Those new to audio engineering might assume that parallel compression is necessarily a form of upwards compression, though it depends on how the parallel compressor is set up. The vast majority of compressors apply downward compression which means, in essence, that loud stuff is made quieter. Signals below the threshold level are left alone, while those above are compressed by an amount determined by the ratio setting. That said, a parallel compressor setup is actually a form of downward compressor, but it is unique in that it is able to behave exactly like an upwards compressor over a defined dynamic range.
PARALLEL COMPRESSIONA parallel compressor setup is actually a form of downward compressor, but it exhibits a unique and very useful characteristic that allows it to behave exactly like an upwards compressor over a defined dynamic range. The idea of parallel compression was conceived to achieve much the same goal as upwards compression. In other words, its aim is to leave the delicate loud transients intact while raising the level of low-level signals.The basic concept is to split the input signal to feed two paths, one being a direct 'through' path, and the other feeding a normal downward compressor. The compressor's output is mixed at unity gain with the direct path to produce the 'parallel compressed' signal.
The compressor is set up with a very high ratio and the threshold adjusted so that it provides a lot of gain reduction when the input signal is at its loudest. The more gain reduction the better so we need a compressor that doesn’t struggle or distort when applying a lot of gain. The attack and release controls are set to suit the material and the effect required. When it comes to input signals which fall below the compressor's threshold the compressor obviously won't be applying any gain reduction there will be identical signals via both the direct and compressor paths. If two identical signals are mixed together, their combined level is 6dB greater than that of each individual signal so this simple parallel compression setup raises the level of quiet signals by 6dB.
In terms of dynamic-range reduction, as this simple form of parallel compression leaves the loud bits unaffected and raises the quiet parts by 6dB, the total reduction in dynamic range is only 6dB. If more compression is required, the solution is to stack up more parallel compressors. Additional compression paths can be added for even more dynamic range reduction.
TYPE 1 - TRANSPARENT PARALLEL COMPRESSIONThere are two different approaches to parallel compression. The first one is the transparent approach in which the compressor is as invisible as possible, producing few if any tonal shifts and leaving transients unharmed. In many cases this technique is indistinguishable from manual fader riding. This transparent parallel compressor raises gain at very low levels and contributes less to the total sound as the signal gets hotter.
Threshold - Set the parallel compressor’s threshold very low perhaps around -40 dBFS. This will put the compressor into a constant state of gain reduction and will assure that the compressor will be applying heavy gain reduction during loud passages in particular. In theory if you add a second signal that is 20 dB or more below the main signal, this second component will not noticeably contribute to the total level or sound. Because the output of the parallel compressor is pushed significantly down during loud passages, it contributes only a small amount at high levels.
Attack - Set as fast as possible, 1ms or less if you are able. This will preserve the transient impact of the original sound: as soon as a loud transient hits, the compressor will instantly go into gain reduction. The faster the attack, the more invisible the parallel compressor, and more transients are preserved.
Ratio - Set the ratio low, to either 2:1 or 2.5:1 at the most. This is the root setting of the compressor, but actual achieved ratio in parallel depends on the output level of the parallel compressor.
Release - Set the release time somewhere in the middle, typically a setting between 250-350ms works best and will help you avoid pumping the signal. Some material may require release times as long as 500ms. Pay attention to reverb tails to determine settings if you are uncertain.
Crest Factor - Make sure this is set to Peak and not RMS to assure transparency.
TYPE 2 - PUNCHY PARALLEL COMPRESSIONIn this second approach to parallel compression, we attempt to inject some attitude or punch into the mix, by clarifying the low and mid frequencies with a bit of warmth. By bringing up the mid levels, where the core of the music lives, we can produce a fat sound without harming the higher frequencies.
Threshold - Set in the middle of the musical action, with up to 5-7 dB of gain reduction.
Attack - Set to medium, around 125ms works to keep transients in tact. No look-ahead necessary.
Ratio - Set to taste, somewhere between 1.3:1 and 3:1 depending on the material.
Release - Set to taste, usually in concert with the attack time and the music’s movement, to obtain rhythm and punch.
Crest Factor - Set to RMS and not Peak.