This is undoubtedly the most important processor. The compressor works on the dynamic range [Dynamic range ] of the input signal and reduces its amplitude when it goes beyond a certain limit. This reduction is expressed with a ratio; for example a 3:1 ratio means that when the signal goes over a certain threshold, the part of the signal above it gets reduced by 1/3:
On the left hand side of the previous figure we have the input signal before it gets compressed. Also we see the reference amplitudes measured in dBu and we notice that the signal's full dynamic range is 50 dB. The diagram also shows the threshold beyond which the compressor kicks into action. On the right hand diagram we see the result of a 3:1 compression. The part of the signal below the threshold has remained unvaried whilst the part above it has been reduced by 1/3, and has thus been lowered from 30 dB to 10 dB. The dynamic range of the overall signal has therefore been reduced from 50 dB to 30 dB.
Let's now take a detailed look at the compressor's controls:
Threshold: this value is expressed in dB and is the limit beyond which the compressor gets activated.
Ratio: quantifies the reduction in signal amplitude above the threshold. Some typical ratios are:
1:1 - no compression, the output signal is the same as the input signal.
2:1 - the signal above the threshold is halved. For example, if the signal goes above the threshold by 10 dB its value will be reduced to 5 dB.
other values include: 3:1, 4:1 etc. For values higher than 10:1, the compressor behaves practically like a limiter [Limiter ] .
In the following diagram a compression curve of a compressor is shown, for different compression ratio values:
The diagram shows the amplitude of the output signal in relation to the input signal. We can see how up to the threshold value the signal's amplitude is the same as that of the input signal. Beyond the threshold, compression takes place according to the set ratio.
Attack time: indicates the time taken by the compressor to be activated after the signal has gone beyond its threshold. It is stated in milliseconds. In the following diagram two situations, one with a short and with a long attack time, are compared.
Having a long attack time means that the signal has gone beyond the preset threshold, but won't get compressed until the attack time has passed. Once the attack time has passed, the compressor reduces the signal's amplitude: this results in the initial part of sounds being highlighted.
Let's consider a bass drum whose sound envelope [Sound envelope ] initially has the form indicated in green:
By applying compression, the envelope becomes the one indicated in red. This strongly highlights the bass drum's attack, giving it a sharper sound.
Two opposite examples of the sound of a bass drum can be found in techno music and jazz. In techno music the sound of the bass drum must be very sharp, dry, aggressive, and therefore a high degree of compression takes place (4:1, for example) with a slow attack time (100 ms, for example). In jazz music the sound of the bass drum can be considered almost like an actual instrument, and therefore has a long tail sound, almost like a booming. In this case we'd use a lighter compression ratio (2:1, for example) and a very brief attack time (10 ms) to capture the whole sound envelope. For physical reasons, it is impossible to produce analogue compressors which have a very short or a complete lack of attack time. This is because circuits by their very nature have a reaction time every time the signal varies. An attack time equal to zero can be simulated on a sampled signal which has been stored in a RAM: in this case the compressor already knows the whole of the rate of the signal it will manipulate, and it is therefore possible to operate with zero attack time, although not in real-time.
Release time: this is the time taken by the compressor to return to absence of compression after the input signal has returned below the threshold, in other words to a 1:1 ratio. Its purpose is to soften the compressor's action.
Hold time: after the input signal's amplitude has returned beneath the threshold, the compressor reduces its action during release time until it reaches a 1:1 compression ratio. The hold time allows release time to be delayed after the signal has gone beneath the threshold. It practically keeps the compressor activated for longer.
In the following diagram the overall action of a compressor is demonstrated:
Let's now listen to the pure sound of a bass drum (the one you play with a pedal) followed by the same sound passed through a compressor which modifies its ADSR envelope:
To get a better idea of the compressor's effects it is useful to observe its actions on the ADSR envelope. In the following figure we see the envelope of a bass drum beat extracted from the previous sound, followed by the very same envelope after it has undergone compression. A comparison between the two figures clearly highlights the compression operation.
The compressor circuit can be considered as a voltage-controlled amplifier whereby the controlling voltage is the input signal. If the input signal's voltage goes beyond the threshold, the compressor is activated. It is not necessary for the compressor to be controlled by the input signal's voltage, any controlling signal will do. This peculiar trait of compressors opens the doors to many interesting techniques.
Let's see an example: when a bass drum and a bass guitar note are played at the same time (which is quite often the case, since the rhythm section of a band should always be tight), in particular on even beats (1 and 3 in 4/4 notation music), their similar frequency content causes them to be easily confused with one another. Let's see how we can bring out the sound of the bass drum in the moment in which it is kicked. Firstly let's compress the bass drum, as we saw earlier, with a high compression ratio and a slow attack time, with the intent of emphasizing the bass drum's attack, its "punch". Then let's take another compressor and let's apply it to the bass guitar's signal, using the signal from the bass drum as a sidechain input. This lowers the bass guitar's volume when the bass drum is kicked and therefore the sound of the latter will be the predominant one of the two. After the attack, the compressor goes into its release phase, which means that the bass guitar's volume gently increases: when the bass drum's sound is gone, the compressor ceases its action and the bass guitar returns to its original volume.
We've seen how the shape of a compression curve changes as compression ratios vary. This kind of curve is called hard knee is characterized by rapid gain slope variations. Another mode called soft knee has a mellower slope variation and softens the compressor's action. The following are the two modes of a compression curve:
Compressors act upon the signal depending on the input voltage's rate in the two following ways:
Peak: the compressor responds to signal peaks and therefore measures exactly the amplitude of the input voltage.
RMS: the compressor responds to the RMS (Root Mean Square) of the signal, in other words its effective value, thereby softening its action and making it less jerky.
For these kind of devices, when no compression takes place, the compression curve is a unity gain straight line. When the compression curve is rotated we notice how above the threshold a compression takes place whilst the signal below the threshold gets expanded (amplified):
This device subdivides the signal into frequency bands and applies a different compression on each of the bands. To do so, the module includes a crossover circuit [The crossover ] which subdivides the signal into separate bands before it gets compressed. Every output of the crossover is sent to a separate compressor, each of which has its independent controls.
This allows compressions to be a lot more refined. Generally, high-frequency signals get compressed with rapid attack times and slower release times. This ensures that compression follows the input signal's characteristics more precisely.
The following diagram shows the signal's path within the multiband compressor:
[7 ] Low Frequency Oscillator is an oscillator capable of generating low-frequency waveforms (0 - 10 Hz)