from becoming too pronounced. High transients tend to decay faster than low frequencies, so you may be able to use a faster release time than you did in the mid-range, but listen carefully for any pumping effects or other unnatural gain changes, and increase the release time as necessary to get rid of them. If you're using a compressor that allows you to solo each of the frequency bands, setting the release time may be easier, but as a 'ball park' figure, use half the mid-range release time as a starting point.
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Even if you use a complex multi-band compressor such as the Waves C4, it's still worth following it with a dedicated peak limiter, such as the Waves L1, to avoid digital overloads. |
So far I've talked mostly about compressing complete mixes at the mastering stage, and the same criteria apply to mixing if you're processing subgroups of two or more instruments. However, there's no reason not to use multi-band compression on individual tracks if you want to. I must admit that I seldom use multi-band compression on single tracks, because the side effects of full-band compression are often musically flattering to single sounds, but where you need gain control without obvious compression, a multi-band model is more likely to deliver the results you need. In this case, I'd tend to start with identical settings for all three bands and set the values in a similar way as when using a full-band model. The main difference is that you probably won't have to set an artificially long attack time to keep transients intact. Complex instruments, such as acoustic pianos or drum kits miked with a limited number of microphones, probably benefit most from multi-band compression, but I know engineers who also use them on vocals or sources like guitar and wind instruments.
While compressing mixes, we tend to keep the crossover point away from the mid-band, but different instruments may require different settings. For example, a bass guitar might benefit from a low crossover point set to around 50 or 60Hz so you can control the really deep bass more precisely. At the same time, the upper crossover point could be reduced to just a few hundred hertz as the 50Hz to 800Hz band is quite critical to the way a bass guitar sounds.
Above that, the energy tails off quite significantly, so the high band (800Hz and up) is quite capable of looking after the rest. If you feel the sound has got a boxy coloration that needs dealing with, then you can move the bands in closer to bracket the offending area and treat it differently to the bands either side, either by pulling its level down, or by applying less compression so as to avoid increasing its energy content. As with any conventional compressor, the more compression you apply, the more make-up gain you need to get the peak level back to where it was, but you have the option of using less make-up gain in a specific band if you want to suppress the contribution of that band to the overall sound.
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