Audio compression is a fairly simple, yet often misunderstood process. At its simplest a compressor reduces the difference between the loudest and quietest levels in a signal (or piece of music). It does this by reducing the gain (volume) of the louder parts of the signal without changing the quieter parts.
It is this ability to leave the quieter parts untouched that makes it different from merely adjusting the overall volume of the signal. If you simply used the volume control to reduce the louder parts of the signal, then the quieter parts would become even quieter. Similarly, if you use the volume control to raise the quiet parts in volume then the louder parts would also be raised, becoming too loud.
The level above which the compressor operates is known as the threshold. If the signal level rises above the threshold, then the compressor will kick in and reduce the level of the signal back down towards the threshold level. If, however, the signal level remains below the threshold then the compressor will do nothing.
The diagram above shows an audio signal on a graph. The signal level (loudness) is shown in dB (decibels) on the vertical axis and time passes along the horizontal axis. The red lines show the threshold for the compressor. Only when the signal goes beyond the red lines will these peaks will be compressed. The blue line (-7dB) highlights a peak in the signal that is below the threshold.
The ratio is the amount of compression applied. This is expressed as two numbers (e.g. 3:1 or 4:1 etc.). The first number shows by how much the incoming signal must increase before the compressor will allow the outgoing signal to increase by the amount of the second number.
The diagram above shows the effect of different compression ratios on the output level of a signal as it rises above the threshold. A 4:1 ratio means that the incoming signal must increase by 4 dB to get an increase of 1 dB at the output. The higher the ratio, the more that loud signals will be reduced in volume.
The diagram above shows the result of compression. The threshold has been set to 10 dB (so that only signals above 10dB are affected) and the ratio is set to 4:1. This means an input signal of 14 dB (the threshold value + 4 dB) will be reduced to 11 dB (the threshold value + 1 dB).
Note that the compressed signal is allowed to go above the 10dB threshold and that the signal peak at –7dB is not changed.
This is the clever bit. After the signal has been compressed the overall gain (volume) of the signal can be increased so that the loudest part of the compressed signal is restored back to its original level. This is applied to the whole signal so the end result is that the loud sections are restored to (or near) their original volume but the quiet sections become louder.
The diagram above shows the compressed signal after make-up gain has been applied.
The volume of the loudest parts of the signal have been restored to original levels, but note that the peak originally at 7dB is now also louder as the make-up gain has been applied to the whole signal.
The result of this is that the loudest peak of the compressed signal is no louder than the original signal but the quieter parts of the signal are louder. This signal will sound punchier and more consistent in volume, with less variation in loudness.
The attack time defines how quickly the compressor will act and reduce the volume of any signals above the threshold level. With a fast attack the signal is compressed almost immediately whereas a slower attack will allow the initial loud pulse to pass through before the level is reduced.
The release time controls how quickly the compressor will stop working and release the signal back to its original level after it has dropped below the threshold. If the release is too fast then the compressor may “pump” or “breathe” up and down as the compressor comes on and off quickly in time with the music.
Fully featured compressors may have a control to adjust the “knee” of the compressor. A “hard knee” compressor applies the full amount a gain reduction as soon as the signal level exceeds the threshold. A “soft knee” compressor progressively applies gain reduction before the threshold, as the signal approaches the threshold, producing a softer more subtle effect.
Low frequency elements of a sound tend to be higher in levels and will often reach the threshold and trigger the compressor, but any high frequencies that occur at the same time will also be compressed. Some compressors have enhancer circuitry that boosts these higher frequencies to compensate for any loss in brightness.
Many compressors, especially stomp-box ones, will often have preset attack and release times, or will combine the operation of several parameters within one control to simplify operation. Likewise, some devices will automatically apply make-up gain to compensate for the reduction in the compressed signal so that the overall output level is always restored.
A Limiter is simply a compressor set up with an infinity:1 ratio and a very fast attack time. This means that no amount of input signal will allow the output signal to rise above the threshold. The output is effectively limited to the level set by the threshold control.
Limiters are often used as protection to avoid signal overload into a device. The threshold will be set to a level that the device can safely handle and the limiter will prevent the signal ever exceeding that level.
So what effect does a compressor have on your bass sound ?
Generally the effect is usually described as an “evening out” of the overall sound and an increase in “punch”– but note that fret noise and other unwanted background noise might also be increased.
Harmonic undertones of a note will usually be well below the sound level of the fundamental note. A compressor can increase the level of these harmonics making them louder and thickening up the sound and making it more even and “punchy”.
If you play slap bass then the slaps and pops will often be louder and leap out more than normal bass notes. A compressor can reduce the difference in volume between these so that the overall sound is more even.
A comprehensive review of compressors with handy faq's is available at http://www.ovnilab.com/
