Home Studio: Mixing - Part 5

Introduction

In the previous blog entry we have introduced the concepts of sound dynamics and dynamic range. Now, we can start to talk about the tools that modify the dynamics of the sound, which are broadly used during mixing an masterizing tasks.

Compressor

Imagine that we did our mix (following the technique of the 3 dimensions, mentioned before) and we achieve some final volume similar to the picture below (click on the picture for full size).

We already discussed how this TT Dynamic Range meter works so, now, we should be able to understand this picture.
We can see that average RMS are around -27 dB, while peaks RMS are around -14 db, while the dynamic range is 12.5 and 10.5 dB, left and right channels, respectively.
Definitively, not a very commercial loudness. Our mix will sound very quiet, compared to any commercial mix.

We could simply move the fader control of the mix, raising the gain about 13.5 dB, to make those RMS to be placed around -13.5 dB, what will leave the mix with peaks RMS around -0.5 dB, more or less.

But, since we are talking about Peak RMS, some peaks will "overflow" that target Ceil of -0.5 dB, producing clipping. This can be solved with the use of a Limiter but, once again, we will clip the peaks, anyway (because the compression ratio of a limiter is infinite).

So, the natural solution would be to lower a bit the channel fader until we have no clipping anymore (that red light that lights from time to time), but this will lower again the average RMS of the program and, therefore, the loudness of our mix. So, we will achieve a nicely sounding mix (dynamic range enough) but, sounding clearly lower than the commercial mixes of our competitors.

The set of peaks of the attack phase of a track are being called transients. In a track or the whole mix, not all the peaks have the same height (volume). We could say that a very low percentage are the ones that "overflow" and, those are the ones that would force us to lower that fader, loosing the mix some strength.

The Compressor is a tool that "compresses" the peaks, that are over a certain loudness level, to make them suit a determined dynamic range. Even that this is its main function, it has several other functions, since it's able to alter the whole dynamic curve of the material being processed, accenting more those parts that we are interested on more.

Let see a compressor plugin, by example:

Not all compressors have same names for their controls, but all more or less (some have some more knob than others) have same basic controls.

The gain or make-up is a control that allows us to raise the overall loudness of every bit of the material before to start the compression task.
Following with the previous example:

Imagine that we would like to achieve a dynamic range of about 12 dB. We wanted to put our average RMS more or less around -12dB. Before starting to compress, we saw that the average RMS were around -27, therefore, we should raise everything 15 dB (27 - 12 = 15), so we would put that gain control on +15 dB.

If our Ceil was set at -0.5 dB, to avoid clipping and overs when translating the mix to lower resolutions (16 bits, MP3, etc), by raising the average RMS to -12 dB, we will have a dynamic range reduced to 11.5 dB and, the average peaks will "overflow".

The control named threshold determines which is the minimum loudness level that the sound must reach for that compressor to start compressing the peaks. Everything that falls below the threshold level remains uncompressed and with a higher gain (determined by the gain or makeup control) but, everything that is over the threshold will be compressed. How it will be compressed depends on the rest of controls that we are going to describe.

To determine the threshold loudness, the better is try and fail. If the threshold was very low, we will compress practically everything. If the threshold was very high, we will compress just the higher peaks.
The correct threshold is that loudness were the interesting things are happening in our mix. If we raise or lower the threshold we will notice that radically changes the result, the accent of the song. Therefore, it is of high importance to exactly determine that threshold value to give to our mix the exact accent we wanted to achieve.

The control named attack determines how long should the compressor to wait, from the exact instant that a signal crossed the threshold level until it starts to compress such a signal. If we lower the attack, the compressor works earlier, lowering every single peak. Overdone, we can lose the percussive characteristic of our mix.

With high attack settings, we will allow the most of transients to cross the threshold. We will retain the punch but, we will go back to the initial issue: some peaks will overflow.

Normally, we need short attacks to tame the peaks (by example excessive percussive sounds as a bass guitar, kick drum or snare) and, we use longer attacks to maintain the punch in tracks with weak punch.
The release control takes care of the last phase of the ADSR curve of the compression task. When we reach the threshold level, and the time established with the attack is over, the compression will take place and, this compression will take the time determined by the release control. Therefore, long releases add sustain to the material, while short releases allow to work just in the starting parts of the sound (attack phase). To give more sustain to a weak bass guitar, or to give it some density, we usually raise the release time.

The combination of attack and release controls, define how the peaks are being tamed and, they are able to give some punch or to smooth the material.

The Sonnox Dynamics compressor has one more additional control, named hold that works before the release time, in an analog way as we already discussed when we introduced the ADSHR curves. This control isn't so usual in other compressors.

Finally, we have the ratio of compression, that determines the proportion to which the original loudness will be reduced. In this image, we see a ratio o 3.02:1, that means that any peak over the threshold level will reduce its loudness 3.02 times. So, the excessive 3.02 dB will be converted to just 1.0 dB, 6.04 dB will be reduced to 2 dB, and so on.

Some compressors include some algorithms that handle in different ways the transition area around the threshold, often named the "knee" of the compression curve. You can see in the above picture, that for this example, we are using a soft 5 dB knee, which smooths nicely the compression effect in this transition area.

Even that the Compression concept seems to be unique, the way as it is being implemented by each designer is always different. Not all compressor react at the same speed, some introduce their own fingerprint to the sound (and very specially, tube compressors).

There are some mythical compressors, like the UA 1175 or the LA-2A (used in drum kits and bass guitars, by example), that clearly add some color to the signal. The Fairchild 670 (used in electric guitar tracks, by example) is also coloring the sound. There are many other mythical compressors, usually forming part of a famous channel strip or mixing board.

Optocompressors (with optical cells) are often used for vocals, because they have a faster reaction time.

