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SESSION NOTES 13 “Mastering” (part 3 “Compressors & Limiters”)



Hi folks, this month I want to look at two of the most powerful processors in the mastering chain, compressors & limiters. They are usually combined into one piece of “outboard” gear in most studios but as software processors they are separated in to individual “plug-ins”. They are in fact two distinctly different things and are used to control dynamic range. Simply put dynamic range is the difference between the quietest and loudest signal levels. Control of these levels is essential. You can use them creatively or as we shall find out later … destructively. When tracking in the studio compressors are used to prevent overload of the signal path due to the nature of transients. Transients are those huge bursts of acoustic energy that happen over a VERY short space in time at the front of a waveform. Examples of these are snare & bass drum hits along with hard edged guitar chords and brass instruments. Compressors are also used creatively to sculpt a particular sound or to add some cohesiveness to the mix or “glue” as some engineers are prone to say.


The typical controls on a compressor after the input level are “threshold”. This is the point that the compressor starts working. Any sound that goes beyond the threshold will be compressed. Next up is “attack” … this determines how fast the compressor will respond to the signal once the threshold has been breached. If you want to tame those huge snare hits you will set this to a very short time, usually measured in single milliseconds, sometimes microseconds. Next up is the “ratio”. This can seem a bit confusing at first but the ratio determines the output in relation to the input. For example a 4:1 ratio will mean that for a 4db change at the input only a 1db change will occur at the output. Most compressors will have a variable ratio setting that will go up to 20:1 or even ∞(infinity) : 1. Next control is the “release”. The time it takes for the compressor to return to its starting point once the signal has fallen below the threshold. You can set this to be very quick or quite slow. Finally you have the “make-up gain”. When you compress signals you will end up with a quieter result and the “make-up” gain will restore that level.


Limiting (as the name suggests) limits peaks absolutely, to ensure no clipping or distortion of the signal. This is the ∞ : 1 setting of the ratio control.


So, how do these processes work in the mastering world? For a start, when mastering you are dealing with a finished stereo mix of a track not individual instruments. I always use VERY gentle compression settings when mastering audio. The idea is to impart some analogue valve warmth to the sound given that most material i work on has been digitally recorded and mixed on DAW’s (digital audio workstations). In some cases mainly with Hip Hop, R&B and Rap i will make the compressor “pump” to the beat using the attack and release times in relation to the tempo. Again it’s a subtle process. The unsubtle process begins with the dreaded limiter. This device in its software form is used by pretty much everyone as a “Loudness” machine. The more you pull down the threshold the louder the sound will be. This is how the loudness wars began in the mid 90’s. The sonic damage these limiters can cause is excruciating at best. In a lot of cases i receive audio for mastering that has been limited to within an inch of its life. The first thing i have to do before i can do any useful work is turn the level down. The waveforms look like pieces of LEGO when viewed on the screen. There is NO dynamic range and everything is at one punishing level. The true use of a limiter (there’s a clue in the name here) is to limit the peaks so there is no digital clipping. Thankfully there is the new LUFS (loudness units full scale) requirements for streaming platforms that ensure audio is mastered to sensible levels. Typically -14db.


So please don’t submit “crushed” mixes for mastering. As Bob Katz says in his mastering book “It’s not how loud you make it … it’s HOW you make it loud”.


Next month I’ll look at the fascinating world of equalisation or as it’s known commonly … EQ.

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