What is compression and how do you use it?

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What is compression and how do you use it?

Compression-full band

The simplest way to describe the function of a compressor is as follows: a compressor makes the quiet bits louder and the loud bits quieter. It acts as an automatic volume control.

In the old days…ah, dem days, we used to adopt a technique called ‘gain riding’. Gain was a nice Welsh girl that….no seriously though.

Gain riding: When you had audio passing through the mixer, for example: vocals, we used to pull the volume (gain) fader down when the vocals were being sung too loud, and push it up when the vocals was dropping in level. But this was never too accurate, as you had to anticipate when the vocals would rise or drop in gain.

So, enter the compressor.

There was much joy to be had. Grapes came out, wanton wenches were brought in, and the odd cow was slaughtered and barbecued there, right in front of the Neve. It was truly a happy time for studio engineers.

By using the compressor, we were able to achieve a far more consistent and stable gain across the audio. By using the threshold level, we were able to control when the compressor would kick into action, and the amount of compression (ratio) to be used on the material. We were also able to set the speed of the compressor kicking in (attack) and how quickly for it to go back to normal (release). Add to that the ability to set the amount of gain reduction, and we had a potent tool.

Today, compressors have become creative tools as well as dynamic controllers, and it is fair to say, at least in the commercial dance markets, that using a compressor is a must.

Unfortunately, the compressor is also being abused. It has become a ‘boost’ tool as opposed to a dynamic controller. I often see audio data that is so compressed that it breaks down into a square wave due to so much compression being applied.

Please look at the following figure of what happens when audio is compressed.
Compression: hard and soft knee.

Please apply the information below to the figure above.

You will soon get to grips as to what is going on.

Let us understand and thus use the compressor for what it should be used for, and to do this, let us take a closer look at it’s features.

Threshold

This is the input level above which compression occurs. Above this level, the output increases at a lesser rate than the corresponding input stage.
Set the threshold high to compress only the loudest part of the signal, set it low to compress more of the signal.

Ratio

This is the ratio of the change in input level to the change in output level.
For example, a 2:1 ratio means that for every 2dB change in input level, the output changes 1dB. A ‘soft-knee’ characteristic is a low compression ratio for low-level signals and a high ratio for high-level signals.
With infinite setting, the output stays the same no matter how much you increase the input.

Attack

This is how fast the compressor reduces the gain when a signal is input.
Basically, the time it takes to kick in.
Longer attack times mean that more of the signal goes through, before it starts to get compressed.

Release

This is how fast the compressor returns to neutral, or how fast the gain returns to normal. Short release times give the famous ‘pumping’ or ‘breathing’ sound, and are good for following rapid gain changes.
Long release times sound more natural and don’t interfere with the sound’s harmonics.

Gain Reduction

This is the number of dB that the gain is reduced by the compressor, and varies with the input level. This is displayed on the meter.

Side Chain

Mainly available on hardware compressors, and is used for inserting an EQ or filter, or any device, into the signal path, independently of the main input signal, so that the compressor responds only to frequencies boosted by the input device/signal, in this case the EQ.
The EQ does not affect the actual input signal...only the controls of the unit.....the controls then adjust the actual Main Input Signal
They can also be used creatively and make for some great effects.

Output Control or Gain Makeup

Because we are squashing peaks in the signal, we are actually reducing the overall peak level, increasing the output level compensates for the volume drop. Turn this level up until the peak levels of the compressed signal match the bypassed signal peaks.

Peak/RMS

RMS stands for Root Mean Square and is a mathematical term for a method of averaging the level of a complex waveform. If your compressor has a Peak/RMS switch, this will determine how the compressor evaluates the incoming sound level and your choice for selection is dependent on the type of material you will be compressing.
The beauty of using RMS is that we, as humans (some of us anyway), tend to use this method for listening. Our ears average out incoming audio, so RMS works in the same way.
But, as stated, the method chosen is dependent on the audio being processed. For short signals, such as drum sounds, Peak will work much better. In Peak mode, the compressor takes action based on the peak level of the input signal, no matter how long or short the sound is. In this instance, using a fast attack time and the Peak setting will afford far better sonic control over the audio than RMS I tend to find that RMS works really well on longer, undulating sounds, like vocals, and Peak works well on short sounds, like percussion.

Knee

Knee refers to the way the compressor reacts when the input level reaches the threshold. A hard-knee compressor brings in all the gain reduction as soon as the signal crosses the threshold.
A soft knee on the other hand brings in the compression more progressively by gradually increasing the compression ratio as the signal level approaches the threshold. Again, the decision in choice is down to the material being processed.

Now let us take a brief look at the different types of compressors.

Stereo/Dual Channel Compressor

If a compressor is to be used on a stereo track, it is important that a stereo compressor or dual channel compressor be used. Dual channel compressors feature a stereo link switch that effectively sums the two channel levels and then uses this combined signal to control both channels. In this way, the same gain reduction is applied to both channels at all times. If the two sides worked independently, then the compressor would sound as if it were shifting from side to side, as audio will vary in loudness from channel to channel. When linked for stereo operation, both channels of the compressor react to a mix of the sound passing through the two channels so both always react together, regardless of the level balance between the two channels.

Multiband Compressor

These divide the incoming audio signal into multiple bands, with each band being compressed independently from the other.
The beauty of this is that with full band compressors, that we have been discussing till now, the whole signal is treated, so when a peak is detected, the whole signal is compressed and so other frequencies are also subject to compression.

Multiband compression only compresses the frequency bands chosen, so a more fluid and less abrupt result is gained. Instead of having one peak trigger the compressor into compressing the entire signal, the multiband allows for individual bands to be compressed.
On some compressors, you even have the option of selecting bands that will not undergo any treatment.

In essence, a multi-band compressor comprises a set of filters that splits the audio signal into two or more frequency bands.
After passing through the filters, each frequency band is fed into its own compressor, after which the signals are recombined at the output.
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.
Another feature of the multiband compressor is that you are offered crossover points. This is crucial, as you are given control over where to place the frequency band. Setting these crossover points is the heart of the compressor and crucial in processing the right frequency spectrum with the right settings.
For example: if you are treating the vocals in the mid range but put your low end crossover too far into the middle range, then the low end compression settings will also affect the mid range vocals.

Limiter

A limiter keeps signal peaks from exceeding a pre determined level. While a compressor reduces the overall dynamic range, a limiter affects only the highest peaks. Limiters have very fast attack times, very high compression ratios and a high threshold.
You can turn your compressor into a limiter by using a very high threshold and ratio.

The ‘classic’ definition is that a limiter ‘flattens’ all peaks above a certain level, but leaves lower-level sounds intact.

I think this is enough as far as compressors are concerned, until we come to practical uses, and it is then that we will discuss Upward Compression, Parallel Compression and Noise.

Excerpt taken from Mixing Simplified e-book. Get your copy here.

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