On bended knee
Mix bus compression would be fairly simple if only the ratio, attack, release and gain reduction played a role in defining the sound. Of course, there’s a lot more to it than that. Next, we need to consider the way that the transition from uncompressed to compressed audio around the threshold point has a significant bearing on how noticeable the process is.
Traditionally this is fixed in the design, though nowadays there are many items of hardware and software that allow you to adjust this so-called ‘knee’ (named after the way it looks on input/output mapping graphs). A ‘soft’ knee rounds off the threshold point to make the transition to full compression softer.
Here’s how the same compressor sounds on a hard knee setting:
And with a soft knee:
It’s also essential to realise that not all compressors are created equal. Do you ever wonder why successful engineers regularly use more than one type of compressor, or why certain models of compressor are favoured for particular scenarios, even in the software realm? Classic hardware including the SSL G Series master compressor, Neve 33609 and API 2500 (and, indeed, emulations of these and similar models) remain popular choices for mix compression despite apparently being somewhat limited in comparison to newer alternatives.
Much of the characteristic sound of these models is down to the different methods and components used to sense the input level, and then instigate the gain reduction process (taken care of by analysis in the sidechain).
Even many free options found in DAWs – such as Logic’s Platinum Compressor shown here – allow the user to dial in a range of alternative flavours, based on various traditional hardware designs. Notice the way that, even with the same settings, the sound and shape of the compression varies greatly.
First off, here’s a demo using the default Platinum mode. This is a fairly dull, slow model without much character:
And then with the Opto mode, a very fast model based on classic optical compressors:
And finally VCA mode, with a coloured sound based on classic voltage-controlled attenuator circuits:
Increasingly, plugins are called on to emulate classic hardware units. In the case of mix bus compression, the most commonly emulated model is undoubtedly the SSL master compressor, used on countless great records. To many producers, the SSL is the sound of a hit record. Manufacturers including Universal Audio with their 4k Buss Compressor and Cytomic with The Glue have tried to capture this classic sound in software form.
Compression vs Limiting
A discussion of mix bus compression would not be complete without a word on limiting – the process that has probably contributed more to the so-called loudness war than any other. Limiting is in fact just an extreme form of dynamic compression with a ratio of 10:1 or greater and (very) fast attack times.
We’ll explore limiters in detail another time, but bear in mind that while high ratio compression can massively boost the perceived loudness of your track, it can also add distortion to your mix, destroy dynamic range and create an incredibly fatiguing sound. For mix bus compression rather than out and out loudness, it’s best to stick to lower ratios (i.e. below 10:1) and ensure that attack times are slow enough not to destroy the dynamics of your mix.
For the time being, try experimenting with different types of moderate compression. Think colour and shape – not loudness – and you should be on the right lines.
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