At the end, there are several types of compressors, that react very differently. Some seem to work better for certain instrument tracks, while other are more universal.

For every track, try as many compressors as you have and, choose the one that brings to you the most satisfactory result for your needs.

In this last picture, we can see the results of the compressor, in the TT Dynamic Range meter.
We achieve to raise our average RMS around -12 dB (as we wanted).
We balanced both channels (-12.7 and -12.5, against the initial -12.5 and -10.5).
We lowered the peaks up to -0.2 dB.
And, we achieved a dynamic range of 11.7 and 12.1 dB (very close to our target 12 dB).

For sure, those numbers say nothing if we don't hear and like the resulting compression effect, but they help us to understand how are we using the compressor and, if we are overdoing or underdoing the effect, while achieving a reasonable dynamic range and, removing the excessive overflowing peaks.


The Limiter

The Limiter is a particular case o a compressor, which compression ratio takes an infinite value. As we saw above, one compressor can allow to some instantaneous peaks to escape (to maintain the punch) but, in our final mix, we need that no peaks can produce a digital clipping, to avoid introducing distortion in our final mix.
The limiter fixes a Ceil, or the maximum loudness that every single bit of sound can reach and, any signal exceeding such a Ceil, will be lowered up to the Ceil value.

As it is an specialized compressor, it shares great part of their functions (control knobs).

The input gain is the same gain or make-up control already discussed. We already saw the attack and release controls and the knee.
The most characteristic value of a limiter is the output level or Ceil. Any signal over such a value will be lowered down up to the Ceil value.
Simplifying a lot things, when there are two or more peaks in the same row, with same volume, overs and overs will occur when we transport the digital mix to other equipment or audio processors.
Many limiters aren't able to recognize such an overs and to react accordingly (by example, lowering more one of the three peaks in a row with same volume).

Brickwall Limiters have often special algorithms that analyze the input material with some advance (look ahead) to introduce the right corrections to avoid such an issues.
The limiter in the below picture has some other functions that we aren't going to discuss here, because they are uncommon to most of limiters, with the exception of the auto-gain control, that can be found in more cases.

Decompressors o expanders

The expanders are dynamics tools that try to do the inverse work to a compressor.
If the function of a compressor is to raise the average loudness, compressing the dynamic range; the expander tries to increase the dynamic range, lowering the average loudness and, increasing the difference between Peaks RMS and average RMS.

They are used often as an attempt to return back some dynamics to a material that was previously compressed in excess and, they are being used in that way just before applying a softer compression to the material.

Probably, expanders are more habitual during masterizing, by example, to decompress a recording that passed through the channel strip of a mixing board and, that resulted in an excessive compression.

Its uses can seem a tad arcane. First, you want to control compressors, before you can understand how to correctly decompress.


Gates

If the Limiter established the maximum Ceil for loudness, the Gate establishes the minimum floor.
The Gate ensures that only those signals with a loudness over its threshold will be accepted (they cross the gate).

This concept seems very good to, by example, avoid the floor noises of the amp to cross the gate but, the real thing is that a perfect control of a Gate is a real headache.

Shares with a compressor the threshold, the attack (time that the signal must be over the threshold to open the door) and the release (time that the gate will stay open when the sound falls below the threshold).

Tweaking such a controls is a delicate task and, you can even ruin the track, if the softer passages are very close to the floor noise.
But gates are being used for more creative things also. By example, to control the amount of snare "pap" that you send to the reverberation effect. We can control that just the loudest beats go to reverberation, avoiding that the quietest parts could create unnecessary echoes. This can help also to better define the beat.

Transient modifiers

They take nice names as transient modelers, transient designers among other similar names.
Their main goal is to alter just the transients (attack) of the material, without affecting the rest of the ADSR curve. They can reduce (without clipping) or enhance the peaks of the attack phase, increasing or reducing the punch of the sound.

As we have seen, a compressor could leave some peaks to escape, to maintain the punch. The transient modifiers are able to modify their volume and duration in time.

We will see again some controls, as the threshold, gain and ratio but, this time, the compression effect will be oriented to just the transients, not to the whole signal.

De-Esser

A de-esser is a ultra-specialized compressor. Instead of working over the whole material, it works over a very narrow band of frequencies. By default, they are programmed to work over the range of frequencies where the human voice produces that sibilant sound.
The goal is to detect such an "s" and to compress them, reducing their loudness, making them smoothly sounding.

Most of the channel strips in the professional studio mixing boards, or in professional preamps dedicated to vocals, can include a de-esser module.

Even that they were designed to get rid of sibilant sounds, their particular ability to compress just certain range of frequencies, make them very useful to accurately act over certain ugly noises (fret noise in bass guitars, by example), reducing their impact without heavily affecting the rest of the material.


Other dynamics tools

They will be always very specializes versions of a compressor.
There are multi-band compressors, that differentiate the material in 3 or more ranges of frequencies, allowing individual compression control for each range.

Other compressors will work over the stereo image, dividing the signal into ghost center and side signals, allowing individual compression to each part.

RNDigital has compressors even more Gothic.
The Dynamizer is some kind of multi-band compressor but, instead of to divide the material in several bands of frequencies, it works dividing the material in several bands of loudness, instead. So, we can compress very differently the quieter and the louder parts. It's a very interesting concept.

At the end, any tool able to alter the sound dynamic or its dynamic range is a dynamics tool.


To be continued...

Well, I think I've covered mostly everything.
One of the darkest areas of mixing is the use of dynamics tools and, I guess I was of help in this area.
As most of the dynamics tools are just specialized compressors, to clearly understand how any usual compressor works will help you to understand how those specialized versions work.

I think i will end this set of entries with some tricks to enhance your mixes, in the next entry. Stay tuned.